\input texinfo @c -*-texinfo-*- @c $NetBSD: awk.texi,v 1.1 2010/12/13 06:21:53 mrg Exp $ @c %**start of header (This is for running Texinfo on a region.) @setfilename awk.info @settitle The GNU Awk User's Guide @c %**end of header (This is for running Texinfo on a region.) @dircategory Text creation and manipulation @direntry * Gawk: (awk). A text scanning and processing language. @end direntry @dircategory Individual utilities @direntry * awk: (awk)Invoking gawk. Text scanning and processing. @end direntry @set xref-automatic-section-title @c The following information should be updated here only! @c This sets the edition of the document, the version of gawk it @c applies to and all the info about who's publishing this edition @c These apply across the board. @set UPDATE-MONTH June, 2003 @set VERSION 3.1 @set PATCHLEVEL 3 @set FSF @set TITLE GAWK: Effective AWK Programming @set SUBTITLE A User's Guide for GNU Awk @set EDITION 3 @iftex @set DOCUMENT book @set CHAPTER chapter @set APPENDIX appendix @set SECTION section @set SUBSECTION subsection @set DARKCORNER @inmargin{@image{lflashlight,1cm}, @image{rflashlight,1cm}} @end iftex @ifinfo @set DOCUMENT Info file @set CHAPTER major node @set APPENDIX major node @set SECTION minor node @set SUBSECTION node @set DARKCORNER (d.c.) @end ifinfo @ifhtml @set DOCUMENT Web page @set CHAPTER chapter @set APPENDIX appendix @set SECTION section @set SUBSECTION subsection @set DARKCORNER (d.c.) @end ifhtml @ifxml @set DOCUMENT book @set CHAPTER chapter @set APPENDIX appendix @set SECTION section @set SUBSECTION subsection @set DARKCORNER (d.c.) @end ifxml @c some special symbols @iftex @set LEQ @math{@leq} @end iftex @ifnottex @set LEQ <= @end ifnottex @set FN file name @set FFN File Name @set DF data file @set DDF Data File @set PVERSION version @set CTL Ctrl @ignore Some comments on the layout for TeX. 1. Use at least texinfo.tex 2000-09-06.09 2. I have done A LOT of work to make this look good. There are `@page' commands and use of `@group ... @end group' in a number of places. If you muck with anything, it's your responsibility not to break the layout. @end ignore @c merge the function and variable indexes into the concept index @ifinfo @synindex fn cp @synindex vr cp @end ifinfo @iftex @syncodeindex fn cp @syncodeindex vr cp @end iftex @ifxml @syncodeindex fn cp @syncodeindex vr cp @end ifxml @c If "finalout" is commented out, the printed output will show @c black boxes that mark lines that are too long. Thus, it is @c unwise to comment it out when running a master in case there are @c overfulls which are deemed okay. @iftex @finalout @end iftex @copying Copyright @copyright{} 1989, 1991, 1992, 1993, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003 Free Software Foundation, Inc. @sp 2 This is Edition @value{EDITION} of @cite{@value{TITLE}: @value{SUBTITLE}}, for the @value{VERSION}.@value{PATCHLEVEL} (or later) version of the GNU implementation of AWK. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with the Invariant Sections being ``GNU General Public License'', the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the section entitled ``GNU Free Documentation License''. @enumerate a @item ``A GNU Manual'' @item ``You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.'' @end enumerate @end copying @c Comment out the "smallbook" for technical review. Saves @c considerable paper. Remember to turn it back on *before* @c starting the page-breaking work. @c 4/2002: Karl Berry recommends commenting out this and the @c `@setchapternewpage odd', and letting users use `texi2dvi -t' @c if they want to waste paper. @c @smallbook @c Uncomment this for the release. Leaving it off saves paper @c during editing and review. @c @setchapternewpage odd @titlepage @title @value{TITLE} @subtitle @value{SUBTITLE} @subtitle Edition @value{EDITION} @subtitle @value{UPDATE-MONTH} @author Arnold D. Robbins @c Include the Distribution inside the titlepage environment so @c that headings are turned off. Headings on and off do not work. @page @vskip 0pt plus 1filll @ignore The programs and applications presented in this book have been included for their instructional value. They have been tested with care but are not guaranteed for any particular purpose. The publisher does not offer any warranties or representations, nor does it accept any liabilities with respect to the programs or applications. So there. @sp 2 UNIX is a registered trademark of The Open Group in the United States and other countries. @* Microsoft, MS and MS-DOS are registered trademarks, and Windows is a trademark of Microsoft Corporation in the United States and other countries. @* Atari, 520ST, 1040ST, TT, STE, Mega and Falcon are registered trademarks or trademarks of Atari Corporation. @* DEC, Digital, OpenVMS, ULTRIX and VMS are trademarks of Digital Equipment Corporation. @* @end ignore ``To boldly go where no man has gone before'' is a Registered Trademark of Paramount Pictures Corporation. @* @c sorry, i couldn't resist @sp 3 Published by: @sp 1 Free Software Foundation @* 59 Temple Place --- Suite 330 @* Boston, MA 02111-1307 USA @* Phone: +1-617-542-5942 @* Fax: +1-617-542-2652 @* Email: @email{gnu@@gnu.org} @* URL: @uref{http://www.gnu.org/} @* @c This one is correct for gawk 3.1.0 from the FSF ISBN 1-882114-28-0 @* @sp 2 @insertcopying @sp 2 Cover art by Etienne Suvasa. @end titlepage @c Thanks to Bob Chassell for directions on doing dedications. @iftex @headings off @page @w{ } @sp 9 @center @i{To Miriam, for making me complete.} @sp 1 @center @i{To Chana, for the joy you bring us.} @sp 1 @center @i{To Rivka, for the exponential increase.} @sp 1 @center @i{To Nachum, for the added dimension.} @sp 1 @center @i{To Malka, for the new beginning.} @w{ } @page @w{ } @page @headings on @end iftex @iftex @headings off @evenheading @thispage@ @ @ @strong{@value{TITLE}} @| @| @oddheading @| @| @strong{@thischapter}@ @ @ @thispage @end iftex @ifnottex @ifnotxml @node Top @top General Introduction @c Preface node should come right after the Top @c node, in `unnumbered' sections, then the chapter, `What is gawk'. @c Licensing nodes are appendices, they're not central to AWK. This file documents @command{awk}, a program that you can use to select particular records in a file and perform operations upon them. @insertcopying @end ifnotxml @end ifnottex @menu * Foreword:: Some nice words about this @value{DOCUMENT}. * Preface:: What this @value{DOCUMENT} is about; brief history and acknowledgments. * Getting Started:: A basic introduction to using @command{awk}. How to run an @command{awk} program. Command-line syntax. * Regexp:: All about matching things using regular expressions. * Reading Files:: How to read files and manipulate fields. * Printing:: How to print using @command{awk}. Describes the @code{print} and @code{printf} statements. Also describes redirection of output. * Expressions:: Expressions are the basic building blocks of statements. * Patterns and Actions:: Overviews of patterns and actions. * Arrays:: The description and use of arrays. Also includes array-oriented control statements. * Functions:: Built-in and user-defined functions. * Internationalization:: Getting @command{gawk} to speak your language. * Advanced Features:: Stuff for advanced users, specific to @command{gawk}. * Invoking Gawk:: How to run @command{gawk}. * Library Functions:: A Library of @command{awk} Functions. * Sample Programs:: Many @command{awk} programs with complete explanations. * Language History:: The evolution of the @command{awk} language. * Installation:: Installing @command{gawk} under various operating systems. * Notes:: Notes about @command{gawk} extensions and possible future work. * Basic Concepts:: A very quick intoduction to programming concepts. * Glossary:: An explanation of some unfamiliar terms. * Copying:: Your right to copy and distribute @command{gawk}. * GNU Free Documentation License:: The license for this @value{DOCUMENT}. * Index:: Concept and Variable Index. @detailmenu * History:: The history of @command{gawk} and @command{awk}. * Names:: What name to use to find @command{awk}. * This Manual:: Using this @value{DOCUMENT}. Includes sample input files that you can use. * Conventions:: Typographical Conventions. * Manual History:: Brief history of the GNU project and this @value{DOCUMENT}. * How To Contribute:: Helping to save the world. * Acknowledgments:: Acknowledgments. * Running gawk:: How to run @command{gawk} programs; includes command-line syntax. * One-shot:: Running a short throwaway @command{awk} program. * Read Terminal:: Using no input files (input from terminal instead). * Long:: Putting permanent @command{awk} programs in files. * Executable Scripts:: Making self-contained @command{awk} programs. * Comments:: Adding documentation to @command{gawk} programs. * Quoting:: More discussion of shell quoting issues. * Sample Data Files:: Sample data files for use in the @command{awk} programs illustrated in this @value{DOCUMENT}. * Very Simple:: A very simple example. * Two Rules:: A less simple one-line example using two rules. * More Complex:: A more complex example. * Statements/Lines:: Subdividing or combining statements into lines. * Other Features:: Other Features of @command{awk}. * When:: When to use @command{gawk} and when to use other things. * Regexp Usage:: How to Use Regular Expressions. * Escape Sequences:: How to write nonprinting characters. * Regexp Operators:: Regular Expression Operators. * Character Lists:: What can go between @samp{[...]}. * GNU Regexp Operators:: Operators specific to GNU software. * Case-sensitivity:: How to do case-insensitive matching. * Leftmost Longest:: How much text matches. * Computed Regexps:: Using Dynamic Regexps. * Locales:: How the locale affects things. * Records:: Controlling how data is split into records. * Fields:: An introduction to fields. * Nonconstant Fields:: Nonconstant Field Numbers. * Changing Fields:: Changing the Contents of a Field. * Field Separators:: The field separator and how to change it. * Regexp Field Splitting:: Using regexps as the field separator. * Single Character Fields:: Making each character a separate field. * Command Line Field Separator:: Setting @code{FS} from the command-line. * Field Splitting Summary:: Some final points and a summary table. * Constant Size:: Reading constant width data. * Multiple Line:: Reading multi-line records. * Getline:: Reading files under explicit program control using the @code{getline} function. * Plain Getline:: Using @code{getline} with no arguments. * Getline/Variable:: Using @code{getline} into a variable. * Getline/File:: Using @code{getline} from a file. * Getline/Variable/File:: Using @code{getline} into a variable from a file. * Getline/Pipe:: Using @code{getline} from a pipe. * Getline/Variable/Pipe:: Using @code{getline} into a variable from a pipe. * Getline/Coprocess:: Using @code{getline} from a coprocess. * Getline/Variable/Coprocess:: Using @code{getline} into a variable from a coprocess. * Getline Notes:: Important things to know about @code{getline}. * Getline Summary:: Summary of @code{getline} Variants. * Print:: The @code{print} statement. * Print Examples:: Simple examples of @code{print} statements. * Output Separators:: The output separators and how to change them. * OFMT:: Controlling Numeric Output With @code{print}. * Printf:: The @code{printf} statement. * Basic Printf:: Syntax of the @code{printf} statement. * Control Letters:: Format-control letters. * Format Modifiers:: Format-specification modifiers. * Printf Examples:: Several examples. * Redirection:: How to redirect output to multiple files and pipes. * Special Files:: File name interpretation in @command{gawk}. @command{gawk} allows access to inherited file descriptors. * Special FD:: Special files for I/O. * Special Process:: Special files for process information. * Special Network:: Special files for network communications. * Special Caveats:: Things to watch out for. * Close Files And Pipes:: Closing Input and Output Files and Pipes. * Constants:: String, numeric and regexp constants. * Scalar Constants:: Numeric and string constants. * Nondecimal-numbers:: What are octal and hex numbers. * Regexp Constants:: Regular Expression constants. * Using Constant Regexps:: When and how to use a regexp constant. * Variables:: Variables give names to values for later use. * Using Variables:: Using variables in your programs. * Assignment Options:: Setting variables on the command-line and a summary of command-line syntax. This is an advanced method of input. * Conversion:: The conversion of strings to numbers and vice versa. * Arithmetic Ops:: Arithmetic operations (@samp{+}, @samp{-}, etc.) * Concatenation:: Concatenating strings. * Assignment Ops:: Changing the value of a variable or a field. * Increment Ops:: Incrementing the numeric value of a variable. * Truth Values:: What is ``true'' and what is ``false''. * Typing and Comparison:: How variables acquire types and how this affects comparison of numbers and strings with @samp{<}, etc. * Boolean Ops:: Combining comparison expressions using boolean operators @samp{||} (``or''), @samp{&&} (``and'') and @samp{!} (``not''). * Conditional Exp:: Conditional expressions select between two subexpressions under control of a third subexpression. * Function Calls:: A function call is an expression. * Precedence:: How various operators nest. * Pattern Overview:: What goes into a pattern. * Regexp Patterns:: Using regexps as patterns. * Expression Patterns:: Any expression can be used as a pattern. * Ranges:: Pairs of patterns specify record ranges. * BEGIN/END:: Specifying initialization and cleanup rules. * Using BEGIN/END:: How and why to use BEGIN/END rules. * I/O And BEGIN/END:: I/O issues in BEGIN/END rules. * Empty:: The empty pattern, which matches every record. * Using Shell Variables:: How to use shell variables with @command{awk}. * Action Overview:: What goes into an action. * Statements:: Describes the various control statements in detail. * If Statement:: Conditionally execute some @command{awk} statements. * While Statement:: Loop until some condition is satisfied. * Do Statement:: Do specified action while looping until some condition is satisfied. * For Statement:: Another looping statement, that provides initialization and increment clauses. * Switch Statement:: Switch/case evaluation for conditional execution of statements based on a value. * Break Statement:: Immediately exit the innermost enclosing loop. * Continue Statement:: Skip to the end of the innermost enclosing loop. * Next Statement:: Stop processing the current input record. * Nextfile Statement:: Stop processing the current file. * Exit Statement:: Stop execution of @command{awk}. * Built-in Variables:: Summarizes the built-in variables. * User-modified:: Built-in variables that you change to control @command{awk}. * Auto-set:: Built-in variables where @command{awk} gives you information. * ARGC and ARGV:: Ways to use @code{ARGC} and @code{ARGV}. * Array Intro:: Introduction to Arrays * Reference to Elements:: How to examine one element of an array. * Assigning Elements:: How to change an element of an array. * Array Example:: Basic Example of an Array * Scanning an Array:: A variation of the @code{for} statement. It loops through the indices of an array's existing elements. * Delete:: The @code{delete} statement removes an element from an array. * Numeric Array Subscripts:: How to use numbers as subscripts in @command{awk}. * Uninitialized Subscripts:: Using Uninitialized variables as subscripts. * Multi-dimensional:: Emulating multidimensional arrays in @command{awk}. * Multi-scanning:: Scanning multidimensional arrays. * Array Sorting:: Sorting array values and indices. * Built-in:: Summarizes the built-in functions. * Calling Built-in:: How to call built-in functions. * Numeric Functions:: Functions that work with numbers, including @code{int}, @code{sin} and @code{rand}. * String Functions:: Functions for string manipulation, such as @code{split}, @code{match} and @code{sprintf}. * Gory Details:: More than you want to know about @samp{\} and @samp{&} with @code{sub}, @code{gsub}, and @code{gensub}. * I/O Functions:: Functions for files and shell commands. * Time Functions:: Functions for dealing with timestamps. * Bitwise Functions:: Functions for bitwise operations. * I18N Functions:: Functions for string translation. * User-defined:: Describes User-defined functions in detail. * Definition Syntax:: How to write definitions and what they mean. * Function Example:: An example function definition and what it does. * Function Caveats:: Things to watch out for. * Return Statement:: Specifying the value a function returns. * Dynamic Typing:: How variable types can change at runtime. * I18N and L10N:: Internationalization and Localization. * Explaining gettext:: How GNU @code{gettext} works. * Programmer i18n:: Features for the programmer. * Translator i18n:: Features for the translator. * String Extraction:: Extracting marked strings. * Printf Ordering:: Rearranging @code{printf} arguments. * I18N Portability:: @command{awk}-level portability issues. * I18N Example:: A simple i18n example. * Gawk I18N:: @command{gawk} is also internationalized. * Nondecimal Data:: Allowing nondecimal input data. * Two-way I/O:: Two-way communications with another process. * TCP/IP Networking:: Using @command{gawk} for network programming. * Portal Files:: Using @command{gawk} with BSD portals. * Profiling:: Profiling your @command{awk} programs. * Command Line:: How to run @command{awk}. * Options:: Command-line options and their meanings. * Other Arguments:: Input file names and variable assignments. * AWKPATH Variable:: Searching directories for @command{awk} programs. * Obsolete:: Obsolete Options and/or features. * Undocumented:: Undocumented Options and Features. * Known Bugs:: Known Bugs in @command{gawk}. * Library Names:: How to best name private global variables in library functions. * General Functions:: Functions that are of general use. * Nextfile Function:: Two implementations of a @code{nextfile} function. * Assert Function:: A function for assertions in @command{awk} programs. * Round Function:: A function for rounding if @code{sprintf} does not do it correctly. * Cliff Random Function:: The Cliff Random Number Generator. * Ordinal Functions:: Functions for using characters as numbers and vice versa. * Join Function:: A function to join an array into a string. * Gettimeofday Function:: A function to get formatted times. * Data File Management:: Functions for managing command-line data files. * Filetrans Function:: A function for handling data file transitions. * Rewind Function:: A function for rereading the current file. * File Checking:: Checking that data files are readable. * Empty Files:: Checking for zero-length files. * Ignoring Assigns:: Treating assignments as file names. * Getopt Function:: A function for processing command-line arguments. * Passwd Functions:: Functions for getting user information. * Group Functions:: Functions for getting group information. * Running Examples:: How to run these examples. * Clones:: Clones of common utilities. * Cut Program:: The @command{cut} utility. * Egrep Program:: The @command{egrep} utility. * Id Program:: The @command{id} utility. * Split Program:: The @command{split} utility. * Tee Program:: The @command{tee} utility. * Uniq Program:: The @command{uniq} utility. * Wc Program:: The @command{wc} utility. * Miscellaneous Programs:: Some interesting @command{awk} programs. * Dupword Program:: Finding duplicated words in a document. * Alarm Program:: An alarm clock. * Translate Program:: A program similar to the @command{tr} utility. * Labels Program:: Printing mailing labels. * Word Sorting:: A program to produce a word usage count. * History Sorting:: Eliminating duplicate entries from a history file. * Extract Program:: Pulling out programs from Texinfo source files. * Simple Sed:: A Simple Stream Editor. * Igawk Program:: A wrapper for @command{awk} that includes files. * V7/SVR3.1:: The major changes between V7 and System V Release 3.1. * SVR4:: Minor changes between System V Releases 3.1 and 4. * POSIX:: New features from the POSIX standard. * BTL:: New features from the Bell Laboratories version of @command{awk}. * POSIX/GNU:: The extensions in @command{gawk} not in POSIX @command{awk}. * Contributors:: The major contributors to @command{gawk}. * Gawk Distribution:: What is in the @command{gawk} distribution. * Getting:: How to get the distribution. * Extracting:: How to extract the distribution. * Distribution contents:: What is in the distribution. * Unix Installation:: Installing @command{gawk} under various versions of Unix. * Quick Installation:: Compiling @command{gawk} under Unix. * Additional Configuration Options:: Other compile-time options. * Configuration Philosophy:: How it's all supposed to work. * Non-Unix Installation:: Installation on Other Operating Systems. * Amiga Installation:: Installing @command{gawk} on an Amiga. * BeOS Installation:: Installing @command{gawk} on BeOS. * PC Installation:: Installing and Compiling @command{gawk} on MS-DOS and OS/2. * PC Binary Installation:: Installing a prepared distribution. * PC Compiling:: Compiling @command{gawk} for MS-DOS, Windows32, and OS/2. * PC Using:: Running @command{gawk} on MS-DOS, Windows32 and OS/2. * PC Dynamic:: Compiling @command{gawk} for dynamic libraries. * Cygwin:: Building and running @command{gawk} for Cygwin. * VMS Installation:: Installing @command{gawk} on VMS. * VMS Compilation:: How to compile @command{gawk} under VMS. * VMS Installation Details:: How to install @command{gawk} under VMS. * VMS Running:: How to run @command{gawk} under VMS. * VMS POSIX:: Alternate instructions for VMS POSIX. * Unsupported:: Systems whose ports are no longer supported. * Atari Installation:: Installing @command{gawk} on the Atari ST. * Atari Compiling:: Compiling @command{gawk} on Atari. * Atari Using:: Running @command{gawk} on Atari. * Tandem Installation:: Installing @command{gawk} on a Tandem. * Bugs:: Reporting Problems and Bugs. * Other Versions:: Other freely available @command{awk} implementations. * Compatibility Mode:: How to disable certain @command{gawk} extensions. * Additions:: Making Additions To @command{gawk}. * Adding Code:: Adding code to the main body of @command{gawk}. * New Ports:: Porting @command{gawk} to a new operating system. * Dynamic Extensions:: Adding new built-in functions to @command{gawk}. * Internals:: A brief look at some @command{gawk} internals. * Sample Library:: A example of new functions. * Internal File Description:: What the new functions will do. * Internal File Ops:: The code for internal file operations. * Using Internal File Ops:: How to use an external extension. * Future Extensions:: New features that may be implemented one day. * Basic High Level:: The high level view. * Basic Data Typing:: A very quick intro to data types. * Floating Point Issues:: Stuff to know about floating-point numbers. @end detailmenu @end menu @c dedication for Info file @ifinfo @center To Miriam, for making me complete. @sp 1 @center To Chana, for the joy you bring us. @sp 1 @center To Rivka, for the exponential increase. @sp 1 @center To Nachum, for the added dimension. @sp 1 @center To Malka, for the new beginning. @end ifinfo @summarycontents @contents @node Foreword @unnumbered Foreword Arnold Robbins and I are good friends. We were introduced 11 years ago by circumstances---and our favorite programming language, AWK. The circumstances started a couple of years earlier. I was working at a new job and noticed an unplugged Unix computer sitting in the corner. No one knew how to use it, and neither did I. However, a couple of days later it was running, and I was @code{root} and the one-and-only user. That day, I began the transition from statistician to Unix programmer. On one of many trips to the library or bookstore in search of books on Unix, I found the gray AWK book, a.k.a. Aho, Kernighan and Weinberger, @cite{The AWK Programming Language}, Addison-Wesley, 1988. AWK's simple programming paradigm---find a pattern in the input and then perform an action---often reduced complex or tedious data manipulations to few lines of code. I was excited to try my hand at programming in AWK. Alas, the @command{awk} on my computer was a limited version of the language described in the AWK book. I discovered that my computer had ``old @command{awk}'' and the AWK book described ``new @command{awk}.'' I learned that this was typical; the old version refused to step aside or relinquish its name. If a system had a new @command{awk}, it was invariably called @command{nawk}, and few systems had it. The best way to get a new @command{awk} was to @command{ftp} the source code for @command{gawk} from @code{prep.ai.mit.edu}. @command{gawk} was a version of new @command{awk} written by David Trueman and Arnold, and available under the GNU General Public License. (Incidentally, it's no longer difficult to find a new @command{awk}. @command{gawk} ships with Linux, and you can download binaries or source code for almost any system; my wife uses @command{gawk} on her VMS box.) My Unix system started out unplugged from the wall; it certainly was not plugged into a network. So, oblivious to the existence of @command{gawk} and the Unix community in general, and desiring a new @command{awk}, I wrote my own, called @command{mawk}. Before I was finished I knew about @command{gawk}, but it was too late to stop, so I eventually posted to a @code{comp.sources} newsgroup. A few days after my posting, I got a friendly email from Arnold introducing himself. He suggested we share design and algorithms and attached a draft of the POSIX standard so that I could update @command{mawk} to support language extensions added after publication of the AWK book. Frankly, if our roles had been reversed, I would not have been so open and we probably would have never met. I'm glad we did meet. He is an AWK expert's AWK expert and a genuinely nice person. Arnold contributes significant amounts of his expertise and time to the Free Software Foundation. This book is the @command{gawk} reference manual, but at its core it is a book about AWK programming that will appeal to a wide audience. It is a definitive reference to the AWK language as defined by the 1987 Bell Labs release and codified in the 1992 POSIX Utilities standard. On the other hand, the novice AWK programmer can study a wealth of practical programs that emphasize the power of AWK's basic idioms: data driven control-flow, pattern matching with regular expressions, and associative arrays. Those looking for something new can try out @command{gawk}'s interface to network protocols via special @file{/inet} files. The programs in this book make clear that an AWK program is typically much smaller and faster to develop than a counterpart written in C. Consequently, there is often a payoff to prototype an algorithm or design in AWK to get it running quickly and expose problems early. Often, the interpreted performance is adequate and the AWK prototype becomes the product. The new @command{pgawk} (profiling @command{gawk}), produces program execution counts. I recently experimented with an algorithm that for @math{n} lines of input, exhibited @tex $\sim\! Cn^2$ @end tex @ifnottex ~ C n^2 @end ifnottex performance, while theory predicted @tex $\sim\! Cn\log n$ @end tex @ifnottex ~ C n log n @end ifnottex behavior. A few minutes poring over the @file{awkprof.out} profile pinpointed the problem to a single line of code. @command{pgawk} is a welcome addition to my programmer's toolbox. Arnold has distilled over a decade of experience writing and using AWK programs, and developing @command{gawk}, into this book. If you use AWK or want to learn how, then read this book. @display Michael Brennan Author of @command{mawk} @end display @node Preface @unnumbered Preface @c I saw a comment somewhere that the preface should describe the book itself, @c and the introduction should describe what the book covers. @c @c 12/2000: Chuck wants the preface & intro combined. Several kinds of tasks occur repeatedly when working with text files. You might want to extract certain lines and discard the rest. Or you may need to make changes wherever certain patterns appear, but leave the rest of the file alone. Writing single-use programs for these tasks in languages such as C, C++, or Pascal is time-consuming and inconvenient. Such jobs are often easier with @command{awk}. The @command{awk} utility interprets a special-purpose programming language that makes it easy to handle simple data-reformatting jobs. The GNU implementation of @command{awk} is called @command{gawk}; it is fully compatible with the System V Release 4 version of @command{awk}. @command{gawk} is also compatible with the POSIX specification of the @command{awk} language. This means that all properly written @command{awk} programs should work with @command{gawk}. Thus, we usually don't distinguish between @command{gawk} and other @command{awk} implementations. @cindex @command{awk}, POSIX and, See Also POSIX @command{awk} @cindex @command{awk}, POSIX and @cindex POSIX, @command{awk} and @cindex @command{gawk}, @command{awk} and @cindex @command{awk}, @command{gawk} and @cindex @command{awk}, uses for Using @command{awk} allows you to: @itemize @bullet @item Manage small, personal databases @item Generate reports @item Validate data @item Produce indexes and perform other document preparation tasks @item Experiment with algorithms that you can adapt later to other computer languages @end itemize @cindex @command{awk}, See Also @command{gawk} @cindex @command{gawk}, See Also @command{awk} @cindex @command{gawk}, uses for In addition, @command{gawk} provides facilities that make it easy to: @itemize @bullet @item Extract bits and pieces of data for processing @item Sort data @item Perform simple network communications @end itemize This @value{DOCUMENT} teaches you about the @command{awk} language and how you can use it effectively. You should already be familiar with basic system commands, such as @command{cat} and @command{ls},@footnote{These commands are available on POSIX-compliant systems, as well as on traditional Unix-based systems. If you are using some other operating system, you still need to be familiar with the ideas of I/O redirection and pipes.} as well as basic shell facilities, such as input/output (I/O) redirection and pipes. @cindex GNU @command{awk}, See @command{gawk} Implementations of the @command{awk} language are available for many different computing environments. This @value{DOCUMENT}, while describing the @command{awk} language in general, also describes the particular implementation of @command{awk} called @command{gawk} (which stands for ``GNU awk''). @command{gawk} runs on a broad range of Unix systems, ranging from 80386 PC-based computers up through large-scale systems, such as Crays. @command{gawk} has also been ported to Mac OS X, MS-DOS, Microsoft Windows (all versions) and OS/2 PCs, Atari and Amiga microcomputers, BeOS, Tandem D20, and VMS. @menu * History:: The history of @command{gawk} and @command{awk}. * Names:: What name to use to find @command{awk}. * This Manual:: Using this @value{DOCUMENT}. Includes sample input files that you can use. * Conventions:: Typographical Conventions. * Manual History:: Brief history of the GNU project and this @value{DOCUMENT}. * How To Contribute:: Helping to save the world. * Acknowledgments:: Acknowledgments. @end menu @node History @unnumberedsec History of @command{awk} and @command{gawk} @cindex recipe for a programming language @cindex programming language, recipe for @center Recipe For A Programming Language @multitable {2 parts} {1 part @code{egrep}} {1 part @code{snobol}} @item @tab 1 part @code{egrep} @tab 1 part @code{snobol} @item @tab 2 parts @code{ed} @tab 3 parts C @end multitable @quotation Blend all parts well using @code{lex} and @code{yacc}. Document minimally and release. After eight years, add another part @code{egrep} and two more parts C. Document very well and release. @end quotation @cindex Aho, Alfred @cindex Weinberger, Peter @cindex Kernighan, Brian @cindex @command{awk}, history of The name @command{awk} comes from the initials of its designers: Alfred V.@: Aho, Peter J.@: Weinberger and Brian W.@: Kernighan. The original version of @command{awk} was written in 1977 at AT&T Bell Laboratories. In 1985, a new version made the programming language more powerful, introducing user-defined functions, multiple input streams, and computed regular expressions. This new version became widely available with Unix System V Release 3.1 (SVR3.1). The version in SVR4 added some new features and cleaned up the behavior in some of the ``dark corners'' of the language. The specification for @command{awk} in the POSIX Command Language and Utilities standard further clarified the language. Both the @command{gawk} designers and the original Bell Laboratories @command{awk} designers provided feedback for the POSIX specification. @cindex Rubin, Paul @cindex Fenlason, Jay @cindex Trueman, David Paul Rubin wrote the GNU implementation, @command{gawk}, in 1986. Jay Fenlason completed it, with advice from Richard Stallman. John Woods contributed parts of the code as well. In 1988 and 1989, David Trueman, with help from me, thoroughly reworked @command{gawk} for compatibility with the newer @command{awk}. Circa 1995, I became the primary maintainer. Current development focuses on bug fixes, performance improvements, standards compliance, and occasionally, new features. In May of 1997, J@"urgen Kahrs felt the need for network access from @command{awk}, and with a little help from me, set about adding features to do this for @command{gawk}. At that time, he also wrote the bulk of @cite{TCP/IP Internetworking with @command{gawk}} (a separate document, available as part of the @command{gawk} distribution). His code finally became part of the main @command{gawk} distribution with @command{gawk} @value{PVERSION} 3.1. @xref{Contributors}, for a complete list of those who made important contributions to @command{gawk}. @node Names @section A Rose by Any Other Name @cindex @command{awk}, new vs. old The @command{awk} language has evolved over the years. Full details are provided in @ref{Language History}. The language described in this @value{DOCUMENT} is often referred to as ``new @command{awk}'' (@command{nawk}). @cindex @command{awk}, versions of Because of this, many systems have multiple versions of @command{awk}. Some systems have an @command{awk} utility that implements the original version of the @command{awk} language and a @command{nawk} utility for the new version. Others have an @command{oawk} version for the ``old @command{awk}'' language and plain @command{awk} for the new one. Still others only have one version, which is usually the new one.@footnote{Often, these systems use @command{gawk} for their @command{awk} implementation!} @cindex @command{nawk} utility @cindex @command{oawk} utility All in all, this makes it difficult for you to know which version of @command{awk} you should run when writing your programs. The best advice I can give here is to check your local documentation. Look for @command{awk}, @command{oawk}, and @command{nawk}, as well as for @command{gawk}. It is likely that you already have some version of new @command{awk} on your system, which is what you should use when running your programs. (Of course, if you're reading this @value{DOCUMENT}, chances are good that you have @command{gawk}!) Throughout this @value{DOCUMENT}, whenever we refer to a language feature that should be available in any complete implementation of POSIX @command{awk}, we simply use the term @command{awk}. When referring to a feature that is specific to the GNU implementation, we use the term @command{gawk}. @node This Manual @section Using This Book @cindex @command{awk}, terms describing The term @command{awk} refers to a particular program as well as to the language you use to tell this program what to do. When we need to be careful, we call the language ``the @command{awk} language,'' and the program ``the @command{awk} utility.'' This @value{DOCUMENT} explains both the @command{awk} language and how to run the @command{awk} utility. The term @dfn{@command{awk} program} refers to a program written by you in the @command{awk} programming language. @cindex @command{gawk}, @command{awk} and @cindex @command{awk}, @command{gawk} and @cindex POSIX @command{awk} Primarily, this @value{DOCUMENT} explains the features of @command{awk}, as defined in the POSIX standard. It does so in the context of the @command{gawk} implementation. While doing so, it also attempts to describe important differences between @command{gawk} and other @command{awk} implementations.@footnote{All such differences appear in the index under the entry ``differences in @command{awk} and @command{gawk}.''} Finally, any @command{gawk} features that are not in the POSIX standard for @command{awk} are noted. @ifnotinfo This @value{DOCUMENT} has the difficult task of being both a tutorial and a reference. If you are a novice, feel free to skip over details that seem too complex. You should also ignore the many cross-references; they are for the expert user and for the online Info version of the document. @end ifnotinfo There are subsections labelled as @strong{Advanced Notes} scattered throughout the @value{DOCUMENT}. They add a more complete explanation of points that are relevant, but not likely to be of interest on first reading. All appear in the index, under the heading ``advanced features.'' Most of the time, the examples use complete @command{awk} programs. In some of the more advanced sections, only the part of the @command{awk} program that illustrates the concept currently being described is shown. While this @value{DOCUMENT} is aimed principally at people who have not been exposed to @command{awk}, there is a lot of information here that even the @command{awk} expert should find useful. In particular, the description of POSIX @command{awk} and the example programs in @ref{Library Functions}, and in @ref{Sample Programs}, should be of interest. @ref{Getting Started}, provides the essentials you need to know to begin using @command{awk}. @ref{Regexp}, introduces regular expressions in general, and in particular the flavors supported by POSIX @command{awk} and @command{gawk}. @ref{Reading Files}, describes how @command{awk} reads your data. It introduces the concepts of records and fields, as well as the @code{getline} command. I/O redirection is first described here. @ref{Printing}, describes how @command{awk} programs can produce output with @code{print} and @code{printf}. @ref{Expressions}, describes expressions, which are the basic building blocks for getting most things done in a program. @ref{Patterns and Actions}, describes how to write patterns for matching records, actions for doing something when a record is matched, and the built-in variables @command{awk} and @command{gawk} use. @ref{Arrays}, covers @command{awk}'s one-and-only data structure: associative arrays. Deleting array elements and whole arrays is also described, as well as sorting arrays in @command{gawk}. @ref{Functions}, describes the built-in functions @command{awk} and @command{gawk} provide, as well as how to define your own functions. @ref{Internationalization}, describes special features in @command{gawk} for translating program messages into different languages at runtime. @ref{Advanced Features}, describes a number of @command{gawk}-specific advanced features. Of particular note are the abilities to have two-way communications with another process, perform TCP/IP networking, and profile your @command{awk} programs. @ref{Invoking Gawk}, describes how to run @command{gawk}, the meaning of its command-line options, and how it finds @command{awk} program source files. @ref{Library Functions}, and @ref{Sample Programs}, provide many sample @command{awk} programs. Reading them allows you to see @command{awk} solving real problems. @ref{Language History}, describes how the @command{awk} language has evolved since first release to present. It also describes how @command{gawk} has acquired features over time. @ref{Installation}, describes how to get @command{gawk}, how to compile it under Unix, and how to compile and use it on different non-Unix systems. It also describes how to report bugs in @command{gawk} and where to get three other freely available implementations of @command{awk}. @ref{Notes}, describes how to disable @command{gawk}'s extensions, as well as how to contribute new code to @command{gawk}, how to write extension libraries, and some possible future directions for @command{gawk} development. @ref{Basic Concepts}, provides some very cursory background material for those who are completely unfamiliar with computer programming. Also centralized there is a discussion of some of the issues surrounding floating-point numbers. The @ref{Glossary}, defines most, if not all, the significant terms used throughout the book. If you find terms that you aren't familiar with, try looking them up here. @ref{Copying}, and @ref{GNU Free Documentation License}, present the licenses that cover the @command{gawk} source code and this @value{DOCUMENT}, respectively. @node Conventions @section Typographical Conventions @cindex Texinfo This @value{DOCUMENT} is written using Texinfo, the GNU documentation formatting language. A single Texinfo source file is used to produce both the printed and online versions of the documentation. @ifnotinfo Because of this, the typographical conventions are slightly different than in other books you may have read. @end ifnotinfo @ifinfo This @value{SECTION} briefly documents the typographical conventions used in Texinfo. @end ifinfo Examples you would type at the command-line are preceded by the common shell primary and secondary prompts, @samp{$} and @samp{>}. Output from the command is preceded by the glyph ``@print{}''. This typically represents the command's standard output. Error messages, and other output on the command's standard error, are preceded by the glyph ``@error{}''. For example: @example $ echo hi on stdout @print{} hi on stdout $ echo hello on stderr 1>&2 @error{} hello on stderr @end example @ifnotinfo In the text, command names appear in @code{this font}, while code segments appear in the same font and quoted, @samp{like this}. Some things are emphasized @emph{like this}, and if a point needs to be made strongly, it is done @strong{like this}. The first occurrence of a new term is usually its @dfn{definition} and appears in the same font as the previous occurrence of ``definition'' in this sentence. @value{FN}s are indicated like this: @file{/path/to/ourfile}. @end ifnotinfo Characters that you type at the keyboard look @kbd{like this}. In particular, there are special characters called ``control characters.'' These are characters that you type by holding down both the @kbd{CONTROL} key and another key, at the same time. For example, a @kbd{@value{CTL}-d} is typed by first pressing and holding the @kbd{CONTROL} key, next pressing the @kbd{d} key and finally releasing both keys. @c fakenode --- for prepinfo @subsubheading Dark Corners @cindex Kernighan, Brian @quotation @i{Dark corners are basically fractal --- no matter how much you illuminate, there's always a smaller but darker one.}@* Brian Kernighan @end quotation @cindex d.c., See dark corner @cindex dark corner Until the POSIX standard (and @cite{The Gawk Manual}), many features of @command{awk} were either poorly documented or not documented at all. Descriptions of such features (often called ``dark corners'') are noted in this @value{DOCUMENT} with @iftex the picture of a flashlight in the margin, as shown here. @value{DARKCORNER} @end iftex @ifnottex ``(d.c.)''. @end ifnottex They also appear in the index under the heading ``dark corner.'' As noted by the opening quote, though, any coverage of dark corners is, by definition, something that is incomplete. @node Manual History @unnumberedsec The GNU Project and This Book @cindex FSF (Free Software Foundation) @cindex Free Software Foundation (FSF) @cindex Stallman, Richard The Free Software Foundation (FSF) is a nonprofit organization dedicated to the production and distribution of freely distributable software. It was founded by Richard M.@: Stallman, the author of the original Emacs editor. GNU Emacs is the most widely used version of Emacs today. @cindex GNU Project @cindex GPL (General Public License) @cindex General Public License, See GPL @cindex documentation, online The GNU@footnote{GNU stands for ``GNU's not Unix.''} Project is an ongoing effort on the part of the Free Software Foundation to create a complete, freely distributable, POSIX-compliant computing environment. The FSF uses the ``GNU General Public License'' (GPL) to ensure that their software's source code is always available to the end user. A copy of the GPL is included @ifnotinfo in this @value{DOCUMENT} @end ifnotinfo for your reference (@pxref{Copying}). The GPL applies to the C language source code for @command{gawk}. To find out more about the FSF and the GNU Project online, see @uref{http://www.gnu.org, the GNU Project's home page}. This @value{DOCUMENT} may also be read from @uref{http://www.gnu.org/manual/gawk/, their web site}. A shell, an editor (Emacs), highly portable optimizing C, C++, and Objective-C compilers, a symbolic debugger and dozens of large and small utilities (such as @command{gawk}), have all been completed and are freely available. The GNU operating system kernel (the HURD), has been released but is still in an early stage of development. @cindex Linux @cindex GNU/Linux @cindex operating systems, BSD-based @cindex Alpha (DEC) Until the GNU operating system is more fully developed, you should consider using GNU/Linux, a freely distributable, Unix-like operating system for Intel 80386, DEC Alpha, Sun SPARC, IBM S/390, and other systems.@footnote{The terminology ``GNU/Linux'' is explained in the @ref{Glossary}.} There are many books on GNU/Linux. One that is freely available is @cite{Linux Installation and Getting Started}, by Matt Welsh. Many GNU/Linux distributions are often available in computer stores or bundled on CD-ROMs with books about Linux. (There are three other freely available, Unix-like operating systems for 80386 and other systems: NetBSD, FreeBSD, and OpenBSD. All are based on the 4.4-Lite Berkeley Software Distribution, and they use recent versions of @command{gawk} for their versions of @command{awk}.) @ifnotinfo The @value{DOCUMENT} you are reading is actually free---at least, the information in it is free to anyone. The machine-readable source code for the @value{DOCUMENT} comes with @command{gawk}; anyone may take this @value{DOCUMENT} to a copying machine and make as many copies as they like. (Take a moment to check the Free Documentation License in @ref{GNU Free Documentation License}.) Although you could just print it out yourself, bound books are much easier to read and use. Furthermore, the proceeds from sales of this book go back to the FSF to help fund development of more free software. @end ifnotinfo @ignore @cindex Close, Diane The @value{DOCUMENT} itself has gone through several previous, preliminary editions. Paul Rubin wrote the very first draft of @cite{The GAWK Manual}; it was around 40 pages in size. Diane Close and Richard Stallman improved it, yielding the version which I started working with in the fall of 1988. It was around 90 pages long and barely described the original, ``old'' version of @command{awk}. After substantial revision, the first version of the @cite{The GAWK Manual} to be released was Edition 0.11 Beta in October of 1989. The manual then underwent more substantial revision for Edition 0.13 of December 1991. David Trueman, Pat Rankin and Michal Jaegermann contributed sections of the manual for Edition 0.13. That edition was published by the FSF as a bound book early in 1992. Since then there were several minor revisions, notably Edition 0.14 of November 1992 that was published by the FSF in January of 1993 and Edition 0.16 of August 1993. Edition 1.0 of @cite{GAWK: The GNU Awk User's Guide} represented a significant re-working of @cite{The GAWK Manual}, with much additional material. The FSF and I agreed that I was now the primary author. @c I also felt that the manual needed a more descriptive title. In January 1996, SSC published Edition 1.0 under the title @cite{Effective AWK Programming}. In February 1997, they published Edition 1.0.3 which had minor changes as a ``second edition.'' In 1999, the FSF published this same version as Edition 2 of @cite{GAWK: The GNU Awk User's Guide}. Edition @value{EDITION} maintains the basic structure of Edition 1.0, but with significant additional material, reflecting the host of new features in @command{gawk} @value{PVERSION} @value{VERSION}. Of particular note is @ref{Array Sorting}, @ref{Bitwise Functions}, @ref{Internationalization}, @ref{Advanced Features}, and @ref{Dynamic Extensions}. @end ignore @cindex Close, Diane The @value{DOCUMENT} itself has gone through a number of previous editions. Paul Rubin wrote the very first draft of @cite{The GAWK Manual}; it was around 40 pages in size. Diane Close and Richard Stallman improved it, yielding a version that was around 90 pages long and barely described the original, ``old'' version of @command{awk}. I started working with that version in the fall of 1988. As work on it progressed, the FSF published several preliminary versions (numbered 0.@var{x}). In 1996, Edition 1.0 was released with @command{gawk} 3.0.0. The FSF published the first two editions under the title @cite{The GNU Awk User's Guide}. This edition maintains the basic structure of Edition 1.0, but with significant additional material, reflecting the host of new features in @command{gawk} @value{PVERSION} @value{VERSION}. Of particular note is @ref{Array Sorting}, as well as @ref{Bitwise Functions}, @ref{Internationalization}, and also @ref{Advanced Features}, and @ref{Dynamic Extensions}. @cite{@value{TITLE}} will undoubtedly continue to evolve. An electronic version comes with the @command{gawk} distribution from the FSF. If you find an error in this @value{DOCUMENT}, please report it! @xref{Bugs}, for information on submitting problem reports electronically, or write to me in care of the publisher. @node How To Contribute @unnumberedsec How to Contribute As the maintainer of GNU @command{awk}, I am starting a collection of publicly available @command{awk} programs. For more information, see @uref{ftp://ftp.freefriends.org/arnold/Awkstuff}. If you have written an interesting @command{awk} program, or have written a @command{gawk} extension that you would like to share with the rest of the world, please contact me (@email{arnold@@gnu.org}). Making things available on the Internet helps keep the @command{gawk} distribution down to manageable size. @node Acknowledgments @unnumberedsec Acknowledgments The initial draft of @cite{The GAWK Manual} had the following acknowledgments: @quotation Many people need to be thanked for their assistance in producing this manual. Jay Fenlason contributed many ideas and sample programs. Richard Mlynarik and Robert Chassell gave helpful comments on drafts of this manual. The paper @cite{A Supplemental Document for @command{awk}} by John W.@: Pierce of the Chemistry Department at UC San Diego, pinpointed several issues relevant both to @command{awk} implementation and to this manual, that would otherwise have escaped us. @end quotation @cindex Stallman, Richard I would like to acknowledge Richard M.@: Stallman, for his vision of a better world and for his courage in founding the FSF and starting the GNU Project. The following people (in alphabetical order) provided helpful comments on various versions of this book, up to and including this edition. Rick Adams, Nelson H.F. Beebe, Karl Berry, Dr.@: Michael Brennan, Rich Burridge, Claire Cloutier, Diane Close, Scott Deifik, Christopher (``Topher'') Eliot, Jeffrey Friedl, Dr.@: Darrel Hankerson, Michal Jaegermann, Dr.@: Richard J.@: LeBlanc, Michael Lijewski, Pat Rankin, Miriam Robbins, Mary Sheehan, and Chuck Toporek. @cindex Berry, Karl @cindex Chassell, Robert J.@: @c @cindex Texinfo Robert J.@: Chassell provided much valuable advice on the use of Texinfo. He also deserves special thanks for convincing me @emph{not} to title this @value{DOCUMENT} @cite{How To Gawk Politely}. Karl Berry helped significantly with the @TeX{} part of Texinfo. @cindex Hartholz, Marshall @cindex Hartholz, Elaine @cindex Schreiber, Bert @cindex Schreiber, Rita I would like to thank Marshall and Elaine Hartholz of Seattle and Dr.@: Bert and Rita Schreiber of Detroit for large amounts of quiet vacation time in their homes, which allowed me to make significant progress on this @value{DOCUMENT} and on @command{gawk} itself. @cindex Hughes, Phil Phil Hughes of SSC contributed in a very important way by loaning me his laptop GNU/Linux system, not once, but twice, which allowed me to do a lot of work while away from home. @cindex Trueman, David David Trueman deserves special credit; he has done a yeoman job of evolving @command{gawk} so that it performs well and without bugs. Although he is no longer involved with @command{gawk}, working with him on this project was a significant pleasure. @cindex Drepper, Ulrich @cindex GNITS mailing list @cindex mailing list, GNITS The intrepid members of the GNITS mailing list, and most notably Ulrich Drepper, provided invaluable help and feedback for the design of the internationalization features. @cindex Beebe, Nelson @cindex Brown, Martin @cindex Buening, Andreas @cindex Deifik, Scott @cindex Hankerson, Darrel @cindex Hasegawa, Isamu @cindex Jaegermann, Michal @cindex Kahrs, J@"urgen @cindex Rankin, Pat @cindex Rommel, Kai Uwe @cindex Zaretskii, Eli Nelson Beebe, Martin Brown, Andreas Buening, Scott Deifik, Darrel Hankerson, Isamu Hasegawa, Michal Jaegermann, J@"urgen Kahrs, Pat Rankin, Kai Uwe Rommel, and Eli Zaretskii (in alphabetical order) make up the @command{gawk} ``crack portability team.'' Without their hard work and help, @command{gawk} would not be nearly the fine program it is today. It has been and continues to be a pleasure working with this team of fine people. @cindex Kernighan, Brian David and I would like to thank Brian Kernighan of Bell Laboratories for invaluable assistance during the testing and debugging of @command{gawk}, and for help in clarifying numerous points about the language. We could not have done nearly as good a job on either @command{gawk} or its documentation without his help. Chuck Toporek, Mary Sheehan, and Claire Coutier of O'Reilly & Associates contributed significant editorial help for this @value{DOCUMENT} for the 3.1 release of @command{gawk}. @cindex Robbins, Miriam @cindex Robbins, Jean @cindex Robbins, Harry @cindex G-d I must thank my wonderful wife, Miriam, for her patience through the many versions of this project, for her proofreading, and for sharing me with the computer. I would like to thank my parents for their love, and for the grace with which they raised and educated me. Finally, I also must acknowledge my gratitude to G-d, for the many opportunities He has sent my way, as well as for the gifts He has given me with which to take advantage of those opportunities. @sp 2 @noindent Arnold Robbins @* Nof Ayalon @* ISRAEL @* March, 2001 @ignore @c Try this @iftex @page @headings off @majorheading I@ @ @ @ The @command{awk} Language and @command{gawk} Part I describes the @command{awk} language and @command{gawk} program in detail. It starts with the basics, and continues through all of the features of @command{awk} and @command{gawk}. It contains the following chapters: @itemize @bullet @item @ref{Getting Started}. @item @ref{Regexp}. @item @ref{Reading Files}. @item @ref{Printing}. @item @ref{Expressions}. @item @ref{Patterns and Actions}. @item @ref{Arrays}. @item @ref{Functions}. @item @ref{Internationalization}. @item @ref{Advanced Features}. @item @ref{Invoking Gawk}. @end itemize @page @evenheading @thispage@ @ @ @strong{@value{TITLE}} @| @| @oddheading @| @| @strong{@thischapter}@ @ @ @thispage @end iftex @end ignore @node Getting Started @chapter Getting Started with @command{awk} @c @cindex script, definition of @c @cindex rule, definition of @c @cindex program, definition of @c @cindex basic function of @command{awk} @cindex @command{awk}, function of The basic function of @command{awk} is to search files for lines (or other units of text) that contain certain patterns. When a line matches one of the patterns, @command{awk} performs specified actions on that line. @command{awk} keeps processing input lines in this way until it reaches the end of the input files. @cindex @command{awk}, uses for @c comma here is NOT for secondary @cindex programming languages, data-driven vs. procedural @cindex @command{awk} programs Programs in @command{awk} are different from programs in most other languages, because @command{awk} programs are @dfn{data-driven}; that is, you describe the data you want to work with and then what to do when you find it. Most other languages are @dfn{procedural}; you have to describe, in great detail, every step the program is to take. When working with procedural languages, it is usually much harder to clearly describe the data your program will process. For this reason, @command{awk} programs are often refreshingly easy to read and write. @cindex program, definition of @cindex rule, definition of When you run @command{awk}, you specify an @command{awk} @dfn{program} that tells @command{awk} what to do. The program consists of a series of @dfn{rules}. (It may also contain @dfn{function definitions}, an advanced feature that we will ignore for now. @xref{User-defined}.) Each rule specifies one pattern to search for and one action to perform upon finding the pattern. Syntactically, a rule consists of a pattern followed by an action. The action is enclosed in curly braces to separate it from the pattern. Newlines usually separate rules. Therefore, an @command{awk} program looks like this: @example @var{pattern} @{ @var{action} @} @var{pattern} @{ @var{action} @} @dots{} @end example @menu * Running gawk:: How to run @command{gawk} programs; includes command-line syntax. * Sample Data Files:: Sample data files for use in the @command{awk} programs illustrated in this @value{DOCUMENT}. * Very Simple:: A very simple example. * Two Rules:: A less simple one-line example using two rules. * More Complex:: A more complex example. * Statements/Lines:: Subdividing or combining statements into lines. * Other Features:: Other Features of @command{awk}. * When:: When to use @command{gawk} and when to use other things. @end menu @node Running gawk @section How to Run @command{awk} Programs @cindex @command{awk} programs, running There are several ways to run an @command{awk} program. If the program is short, it is easiest to include it in the command that runs @command{awk}, like this: @example awk '@var{program}' @var{input-file1} @var{input-file2} @dots{} @end example @cindex command line, formats When the program is long, it is usually more convenient to put it in a file and run it with a command like this: @example awk -f @var{program-file} @var{input-file1} @var{input-file2} @dots{} @end example This @value{SECTION} discusses both mechanisms, along with several variations of each. @menu * One-shot:: Running a short throwaway @command{awk} program. * Read Terminal:: Using no input files (input from terminal instead). * Long:: Putting permanent @command{awk} programs in files. * Executable Scripts:: Making self-contained @command{awk} programs. * Comments:: Adding documentation to @command{gawk} programs. * Quoting:: More discussion of shell quoting issues. @end menu @node One-shot @subsection One-Shot Throwaway @command{awk} Programs Once you are familiar with @command{awk}, you will often type in simple programs the moment you want to use them. Then you can write the program as the first argument of the @command{awk} command, like this: @example awk '@var{program}' @var{input-file1} @var{input-file2} @dots{} @end example @noindent where @var{program} consists of a series of @var{patterns} and @var{actions}, as described earlier. @cindex single quote (@code{'}) @cindex @code{'} (single quote) This command format instructs the @dfn{shell}, or command interpreter, to start @command{awk} and use the @var{program} to process records in the input file(s). There are single quotes around @var{program} so the shell won't interpret any @command{awk} characters as special shell characters. The quotes also cause the shell to treat all of @var{program} as a single argument for @command{awk}, and allow @var{program} to be more than one line long. @cindex shells, scripts @cindex @command{awk} programs, running, from shell scripts This format is also useful for running short or medium-sized @command{awk} programs from shell scripts, because it avoids the need for a separate file for the @command{awk} program. A self-contained shell script is more reliable because there are no other files to misplace. @ref{Very Simple}, @ifnotinfo later in this @value{CHAPTER}, @end ifnotinfo presents several short, self-contained programs. @c Removed for gawk 3.1, doesn't really add anything here. @ignore As an interesting side point, the command @example awk '/foo/' @var{files} @dots{} @end example @noindent is essentially the same as @cindex @command{egrep} utility @example egrep foo @var{files} @dots{} @end example @end ignore @node Read Terminal @subsection Running @command{awk} Without Input Files @cindex standard input @cindex input, standard @cindex input files, running @command{awk} without You can also run @command{awk} without any input files. If you type the following command line: @example awk '@var{program}' @end example @noindent @command{awk} applies the @var{program} to the @dfn{standard input}, which usually means whatever you type on the terminal. This continues until you indicate end-of-file by typing @kbd{@value{CTL}-d}. (On other operating systems, the end-of-file character may be different. For example, on OS/2 and MS-DOS, it is @kbd{@value{CTL}-z}.) @cindex files, input, See input files @cindex input files, running @command{awk} without @cindex @command{awk} programs, running, without input files As an example, the following program prints a friendly piece of advice (from Douglas Adams's @cite{The Hitchhiker's Guide to the Galaxy}), to keep you from worrying about the complexities of computer programming (@code{BEGIN} is a feature we haven't discussed yet): @example $ awk "BEGIN @{ print \"Don't Panic!\" @}" @print{} Don't Panic! @end example @cindex quoting @cindex double quote (@code{"}) @cindex @code{"} (double quote) @cindex @code{\} (backslash) @cindex backslash (@code{\}) This program does not read any input. The @samp{\} before each of the inner double quotes is necessary because of the shell's quoting rules---in particular because it mixes both single quotes and double quotes.@footnote{Although we generally recommend the use of single quotes around the program text, double quotes are needed here in order to put the single quote into the message.} This next simple @command{awk} program emulates the @command{cat} utility; it copies whatever you type on the keyboard to its standard output (why this works is explained shortly). @example $ awk '@{ print @}' Now is the time for all good men @print{} Now is the time for all good men to come to the aid of their country. @print{} to come to the aid of their country. Four score and seven years ago, ... @print{} Four score and seven years ago, ... What, me worry? @print{} What, me worry? @kbd{@value{CTL}-d} @end example @node Long @subsection Running Long Programs @cindex @command{awk} programs, running @cindex @command{awk} programs, lengthy @cindex files, @command{awk} programs in Sometimes your @command{awk} programs can be very long. In this case, it is more convenient to put the program into a separate file. In order to tell @command{awk} to use that file for its program, you type: @example awk -f @var{source-file} @var{input-file1} @var{input-file2} @dots{} @end example @cindex @code{-f} option @cindex command line, options @cindex options, command-line The @option{-f} instructs the @command{awk} utility to get the @command{awk} program from the file @var{source-file}. Any @value{FN} can be used for @var{source-file}. For example, you could put the program: @example BEGIN @{ print "Don't Panic!" @} @end example @noindent into the file @file{advice}. Then this command: @example awk -f advice @end example @noindent does the same thing as this one: @example awk "BEGIN @{ print \"Don't Panic!\" @}" @end example @cindex quoting @noindent This was explained earlier (@pxref{Read Terminal}). Note that you don't usually need single quotes around the @value{FN} that you specify with @option{-f}, because most @value{FN}s don't contain any of the shell's special characters. Notice that in @file{advice}, the @command{awk} program did not have single quotes around it. The quotes are only needed for programs that are provided on the @command{awk} command line. @c STARTOFRANGE sq1x @cindex single quote (@code{'}) @c STARTOFRANGE qs2x @cindex @code{'} (single quote) If you want to identify your @command{awk} program files clearly as such, you can add the extension @file{.awk} to the @value{FN}. This doesn't affect the execution of the @command{awk} program but it does make ``housekeeping'' easier. @node Executable Scripts @subsection Executable @command{awk} Programs @cindex @command{awk} programs @cindex @code{#} (number sign), @code{#!} (executable scripts) @cindex number sign (@code{#}), @code{#!} (executable scripts) @cindex Unix, @command{awk} scripts and @cindex @code{#} (number sign), @code{#!} (executable scripts), portability issues with @cindex number sign (@code{#}), @code{#!} (executable scripts), portability issues with Once you have learned @command{awk}, you may want to write self-contained @command{awk} scripts, using the @samp{#!} script mechanism. You can do this on many Unix systems@footnote{The @samp{#!} mechanism works on Linux systems, systems derived from the 4.4-Lite Berkeley Software Distribution, and most commercial Unix systems.} as well as on the GNU system. For example, you could update the file @file{advice} to look like this: @example #! /bin/awk -f BEGIN @{ print "Don't Panic!" @} @end example @noindent After making this file executable (with the @command{chmod} utility), simply type @samp{advice} at the shell and the system arranges to run @command{awk}@footnote{The line beginning with @samp{#!} lists the full @value{FN} of an interpreter to run and an optional initial command-line argument to pass to that interpreter. The operating system then runs the interpreter with the given argument and the full argument list of the executed program. The first argument in the list is the full @value{FN} of the @command{awk} program. The rest of the argument list contains either options to @command{awk}, or @value{DF}s, or both.} as if you had typed @samp{awk -f advice}: @example $ chmod +x advice $ advice @print{} Don't Panic! @end example @noindent (We assume you have the current directory in your shell's search path variable (typically @code{$PATH}). If not, you may need to type @samp{./advice} at the shell.) Self-contained @command{awk} scripts are useful when you want to write a program that users can invoke without their having to know that the program is written in @command{awk}. @c fakenode --- for prepinfo @subheading Advanced Notes: Portability Issues with @samp{#!} @cindex portability, @code{#!} (executable scripts) Some systems limit the length of the interpreter name to 32 characters. Often, this can be dealt with by using a symbolic link. You should not put more than one argument on the @samp{#!} line after the path to @command{awk}. It does not work. The operating system treats the rest of the line as a single argument and passes it to @command{awk}. Doing this leads to confusing behavior---most likely a usage diagnostic of some sort from @command{awk}. @cindex @code{ARGC}/@code{ARGV} variables, portability and @cindex portability, @code{ARGV} variable Finally, the value of @code{ARGV[0]} (@pxref{Built-in Variables}) varies depending upon your operating system. Some systems put @samp{awk} there, some put the full pathname of @command{awk} (such as @file{/bin/awk}), and some put the name of your script (@samp{advice}). Don't rely on the value of @code{ARGV[0]} to provide your script name. @node Comments @subsection Comments in @command{awk} Programs @cindex @code{#} (number sign), commenting @cindex number sign (@code{#}), commenting @cindex commenting @cindex @command{awk} programs, documenting A @dfn{comment} is some text that is included in a program for the sake of human readers; it is not really an executable part of the program. Comments can explain what the program does and how it works. Nearly all programming languages have provisions for comments, as programs are typically hard to understand without them. In the @command{awk} language, a comment starts with the sharp sign character (@samp{#}) and continues to the end of the line. The @samp{#} does not have to be the first character on the line. The @command{awk} language ignores the rest of a line following a sharp sign. For example, we could have put the following into @file{advice}: @example # This program prints a nice friendly message. It helps # keep novice users from being afraid of the computer. BEGIN @{ print "Don't Panic!" @} @end example You can put comment lines into keyboard-composed throwaway @command{awk} programs, but this usually isn't very useful; the purpose of a comment is to help you or another person understand the program when reading it at a later time. @cindex quoting @cindex single quote (@code{'}), vs. apostrophe @cindex @code{'} (single quote), vs. apostrophe @strong{Caution:} As mentioned in @ref{One-shot}, you can enclose small to medium programs in single quotes, in order to keep your shell scripts self-contained. When doing so, @emph{don't} put an apostrophe (i.e., a single quote) into a comment (or anywhere else in your program). The shell interprets the quote as the closing quote for the entire program. As a result, usually the shell prints a message about mismatched quotes, and if @command{awk} actually runs, it will probably print strange messages about syntax errors. For example, look at the following: @example $ awk '@{ print "hello" @} # let's be cute' > @end example The shell sees that the first two quotes match, and that a new quoted object begins at the end of the command line. It therefore prompts with the secondary prompt, waiting for more input. With Unix @command{awk}, closing the quoted string produces this result: @example $ awk '@{ print "hello" @} # let's be cute' > ' @error{} awk: can't open file be @error{} source line number 1 @end example @cindex @code{\} (backslash) @cindex backslash (@code{\}) Putting a backslash before the single quote in @samp{let's} wouldn't help, since backslashes are not special inside single quotes. The next @value{SUBSECTION} describes the shell's quoting rules. @node Quoting @subsection Shell-Quoting Issues @cindex quoting, rules for For short to medium length @command{awk} programs, it is most convenient to enter the program on the @command{awk} command line. This is best done by enclosing the entire program in single quotes. This is true whether you are entering the program interactively at the shell prompt, or writing it as part of a larger shell script: @example awk '@var{program text}' @var{input-file1} @var{input-file2} @dots{} @end example @cindex shells, quoting, rules for @cindex Bourne shell, quoting rules for Once you are working with the shell, it is helpful to have a basic knowledge of shell quoting rules. The following rules apply only to POSIX-compliant, Bourne-style shells (such as @command{bash}, the GNU Bourne-Again Shell). If you use @command{csh}, you're on your own. @itemize @bullet @item Quoted items can be concatenated with nonquoted items as well as with other quoted items. The shell turns everything into one argument for the command. @item Preceding any single character with a backslash (@samp{\}) quotes that character. The shell removes the backslash and passes the quoted character on to the command. @item @cindex @code{\} (backslash) @cindex backslash (@code{\}) @cindex single quote (@code{'}) @cindex @code{'} (single quote) Single quotes protect everything between the opening and closing quotes. The shell does no interpretation of the quoted text, passing it on verbatim to the command. It is @emph{impossible} to embed a single quote inside single-quoted text. Refer back to @ref{Comments}, for an example of what happens if you try. @item @cindex double quote (@code{"}) @cindex @code{"} (double quote) Double quotes protect most things between the opening and closing quotes. The shell does at least variable and command substitution on the quoted text. Different shells may do additional kinds of processing on double-quoted text. Since certain characters within double-quoted text are processed by the shell, they must be @dfn{escaped} within the text. Of note are the characters @samp{$}, @samp{`}, @samp{\}, and @samp{"}, all of which must be preceded by a backslash within double-quoted text if they are to be passed on literally to the program. (The leading backslash is stripped first.) Thus, the example seen @ifnotinfo previously @end ifnotinfo in @ref{Read Terminal}, is applicable: @example $ awk "BEGIN @{ print \"Don't Panic!\" @}" @print{} Don't Panic! @end example @cindex single quote (@code{'}), with double quotes @cindex @code{'} (single quote), with double quotes Note that the single quote is not special within double quotes. @item Null strings are removed when they occur as part of a non-null command-line argument, while explicit non-null objects are kept. For example, to specify that the field separator @code{FS} should be set to the null string, use: @example awk -F "" '@var{program}' @var{files} # correct @end example @noindent @cindex null strings, quoting and Don't use this: @example awk -F"" '@var{program}' @var{files} # wrong! @end example @noindent In the second case, @command{awk} will attempt to use the text of the program as the value of @code{FS}, and the first @value{FN} as the text of the program! This results in syntax errors at best, and confusing behavior at worst. @end itemize @cindex quoting, tricks for Mixing single and double quotes is difficult. You have to resort to shell quoting tricks, like this: @example $ awk 'BEGIN @{ print "Here is a single quote <'"'"'>" @}' @print{} Here is a single quote <'> @end example @noindent This program consists of three concatenated quoted strings. The first and the third are single-quoted, the second is double-quoted. This can be ``simplified'' to: @example $ awk 'BEGIN @{ print "Here is a single quote <'\''>" @}' @print{} Here is a single quote <'> @end example @noindent Judge for yourself which of these two is the more readable. Another option is to use double quotes, escaping the embedded, @command{awk}-level double quotes: @example $ awk "BEGIN @{ print \"Here is a single quote <'>\" @}" @print{} Here is a single quote <'> @end example @noindent @c ENDOFRANGE sq1x @c ENDOFRANGE qs2x This option is also painful, because double quotes, backslashes, and dollar signs are very common in @command{awk} programs. If you really need both single and double quotes in your @command{awk} program, it is probably best to move it into a separate file, where the shell won't be part of the picture, and you can say what you mean. @node Sample Data Files @section @value{DDF}s for the Examples @c For gawk >= 3.2, update these data files. No-one has such slow modems! @cindex input files, examples @cindex @code{BBS-list} file Many of the examples in this @value{DOCUMENT} take their input from two sample @value{DF}s. The first, @file{BBS-list}, represents a list of computer bulletin board systems together with information about those systems. The second @value{DF}, called @file{inventory-shipped}, contains information about monthly shipments. In both files, each line is considered to be one @dfn{record}. In the @value{DF} @file{BBS-list}, each record contains the name of a computer bulletin board, its phone number, the board's baud rate(s), and a code for the number of hours it is operational. An @samp{A} in the last column means the board operates 24 hours a day. A @samp{B} in the last column means the board only operates on evening and weekend hours. A @samp{C} means the board operates only on weekends: @c 2e: Update the baud rates to reflect today's faster modems @example @c system if test ! -d eg ; then mkdir eg ; fi @c system if test ! -d eg/lib ; then mkdir eg/lib ; fi @c system if test ! -d eg/data ; then mkdir eg/data ; fi @c system if test ! -d eg/prog ; then mkdir eg/prog ; fi @c system if test ! -d eg/misc ; then mkdir eg/misc ; fi @c file eg/data/BBS-list aardvark 555-5553 1200/300 B alpo-net 555-3412 2400/1200/300 A barfly 555-7685 1200/300 A bites 555-1675 2400/1200/300 A camelot 555-0542 300 C core 555-2912 1200/300 C fooey 555-1234 2400/1200/300 B foot 555-6699 1200/300 B macfoo 555-6480 1200/300 A sdace 555-3430 2400/1200/300 A sabafoo 555-2127 1200/300 C @c endfile @end example @cindex @code{inventory-shipped} file The @value{DF} @file{inventory-shipped} represents information about shipments during the year. Each record contains the month, the number of green crates shipped, the number of red boxes shipped, the number of orange bags shipped, and the number of blue packages shipped, respectively. There are 16 entries, covering the 12 months of last year and the first four months of the current year. @example @c file eg/data/inventory-shipped Jan 13 25 15 115 Feb 15 32 24 226 Mar 15 24 34 228 Apr 31 52 63 420 May 16 34 29 208 Jun 31 42 75 492 Jul 24 34 67 436 Aug 15 34 47 316 Sep 13 55 37 277 Oct 29 54 68 525 Nov 20 87 82 577 Dec 17 35 61 401 Jan 21 36 64 620 Feb 26 58 80 652 Mar 24 75 70 495 Apr 21 70 74 514 @c endfile @end example @ifinfo If you are reading this in GNU Emacs using Info, you can copy the regions of text showing these sample files into your own test files. This way you can try out the examples shown in the remainder of this document. You do this by using the command @kbd{M-x write-region} to copy text from the Info file into a file for use with @command{awk} (@xref{Misc File Ops, , Miscellaneous File Operations, emacs, GNU Emacs Manual}, for more information). Using this information, create your own @file{BBS-list} and @file{inventory-shipped} files and practice what you learn in this @value{DOCUMENT}. @cindex Texinfo If you are using the stand-alone version of Info, see @ref{Extract Program}, for an @command{awk} program that extracts these @value{DF}s from @file{gawk.texi}, the Texinfo source file for this Info file. @end ifinfo @node Very Simple @section Some Simple Examples The following command runs a simple @command{awk} program that searches the input file @file{BBS-list} for the character string @samp{foo} (a grouping of characters is usually called a @dfn{string}; the term @dfn{string} is based on similar usage in English, such as ``a string of pearls,'' or ``a string of cars in a train''): @example awk '/foo/ @{ print $0 @}' BBS-list @end example @noindent When lines containing @samp{foo} are found, they are printed because @w{@samp{print $0}} means print the current line. (Just @samp{print} by itself means the same thing, so we could have written that instead.) You will notice that slashes (@samp{/}) surround the string @samp{foo} in the @command{awk} program. The slashes indicate that @samp{foo} is the pattern to search for. This type of pattern is called a @dfn{regular expression}, which is covered in more detail later (@pxref{Regexp}). The pattern is allowed to match parts of words. There are single quotes around the @command{awk} program so that the shell won't interpret any of it as special shell characters. Here is what this program prints: @example $ awk '/foo/ @{ print $0 @}' BBS-list @print{} fooey 555-1234 2400/1200/300 B @print{} foot 555-6699 1200/300 B @print{} macfoo 555-6480 1200/300 A @print{} sabafoo 555-2127 1200/300 C @end example @cindex actions, default @cindex patterns, default In an @command{awk} rule, either the pattern or the action can be omitted, but not both. If the pattern is omitted, then the action is performed for @emph{every} input line. If the action is omitted, the default action is to print all lines that match the pattern. @cindex actions, empty Thus, we could leave out the action (the @code{print} statement and the curly braces) in the previous example and the result would be the same: all lines matching the pattern @samp{foo} are printed. By comparison, omitting the @code{print} statement but retaining the curly braces makes an empty action that does nothing (i.e., no lines are printed). @cindex @command{awk} programs, one-line examples Many practical @command{awk} programs are just a line or two. Following is a collection of useful, short programs to get you started. Some of these programs contain constructs that haven't been covered yet. (The description of the program will give you a good idea of what is going on, but please read the rest of the @value{DOCUMENT} to become an @command{awk} expert!) Most of the examples use a @value{DF} named @file{data}. This is just a placeholder; if you use these programs yourself, substitute your own @value{FN}s for @file{data}. For future reference, note that there is often more than one way to do things in @command{awk}. At some point, you may want to look back at these examples and see if you can come up with different ways to do the same things shown here: @itemize @bullet @item Print the length of the longest input line: @example awk '@{ if (length($0) > max) max = length($0) @} END @{ print max @}' data @end example @item Print every line that is longer than 80 characters: @example awk 'length($0) > 80' data @end example The sole rule has a relational expression as its pattern and it has no action---so the default action, printing the record, is used. @cindex @command{expand} utility @item Print the length of the longest line in @file{data}: @example expand data | awk '@{ if (x < length()) x = length() @} END @{ print "maximum line length is " x @}' @end example The input is processed by the @command{expand} utility to change tabs into spaces, so the widths compared are actually the right-margin columns. @item Print every line that has at least one field: @example awk 'NF > 0' data @end example This is an easy way to delete blank lines from a file (or rather, to create a new file similar to the old file but from which the blank lines have been removed). @item Print seven random numbers from 0 to 100, inclusive: @example awk 'BEGIN @{ for (i = 1; i <= 7; i++) print int(101 * rand()) @}' @end example @item Print the total number of bytes used by @var{files}: @example ls -l @var{files} | awk '@{ x += $5 @} END @{ print "total bytes: " x @}' @end example @item Print the total number of kilobytes used by @var{files}: @c Don't use \ continuation, not discussed yet @example ls -l @var{files} | awk '@{ x += $5 @} END @{ print "total K-bytes: " (x + 1023)/1024 @}' @end example @item Print a sorted list of the login names of all users: @example awk -F: '@{ print $1 @}' /etc/passwd | sort @end example @item Count the lines in a file: @example awk 'END @{ print NR @}' data @end example @item Print the even-numbered lines in the @value{DF}: @example awk 'NR % 2 == 0' data @end example If you use the expression @samp{NR % 2 == 1} instead, the program would print the odd-numbered lines. @end itemize @node Two Rules @section An Example with Two Rules @cindex @command{awk} programs The @command{awk} utility reads the input files one line at a time. For each line, @command{awk} tries the patterns of each of the rules. If several patterns match, then several actions are run in the order in which they appear in the @command{awk} program. If no patterns match, then no actions are run. After processing all the rules that match the line (and perhaps there are none), @command{awk} reads the next line. (However, @pxref{Next Statement}, and also @pxref{Nextfile Statement}). This continues until the program reaches the end of the file. For example, the following @command{awk} program contains two rules: @example /12/ @{ print $0 @} /21/ @{ print $0 @} @end example @noindent The first rule has the string @samp{12} as the pattern and @samp{print $0} as the action. The second rule has the string @samp{21} as the pattern and also has @samp{print $0} as the action. Each rule's action is enclosed in its own pair of braces. This program prints every line that contains the string @samp{12} @emph{or} the string @samp{21}. If a line contains both strings, it is printed twice, once by each rule. This is what happens if we run this program on our two sample @value{DF}s, @file{BBS-list} and @file{inventory-shipped}: @example $ awk '/12/ @{ print $0 @} > /21/ @{ print $0 @}' BBS-list inventory-shipped @print{} aardvark 555-5553 1200/300 B @print{} alpo-net 555-3412 2400/1200/300 A @print{} barfly 555-7685 1200/300 A @print{} bites 555-1675 2400/1200/300 A @print{} core 555-2912 1200/300 C @print{} fooey 555-1234 2400/1200/300 B @print{} foot 555-6699 1200/300 B @print{} macfoo 555-6480 1200/300 A @print{} sdace 555-3430 2400/1200/300 A @print{} sabafoo 555-2127 1200/300 C @print{} sabafoo 555-2127 1200/300 C @print{} Jan 21 36 64 620 @print{} Apr 21 70 74 514 @end example @noindent Note how the line beginning with @samp{sabafoo} in @file{BBS-list} was printed twice, once for each rule. @node More Complex @section A More Complex Example Now that we've mastered some simple tasks, let's look at what typical @command{awk} programs do. This example shows how @command{awk} can be used to summarize, select, and rearrange the output of another utility. It uses features that haven't been covered yet, so don't worry if you don't understand all the details: @example ls -l | awk '$6 == "Nov" @{ sum += $5 @} END @{ print sum @}' @end example @cindex @command{csh} utility, backslash continuation and @cindex @command{ls} utility @cindex backslash (@code{\}), continuing lines and, in @command{csh} @cindex @code{\} (backslash), continuing lines and, in @command{csh} This command prints the total number of bytes in all the files in the current directory that were last modified in November (of any year). @footnote{In the C shell (@command{csh}), you need to type a semicolon and then a backslash at the end of the first line; see @ref{Statements/Lines}, for an explanation. In a POSIX-compliant shell, such as the Bourne shell or @command{bash}, you can type the example as shown. If the command @samp{echo $path} produces an empty output line, you are most likely using a POSIX-compliant shell. Otherwise, you are probably using the C shell or a shell derived from it.} The @w{@samp{ls -l}} part of this example is a system command that gives you a listing of the files in a directory, including each file's size and the date the file was last modified. Its output looks like this: @example -rw-r--r-- 1 arnold user 1933 Nov 7 13:05 Makefile -rw-r--r-- 1 arnold user 10809 Nov 7 13:03 awk.h -rw-r--r-- 1 arnold user 983 Apr 13 12:14 awk.tab.h -rw-r--r-- 1 arnold user 31869 Jun 15 12:20 awk.y -rw-r--r-- 1 arnold user 22414 Nov 7 13:03 awk1.c -rw-r--r-- 1 arnold user 37455 Nov 7 13:03 awk2.c -rw-r--r-- 1 arnold user 27511 Dec 9 13:07 awk3.c -rw-r--r-- 1 arnold user 7989 Nov 7 13:03 awk4.c @end example @noindent @cindex line continuations, with C shell The first field contains read-write permissions, the second field contains the number of links to the file, and the third field identifies the owner of the file. The fourth field identifies the group of the file. The fifth field contains the size of the file in bytes. The sixth, seventh, and eighth fields contain the month, day, and time, respectively, that the file was last modified. Finally, the ninth field contains the name of the file.@footnote{On some very old systems, you may need to use @samp{ls -lg} to get this output.} @c @cindex automatic initialization @cindex initialization, automatic The @samp{$6 == "Nov"} in our @command{awk} program is an expression that tests whether the sixth field of the output from @w{@samp{ls -l}} matches the string @samp{Nov}. Each time a line has the string @samp{Nov} for its sixth field, the action @samp{sum += $5} is performed. This adds the fifth field (the file's size) to the variable @code{sum}. As a result, when @command{awk} has finished reading all the input lines, @code{sum} is the total of the sizes of the files whose lines matched the pattern. (This works because @command{awk} variables are automatically initialized to zero.) After the last line of output from @command{ls} has been processed, the @code{END} rule executes and prints the value of @code{sum}. In this example, the value of @code{sum} is 80600. These more advanced @command{awk} techniques are covered in later sections (@pxref{Action Overview}). Before you can move on to more advanced @command{awk} programming, you have to know how @command{awk} interprets your input and displays your output. By manipulating fields and using @code{print} statements, you can produce some very useful and impressive-looking reports. @node Statements/Lines @section @command{awk} Statements Versus Lines @cindex line breaks @cindex newlines Most often, each line in an @command{awk} program is a separate statement or separate rule, like this: @example awk '/12/ @{ print $0 @} /21/ @{ print $0 @}' BBS-list inventory-shipped @end example @cindex @command{gawk}, newlines in However, @command{gawk} ignores newlines after any of the following symbols and keywords: @example , @{ ? : || && do else @end example @noindent A newline at any other point is considered the end of the statement.@footnote{The @samp{?} and @samp{:} referred to here is the three-operand conditional expression described in @ref{Conditional Exp}. Splitting lines after @samp{?} and @samp{:} is a minor @command{gawk} extension; if @option{--posix} is specified (@pxref{Options}), then this extension is disabled.} @cindex @code{\} (backslash), continuing lines and @cindex backslash (@code{\}), continuing lines and If you would like to split a single statement into two lines at a point where a newline would terminate it, you can @dfn{continue} it by ending the first line with a backslash character (@samp{\}). The backslash must be the final character on the line in order to be recognized as a continuation character. A backslash is allowed anywhere in the statement, even in the middle of a string or regular expression. For example: @example awk '/This regular expression is too long, so continue it\ on the next line/ @{ print $1 @}' @end example @noindent @cindex portability, backslash continuation and We have generally not used backslash continuation in the sample programs in this @value{DOCUMENT}. In @command{gawk}, there is no limit on the length of a line, so backslash continuation is never strictly necessary; it just makes programs more readable. For this same reason, as well as for clarity, we have kept most statements short in the sample programs presented throughout the @value{DOCUMENT}. Backslash continuation is most useful when your @command{awk} program is in a separate source file instead of entered from the command line. You should also note that many @command{awk} implementations are more particular about where you may use backslash continuation. For example, they may not allow you to split a string constant using backslash continuation. Thus, for maximum portability of your @command{awk} programs, it is best not to split your lines in the middle of a regular expression or a string. @c 10/2000: gawk, mawk, and current bell labs awk allow it, @c solaris 2.7 nawk does not. Solaris /usr/xpg4/bin/awk does though! sigh. @cindex @command{csh} utility @cindex backslash (@code{\}), continuing lines and, in @command{csh} @cindex @code{\} (backslash), continuing lines and, in @command{csh} @strong{Caution:} @emph{Backslash continuation does not work as described with the C shell.} It works for @command{awk} programs in files and for one-shot programs, @emph{provided} you are using a POSIX-compliant shell, such as the Unix Bourne shell or @command{bash}. But the C shell behaves differently! There, you must use two backslashes in a row, followed by a newline. Note also that when using the C shell, @emph{every} newline in your awk program must be escaped with a backslash. To illustrate: @example % awk 'BEGIN @{ \ ? print \\ ? "hello, world" \ ? @}' @print{} hello, world @end example @noindent Here, the @samp{%} and @samp{?} are the C shell's primary and secondary prompts, analogous to the standard shell's @samp{$} and @samp{>}. Compare the previous example to how it is done with a POSIX-compliant shell: @example $ awk 'BEGIN @{ > print \ > "hello, world" > @}' @print{} hello, world @end example @command{awk} is a line-oriented language. Each rule's action has to begin on the same line as the pattern. To have the pattern and action on separate lines, you @emph{must} use backslash continuation; there is no other option. @cindex backslash (@code{\}), continuing lines and, comments and @cindex @code{\} (backslash), continuing lines and, comments and @cindex commenting, backslash continuation and Another thing to keep in mind is that backslash continuation and comments do not mix. As soon as @command{awk} sees the @samp{#} that starts a comment, it ignores @emph{everything} on the rest of the line. For example: @example $ gawk 'BEGIN @{ print "dont panic" # a friendly \ > BEGIN rule > @}' @error{} gawk: cmd. line:2: BEGIN rule @error{} gawk: cmd. line:2: ^ parse error @end example @noindent In this case, it looks like the backslash would continue the comment onto the next line. However, the backslash-newline combination is never even noticed because it is ``hidden'' inside the comment. Thus, the @code{BEGIN} is noted as a syntax error. @cindex statements, multiple @cindex @code{;} (semicolon) @cindex semicolon (@code{;}) When @command{awk} statements within one rule are short, you might want to put more than one of them on a line. This is accomplished by separating the statements with a semicolon (@samp{;}). This also applies to the rules themselves. Thus, the program shown at the start of this @value{SECTION} could also be written this way: @example /12/ @{ print $0 @} ; /21/ @{ print $0 @} @end example @noindent @strong{Note:} The requirement that states that rules on the same line must be separated with a semicolon was not in the original @command{awk} language; it was added for consistency with the treatment of statements within an action. @node Other Features @section Other Features of @command{awk} @cindex variables The @command{awk} language provides a number of predefined, or @dfn{built-in}, variables that your programs can use to get information from @command{awk}. There are other variables your program can set as well to control how @command{awk} processes your data. In addition, @command{awk} provides a number of built-in functions for doing common computational and string-related operations. @command{gawk} provides built-in functions for working with timestamps, performing bit manipulation, and for runtime string translation. As we develop our presentation of the @command{awk} language, we introduce most of the variables and many of the functions. They are defined systematically in @ref{Built-in Variables}, and @ref{Built-in}. @node When @section When to Use @command{awk} @cindex @command{awk}, uses for Now that you've seen some of what @command{awk} can do, you might wonder how @command{awk} could be useful for you. By using utility programs, advanced patterns, field separators, arithmetic statements, and other selection criteria, you can produce much more complex output. The @command{awk} language is very useful for producing reports from large amounts of raw data, such as summarizing information from the output of other utility programs like @command{ls}. (@xref{More Complex}.) Programs written with @command{awk} are usually much smaller than they would be in other languages. This makes @command{awk} programs easy to compose and use. Often, @command{awk} programs can be quickly composed at your terminal, used once, and thrown away. Because @command{awk} programs are interpreted, you can avoid the (usually lengthy) compilation part of the typical edit-compile-test-debug cycle of software development. Complex programs have been written in @command{awk}, including a complete retargetable assembler for eight-bit microprocessors (@pxref{Glossary}, for more information), and a microcode assembler for a special-purpose Prolog computer. However, @command{awk}'s capabilities are strained by tasks of such complexity. @cindex @command{awk} programs, complex If you find yourself writing @command{awk} scripts of more than, say, a few hundred lines, you might consider using a different programming language. Emacs Lisp is a good choice if you need sophisticated string or pattern matching capabilities. The shell is also good at string and pattern matching; in addition, it allows powerful use of the system utilities. More conventional languages, such as C, C++, and Java, offer better facilities for system programming and for managing the complexity of large programs. Programs in these languages may require more lines of source code than the equivalent @command{awk} programs, but they are easier to maintain and usually run more efficiently. @node Regexp @chapter Regular Expressions @cindex regexp, See regular expressions @c STARTOFRANGE regexp @cindex regular expressions A @dfn{regular expression}, or @dfn{regexp}, is a way of describing a set of strings. Because regular expressions are such a fundamental part of @command{awk} programming, their format and use deserve a separate @value{CHAPTER}. @cindex forward slash (@code{/}) @cindex @code{/} (forward slash) A regular expression enclosed in slashes (@samp{/}) is an @command{awk} pattern that matches every input record whose text belongs to that set. The simplest regular expression is a sequence of letters, numbers, or both. Such a regexp matches any string that contains that sequence. Thus, the regexp @samp{foo} matches any string containing @samp{foo}. Therefore, the pattern @code{/foo/} matches any input record containing the three characters @samp{foo} @emph{anywhere} in the record. Other kinds of regexps let you specify more complicated classes of strings. @ifnotinfo Initially, the examples in this @value{CHAPTER} are simple. As we explain more about how regular expressions work, we will present more complicated instances. @end ifnotinfo @menu * Regexp Usage:: How to Use Regular Expressions. * Escape Sequences:: How to write nonprinting characters. * Regexp Operators:: Regular Expression Operators. * Character Lists:: What can go between @samp{[...]}. * GNU Regexp Operators:: Operators specific to GNU software. * Case-sensitivity:: How to do case-insensitive matching. * Leftmost Longest:: How much text matches. * Computed Regexps:: Using Dynamic Regexps. * Locales:: How the locale affects things. @end menu @node Regexp Usage @section How to Use Regular Expressions @cindex regular expressions, as patterns A regular expression can be used as a pattern by enclosing it in slashes. Then the regular expression is tested against the entire text of each record. (Normally, it only needs to match some part of the text in order to succeed.) For example, the following prints the second field of each record that contains the string @samp{foo} anywhere in it: @example $ awk '/foo/ @{ print $2 @}' BBS-list @print{} 555-1234 @print{} 555-6699 @print{} 555-6480 @print{} 555-2127 @end example @cindex regular expressions, operators @cindex operators, string-matching @c @cindex operators, @code{~} @cindex string-matching operators @code{~} (tilde), @code{~} operator @cindex tilde (@code{~}), @code{~} operator @cindex @code{!} (exclamation point), @code{!~} operator @cindex exclamation point (@code{!}), @code{!~} operator @c @cindex operators, @code{!~} @cindex @code{if} statement @cindex @code{while} statement @cindex @code{do}-@code{while} statement @c @cindex statements, @code{if} @c @cindex statements, @code{while} @c @cindex statements, @code{do} Regular expressions can also be used in matching expressions. These expressions allow you to specify the string to match against; it need not be the entire current input record. The two operators @samp{~} and @samp{!~} perform regular expression comparisons. Expressions using these operators can be used as patterns, or in @code{if}, @code{while}, @code{for}, and @code{do} statements. (@xref{Statements}.) For example: @example @var{exp} ~ /@var{regexp}/ @end example @noindent is true if the expression @var{exp} (taken as a string) matches @var{regexp}. The following example matches, or selects, all input records with the uppercase letter @samp{J} somewhere in the first field: @example $ awk '$1 ~ /J/' inventory-shipped @print{} Jan 13 25 15 115 @print{} Jun 31 42 75 492 @print{} Jul 24 34 67 436 @print{} Jan 21 36 64 620 @end example So does this: @example awk '@{ if ($1 ~ /J/) print @}' inventory-shipped @end example This next example is true if the expression @var{exp} (taken as a character string) does @emph{not} match @var{regexp}: @example @var{exp} !~ /@var{regexp}/ @end example The following example matches, or selects, all input records whose first field @emph{does not} contain the uppercase letter @samp{J}: @example $ awk '$1 !~ /J/' inventory-shipped @print{} Feb 15 32 24 226 @print{} Mar 15 24 34 228 @print{} Apr 31 52 63 420 @print{} May 16 34 29 208 @dots{} @end example @cindex regexp constants @cindex regular expressions, constants, See regexp constants When a regexp is enclosed in slashes, such as @code{/foo/}, we call it a @dfn{regexp constant}, much like @code{5.27} is a numeric constant and @code{"foo"} is a string constant. @node Escape Sequences @section Escape Sequences @cindex escape sequences @cindex backslash (@code{\}), in escape sequences @cindex @code{\} (backslash), in escape sequences Some characters cannot be included literally in string constants (@code{"foo"}) or regexp constants (@code{/foo/}). Instead, they should be represented with @dfn{escape sequences}, which are character sequences beginning with a backslash (@samp{\}). One use of an escape sequence is to include a double-quote character in a string constant. Because a plain double quote ends the string, you must use @samp{\"} to represent an actual double-quote character as a part of the string. For example: @example $ awk 'BEGIN @{ print "He said \"hi!\" to her." @}' @print{} He said "hi!" to her. @end example The backslash character itself is another character that cannot be included normally; you must write @samp{\\} to put one backslash in the string or regexp. Thus, the string whose contents are the two characters @samp{"} and @samp{\} must be written @code{"\"\\"}. Backslash also represents unprintable characters such as TAB or newline. While there is nothing to stop you from entering most unprintable characters directly in a string constant or regexp constant, they may look ugly. The following table lists all the escape sequences used in @command{awk} and what they represent. Unless noted otherwise, all these escape sequences apply to both string constants and regexp constants: @table @code @item \\ A literal backslash, @samp{\}. @c @cindex @command{awk} language, V.4 version @cindex @code{\} (backslash), @code{\a} escape sequence @cindex backslash (@code{\}), @code{\a} escape sequence @item \a The ``alert'' character, @kbd{@value{CTL}-g}, ASCII code 7 (BEL). (This usually makes some sort of audible noise.) @cindex @code{\} (backslash), @code{\b} escape sequence @cindex backslash (@code{\}), @code{\b} escape sequence @item \b Backspace, @kbd{@value{CTL}-h}, ASCII code 8 (BS). @cindex @code{\} (backslash), @code{\f} escape sequence @cindex backslash (@code{\}), @code{\f} escape sequence @item \f Formfeed, @kbd{@value{CTL}-l}, ASCII code 12 (FF). @cindex @code{\} (backslash), @code{\n} escape sequence @cindex backslash (@code{\}), @code{\n} escape sequence @item \n Newline, @kbd{@value{CTL}-j}, ASCII code 10 (LF). @cindex @code{\} (backslash), @code{\r} escape sequence @cindex backslash (@code{\}), @code{\r} escape sequence @item \r Carriage return, @kbd{@value{CTL}-m}, ASCII code 13 (CR). @cindex @code{\} (backslash), @code{\t} escape sequence @cindex backslash (@code{\}), @code{\t} escape sequence @item \t Horizontal TAB, @kbd{@value{CTL}-i}, ASCII code 9 (HT). @c @cindex @command{awk} language, V.4 version @cindex @code{\} (backslash), @code{\v} escape sequence @cindex backslash (@code{\}), @code{\v} escape sequence @item \v Vertical tab, @kbd{@value{CTL}-k}, ASCII code 11 (VT). @cindex @code{\} (backslash), @code{\}@var{nnn} escape sequence @cindex backslash (@code{\}), @code{\}@var{nnn} escape sequence @item \@var{nnn} The octal value @var{nnn}, where @var{nnn} stands for 1 to 3 digits between @samp{0} and @samp{7}. For example, the code for the ASCII ESC (escape) character is @samp{\033}. @c @cindex @command{awk} language, V.4 version @c @cindex @command{awk} language, POSIX version @cindex @code{\} (backslash), @code{\x} escape sequence @cindex backslash (@code{\}), @code{\x} escape sequence @item \x@var{hh}@dots{} The hexadecimal value @var{hh}, where @var{hh} stands for a sequence of hexadecimal digits (@samp{0}--@samp{9}, and either @samp{A}--@samp{F} or @samp{a}--@samp{f}). Like the same construct in ISO C, the escape sequence continues until the first nonhexadecimal digit is seen. However, using more than two hexadecimal digits produces undefined results. (The @samp{\x} escape sequence is not allowed in POSIX @command{awk}.) @cindex @code{\} (backslash), @code{\/} escape sequence @cindex backslash (@code{\}), @code{\/} escape sequence @item \/ A literal slash (necessary for regexp constants only). This expression is used when you want to write a regexp constant that contains a slash. Because the regexp is delimited by slashes, you need to escape the slash that is part of the pattern, in order to tell @command{awk} to keep processing the rest of the regexp. @cindex @code{\} (backslash), @code{\"} escape sequence @cindex backslash (@code{\}), @code{\"} escape sequence @item \" A literal double quote (necessary for string constants only). This expression is used when you want to write a string constant that contains a double quote. Because the string is delimited by double quotes, you need to escape the quote that is part of the string, in order to tell @command{awk} to keep processing the rest of the string. @end table In @command{gawk}, a number of additional two-character sequences that begin with a backslash have special meaning in regexps. @xref{GNU Regexp Operators}. In a regexp, a backslash before any character that is not in the previous list and not listed in @ref{GNU Regexp Operators}, means that the next character should be taken literally, even if it would normally be a regexp operator. For example, @code{/a\+b/} matches the three characters @samp{a+b}. @cindex backslash (@code{\}), in escape sequences @cindex @code{\} (backslash), in escape sequences @cindex portability For complete portability, do not use a backslash before any character not shown in the previous list. To summarize: @itemize @bullet @item The escape sequences in the table above are always processed first, for both string constants and regexp constants. This happens very early, as soon as @command{awk} reads your program. @item @command{gawk} processes both regexp constants and dynamic regexps (@pxref{Computed Regexps}), for the special operators listed in @ref{GNU Regexp Operators}. @item A backslash before any other character means to treat that character literally. @end itemize @c fakenode --- for prepinfo @subheading Advanced Notes: Backslash Before Regular Characters @cindex portability, backslash in escape sequences @cindex POSIX @command{awk}, backslashes in string constants @cindex backslash (@code{\}), in escape sequences, POSIX and @cindex @code{\} (backslash), in escape sequences, POSIX and @cindex troubleshooting, backslash before nonspecial character If you place a backslash in a string constant before something that is not one of the characters previously listed, POSIX @command{awk} purposely leaves what happens as undefined. There are two choices: @c @cindex automatic warnings @c @cindex warnings, automatic @table @asis @item Strip the backslash out This is what Unix @command{awk} and @command{gawk} both do. For example, @code{"a\qc"} is the same as @code{"aqc"}. (Because this is such an easy bug both to introduce and to miss, @command{gawk} warns you about it.) Consider @samp{FS = @w{"[ \t]+\|[ \t]+"}} to use vertical bars surrounded by whitespace as the field separator. There should be two backslashes in the string @samp{FS = @w{"[ \t]+\\|[ \t]+"}}.) @c I did this! This is why I added the warning. @cindex @command{gawk}, escape sequences @cindex Unix @command{awk}, backslashes in escape sequences @item Leave the backslash alone Some other @command{awk} implementations do this. In such implementations, typing @code{"a\qc"} is the same as typing @code{"a\\qc"}. @end table @c fakenode --- for prepinfo @subheading Advanced Notes: Escape Sequences for Metacharacters @cindex metacharacters, escape sequences for Suppose you use an octal or hexadecimal escape to represent a regexp metacharacter. (See @ref{Regexp Operators}.) Does @command{awk} treat the character as a literal character or as a regexp operator? @cindex dark corner, escape sequences, for metacharacters Historically, such characters were taken literally. @value{DARKCORNER} However, the POSIX standard indicates that they should be treated as real metacharacters, which is what @command{gawk} does. In compatibility mode (@pxref{Options}), @command{gawk} treats the characters represented by octal and hexadecimal escape sequences literally when used in regexp constants. Thus, @code{/a\52b/} is equivalent to @code{/a\*b/}. @node Regexp Operators @section Regular Expression Operators @c STARTOFRANGE regexpo @cindex regular expressions, operators You can combine regular expressions with special characters, called @dfn{regular expression operators} or @dfn{metacharacters}, to increase the power and versatility of regular expressions. The escape sequences described @ifnotinfo earlier @end ifnotinfo in @ref{Escape Sequences}, are valid inside a regexp. They are introduced by a @samp{\} and are recognized and converted into corresponding real characters as the very first step in processing regexps. Here is a list of metacharacters. All characters that are not escape sequences and that are not listed in the table stand for themselves: @table @code @cindex backslash (@code{\}) @cindex @code{\} (backslash) @item \ This is used to suppress the special meaning of a character when matching. For example, @samp{\$} matches the character @samp{$}. @cindex regular expressions, anchors in @cindex Texinfo, chapter beginnings in files @cindex @code{^} (caret) @cindex caret (@code{^}) @item ^ This matches the beginning of a string. For example, @samp{^@@chapter} matches @samp{@@chapter} at the beginning of a string and can be used to identify chapter beginnings in Texinfo source files. The @samp{^} is known as an @dfn{anchor}, because it anchors the pattern to match only at the beginning of the string. It is important to realize that @samp{^} does not match the beginning of a line embedded in a string. The condition is not true in the following example: @example if ("line1\nLINE 2" ~ /^L/) @dots{} @end example @cindex @code{$} (dollar sign) @cindex dollar sign (@code{$}) @item $ This is similar to @samp{^}, but it matches only at the end of a string. For example, @samp{p$} matches a record that ends with a @samp{p}. The @samp{$} is an anchor and does not match the end of a line embedded in a string. The condition in the following example is not true: @example if ("line1\nLINE 2" ~ /1$/) @dots{} @end example @cindex @code{.} (period) @cindex period (@code{.}) @item . This matches any single character, @emph{including} the newline character. For example, @samp{.P} matches any single character followed by a @samp{P} in a string. Using concatenation, we can make a regular expression such as @samp{U.A}, which matches any three-character sequence that begins with @samp{U} and ends with @samp{A}. @c comma before using does NOT do tertiary @cindex POSIX @command{awk}, period (@code{.}), using In strict POSIX mode (@pxref{Options}), @samp{.} does not match the @sc{nul} character, which is a character with all bits equal to zero. Otherwise, @sc{nul} is just another character. Other versions of @command{awk} may not be able to match the @sc{nul} character. @cindex @code{[]} (square brackets) @cindex square brackets (@code{[]}) @cindex character lists @cindex character sets, See Also character lists @cindex bracket expressions, See character lists @item [@dots{}] This is called a @dfn{character list}.@footnote{In other literature, you may see a character list referred to as either a @dfn{character set}, a @dfn{character class}, or a @dfn{bracket expression}.} It matches any @emph{one} of the characters that are enclosed in the square brackets. For example, @samp{[MVX]} matches any one of the characters @samp{M}, @samp{V}, or @samp{X} in a string. A full discussion of what can be inside the square brackets of a character list is given in @ref{Character Lists}. @cindex character lists, complemented @item [^ @dots{}] This is a @dfn{complemented character list}. The first character after the @samp{[} @emph{must} be a @samp{^}. It matches any characters @emph{except} those in the square brackets. For example, @samp{[^awk]} matches any character that is not an @samp{a}, @samp{w}, or @samp{k}. @cindex @code{|} (vertical bar) @cindex vertical bar (@code{|}) @item | This is the @dfn{alternation operator} and it is used to specify alternatives. The @samp{|} has the lowest precedence of all the regular expression operators. For example, @samp{^P|[[:digit:]]} matches any string that matches either @samp{^P} or @samp{[[:digit:]]}. This means it matches any string that starts with @samp{P} or contains a digit. The alternation applies to the largest possible regexps on either side. @cindex @code{()} (parentheses) @cindex parentheses @code{()} @item (@dots{}) Parentheses are used for grouping in regular expressions, as in arithmetic. They can be used to concatenate regular expressions containing the alternation operator, @samp{|}. For example, @samp{@@(samp|code)\@{[^@}]+\@}} matches both @samp{@@code@{foo@}} and @samp{@@samp@{bar@}}. (These are Texinfo formatting control sequences. The @samp{+} is explained further on in this list.) @cindex @code{*} (asterisk), @code{*} operator, as regexp operator @cindex asterisk (@code{*}), @code{*} operator, as regexp operator @item * This symbol means that the preceding regular expression should be repeated as many times as necessary to find a match. For example, @samp{ph*} applies the @samp{*} symbol to the preceding @samp{h} and looks for matches of one @samp{p} followed by any number of @samp{h}s. This also matches just @samp{p} if no @samp{h}s are present. The @samp{*} repeats the @emph{smallest} possible preceding expression. (Use parentheses if you want to repeat a larger expression.) It finds as many repetitions as possible. For example, @samp{awk '/\(c[ad][ad]*r x\)/ @{ print @}' sample} prints every record in @file{sample} containing a string of the form @samp{(car x)}, @samp{(cdr x)}, @samp{(cadr x)}, and so on. Notice the escaping of the parentheses by preceding them with backslashes. @cindex @code{+} (plus sign) @cindex plus sign (@code{+}) @item + This symbol is similar to @samp{*}, except that the preceding expression must be matched at least once. This means that @samp{wh+y} would match @samp{why} and @samp{whhy}, but not @samp{wy}, whereas @samp{wh*y} would match all three of these strings. The following is a simpler way of writing the last @samp{*} example: @example awk '/\(c[ad]+r x\)/ @{ print @}' sample @end example @cindex @code{?} (question mark) @cindex question mark (@code{?}) @item ? This symbol is similar to @samp{*}, except that the preceding expression can be matched either once or not at all. For example, @samp{fe?d} matches @samp{fed} and @samp{fd}, but nothing else. @cindex interval expressions @item @{@var{n}@} @itemx @{@var{n},@} @itemx @{@var{n},@var{m}@} One or two numbers inside braces denote an @dfn{interval expression}. If there is one number in the braces, the preceding regexp is repeated @var{n} times. If there are two numbers separated by a comma, the preceding regexp is repeated @var{n} to @var{m} times. If there is one number followed by a comma, then the preceding regexp is repeated at least @var{n} times: @table @code @item wh@{3@}y Matches @samp{whhhy}, but not @samp{why} or @samp{whhhhy}. @item wh@{3,5@}y Matches @samp{whhhy}, @samp{whhhhy}, or @samp{whhhhhy}, only. @item wh@{2,@}y Matches @samp{whhy} or @samp{whhhy}, and so on. @end table @cindex POSIX @command{awk}, interval expressions in Interval expressions were not traditionally available in @command{awk}. They were added as part of the POSIX standard to make @command{awk} and @command{egrep} consistent with each other. @cindex @command{gawk}, interval expressions and However, because old programs may use @samp{@{} and @samp{@}} in regexp constants, by default @command{gawk} does @emph{not} match interval expressions in regexps. If either @option{--posix} or @option{--re-interval} are specified (@pxref{Options}), then interval expressions are allowed in regexps. For new programs that use @samp{@{} and @samp{@}} in regexp constants, it is good practice to always escape them with a backslash. Then the regexp constants are valid and work the way you want them to, using any version of @command{awk}.@footnote{Use two backslashes if you're using a string constant with a regexp operator or function.} @end table @cindex precedence, regexp operators @cindex regular expressions, operators, precedence of In regular expressions, the @samp{*}, @samp{+}, and @samp{?} operators, as well as the braces @samp{@{} and @samp{@}}, have the highest precedence, followed by concatenation, and finally by @samp{|}. As in arithmetic, parentheses can change how operators are grouped. @cindex POSIX @command{awk}, regular expressions and @cindex @command{gawk}, regular expressions, precedence In POSIX @command{awk} and @command{gawk}, the @samp{*}, @samp{+}, and @samp{?} operators stand for themselves when there is nothing in the regexp that precedes them. For example, @samp{/+/} matches a literal plus sign. However, many other versions of @command{awk} treat such a usage as a syntax error. If @command{gawk} is in compatibility mode (@pxref{Options}), POSIX character classes and interval expressions are not available in regular expressions. @c ENDOFRANGE regexpo @node Character Lists @section Using Character Lists @c STARTOFRANGE charlist @cindex character lists @cindex character lists, range expressions @cindex range expressions Within a character list, a @dfn{range expression} consists of two characters separated by a hyphen. It matches any single character that sorts between the two characters, using the locale's collating sequence and character set. For example, in the default C locale, @samp{[a-dx-z]} is equivalent to @samp{[abcdxyz]}. Many locales sort characters in dictionary order, and in these locales, @samp{[a-dx-z]} is typically not equivalent to @samp{[abcdxyz]}; instead it might be equivalent to @samp{[aBbCcDdxXyYz]}, for example. To obtain the traditional interpretation of bracket expressions, you can use the C locale by setting the @env{LC_ALL} environment variable to the value @samp{C}. @cindex @code{\} (backslash), in character lists @cindex backslash (@code{\}), in character lists @cindex @code{^} (caret), in character lists @cindex caret (@code{^}), in character lists @cindex @code{-} (hyphen), in character lists @cindex hyphen (@code{-}), in character lists To include one of the characters @samp{\}, @samp{]}, @samp{-}, or @samp{^} in a character list, put a @samp{\} in front of it. For example: @example [d\]] @end example @noindent matches either @samp{d} or @samp{]}. @cindex POSIX @command{awk}, character lists and @cindex Extended Regular Expressions (EREs) @cindex EREs (Extended Regular Expressions) @cindex @command{egrep} utility This treatment of @samp{\} in character lists is compatible with other @command{awk} implementations and is also mandated by POSIX. The regular expressions in @command{awk} are a superset of the POSIX specification for Extended Regular Expressions (EREs). POSIX EREs are based on the regular expressions accepted by the traditional @command{egrep} utility. @cindex character lists, character classes @cindex POSIX @command{awk}, character lists and, character classes @dfn{Character classes} are a new feature introduced in the POSIX standard. A character class is a special notation for describing lists of characters that have a specific attribute, but the actual characters can vary from country to country and/or from character set to character set. For example, the notion of what is an alphabetic character differs between the United States and France. A character class is only valid in a regexp @emph{inside} the brackets of a character list. Character classes consist of @samp{[:}, a keyword denoting the class, and @samp{:]}. Here are the character classes defined by the POSIX standard. @c the regular table is commented out while trying out the multitable. @c leave it here in case we need to go back, but make sure the text @c still corresponds! @ignore @table @code @item [:alnum:] Alphanumeric characters. @item [:alpha:] Alphabetic characters. @item [:blank:] Space and TAB characters. @item [:cntrl:] Control characters. @item [:digit:] Numeric characters. @item [:graph:] Characters that are printable and visible. (A space is printable but not visible, whereas an @samp{a} is both.) @item [:lower:] Lowercase alphabetic characters. @item [:print:] Printable characters (characters that are not control characters). @item [:punct:] Punctuation characters (characters that are not letters, digits, control characters, or space characters). @item [:space:] Space characters (such as space, TAB, and formfeed, to name a few). @item [:upper:] Uppercase alphabetic characters. @item [:xdigit:] Characters that are hexadecimal digits. @end table @end ignore @multitable {@code{[:xdigit:]}} {Characters that are both printable and visible. (A space is} @item @code{[:alnum:]} @tab Alphanumeric characters. @item @code{[:alpha:]} @tab Alphabetic characters. @item @code{[:blank:]} @tab Space and TAB characters. @item @code{[:cntrl:]} @tab Control characters. @item @code{[:digit:]} @tab Numeric characters. @item @code{[:graph:]} @tab Characters that are both printable and visible. (A space is printable but not visible, whereas an @samp{a} is both.) @item @code{[:lower:]} @tab Lowercase alphabetic characters. @item @code{[:print:]} @tab Printable characters (characters that are not control characters). @item @code{[:punct:]} @tab Punctuation characters (characters that are not letters, digits, control characters, or space characters). @item @code{[:space:]} @tab Space characters (such as space, TAB, and formfeed, to name a few). @item @code{[:upper:]} @tab Uppercase alphabetic characters. @item @code{[:xdigit:]} @tab Characters that are hexadecimal digits. @end multitable For example, before the POSIX standard, you had to write @code{/[A-Za-z0-9]/} to match alphanumeric characters. If your character set had other alphabetic characters in it, this would not match them, and if your character set collated differently from ASCII, this might not even match the ASCII alphanumeric characters. With the POSIX character classes, you can write @code{/[[:alnum:]]/} to match the alphabetic and numeric characters in your character set. @cindex character lists, collating elements @cindex character lists, non-ASCII @cindex collating elements Two additional special sequences can appear in character lists. These apply to non-ASCII character sets, which can have single symbols (called @dfn{collating elements}) that are represented with more than one character. They can also have several characters that are equivalent for @dfn{collating}, or sorting, purposes. (For example, in French, a plain ``e'' and a grave-accented ``@`e'' are equivalent.) These sequences are: @table @asis @cindex character lists, collating symbols @cindex collating symbols @item Collating symbols Multicharacter collating elements enclosed between @samp{[.} and @samp{.]}. For example, if @samp{ch} is a collating element, then @code{[[.ch.]]} is a regexp that matches this collating element, whereas @code{[ch]} is a regexp that matches either @samp{c} or @samp{h}. @cindex character lists, equivalence classes @item Equivalence classes Locale-specific names for a list of characters that are equal. The name is enclosed between @samp{[=} and @samp{=]}. For example, the name @samp{e} might be used to represent all of ``e,'' ``@`e,'' and ``@'e.'' In this case, @code{[[=e=]]} is a regexp that matches any of @samp{e}, @samp{@'e}, or @samp{@`e}. @end table These features are very valuable in non-English-speaking locales. @cindex internationalization, localization, character classes @cindex @command{gawk}, character classes and @cindex POSIX @command{awk}, character lists and, character classes @strong{Caution:} The library functions that @command{gawk} uses for regular expression matching currently recognize only POSIX character classes; they do not recognize collating symbols or equivalence classes. @c maybe one day ... @c ENDOFRANGE charlist @node GNU Regexp Operators @section @command{gawk}-Specific Regexp Operators @c This section adapted (long ago) from the regex-0.12 manual @c STARTOFRANGE regexpg @cindex regular expressions, operators, @command{gawk} @c STARTOFRANGE gregexp @cindex @command{gawk}, regular expressions, operators @cindex operators, GNU-specific @cindex regular expressions, operators, for words @cindex word, regexp definition of GNU software that deals with regular expressions provides a number of additional regexp operators. These operators are described in this @value{SECTION} and are specific to @command{gawk}; they are not available in other @command{awk} implementations. Most of the additional operators deal with word matching. For our purposes, a @dfn{word} is a sequence of one or more letters, digits, or underscores (@samp{_}): @table @code @c @cindex operators, @code{\w} (@command{gawk}) @cindex backslash (@code{\}), @code{\w} operator (@command{gawk}) @cindex @code{\} (backslash), @code{\w} operator (@command{gawk}) @item \w Matches any word-constituent character---that is, it matches any letter, digit, or underscore. Think of it as shorthand for @w{@code{[[:alnum:]_]}}. @c @cindex operators, @code{\W} (@command{gawk}) @cindex backslash (@code{\}), @code{\W} operator (@command{gawk}) @cindex @code{\} (backslash), @code{\W} operator (@command{gawk}) @item \W Matches any character that is not word-constituent. Think of it as shorthand for @w{@code{[^[:alnum:]_]}}. @c @cindex operators, @code{\<} (@command{gawk}) @cindex backslash (@code{\}), @code{\<} operator (@command{gawk}) @cindex @code{\} (backslash), @code{\<} operator (@command{gawk}) @item \< Matches the empty string at the beginning of a word. For example, @code{/\} (@command{gawk}) @cindex backslash (@code{\}), @code{\>} operator (@command{gawk}) @cindex @code{\} (backslash), @code{\>} operator (@command{gawk}) @item \> Matches the empty string at the end of a word. For example, @code{/stow\>/} matches @samp{stow} but not @samp{stowaway}. @c @cindex operators, @code{\y} (@command{gawk}) @cindex backslash (@code{\}), @code{\y} operator (@command{gawk}) @cindex @code{\} (backslash), @code{\y} operator (@command{gawk}) @c comma before using does NOT do secondary @cindex word boundaries, matching @item \y Matches the empty string at either the beginning or the end of a word (i.e., the word boundar@strong{y}). For example, @samp{\yballs?\y} matches either @samp{ball} or @samp{balls}, as a separate word. @c @cindex operators, @code{\B} (@command{gawk}) @cindex backslash (@code{\}), @code{\B} operator (@command{gawk}) @cindex @code{\} (backslash), @code{\B} operator (@command{gawk}) @item \B Matches the empty string that occurs between two word-constituent characters. For example, @code{/\Brat\B/} matches @samp{crate} but it does not match @samp{dirty rat}. @samp{\B} is essentially the opposite of @samp{\y}. @end table @cindex buffers, operators for @cindex regular expressions, operators, for buffers @cindex operators, string-matching, for buffers There are two other operators that work on buffers. In Emacs, a @dfn{buffer} is, naturally, an Emacs buffer. For other programs, @command{gawk}'s regexp library routines consider the entire string to match as the buffer. The operators are: @table @code @item \` @c @cindex operators, @code{\`} (@command{gawk}) @cindex backslash (@code{\}), @code{\`} operator (@command{gawk}) @cindex @code{\} (backslash), @code{\`} operator (@command{gawk}) Matches the empty string at the beginning of a buffer (string). @c @cindex operators, @code{\'} (@command{gawk}) @cindex backslash (@code{\}), @code{\'} operator (@command{gawk}) @cindex @code{\} (backslash), @code{\'} operator (@command{gawk}) @item \' Matches the empty string at the end of a buffer (string). @end table @cindex @code{^} (caret) @cindex caret (@code{^}) @cindex @code{?} (question mark) @cindex question mark (@code{?}) Because @samp{^} and @samp{$} always work in terms of the beginning and end of strings, these operators don't add any new capabilities for @command{awk}. They are provided for compatibility with other GNU software. @cindex @command{gawk}, word-boundary operator @cindex word-boundary operator (@command{gawk}) @cindex operators, word-boundary (@command{gawk}) In other GNU software, the word-boundary operator is @samp{\b}. However, that conflicts with the @command{awk} language's definition of @samp{\b} as backspace, so @command{gawk} uses a different letter. An alternative method would have been to require two backslashes in the GNU operators, but this was deemed too confusing. The current method of using @samp{\y} for the GNU @samp{\b} appears to be the lesser of two evils. @c NOTE!!! Keep this in sync with the same table in the summary appendix! @c @c Should really do this with file inclusion. @cindex regular expressions, @command{gawk}, command-line options @cindex @command{gawk}, command-line options The various command-line options (@pxref{Options}) control how @command{gawk} interprets characters in regexps: @table @asis @item No options In the default case, @command{gawk} provides all the facilities of POSIX regexps and the @ifnotinfo previously described GNU regexp operators. @end ifnotinfo @ifnottex GNU regexp operators described in @ref{Regexp Operators}. @end ifnottex However, interval expressions are not supported. @item @code{--posix} Only POSIX regexps are supported; the GNU operators are not special (e.g., @samp{\w} matches a literal @samp{w}). Interval expressions are allowed. @item @code{--traditional} Traditional Unix @command{awk} regexps are matched. The GNU operators are not special, interval expressions are not available, nor are the POSIX character classes (@code{[[:alnum:]]}, etc.). Characters described by octal and hexadecimal escape sequences are treated literally, even if they represent regexp metacharacters. @item @code{--re-interval} Allow interval expressions in regexps, even if @option{--traditional} has been provided. (@option{--posix} automatically enables interval expressions, so @option{--re-interval} is redundant when @option{--posix} is is used.) @end table @c ENDOFRANGE gregexp @c ENDOFRANGE regexpg @node Case-sensitivity @section Case Sensitivity in Matching @c STARTOFRANGE regexpcs @cindex regular expressions, case sensitivity @c STARTOFRANGE csregexp @cindex case sensitivity, regexps and Case is normally significant in regular expressions, both when matching ordinary characters (i.e., not metacharacters) and inside character sets. Thus, a @samp{w} in a regular expression matches only a lowercase @samp{w} and not an uppercase @samp{W}. The simplest way to do a case-independent match is to use a character list---for example, @samp{[Ww]}. However, this can be cumbersome if you need to use it often, and it can make the regular expressions harder to read. There are two alternatives that you might prefer. One way to perform a case-insensitive match at a particular point in the program is to convert the data to a single case, using the @code{tolower} or @code{toupper} built-in string functions (which we haven't discussed yet; @pxref{String Functions}). For example: @example tolower($1) ~ /foo/ @{ @dots{} @} @end example @noindent converts the first field to lowercase before matching against it. This works in any POSIX-compliant @command{awk}. @cindex @command{gawk}, regular expressions, case sensitivity @cindex case sensitivity, @command{gawk} @cindex differences in @command{awk} and @command{gawk}, regular expressions @cindex @code{~} (tilde), @code{~} operator @cindex tilde (@code{~}), @code{~} operator @cindex @code{!} (exclamation point), @code{!~} operator @cindex exclamation point (@code{!}), @code{!~} operator @cindex @code{IGNORECASE} variable @c @cindex variables, @code{IGNORECASE} Another method, specific to @command{gawk}, is to set the variable @code{IGNORECASE} to a nonzero value (@pxref{Built-in Variables}). When @code{IGNORECASE} is not zero, @emph{all} regexp and string operations ignore case. Changing the value of @code{IGNORECASE} dynamically controls the case-sensitivity of the program as it runs. Case is significant by default because @code{IGNORECASE} (like most variables) is initialized to zero: @example x = "aB" if (x ~ /ab/) @dots{} # this test will fail IGNORECASE = 1 if (x ~ /ab/) @dots{} # now it will succeed @end example In general, you cannot use @code{IGNORECASE} to make certain rules case-insensitive and other rules case-sensitive, because there is no straightforward way to set @code{IGNORECASE} just for the pattern of a particular rule.@footnote{Experienced C and C++ programmers will note that it is possible, using something like @samp{IGNORECASE = 1 && /foObAr/ @{ @dots{} @}} and @samp{IGNORECASE = 0 || /foobar/ @{ @dots{} @}}. However, this is somewhat obscure and we don't recommend it.} To do this, use either character lists or @code{tolower}. However, one thing you can do with @code{IGNORECASE} only is dynamically turn case-sensitivity on or off for all the rules at once. @code{IGNORECASE} can be set on the command line or in a @code{BEGIN} rule (@pxref{Other Arguments}; also @pxref{Using BEGIN/END}). Setting @code{IGNORECASE} from the command line is a way to make a program case-insensitive without having to edit it. Prior to @command{gawk} 3.0, the value of @code{IGNORECASE} affected regexp operations only. It did not affect string comparison with @samp{==}, @samp{!=}, and so on. Beginning with @value{PVERSION} 3.0, both regexp and string comparison operations are also affected by @code{IGNORECASE}. @c @cindex ISO 8859-1 @c @cindex ISO Latin-1 Beginning with @command{gawk} 3.0, the equivalences between upper- and lowercase characters are based on the ISO-8859-1 (ISO Latin-1) character set. This character set is a superset of the traditional 128 ASCII characters, which also provides a number of characters suitable for use with European languages. The value of @code{IGNORECASE} has no effect if @command{gawk} is in compatibility mode (@pxref{Options}). Case is always significant in compatibility mode. @c ENDOFRANGE csregexp @c ENDOFRANGE regexpcs @node Leftmost Longest @section How Much Text Matches? @cindex regular expressions, leftmost longest match @c @cindex matching, leftmost longest Consider the following: @example echo aaaabcd | awk '@{ sub(/a+/, ""); print @}' @end example This example uses the @code{sub} function (which we haven't discussed yet; @pxref{String Functions}) to make a change to the input record. Here, the regexp @code{/a+/} indicates ``one or more @samp{a} characters,'' and the replacement text is @samp{}. The input contains four @samp{a} characters. @command{awk} (and POSIX) regular expressions always match the leftmost, @emph{longest} sequence of input characters that can match. Thus, all four @samp{a} characters are replaced with @samp{} in this example: @example $ echo aaaabcd | awk '@{ sub(/a+/, ""); print @}' @print{} bcd @end example For simple match/no-match tests, this is not so important. But when doing text matching and substitutions with the @code{match}, @code{sub}, @code{gsub}, and @code{gensub} functions, it is very important. @ifinfo @xref{String Functions}, for more information on these functions. @end ifinfo Understanding this principle is also important for regexp-based record and field splitting (@pxref{Records}, and also @pxref{Field Separators}). @node Computed Regexps @section Using Dynamic Regexps @c STARTOFRANGE dregexp @cindex regular expressions, computed @c STARTOFRANGE regexpd @cindex regular expressions, dynamic @cindex @code{~} (tilde), @code{~} operator @cindex tilde (@code{~}), @code{~} operator @cindex @code{!} (exclamation point), @code{!~} operator @cindex exclamation point (@code{!}), @code{!~} operator @c @cindex operators, @code{~} @c @cindex operators, @code{!~} The righthand side of a @samp{~} or @samp{!~} operator need not be a regexp constant (i.e., a string of characters between slashes). It may be any expression. The expression is evaluated and converted to a string if necessary; the contents of the string are used as the regexp. A regexp that is computed in this way is called a @dfn{dynamic regexp}: @example BEGIN @{ digits_regexp = "[[:digit:]]+" @} $0 ~ digits_regexp @{ print @} @end example @noindent This sets @code{digits_regexp} to a regexp that describes one or more digits, and tests whether the input record matches this regexp. @c @strong{Caution:} When using the @samp{~} and @samp{!~} @strong{Caution:} When using the @samp{~} and @samp{!~} operators, there is a difference between a regexp constant enclosed in slashes and a string constant enclosed in double quotes. If you are going to use a string constant, you have to understand that the string is, in essence, scanned @emph{twice}: the first time when @command{awk} reads your program, and the second time when it goes to match the string on the lefthand side of the operator with the pattern on the right. This is true of any string-valued expression (such as @code{digits_regexp}, shown previously), not just string constants. @cindex regexp constants, slashes vs. quotes @cindex @code{\} (backslash), regexp constants @cindex backslash (@code{\}), regexp constants @cindex @code{"} (double quote), regexp constants @cindex double quote (@code{"}), regexp constants What difference does it make if the string is scanned twice? The answer has to do with escape sequences, and particularly with backslashes. To get a backslash into a regular expression inside a string, you have to type two backslashes. For example, @code{/\*/} is a regexp constant for a literal @samp{*}. Only one backslash is needed. To do the same thing with a string, you have to type @code{"\\*"}. The first backslash escapes the second one so that the string actually contains the two characters @samp{\} and @samp{*}. @cindex troubleshooting, regexp constants vs. string constants @cindex regexp constants, vs. string constants @cindex string constants, vs. regexp constants Given that you can use both regexp and string constants to describe regular expressions, which should you use? The answer is ``regexp constants,'' for several reasons: @itemize @bullet @item String constants are more complicated to write and more difficult to read. Using regexp constants makes your programs less error-prone. Not understanding the difference between the two kinds of constants is a common source of errors. @item It is more efficient to use regexp constants. @command{awk} can note that you have supplied a regexp and store it internally in a form that makes pattern matching more efficient. When using a string constant, @command{awk} must first convert the string into this internal form and then perform the pattern matching. @item Using regexp constants is better form; it shows clearly that you intend a regexp match. @end itemize @c fakenode --- for prepinfo @subheading Advanced Notes: Using @code{\n} in Character Lists of Dynamic Regexps @cindex regular expressions, dynamic, with embedded newlines @cindex newlines, in dynamic regexps Some commercial versions of @command{awk} do not allow the newline character to be used inside a character list for a dynamic regexp: @example $ awk '$0 ~ "[ \t\n]"' @error{} awk: newline in character class [ @error{} ]... @error{} source line number 1 @error{} context is @error{} >>> <<< @end example @cindex newlines, in regexp constants But a newline in a regexp constant works with no problem: @example $ awk '$0 ~ /[ \t\n]/' here is a sample line @print{} here is a sample line @kbd{@value{CTL}-d} @end example @command{gawk} does not have this problem, and it isn't likely to occur often in practice, but it's worth noting for future reference. @c ENDOFRANGE dregexp @c ENDOFRANGE regexpd @c ENDOFRANGE regexp @node Locales @section Where You Are Makes A Difference Modern systems support the notion of @dfn{locales}: a way to tell the system about the local character set and language. The current locale setting can affect the way regexp matching works, often in surprising ways. In particular, many locales do case-insensitive matching, even when you may have specified characters of only one particular case. The following example uses the @code{sub} function, which does text replacement (@pxref{String Functions}). Here, the intent is to remove trailing uppercase characters: @example $ echo something1234abc | gawk '@{ sub("[A-Z]*$", ""); print @}' @print{} something1234 @end example @noindent This output is unexpected, since the @samp{abc} at the end of @samp{something1234abc} should not normally match @samp{[A-Z]*}. This result is due to the locale setting (and thus you may not see it on your system). There are two fixes. The first is to use the POSIX character class @samp{[[:upper:]]}, instead of @samp{[A-Z]}. The second is to change the locale setting in the environment, before running @command{gawk}, by using the shell statements: @example LANG=C LC_ALL=C export LANG LC_ALL @end example The setting @samp{C} forces @command{gawk} to behave in the traditional Unix manner, where case distinctions do matter. You may wish to put these statements into your shell startup file, e.g., @file{$HOME/.profile}. Similar considerations apply to other ranges. For example, @samp{["-/]} is perfectly valid in ASCII, but is not valid in many Unicode locales, such as @samp{en_US.UTF-8}. (In general, such ranges should be avoided; either list the characters individually, or use a POSIX character class such as @samp{[[:punct:]]}.) For the normal case of @samp{RS = "\n"}, the locale is largely irrelevant. For other single byte record separators, using @samp{LC_ALL=C} will give you much better performance when reading records. Otherwise, @command{gawk} has to make several function calls, @emph{per input character} to find the record terminator. @node Reading Files @chapter Reading Input Files @c STARTOFRANGE infir @cindex input files, reading @cindex input files @cindex @code{FILENAME} variable In the typical @command{awk} program, all input is read either from the standard input (by default, this is the keyboard, but often it is a pipe from another command) or from files whose names you specify on the @command{awk} command line. If you specify input files, @command{awk} reads them in order, processing all the data from one before going on to the next. The name of the current input file can be found in the built-in variable @code{FILENAME} (@pxref{Built-in Variables}). @cindex records @cindex fields The input is read in units called @dfn{records}, and is processed by the rules of your program one record at a time. By default, each record is one line. Each record is automatically split into chunks called @dfn{fields}. This makes it more convenient for programs to work on the parts of a record. @cindex @code{getline} command On rare occasions, you may need to use the @code{getline} command. The @code{getline} command is valuable, both because it can do explicit input from any number of files, and because the files used with it do not have to be named on the @command{awk} command line (@pxref{Getline}). @menu * Records:: Controlling how data is split into records. * Fields:: An introduction to fields. * Nonconstant Fields:: Nonconstant Field Numbers. * Changing Fields:: Changing the Contents of a Field. * Field Separators:: The field separator and how to change it. * Constant Size:: Reading constant width data. * Multiple Line:: Reading multi-line records. * Getline:: Reading files under explicit program control using the @code{getline} function. @end menu @node Records @section How Input Is Split into Records @c STARTOFRANGE inspl @cindex input, splitting into records @c STARTOFRANGE recspl @cindex records, splitting input into @cindex @code{NR} variable @cindex @code{FNR} variable The @command{awk} utility divides the input for your @command{awk} program into records and fields. @command{awk} keeps track of the number of records that have been read so far from the current input file. This value is stored in a built-in variable called @code{FNR}. It is reset to zero when a new file is started. Another built-in variable, @code{NR}, is the total number of input records read so far from all @value{DF}s. It starts at zero, but is never automatically reset to zero. @cindex separators, for records @cindex record separators Records are separated by a character called the @dfn{record separator}. By default, the record separator is the newline character. This is why records are, by default, single lines. A different character can be used for the record separator by assigning the character to the built-in variable @code{RS}. @cindex newlines, as record separators @cindex @code{RS} variable Like any other variable, the value of @code{RS} can be changed in the @command{awk} program with the assignment operator, @samp{=} (@pxref{Assignment Ops}). The new record-separator character should be enclosed in quotation marks, which indicate a string constant. Often the right time to do this is at the beginning of execution, before any input is processed, so that the very first record is read with the proper separator. To do this, use the special @code{BEGIN} pattern (@pxref{BEGIN/END}). For example: @cindex @code{BEGIN} pattern @example awk 'BEGIN @{ RS = "/" @} @{ print $0 @}' BBS-list @end example @noindent changes the value of @code{RS} to @code{"/"}, before reading any input. This is a string whose first character is a slash; as a result, records are separated by slashes. Then the input file is read, and the second rule in the @command{awk} program (the action with no pattern) prints each record. Because each @code{print} statement adds a newline at the end of its output, this @command{awk} program copies the input with each slash changed to a newline. Here are the results of running the program on @file{BBS-list}: @example $ awk 'BEGIN @{ RS = "/" @} > @{ print $0 @}' BBS-list @print{} aardvark 555-5553 1200 @print{} 300 B @print{} alpo-net 555-3412 2400 @print{} 1200 @print{} 300 A @print{} barfly 555-7685 1200 @print{} 300 A @print{} bites 555-1675 2400 @print{} 1200 @print{} 300 A @print{} camelot 555-0542 300 C @print{} core 555-2912 1200 @print{} 300 C @print{} fooey 555-1234 2400 @print{} 1200 @print{} 300 B @print{} foot 555-6699 1200 @print{} 300 B @print{} macfoo 555-6480 1200 @print{} 300 A @print{} sdace 555-3430 2400 @print{} 1200 @print{} 300 A @print{} sabafoo 555-2127 1200 @print{} 300 C @print{} @end example @noindent Note that the entry for the @samp{camelot} BBS is not split. In the original @value{DF} (@pxref{Sample Data Files}), the line looks like this: @example camelot 555-0542 300 C @end example @noindent It has one baud rate only, so there are no slashes in the record, unlike the others which have two or more baud rates. In fact, this record is treated as part of the record for the @samp{core} BBS; the newline separating them in the output is the original newline in the @value{DF}, not the one added by @command{awk} when it printed the record! @cindex record separators, changing @cindex separators, for records Another way to change the record separator is on the command line, using the variable-assignment feature (@pxref{Other Arguments}): @example awk '@{ print $0 @}' RS="/" BBS-list @end example @noindent This sets @code{RS} to @samp{/} before processing @file{BBS-list}. Using an unusual character such as @samp{/} for the record separator produces correct behavior in the vast majority of cases. However, the following (extreme) pipeline prints a surprising @samp{1}: @example $ echo | awk 'BEGIN @{ RS = "a" @} ; @{ print NF @}' @print{} 1 @end example There is one field, consisting of a newline. The value of the built-in variable @code{NF} is the number of fields in the current record. @cindex dark corner, input files Reaching the end of an input file terminates the current input record, even if the last character in the file is not the character in @code{RS}. @value{DARKCORNER} @cindex null strings @cindex strings, empty, See null strings The empty string @code{""} (a string without any characters) has a special meaning as the value of @code{RS}. It means that records are separated by one or more blank lines and nothing else. @xref{Multiple Line}, for more details. If you change the value of @code{RS} in the middle of an @command{awk} run, the new value is used to delimit subsequent records, but the record currently being processed, as well as records already processed, are not affected. @cindex @code{RT} variable @cindex records, terminating @cindex terminating records @cindex differences in @command{awk} and @command{gawk}, record separators @cindex regular expressions, as record separators @cindex record separators, regular expressions as @cindex separators, for records, regular expressions as After the end of the record has been determined, @command{gawk} sets the variable @code{RT} to the text in the input that matched @code{RS}. When using @command{gawk}, the value of @code{RS} is not limited to a one-character string. It can be any regular expression (@pxref{Regexp}). In general, each record ends at the next string that matches the regular expression; the next record starts at the end of the matching string. This general rule is actually at work in the usual case, where @code{RS} contains just a newline: a record ends at the beginning of the next matching string (the next newline in the input), and the following record starts just after the end of this string (at the first character of the following line). The newline, because it matches @code{RS}, is not part of either record. When @code{RS} is a single character, @code{RT} contains the same single character. However, when @code{RS} is a regular expression, @code{RT} contains the actual input text that matched the regular expression. The following example illustrates both of these features. It sets @code{RS} equal to a regular expression that matches either a newline or a series of one or more uppercase letters with optional leading and/or trailing whitespace: @example $ echo record 1 AAAA record 2 BBBB record 3 | > gawk 'BEGIN @{ RS = "\n|( *[[:upper:]]+ *)" @} > @{ print "Record =", $0, "and RT =", RT @}' @print{} Record = record 1 and RT = AAAA @print{} Record = record 2 and RT = BBBB @print{} Record = record 3 and RT = @print{} @end example @noindent The final line of output has an extra blank line. This is because the value of @code{RT} is a newline, and the @code{print} statement supplies its own terminating newline. @xref{Simple Sed}, for a more useful example of @code{RS} as a regexp and @code{RT}. If you set @code{RS} to a regular expression that allows optional trailing text, such as @samp{RS = "abc(XYZ)?"} it is possible, due to implementation constraints, that @command{gawk} may match the leading part of the regular expression, but not the trailing part, particularly if the input text that could match the trailing part is fairly long. @command{gawk} attempts to avoid this problem, but currently, there's no guarantee that this will never happen. @cindex differences in @command{awk} and @command{gawk}, @code{RS}/@code{RT} variables The use of @code{RS} as a regular expression and the @code{RT} variable are @command{gawk} extensions; they are not available in compatibility mode (@pxref{Options}). In compatibility mode, only the first character of the value of @code{RS} is used to determine the end of the record. @c fakenode --- for prepinfo @subheading Advanced Notes: @code{RS = "\0"} Is Not Portable @cindex advanced features, @value{DF}s as single record @cindex portability, @value{DF}s as single record There are times when you might want to treat an entire @value{DF} as a single record. The only way to make this happen is to give @code{RS} a value that you know doesn't occur in the input file. This is hard to do in a general way, such that a program always works for arbitrary input files. @c can you say `understatement' boys and girls? You might think that for text files, the @sc{nul} character, which consists of a character with all bits equal to zero, is a good value to use for @code{RS} in this case: @example BEGIN @{ RS = "\0" @} # whole file becomes one record? @end example @cindex differences in @command{awk} and @command{gawk}, strings, storing @command{gawk} in fact accepts this, and uses the @sc{nul} character for the record separator. However, this usage is @emph{not} portable to other @command{awk} implementations. @cindex dark corner, strings, storing All other @command{awk} implementations@footnote{At least that we know about.} store strings internally as C-style strings. C strings use the @sc{nul} character as the string terminator. In effect, this means that @samp{RS = "\0"} is the same as @samp{RS = ""}. @value{DARKCORNER} @cindex records, treating files as @cindex files, as single records The best way to treat a whole file as a single record is to simply read the file in, one record at a time, concatenating each record onto the end of the previous ones. @c ENDOFRANGE inspl @c ENDOFRANGE recspl @node Fields @section Examining Fields @cindex examining fields @cindex fields @cindex accessing fields @c STARTOFRANGE fiex @cindex fields, examining @cindex POSIX @command{awk}, field separators and @cindex field separators, POSIX and @cindex separators, field, POSIX and When @command{awk} reads an input record, the record is automatically @dfn{parsed} or separated by the interpreter into chunks called @dfn{fields}. By default, fields are separated by @dfn{whitespace}, like words in a line. Whitespace in @command{awk} means any string of one or more spaces, tabs, or newlines;@footnote{In POSIX @command{awk}, newlines are not considered whitespace for separating fields.} other characters, such as formfeed, vertical tab, etc.@: that are considered whitespace by other languages, are @emph{not} considered whitespace by @command{awk}. The purpose of fields is to make it more convenient for you to refer to these pieces of the record. You don't have to use them---you can operate on the whole record if you want---but fields are what make simple @command{awk} programs so powerful. @cindex @code{$} field operator @cindex field operator @code{$} @cindex @code{$} (dollar sign), @code{$} field operator @cindex dollar sign (@code{$}), @code{$} field operator @c The comma here does NOT mark a secondary term: @cindex field operators, dollar sign as A dollar-sign (@samp{$}) is used to refer to a field in an @command{awk} program, followed by the number of the field you want. Thus, @code{$1} refers to the first field, @code{$2} to the second, and so on. (Unlike the Unix shells, the field numbers are not limited to single digits. @code{$127} is the one hundred twenty-seventh field in the record.) For example, suppose the following is a line of input: @example This seems like a pretty nice example. @end example @noindent Here the first field, or @code{$1}, is @samp{This}, the second field, or @code{$2}, is @samp{seems}, and so on. Note that the last field, @code{$7}, is @samp{example.}. Because there is no space between the @samp{e} and the @samp{.}, the period is considered part of the seventh field. @cindex @code{NF} variable @cindex fields, number of @code{NF} is a built-in variable whose value is the number of fields in the current record. @command{awk} automatically updates the value of @code{NF} each time it reads a record. No matter how many fields there are, the last field in a record can be represented by @code{$NF}. So, @code{$NF} is the same as @code{$7}, which is @samp{example.}. If you try to reference a field beyond the last one (such as @code{$8} when the record has only seven fields), you get the empty string. (If used in a numeric operation, you get zero.) The use of @code{$0}, which looks like a reference to the ``zero-th'' field, is a special case: it represents the whole input record when you are not interested in specific fields. Here are some more examples: @example $ awk '$1 ~ /foo/ @{ print $0 @}' BBS-list @print{} fooey 555-1234 2400/1200/300 B @print{} foot 555-6699 1200/300 B @print{} macfoo 555-6480 1200/300 A @print{} sabafoo 555-2127 1200/300 C @end example @noindent This example prints each record in the file @file{BBS-list} whose first field contains the string @samp{foo}. The operator @samp{~} is called a @dfn{matching operator} (@pxref{Regexp Usage}); it tests whether a string (here, the field @code{$1}) matches a given regular expression. By contrast, the following example looks for @samp{foo} in @emph{the entire record} and prints the first field and the last field for each matching input record: @example $ awk '/foo/ @{ print $1, $NF @}' BBS-list @print{} fooey B @print{} foot B @print{} macfoo A @print{} sabafoo C @end example @c ENDOFRANGE fiex @node Nonconstant Fields @section Nonconstant Field Numbers @cindex fields, numbers @cindex field numbers The number of a field does not need to be a constant. Any expression in the @command{awk} language can be used after a @samp{$} to refer to a field. The value of the expression specifies the field number. If the value is a string, rather than a number, it is converted to a number. Consider this example: @example awk '@{ print $NR @}' @end example @noindent Recall that @code{NR} is the number of records read so far: one in the first record, two in the second, etc. So this example prints the first field of the first record, the second field of the second record, and so on. For the twentieth record, field number 20 is printed; most likely, the record has fewer than 20 fields, so this prints a blank line. Here is another example of using expressions as field numbers: @example awk '@{ print $(2*2) @}' BBS-list @end example @command{awk} evaluates the expression @samp{(2*2)} and uses its value as the number of the field to print. The @samp{*} sign represents multiplication, so the expression @samp{2*2} evaluates to four. The parentheses are used so that the multiplication is done before the @samp{$} operation; they are necessary whenever there is a binary operator in the field-number expression. This example, then, prints the hours of operation (the fourth field) for every line of the file @file{BBS-list}. (All of the @command{awk} operators are listed, in order of decreasing precedence, in @ref{Precedence}.) If the field number you compute is zero, you get the entire record. Thus, @samp{$(2-2)} has the same value as @code{$0}. Negative field numbers are not allowed; trying to reference one usually terminates the program. (The POSIX standard does not define what happens when you reference a negative field number. @command{gawk} notices this and terminates your program. Other @command{awk} implementations may behave differently.) As mentioned in @ref{Fields}, @command{awk} stores the current record's number of fields in the built-in variable @code{NF} (also @pxref{Built-in Variables}). The expression @code{$NF} is not a special feature---it is the direct consequence of evaluating @code{NF} and using its value as a field number. @node Changing Fields @section Changing the Contents of a Field @c STARTOFRANGE ficon @cindex fields, changing contents of The contents of a field, as seen by @command{awk}, can be changed within an @command{awk} program; this changes what @command{awk} perceives as the current input record. (The actual input is untouched; @command{awk} @emph{never} modifies the input file.) Consider the following example and its output: @example $ awk '@{ nboxes = $3 ; $3 = $3 - 10 > print nboxes, $3 @}' inventory-shipped @print{} 25 15 @print{} 32 22 @print{} 24 14 @dots{} @end example @noindent The program first saves the original value of field three in the variable @code{nboxes}. The @samp{-} sign represents subtraction, so this program reassigns field three, @code{$3}, as the original value of field three minus ten: @samp{$3 - 10}. (@xref{Arithmetic Ops}.) Then it prints the original and new values for field three. (Someone in the warehouse made a consistent mistake while inventorying the red boxes.) For this to work, the text in field @code{$3} must make sense as a number; the string of characters must be converted to a number for the computer to do arithmetic on it. The number resulting from the subtraction is converted back to a string of characters that then becomes field three. @xref{Conversion}. When the value of a field is changed (as perceived by @command{awk}), the text of the input record is recalculated to contain the new field where the old one was. In other words, @code{$0} changes to reflect the altered field. Thus, this program prints a copy of the input file, with 10 subtracted from the second field of each line: @example $ awk '@{ $2 = $2 - 10; print $0 @}' inventory-shipped @print{} Jan 3 25 15 115 @print{} Feb 5 32 24 226 @print{} Mar 5 24 34 228 @dots{} @end example It is also possible to also assign contents to fields that are out of range. For example: @example $ awk '@{ $6 = ($5 + $4 + $3 + $2) > print $6 @}' inventory-shipped @print{} 168 @print{} 297 @print{} 301 @dots{} @end example @cindex adding, fields @cindex fields, adding @noindent We've just created @code{$6}, whose value is the sum of fields @code{$2}, @code{$3}, @code{$4}, and @code{$5}. The @samp{+} sign represents addition. For the file @file{inventory-shipped}, @code{$6} represents the total number of parcels shipped for a particular month. Creating a new field changes @command{awk}'s internal copy of the current input record, which is the value of @code{$0}. Thus, if you do @samp{print $0} after adding a field, the record printed includes the new field, with the appropriate number of field separators between it and the previously existing fields. @cindex @code{OFS} variable @cindex output field separator, See @code{OFS} variable @cindex field separators, See Also @code{OFS} This recomputation affects and is affected by @code{NF} (the number of fields; @pxref{Fields}). For example, the value of @code{NF} is set to the number of the highest field you create. The exact format of @code{$0} is also affected by a feature that has not been discussed yet: the @dfn{output field separator}, @code{OFS}, used to separate the fields (@pxref{Output Separators}). Note, however, that merely @emph{referencing} an out-of-range field does @emph{not} change the value of either @code{$0} or @code{NF}. Referencing an out-of-range field only produces an empty string. For example: @example if ($(NF+1) != "") print "can't happen" else print "everything is normal" @end example @noindent should print @samp{everything is normal}, because @code{NF+1} is certain to be out of range. (@xref{If Statement}, for more information about @command{awk}'s @code{if-else} statements. @xref{Typing and Comparison}, for more information about the @samp{!=} operator.) It is important to note that making an assignment to an existing field changes the value of @code{$0} but does not change the value of @code{NF}, even when you assign the empty string to a field. For example: @example $ echo a b c d | awk '@{ OFS = ":"; $2 = "" > print $0; print NF @}' @print{} a::c:d @print{} 4 @end example @noindent The field is still there; it just has an empty value, denoted by the two colons between @samp{a} and @samp{c}. This example shows what happens if you create a new field: @example $ echo a b c d | awk '@{ OFS = ":"; $2 = ""; $6 = "new" > print $0; print NF @}' @print{} a::c:d::new @print{} 6 @end example @noindent The intervening field, @code{$5}, is created with an empty value (indicated by the second pair of adjacent colons), and @code{NF} is updated with the value six. @c FIXME: Verify that this is in POSIX @cindex dark corner, @code{NF} variable, decrementing @cindex @code{NF} variable, decrementing Decrementing @code{NF} throws away the values of the fields after the new value of @code{NF} and recomputes @code{$0}. @value{DARKCORNER} Here is an example: @example $ echo a b c d e f | awk '@{ print "NF =", NF; > NF = 3; print $0 @}' @print{} NF = 6 @print{} a b c @end example @c the comma before decrementing does NOT represent a tertiary entry @cindex portability, @code{NF} variable, decrementing @strong{Caution:} Some versions of @command{awk} don't rebuild @code{$0} when @code{NF} is decremented. Caveat emptor. Finally, there are times when it is convenient to force @command{awk} to rebuild the entire record, using the current value of the fields and @code{OFS}. To do this, use the seemingly innocuous assignment: @example $1 = $1 # force record to be reconstituted print $0 # or whatever else with $0 @end example @noindent This forces @command{awk} rebuild the record. It does help to add a comment, as we've shown here. There is a flip side to the relationship between @code{$0} and the fields. Any assignment to @code{$0} causes the record to be reparsed into fields using the @emph{current} value of @code{FS}. This also applies to any built-in function that updates @code{$0}, such as @code{sub} and @code{gsub} (@pxref{String Functions}). @c ENDOFRANGE ficon @node Field Separators @section Specifying How Fields Are Separated @menu * Regexp Field Splitting:: Using regexps as the field separator. * Single Character Fields:: Making each character a separate field. * Command Line Field Separator:: Setting @code{FS} from the command-line. * Field Splitting Summary:: Some final points and a summary table. @end menu @cindex @code{FS} variable @cindex fields, separating @c STARTOFRANGE fisepr @cindex field separators @c STARTOFRANGE fisepg @cindex fields, separating The @dfn{field separator}, which is either a single character or a regular expression, controls the way @command{awk} splits an input record into fields. @command{awk} scans the input record for character sequences that match the separator; the fields themselves are the text between the matches. In the examples that follow, we use the bullet symbol (@bullet{}) to represent spaces in the output. If the field separator is @samp{oo}, then the following line: @example moo goo gai pan @end example @noindent is split into three fields: @samp{m}, @samp{@bullet{}g}, and @samp{@bullet{}gai@bullet{}pan}. Note the leading spaces in the values of the second and third fields. @cindex troubleshooting, @command{awk} uses @code{FS} not @code{IFS} The field separator is represented by the built-in variable @code{FS}. Shell programmers take note: @command{awk} does @emph{not} use the name @code{IFS} that is used by the POSIX-compliant shells (such as the Unix Bourne shell, @command{sh}, or @command{bash}). @cindex @code{FS} variable, changing value of The value of @code{FS} can be changed in the @command{awk} program with the assignment operator, @samp{=} (@pxref{Assignment Ops}). Often the right time to do this is at the beginning of execution before any input has been processed, so that the very first record is read with the proper separator. To do this, use the special @code{BEGIN} pattern (@pxref{BEGIN/END}). For example, here we set the value of @code{FS} to the string @code{","}: @example awk 'BEGIN @{ FS = "," @} ; @{ print $2 @}' @end example @cindex @code{BEGIN} pattern @noindent Given the input line: @example John Q. Smith, 29 Oak St., Walamazoo, MI 42139 @end example @noindent this @command{awk} program extracts and prints the string @samp{@bullet{}29@bullet{}Oak@bullet{}St.}. @cindex field separators, choice of @cindex regular expressions as field separators @cindex field separators, regular expressions as Sometimes the input data contains separator characters that don't separate fields the way you thought they would. For instance, the person's name in the example we just used might have a title or suffix attached, such as: @example John Q. Smith, LXIX, 29 Oak St., Walamazoo, MI 42139 @end example @noindent The same program would extract @samp{@bullet{}LXIX}, instead of @samp{@bullet{}29@bullet{}Oak@bullet{}St.}. If you were expecting the program to print the address, you would be surprised. The moral is to choose your data layout and separator characters carefully to prevent such problems. (If the data is not in a form that is easy to process, perhaps you can massage it first with a separate @command{awk} program.) @cindex newlines, as field separators @cindex whitespace, as field separators Fields are normally separated by whitespace sequences (spaces, tabs, and newlines), not by single spaces. Two spaces in a row do not delimit an empty field. The default value of the field separator @code{FS} is a string containing a single space, @w{@code{" "}}. If @command{awk} interpreted this value in the usual way, each space character would separate fields, so two spaces in a row would make an empty field between them. The reason this does not happen is that a single space as the value of @code{FS} is a special case---it is taken to specify the default manner of delimiting fields. If @code{FS} is any other single character, such as @code{","}, then each occurrence of that character separates two fields. Two consecutive occurrences delimit an empty field. If the character occurs at the beginning or the end of the line, that too delimits an empty field. The space character is the only single character that does not follow these rules. @node Regexp Field Splitting @subsection Using Regular Expressions to Separate Fields @c STARTOFRANGE regexpfs @cindex regular expressions, as field separators @c STARTOFRANGE fsregexp @cindex field separators, regular expressions as The previous @value{SUBSECTION} discussed the use of single characters or simple strings as the value of @code{FS}. More generally, the value of @code{FS} may be a string containing any regular expression. In this case, each match in the record for the regular expression separates fields. For example, the assignment: @example FS = ", \t" @end example @noindent makes every area of an input line that consists of a comma followed by a space and a TAB into a field separator. @ifinfo (@samp{\t} is an @dfn{escape sequence} that stands for a TAB; @pxref{Escape Sequences}, for the complete list of similar escape sequences.) @end ifinfo For a less trivial example of a regular expression, try using single spaces to separate fields the way single commas are used. @code{FS} can be set to @w{@code{"[@ ]"}} (left bracket, space, right bracket). This regular expression matches a single space and nothing else (@pxref{Regexp}). There is an important difference between the two cases of @samp{FS = @w{" "}} (a single space) and @samp{FS = @w{"[ \t\n]+"}} (a regular expression matching one or more spaces, tabs, or newlines). For both values of @code{FS}, fields are separated by @dfn{runs} (multiple adjacent occurrences) of spaces, tabs, and/or newlines. However, when the value of @code{FS} is @w{@code{" "}}, @command{awk} first strips leading and trailing whitespace from the record and then decides where the fields are. For example, the following pipeline prints @samp{b}: @example $ echo ' a b c d ' | awk '@{ print $2 @}' @print{} b @end example @noindent However, this pipeline prints @samp{a} (note the extra spaces around each letter): @example $ echo ' a b c d ' | awk 'BEGIN @{ FS = "[ \t\n]+" @} > @{ print $2 @}' @print{} a @end example @noindent @cindex null strings @cindex strings, null @cindex empty strings, See null strings In this case, the first field is @dfn{null} or empty. The stripping of leading and trailing whitespace also comes into play whenever @code{$0} is recomputed. For instance, study this pipeline: @example $ echo ' a b c d' | awk '@{ print; $2 = $2; print @}' @print{} a b c d @print{} a b c d @end example @noindent The first @code{print} statement prints the record as it was read, with leading whitespace intact. The assignment to @code{$2} rebuilds @code{$0} by concatenating @code{$1} through @code{$NF} together, separated by the value of @code{OFS}. Because the leading whitespace was ignored when finding @code{$1}, it is not part of the new @code{$0}. Finally, the last @code{print} statement prints the new @code{$0}. @c ENDOFRANGE regexpfs @c ENDOFRANGE fsregexp @node Single Character Fields @subsection Making Each Character a Separate Field @cindex differences in @command{awk} and @command{gawk}, single-character fields @cindex single-character fields @cindex fields, single-character There are times when you may want to examine each character of a record separately. This can be done in @command{gawk} by simply assigning the null string (@code{""}) to @code{FS}. In this case, each individual character in the record becomes a separate field. For example: @example $ echo a b | gawk 'BEGIN @{ FS = "" @} > @{ > for (i = 1; i <= NF; i = i + 1) > print "Field", i, "is", $i > @}' @print{} Field 1 is a @print{} Field 2 is @print{} Field 3 is b @end example @cindex dark corner, @code{FS} as null string @cindex FS variable, as null string Traditionally, the behavior of @code{FS} equal to @code{""} was not defined. In this case, most versions of Unix @command{awk} simply treat the entire record as only having one field. @value{DARKCORNER} In compatibility mode (@pxref{Options}), if @code{FS} is the null string, then @command{gawk} also behaves this way. @node Command Line Field Separator @subsection Setting @code{FS} from the Command Line @cindex @code{-F} option @cindex options, command-line @cindex command line, options @cindex field separators, on command line @c The comma before "setting" does NOT represent a tertiary @cindex command line, @code{FS} on, setting @cindex @code{FS} variable, setting from command line @code{FS} can be set on the command line. Use the @option{-F} option to do so. For example: @example awk -F, '@var{program}' @var{input-files} @end example @noindent sets @code{FS} to the @samp{,} character. Notice that the option uses an uppercase @samp{F} instead of a lowercase @samp{f}. The latter option (@option{-f}) specifies a file containing an @command{awk} program. Case is significant in command-line options: the @option{-F} and @option{-f} options have nothing to do with each other. You can use both options at the same time to set the @code{FS} variable @emph{and} get an @command{awk} program from a file. The value used for the argument to @option{-F} is processed in exactly the same way as assignments to the built-in variable @code{FS}. Any special characters in the field separator must be escaped appropriately. For example, to use a @samp{\} as the field separator on the command line, you would have to type: @example # same as FS = "\\" awk -F\\\\ '@dots{}' files @dots{} @end example @noindent @cindex @code{\} (backslash), as field separators @cindex backslash (@code{\}), as field separators Because @samp{\} is used for quoting in the shell, @command{awk} sees @samp{-F\\}. Then @command{awk} processes the @samp{\\} for escape characters (@pxref{Escape Sequences}), finally yielding a single @samp{\} to use for the field separator. @c @cindex historical features As a special case, in compatibility mode (@pxref{Options}), if the argument to @option{-F} is @samp{t}, then @code{FS} is set to the TAB character. If you type @samp{-F\t} at the shell, without any quotes, the @samp{\} gets deleted, so @command{awk} figures that you really want your fields to be separated with tabs and not @samp{t}s. Use @samp{-v FS="t"} or @samp{-F"[t]"} on the command line if you really do want to separate your fields with @samp{t}s. For example, let's use an @command{awk} program file called @file{baud.awk} that contains the pattern @code{/300/} and the action @samp{print $1}: @example /300/ @{ print $1 @} @end example Let's also set @code{FS} to be the @samp{-} character and run the program on the file @file{BBS-list}. The following command prints a list of the names of the bulletin boards that operate at 300 baud and the first three digits of their phone numbers: @c tweaked to make the tex output look better in @smallbook @example $ awk -F- -f baud.awk BBS-list @print{} aardvark 555 @print{} alpo @print{} barfly 555 @print{} bites 555 @print{} camelot 555 @print{} core 555 @print{} fooey 555 @print{} foot 555 @print{} macfoo 555 @print{} sdace 555 @print{} sabafoo 555 @end example @noindent Note the second line of output. The second line in the original file looked like this: @example alpo-net 555-3412 2400/1200/300 A @end example The @samp{-} as part of the system's name was used as the field separator, instead of the @samp{-} in the phone number that was originally intended. This demonstrates why you have to be careful in choosing your field and record separators. @c The comma after "password files" does NOT start a tertiary @cindex Unix @command{awk}, password files, field separators and Perhaps the most common use of a single character as the field separator occurs when processing the Unix system password file. On many Unix systems, each user has a separate entry in the system password file, one line per user. The information in these lines is separated by colons. The first field is the user's logon name and the second is the user's (encrypted or shadow) password. A password file entry might look like this: @cindex Robbins, Arnold @example arnold:xyzzy:2076:10:Arnold Robbins:/home/arnold:/bin/bash @end example The following program searches the system password file and prints the entries for users who have no password: @example awk -F: '$2 == ""' /etc/passwd @end example @node Field Splitting Summary @subsection Field-Splitting Summary It is important to remember that when you assign a string constant as the value of @code{FS}, it undergoes normal @command{awk} string processing. For example, with Unix @command{awk} and @command{gawk}, the assignment @samp{FS = "\.."} assigns the character string @code{".."} to @code{FS} (the backslash is stripped). This creates a regexp meaning ``fields are separated by occurrences of any two characters.'' If instead you want fields to be separated by a literal period followed by any single character, use @samp{FS = "\\.."}. The following table summarizes how fields are split, based on the value of @code{FS} (@samp{==} means ``is equal to''): @table @code @item FS == " " Fields are separated by runs of whitespace. Leading and trailing whitespace are ignored. This is the default. @item FS == @var{any other single character} Fields are separated by each occurrence of the character. Multiple successive occurrences delimit empty fields, as do leading and trailing occurrences. The character can even be a regexp metacharacter; it does not need to be escaped. @item FS == @var{regexp} Fields are separated by occurrences of characters that match @var{regexp}. Leading and trailing matches of @var{regexp} delimit empty fields. @item FS == "" Each individual character in the record becomes a separate field. (This is a @command{gawk} extension; it is not specified by the POSIX standard.) @end table @c fakenode --- for prepinfo @subheading Advanced Notes: Changing @code{FS} Does Not Affect the Fields @cindex POSIX @command{awk}, field separators and @cindex field separators, POSIX and According to the POSIX standard, @command{awk} is supposed to behave as if each record is split into fields at the time it is read. In particular, this means that if you change the value of @code{FS} after a record is read, the value of the fields (i.e., how they were split) should reflect the old value of @code{FS}, not the new one. @cindex dark corner, field separators @cindex @command{sed} utility @cindex stream editors However, many implementations of @command{awk} do not work this way. Instead, they defer splitting the fields until a field is actually referenced. The fields are split using the @emph{current} value of @code{FS}! @value{DARKCORNER} This behavior can be difficult to diagnose. The following example illustrates the difference between the two methods. (The @command{sed}@footnote{The @command{sed} utility is a ``stream editor.'' Its behavior is also defined by the POSIX standard.} command prints just the first line of @file{/etc/passwd}.) @example sed 1q /etc/passwd | awk '@{ FS = ":" ; print $1 @}' @end example @noindent which usually prints: @example root @end example @noindent on an incorrect implementation of @command{awk}, while @command{gawk} prints something like: @example root:nSijPlPhZZwgE:0:0:Root:/: @end example @c fakenode --- for prepinfo @subheading Advanced Notes: @code{FS} and @code{IGNORECASE} The @code{IGNORECASE} variable (@pxref{User-modified}) affects field splitting @emph{only} when the value of @code{FS} is a regexp. It has no effect when @code{FS} is a single character, even if that character is a letter. Thus, in the following code: @example FS = "c" IGNORECASE = 1 $0 = "aCa" print $1 @end example @noindent The output is @samp{aCa}. If you really want to split fields on an alphabetic character while ignoring case, use a regexp that will do it for you. E.g., @samp{FS = "[c]"}. In this case, @code{IGNORECASE} will take effect. @c ENDOFRANGE fisepr @c ENDOFRANGE fisepg @node Constant Size @section Reading Fixed-Width Data @ifnotinfo @strong{Note:} This @value{SECTION} discusses an advanced feature of @command{gawk}. If you are a novice @command{awk} user, you might want to skip it on the first reading. @end ifnotinfo @ifinfo (This @value{SECTION} discusses an advanced feature of @command{awk}. If you are a novice @command{awk} user, you might want to skip it on the first reading.) @end ifinfo @cindex data, fixed-width @cindex fixed-width data @cindex advanced features, fixed-width data @command{gawk} @value{PVERSION} 2.13 introduced a facility for dealing with fixed-width fields with no distinctive field separator. For example, data of this nature arises in the input for old Fortran programs where numbers are run together, or in the output of programs that did not anticipate the use of their output as input for other programs. An example of the latter is a table where all the columns are lined up by the use of a variable number of spaces and @emph{empty fields are just spaces}. Clearly, @command{awk}'s normal field splitting based on @code{FS} does not work well in this case. Although a portable @command{awk} program can use a series of @code{substr} calls on @code{$0} (@pxref{String Functions}), this is awkward and inefficient for a large number of fields. @c comma before specifying is part of tertiary @cindex troubleshooting, fatal errors, field widths, specifying @cindex @command{w} utility @cindex @code{FIELDWIDTHS} variable The splitting of an input record into fixed-width fields is specified by assigning a string containing space-separated numbers to the built-in variable @code{FIELDWIDTHS}. Each number specifies the width of the field, @emph{including} columns between fields. If you want to ignore the columns between fields, you can specify the width as a separate field that is subsequently ignored. It is a fatal error to supply a field width that is not a positive number. The following data is the output of the Unix @command{w} utility. It is useful to illustrate the use of @code{FIELDWIDTHS}: @example @group 10:06pm up 21 days, 14:04, 23 users User tty login@ idle JCPU PCPU what hzuo ttyV0 8:58pm 9 5 vi p24.tex hzang ttyV3 6:37pm 50 -csh eklye ttyV5 9:53pm 7 1 em thes.tex dportein ttyV6 8:17pm 1:47 -csh gierd ttyD3 10:00pm 1 elm dave ttyD4 9:47pm 4 4 w brent ttyp0 26Jun91 4:46 26:46 4:41 bash dave ttyq4 26Jun9115days 46 46 wnewmail @end group @end example The following program takes the above input, converts the idle time to number of seconds, and prints out the first two fields and the calculated idle time: @strong{Note:} This program uses a number of @command{awk} features that haven't been introduced yet. @example BEGIN @{ FIELDWIDTHS = "9 6 10 6 7 7 35" @} NR > 2 @{ idle = $4 sub(/^ */, "", idle) # strip leading spaces if (idle == "") idle = 0 if (idle ~ /:/) @{ split(idle, t, ":") idle = t[1] * 60 + t[2] @} if (idle ~ /days/) idle *= 24 * 60 * 60 print $1, $2, idle @} @end example Running the program on the data produces the following results: @example hzuo ttyV0 0 hzang ttyV3 50 eklye ttyV5 0 dportein ttyV6 107 gierd ttyD3 1 dave ttyD4 0 brent ttyp0 286 dave ttyq4 1296000 @end example Another (possibly more practical) example of fixed-width input data is the input from a deck of balloting cards. In some parts of the United States, voters mark their choices by punching holes in computer cards. These cards are then processed to count the votes for any particular candidate or on any particular issue. Because a voter may choose not to vote on some issue, any column on the card may be empty. An @command{awk} program for processing such data could use the @code{FIELDWIDTHS} feature to simplify reading the data. (Of course, getting @command{gawk} to run on a system with card readers is another story!) @ignore Exercise: Write a ballot card reading program @end ignore @cindex @command{gawk}, splitting fields and Assigning a value to @code{FS} causes @command{gawk} to use @code{FS} for field splitting again. Use @samp{FS = FS} to make this happen, without having to know the current value of @code{FS}. In order to tell which kind of field splitting is in effect, use @code{PROCINFO["FS"]} (@pxref{Auto-set}). The value is @code{"FS"} if regular field splitting is being used, or it is @code{"FIELDWIDTHS"} if fixed-width field splitting is being used: @example if (PROCINFO["FS"] == "FS") @var{regular field splitting} @dots{} else @var{fixed-width field splitting} @dots{} @end example This information is useful when writing a function that needs to temporarily change @code{FS} or @code{FIELDWIDTHS}, read some records, and then restore the original settings (@pxref{Passwd Functions}, for an example of such a function). @node Multiple Line @section Multiple-Line Records @c STARTOFRANGE recm @cindex records, multiline @c STARTOFRANGE imr @cindex input, multiline records @c STARTOFRANGE frm @cindex files, reading, multiline records @cindex input, files, See input files In some databases, a single line cannot conveniently hold all the information in one entry. In such cases, you can use multiline records. The first step in doing this is to choose your data format. @cindex record separators, with multiline records One technique is to use an unusual character or string to separate records. For example, you could use the formfeed character (written @samp{\f} in @command{awk}, as in C) to separate them, making each record a page of the file. To do this, just set the variable @code{RS} to @code{"\f"} (a string containing the formfeed character). Any other character could equally well be used, as long as it won't be part of the data in a record. @cindex @code{RS} variable, multiline records and Another technique is to have blank lines separate records. By a special dispensation, an empty string as the value of @code{RS} indicates that records are separated by one or more blank lines. When @code{RS} is set to the empty string, each record always ends at the first blank line encountered. The next record doesn't start until the first nonblank line that follows. No matter how many blank lines appear in a row, they all act as one record separator. (Blank lines must be completely empty; lines that contain only whitespace do not count.) @cindex leftmost longest match @cindex matching, leftmost longest You can achieve the same effect as @samp{RS = ""} by assigning the string @code{"\n\n+"} to @code{RS}. This regexp matches the newline at the end of the record and one or more blank lines after the record. In addition, a regular expression always matches the longest possible sequence when there is a choice (@pxref{Leftmost Longest}). So the next record doesn't start until the first nonblank line that follows---no matter how many blank lines appear in a row, they are considered one record separator. @cindex dark corner, multiline records There is an important difference between @samp{RS = ""} and @samp{RS = "\n\n+"}. In the first case, leading newlines in the input @value{DF} are ignored, and if a file ends without extra blank lines after the last record, the final newline is removed from the record. In the second case, this special processing is not done. @value{DARKCORNER} @cindex field separators, in multiline records Now that the input is separated into records, the second step is to separate the fields in the record. One way to do this is to divide each of the lines into fields in the normal manner. This happens by default as the result of a special feature. When @code{RS} is set to the empty string, @emph{and} @code{FS} is a set to a single character, the newline character @emph{always} acts as a field separator. This is in addition to whatever field separations result from @code{FS}.@footnote{When @code{FS} is the null string (@code{""}) or a regexp, this special feature of @code{RS} does not apply. It does apply to the default field separator of a single space: @samp{FS = " "}.} The original motivation for this special exception was probably to provide useful behavior in the default case (i.e., @code{FS} is equal to @w{@code{" "}}). This feature can be a problem if you really don't want the newline character to separate fields, because there is no way to prevent it. However, you can work around this by using the @code{split} function to break up the record manually (@pxref{String Functions}). If you have a single character field separator, you can work around the special feature in a different way, by making @code{FS} into a regexp for that single character. For example, if the field separator is a percent character, instead of @samp{FS = "%"}, use @samp{FS = "[%]"}. Another way to separate fields is to put each field on a separate line: to do this, just set the variable @code{FS} to the string @code{"\n"}. (This single character seperator matches a single newline.) A practical example of a @value{DF} organized this way might be a mailing list, where each entry is separated by blank lines. Consider a mailing list in a file named @file{addresses}, which looks like this: @example Jane Doe 123 Main Street Anywhere, SE 12345-6789 John Smith 456 Tree-lined Avenue Smallville, MW 98765-4321 @dots{} @end example @noindent A simple program to process this file is as follows: @example # addrs.awk --- simple mailing list program # Records are separated by blank lines. # Each line is one field. BEGIN @{ RS = "" ; FS = "\n" @} @{ print "Name is:", $1 print "Address is:", $2 print "City and State are:", $3 print "" @} @end example Running the program produces the following output: @example $ awk -f addrs.awk addresses @print{} Name is: Jane Doe @print{} Address is: 123 Main Street @print{} City and State are: Anywhere, SE 12345-6789 @print{} @print{} Name is: John Smith @print{} Address is: 456 Tree-lined Avenue @print{} City and State are: Smallville, MW 98765-4321 @print{} @dots{} @end example @xref{Labels Program}, for a more realistic program that deals with address lists. The following table summarizes how records are split, based on the value of @ifinfo @code{RS}. (@samp{==} means ``is equal to.'') @end ifinfo @ifnotinfo @code{RS}: @end ifnotinfo @table @code @item RS == "\n" Records are separated by the newline character (@samp{\n}). In effect, every line in the @value{DF} is a separate record, including blank lines. This is the default. @item RS == @var{any single character} Records are separated by each occurrence of the character. Multiple successive occurrences delimit empty records. @item RS == "" Records are separated by runs of blank lines. The newline character always serves as a field separator, in addition to whatever value @code{FS} may have. Leading and trailing newlines in a file are ignored. @item RS == @var{regexp} Records are separated by occurrences of characters that match @var{regexp}. Leading and trailing matches of @var{regexp} delimit empty records. (This is a @command{gawk} extension; it is not specified by the POSIX standard.) @end table @cindex @code{RT} variable In all cases, @command{gawk} sets @code{RT} to the input text that matched the value specified by @code{RS}. @c ENDOFRANGE recm @c ENDOFRANGE imr @c ENDOFRANGE frm @node Getline @section Explicit Input with @code{getline} @c STARTOFRANGE getl @cindex @code{getline} command, explicit input with @cindex input, explicit So far we have been getting our input data from @command{awk}'s main input stream---either the standard input (usually your terminal, sometimes the output from another program) or from the files specified on the command line. The @command{awk} language has a special built-in command called @code{getline} that can be used to read input under your explicit control. The @code{getline} command is used in several different ways and should @emph{not} be used by beginners. The examples that follow the explanation of the @code{getline} command include material that has not been covered yet. Therefore, come back and study the @code{getline} command @emph{after} you have reviewed the rest of this @value{DOCUMENT} and have a good knowledge of how @command{awk} works. @cindex @code{ERRNO} variable @cindex differences in @command{awk} and @command{gawk}, @code{getline} command @cindex @code{getline} command, return values The @code{getline} command returns one if it finds a record and zero if it encounters the end of the file. If there is some error in getting a record, such as a file that cannot be opened, then @code{getline} returns @minus{}1. In this case, @command{gawk} sets the variable @code{ERRNO} to a string describing the error that occurred. In the following examples, @var{command} stands for a string value that represents a shell command. @menu * Plain Getline:: Using @code{getline} with no arguments. * Getline/Variable:: Using @code{getline} into a variable. * Getline/File:: Using @code{getline} from a file. * Getline/Variable/File:: Using @code{getline} into a variable from a file. * Getline/Pipe:: Using @code{getline} from a pipe. * Getline/Variable/Pipe:: Using @code{getline} into a variable from a pipe. * Getline/Coprocess:: Using @code{getline} from a coprocess. * Getline/Variable/Coprocess:: Using @code{getline} into a variable from a coprocess. * Getline Notes:: Important things to know about @code{getline}. * Getline Summary:: Summary of @code{getline} Variants. @end menu @node Plain Getline @subsection Using @code{getline} with No Arguments The @code{getline} command can be used without arguments to read input from the current input file. All it does in this case is read the next input record and split it up into fields. This is useful if you've finished processing the current record, but want to do some special processing on the next record @emph{right now}. For example: @example @{ if ((t = index($0, "/*")) != 0) @{ # value of `tmp' will be "" if t is 1 tmp = substr($0, 1, t - 1) u = index(substr($0, t + 2), "*/") while (u == 0) @{ if (getline <= 0) @{ m = "unexpected EOF or error" m = (m ": " ERRNO) print m > "/dev/stderr" exit @} t = -1 u = index($0, "*/") @} # substr expression will be "" if */ # occurred at end of line $0 = tmp substr($0, u + 2) @} print $0 @} @end example This @command{awk} program deletes all C-style comments (@samp{/* @dots{} */}) from the input. By replacing the @samp{print $0} with other statements, you could perform more complicated processing on the decommented input, such as searching for matches of a regular expression. (This program has a subtle problem---it does not work if one comment ends and another begins on the same line.) @ignore Exercise, write a program that does handle multiple comments on the line. @end ignore This form of the @code{getline} command sets @code{NF}, @code{NR}, @code{FNR}, and the value of @code{$0}. @strong{Note:} The new value of @code{$0} is used to test the patterns of any subsequent rules. The original value of @code{$0} that triggered the rule that executed @code{getline} is lost. By contrast, the @code{next} statement reads a new record but immediately begins processing it normally, starting with the first rule in the program. @xref{Next Statement}. @node Getline/Variable @subsection Using @code{getline} into a Variable @c comma before using is NOT for tertiary @cindex variables, @code{getline} command into, using You can use @samp{getline @var{var}} to read the next record from @command{awk}'s input into the variable @var{var}. No other processing is done. For example, suppose the next line is a comment or a special string, and you want to read it without triggering any rules. This form of @code{getline} allows you to read that line and store it in a variable so that the main read-a-line-and-check-each-rule loop of @command{awk} never sees it. The following example swaps every two lines of input: @example @{ if ((getline tmp) > 0) @{ print tmp print $0 @} else print $0 @} @end example @noindent It takes the following list: @example wan tew free phore @end example @noindent and produces these results: @example tew wan phore free @end example The @code{getline} command used in this way sets only the variables @code{NR} and @code{FNR} (and of course, @var{var}). The record is not split into fields, so the values of the fields (including @code{$0}) and the value of @code{NF} do not change. @node Getline/File @subsection Using @code{getline} from a File @cindex input redirection @cindex redirection of input @cindex @code{<} (left angle bracket), @code{<} operator (I/O) @cindex left angle bracket (@code{<}), @code{<} operator (I/O) @cindex operators, input/output Use @samp{getline < @var{file}} to read the next record from @var{file}. Here @var{file} is a string-valued expression that specifies the @value{FN}. @samp{< @var{file}} is called a @dfn{redirection} because it directs input to come from a different place. For example, the following program reads its input record from the file @file{secondary.input} when it encounters a first field with a value equal to 10 in the current input file: @example @{ if ($1 == 10) @{ getline < "secondary.input" print @} else print @} @end example Because the main input stream is not used, the values of @code{NR} and @code{FNR} are not changed. However, the record it reads is split into fields in the normal manner, so the values of @code{$0} and the other fields are changed, resulting in a new value of @code{NF}. @cindex POSIX @command{awk}, @code{<} operator and @c Thanks to Paul Eggert for initial wording here According to POSIX, @samp{getline < @var{expression}} is ambiguous if @var{expression} contains unparenthesized operators other than @samp{$}; for example, @samp{getline < dir "/" file} is ambiguous because the concatenation operator is not parenthesized. You should write it as @samp{getline < (dir "/" file)} if you want your program to be portable to other @command{awk} implementations. @node Getline/Variable/File @subsection Using @code{getline} into a Variable from a File @c comma before using is NOT for tertiary @cindex variables, @code{getline} command into, using Use @samp{getline @var{var} < @var{file}} to read input from the file @var{file}, and put it in the variable @var{var}. As above, @var{file} is a string-valued expression that specifies the file from which to read. In this version of @code{getline}, none of the built-in variables are changed and the record is not split into fields. The only variable changed is @var{var}. For example, the following program copies all the input files to the output, except for records that say @w{@samp{@@include @var{filename}}}. Such a record is replaced by the contents of the file @var{filename}: @example @{ if (NF == 2 && $1 == "@@include") @{ while ((getline line < $2) > 0) print line close($2) @} else print @} @end example Note here how the name of the extra input file is not built into the program; it is taken directly from the data, specifically from the second field on the @samp{@@include} line. @cindex @code{close} function The @code{close} function is called to ensure that if two identical @samp{@@include} lines appear in the input, the entire specified file is included twice. @xref{Close Files And Pipes}. One deficiency of this program is that it does not process nested @samp{@@include} statements (i.e., @samp{@@include} statements in included files) the way a true macro preprocessor would. @xref{Igawk Program}, for a program that does handle nested @samp{@@include} statements. @node Getline/Pipe @subsection Using @code{getline} from a Pipe @cindex @code{|} (vertical bar), @code{|} operator (I/O) @cindex vertical bar (@code{|}), @code{|} operator (I/O) @cindex input pipeline @cindex pipes, input @cindex operators, input/output The output of a command can also be piped into @code{getline}, using @samp{@var{command} | getline}. In this case, the string @var{command} is run as a shell command and its output is piped into @command{awk} to be used as input. This form of @code{getline} reads one record at a time from the pipe. For example, the following program copies its input to its output, except for lines that begin with @samp{@@execute}, which are replaced by the output produced by running the rest of the line as a shell command: @example @{ if ($1 == "@@execute") @{ tmp = substr($0, 10) while ((tmp | getline) > 0) print close(tmp) @} else print @} @end example @noindent @cindex @code{close} function The @code{close} function is called to ensure that if two identical @samp{@@execute} lines appear in the input, the command is run for each one. @ifnottex @xref{Close Files And Pipes}. @end ifnottex @c Exercise!! @c This example is unrealistic, since you could just use system Given the input: @example foo bar baz @@execute who bletch @end example @noindent the program might produce: @cindex Robbins, Bill @cindex Robbins, Miriam @cindex Robbins, Arnold @example foo bar baz arnold ttyv0 Jul 13 14:22 miriam ttyp0 Jul 13 14:23 (murphy:0) bill ttyp1 Jul 13 14:23 (murphy:0) bletch @end example @noindent Notice that this program ran the command @command{who} and printed the previous result. (If you try this program yourself, you will of course get different results, depending upon who is logged in on your system.) This variation of @code{getline} splits the record into fields, sets the value of @code{NF}, and recomputes the value of @code{$0}. The values of @code{NR} and @code{FNR} are not changed. @cindex POSIX @command{awk}, @code{|} I/O operator and @c Thanks to Paul Eggert for initial wording here According to POSIX, @samp{@var{expression} | getline} is ambiguous if @var{expression} contains unparenthesized operators other than @samp{$}---for example, @samp{@w{"echo "} "date" | getline} is ambiguous because the concatenation operator is not parenthesized. You should write it as @samp{(@w{"echo "} "date") | getline} if you want your program to be portable to other @command{awk} implementations. @node Getline/Variable/Pipe @subsection Using @code{getline} into a Variable from a Pipe @c comma before using is NOT for tertiary @cindex variables, @code{getline} command into, using When you use @samp{@var{command} | getline @var{var}}, the output of @var{command} is sent through a pipe to @code{getline} and into the variable @var{var}. For example, the following program reads the current date and time into the variable @code{current_time}, using the @command{date} utility, and then prints it: @example BEGIN @{ "date" | getline current_time close("date") print "Report printed on " current_time @} @end example In this version of @code{getline}, none of the built-in variables are changed and the record is not split into fields. @ifinfo @c Thanks to Paul Eggert for initial wording here According to POSIX, @samp{@var{expression} | getline @var{var}} is ambiguous if @var{expression} contains unparenthesized operators other than @samp{$}; for example, @samp{@w{"echo "} "date" | getline @var{var}} is ambiguous because the concatenation operator is not parenthesized. You should write it as @samp{(@w{"echo "} "date") | getline @var{var}} if you want your program to be portable to other @command{awk} implementations. @end ifinfo @node Getline/Coprocess @subsection Using @code{getline} from a Coprocess @cindex coprocesses, @code{getline} from @c comma before using is NOT for tertiary @cindex @code{getline} command, coprocesses, using from @cindex @code{|} (vertical bar), @code{|&} operator (I/O) @cindex vertical bar (@code{|}), @code{|&} operator (I/O) @cindex operators, input/output @cindex differences in @command{awk} and @command{gawk}, input/output operators Input into @code{getline} from a pipe is a one-way operation. The command that is started with @samp{@var{command} | getline} only sends data @emph{to} your @command{awk} program. On occasion, you might want to send data to another program for processing and then read the results back. @command{gawk} allows you start a @dfn{coprocess}, with which two-way communications are possible. This is done with the @samp{|&} operator. Typically, you write data to the coprocess first and then read results back, as shown in the following: @example print "@var{some query}" |& "db_server" "db_server" |& getline @end example @noindent which sends a query to @command{db_server} and then reads the results. The values of @code{NR} and @code{FNR} are not changed, because the main input stream is not used. However, the record is split into fields in the normal manner, thus changing the values of @code{$0}, of the other fields, and of @code{NF}. Coprocesses are an advanced feature. They are discussed here only because this is the @value{SECTION} on @code{getline}. @xref{Two-way I/O}, where coprocesses are discussed in more detail. @node Getline/Variable/Coprocess @subsection Using @code{getline} into a Variable from a Coprocess @c comma before using is NOT for tertiary @cindex variables, @code{getline} command into, using When you use @samp{@var{command} |& getline @var{var}}, the output from the coprocess @var{command} is sent through a two-way pipe to @code{getline} and into the variable @var{var}. In this version of @code{getline}, none of the built-in variables are changed and the record is not split into fields. The only variable changed is @var{var}. @ifinfo Coprocesses are an advanced feature. They are discussed here only because this is the @value{SECTION} on @code{getline}. @xref{Two-way I/O}, where coprocesses are discussed in more detail. @end ifinfo @node Getline Notes @subsection Points to Remember About @code{getline} Here are some miscellaneous points about @code{getline} that you should bear in mind: @itemize @bullet @item When @code{getline} changes the value of @code{$0} and @code{NF}, @command{awk} does @emph{not} automatically jump to the start of the program and start testing the new record against every pattern. However, the new record is tested against any subsequent rules. @cindex differences in @command{awk} and @command{gawk}, implementation limitations @cindex implementation issues, @command{gawk}, limits @cindex @command{awk}, implementations, limits @cindex @command{gawk}, implementation issues, limits @item Many @command{awk} implementations limit the number of pipelines that an @command{awk} program may have open to just one. In @command{gawk}, there is no such limit. You can open as many pipelines (and coprocesses) as the underlying operating system permits. @cindex side effects, @code{FILENAME} variable @c The comma before "setting with" does NOT represent a tertiary @cindex @code{FILENAME} variable, @code{getline}, setting with @cindex dark corner, @code{FILENAME} variable @cindex @code{getline} command, @code{FILENAME} variable and @cindex @code{BEGIN} pattern, @code{getline} and @item An interesting side effect occurs if you use @code{getline} without a redirection inside a @code{BEGIN} rule. Because an unredirected @code{getline} reads from the command-line @value{DF}s, the first @code{getline} command causes @command{awk} to set the value of @code{FILENAME}. Normally, @code{FILENAME} does not have a value inside @code{BEGIN} rules, because you have not yet started to process the command-line @value{DF}s. @value{DARKCORNER} (@xref{BEGIN/END}, also @pxref{Auto-set}.) @item Using @code{FILENAME} with @code{getline} (@samp{getline < FILENAME}) is likely to be a source for confusion. @command{awk} opens a separate input stream from the current input file. However, by not using a variable, @code{$0} and @code{NR} are still updated. If you're doing this, it's probably by accident, and you should reconsider what it is you're trying to accomplish. @end itemize @node Getline Summary @subsection Summary of @code{getline} Variants @cindex @code{getline} command, variants The following table summarizes the eight variants of @code{getline}, listing which built-in variables are set by each one. @multitable {@var{command} @code{|& getline} @var{var}} {1234567890123456789012345678901234567890} @item @code{getline} @tab Sets @code{$0}, @code{NF}, @code{FNR}, and @code{NR} @item @code{getline} @var{var} @tab Sets @var{var}, @code{FNR}, and @code{NR} @item @code{getline <} @var{file} @tab Sets @code{$0} and @code{NF} @item @code{getline @var{var} < @var{file}} @tab Sets @var{var} @item @var{command} @code{| getline} @tab Sets @code{$0} and @code{NF} @item @var{command} @code{| getline} @var{var} @tab Sets @var{var} @item @var{command} @code{|& getline} @tab Sets @code{$0} and @code{NF}. This is a @command{gawk} extension @item @var{command} @code{|& getline} @var{var} @tab Sets @var{var}. This is a @command{gawk} extension @end multitable @c ENDOFRANGE getl @c ENDOFRANGE inex @c ENDOFRANGE infir @node Printing @chapter Printing Output @c STARTOFRANGE prnt @cindex printing @cindex output, printing, See printing One of the most common programming actions is to @dfn{print}, or output, some or all of the input. Use the @code{print} statement for simple output, and the @code{printf} statement for fancier formatting. The @code{print} statement is not limited when computing @emph{which} values to print. However, with two exceptions, you cannot specify @emph{how} to print them---how many columns, whether to use exponential notation or not, and so on. (For the exceptions, @pxref{Output Separators}, and @ref{OFMT}.) For printing with specifications, you need the @code{printf} statement (@pxref{Printf}). @c STARTOFRANGE prnts @cindex @code{print} statement @cindex @code{printf} statement Besides basic and formatted printing, this @value{CHAPTER} also covers I/O redirections to files and pipes, introduces the special @value{FN}s that @command{gawk} processes internally, and discusses the @code{close} built-in function. @menu * Print:: The @code{print} statement. * Print Examples:: Simple examples of @code{print} statements. * Output Separators:: The output separators and how to change them. * OFMT:: Controlling Numeric Output With @code{print}. * Printf:: The @code{printf} statement. * Redirection:: How to redirect output to multiple files and pipes. * Special Files:: File name interpretation in @command{gawk}. @command{gawk} allows access to inherited file descriptors. * Close Files And Pipes:: Closing Input and Output Files and Pipes. @end menu @node Print @section The @code{print} Statement The @code{print} statement is used to produce output with simple, standardized formatting. Specify only the strings or numbers to print, in a list separated by commas. They are output, separated by single spaces, followed by a newline. The statement looks like this: @example print @var{item1}, @var{item2}, @dots{} @end example @noindent The entire list of items may be optionally enclosed in parentheses. The parentheses are necessary if any of the item expressions uses the @samp{>} relational operator; otherwise it could be confused with a redirection (@pxref{Redirection}). The items to print can be constant strings or numbers, fields of the current record (such as @code{$1}), variables, or any @command{awk} expression. Numeric values are converted to strings and then printed. @cindex records, printing @cindex lines, blank, printing @cindex text, printing The simple statement @samp{print} with no items is equivalent to @samp{print $0}: it prints the entire current record. To print a blank line, use @samp{print ""}, where @code{""} is the empty string. To print a fixed piece of text, use a string constant, such as @w{@code{"Don't Panic"}}, as one item. If you forget to use the double-quote characters, your text is taken as an @command{awk} expression, and you will probably get an error. Keep in mind that a space is printed between any two items. @node Print Examples @section Examples of @code{print} Statements Each @code{print} statement makes at least one line of output. However, it isn't limited to only one line. If an item value is a string that contains a newline, the newline is output along with the rest of the string. A single @code{print} statement can make any number of lines this way. @cindex newlines, printing The following is an example of printing a string that contains embedded newlines (the @samp{\n} is an escape sequence, used to represent the newline character; @pxref{Escape Sequences}): @example $ awk 'BEGIN @{ print "line one\nline two\nline three" @}' @print{} line one @print{} line two @print{} line three @end example @cindex fields, printing The next example, which is run on the @file{inventory-shipped} file, prints the first two fields of each input record, with a space between them: @example $ awk '@{ print $1, $2 @}' inventory-shipped @print{} Jan 13 @print{} Feb 15 @print{} Mar 15 @dots{} @end example @cindex @code{print} statement, commas, omitting @c comma does NOT start tertiary @cindex troubleshooting, @code{print} statement, omitting commas A common mistake in using the @code{print} statement is to omit the comma between two items. This often has the effect of making the items run together in the output, with no space. The reason for this is that juxtaposing two string expressions in @command{awk} means to concatenate them. Here is the same program, without the comma: @example $ awk '@{ print $1 $2 @}' inventory-shipped @print{} Jan13 @print{} Feb15 @print{} Mar15 @dots{} @end example @c comma does NOT start tertiary @cindex @code{BEGIN} pattern, headings, adding To someone unfamiliar with the @file{inventory-shipped} file, neither example's output makes much sense. A heading line at the beginning would make it clearer. Let's add some headings to our table of months (@code{$1}) and green crates shipped (@code{$2}). We do this using the @code{BEGIN} pattern (@pxref{BEGIN/END}) so that the headings are only printed once: @example awk 'BEGIN @{ print "Month Crates" print "----- ------" @} @{ print $1, $2 @}' inventory-shipped @end example @noindent When run, the program prints the following: @example Month Crates ----- ------ Jan 13 Feb 15 Mar 15 @dots{} @end example @noindent The only problem, however, is that the headings and the table data don't line up! We can fix this by printing some spaces between the two fields: @example @group awk 'BEGIN @{ print "Month Crates" print "----- ------" @} @{ print $1, " ", $2 @}' inventory-shipped @end group @end example @c comma does NOT start tertiary @cindex @code{printf} statement, columns, aligning @cindex columns, aligning Lining up columns this way can get pretty complicated when there are many columns to fix. Counting spaces for two or three columns is simple, but any more than this can take up a lot of time. This is why the @code{printf} statement was created (@pxref{Printf}); one of its specialties is lining up columns of data. @cindex line continuations, in @code{print} statement @cindex @code{print} statement, line continuations and @strong{Note:} You can continue either a @code{print} or @code{printf} statement simply by putting a newline after any comma (@pxref{Statements/Lines}). @c ENDOFRANGE prnts @node Output Separators @section Output Separators @cindex @code{OFS} variable As mentioned previously, a @code{print} statement contains a list of items separated by commas. In the output, the items are normally separated by single spaces. However, this doesn't need to be the case; a single space is only the default. Any string of characters may be used as the @dfn{output field separator} by setting the built-in variable @code{OFS}. The initial value of this variable is the string @w{@code{" "}}---that is, a single space. The output from an entire @code{print} statement is called an @dfn{output record}. Each @code{print} statement outputs one output record, and then outputs a string called the @dfn{output record separator} (or @code{ORS}). The initial value of @code{ORS} is the string @code{"\n"}; i.e., a newline character. Thus, each @code{print} statement normally makes a separate line. @cindex output, records @cindex output record separator, See @code{ORS} variable @cindex @code{ORS} variable @cindex @code{BEGIN} pattern, @code{OFS}/@code{ORS} variables, assigning values to In order to change how output fields and records are separated, assign new values to the variables @code{OFS} and @code{ORS}. The usual place to do this is in the @code{BEGIN} rule (@pxref{BEGIN/END}), so that it happens before any input is processed. It can also be done with assignments on the command line, before the names of the input files, or using the @option{-v} command-line option (@pxref{Options}). The following example prints the first and second fields of each input record, separated by a semicolon, with a blank line added after each newline: @ignore Exercise, Rewrite the @example awk 'BEGIN @{ print "Month Crates" print "----- ------" @} @{ print $1, " ", $2 @}' inventory-shipped @end example program by using a new value of @code{OFS}. @end ignore @example $ awk 'BEGIN @{ OFS = ";"; ORS = "\n\n" @} > @{ print $1, $2 @}' BBS-list @print{} aardvark;555-5553 @print{} @print{} alpo-net;555-3412 @print{} @print{} barfly;555-7685 @dots{} @end example If the value of @code{ORS} does not contain a newline, the program's output is run together on a single line. @node OFMT @section Controlling Numeric Output with @code{print} @cindex numeric, output format @c the comma does NOT start a secondary @cindex formats, numeric output When the @code{print} statement is used to print numeric values, @command{awk} internally converts the number to a string of characters and prints that string. @command{awk} uses the @code{sprintf} function to do this conversion (@pxref{String Functions}). For now, it suffices to say that the @code{sprintf} function accepts a @dfn{format specification} that tells it how to format numbers (or strings), and that there are a number of different ways in which numbers can be formatted. The different format specifications are discussed more fully in @ref{Control Letters}. @cindex @code{sprintf} function @cindex @code{OFMT} variable @c the comma before OFMT does NOT start a tertiary @cindex output, format specifier, @code{OFMT} The built-in variable @code{OFMT} contains the default format specification that @code{print} uses with @code{sprintf} when it wants to convert a number to a string for printing. The default value of @code{OFMT} is @code{"%.6g"}. The way @code{print} prints numbers can be changed by supplying different format specifications as the value of @code{OFMT}, as shown in the following example: @example $ awk 'BEGIN @{ > OFMT = "%.0f" # print numbers as integers (rounds) > print 17.23, 17.54 @}' @print{} 17 18 @end example @noindent @cindex dark corner, @code{OFMT} variable @cindex POSIX @command{awk}, @code{OFMT} variable and @cindex @code{OFMT} variable, POSIX @command{awk} and According to the POSIX standard, @command{awk}'s behavior is undefined if @code{OFMT} contains anything but a floating-point conversion specification. @value{DARKCORNER} @node Printf @section Using @code{printf} Statements for Fancier Printing @c STARTOFRANGE printfs @cindex @code{printf} statement @cindex output, formatted @cindex formatting output For more precise control over the output format than what is normally provided by @code{print}, use @code{printf}. @code{printf} can be used to specify the width to use for each item, as well as various formatting choices for numbers (such as what output base to use, whether to print an exponent, whether to print a sign, and how many digits to print after the decimal point). This is done by supplying a string, called the @dfn{format string}, that controls how and where to print the other arguments. @menu * Basic Printf:: Syntax of the @code{printf} statement. * Control Letters:: Format-control letters. * Format Modifiers:: Format-specification modifiers. * Printf Examples:: Several examples. @end menu @node Basic Printf @subsection Introduction to the @code{printf} Statement @cindex @code{printf} statement, syntax of A simple @code{printf} statement looks like this: @example printf @var{format}, @var{item1}, @var{item2}, @dots{} @end example @noindent The entire list of arguments may optionally be enclosed in parentheses. The parentheses are necessary if any of the item expressions use the @samp{>} relational operator; otherwise, it can be confused with a redirection (@pxref{Redirection}). @cindex format strings The difference between @code{printf} and @code{print} is the @var{format} argument. This is an expression whose value is taken as a string; it specifies how to output each of the other arguments. It is called the @dfn{format string}. The format string is very similar to that in the ISO C library function @code{printf}. Most of @var{format} is text to output verbatim. Scattered among this text are @dfn{format specifiers}---one per item. Each format specifier says to output the next item in the argument list at that place in the format. The @code{printf} statement does not automatically append a newline to its output. It outputs only what the format string specifies. So if a newline is needed, you must include one in the format string. The output separator variables @code{OFS} and @code{ORS} have no effect on @code{printf} statements. For example: @example $ awk 'BEGIN @{ > ORS = "\nOUCH!\n"; OFS = "+" > msg = "Dont Panic!" > printf "%s\n", msg > @}' @print{} Dont Panic! @end example @noindent Here, neither the @samp{+} nor the @samp{OUCH} appear when the message is printed. @node Control Letters @subsection Format-Control Letters @cindex @code{printf} statement, format-control characters @cindex format specifiers, @code{printf} statement A format specifier starts with the character @samp{%} and ends with a @dfn{format-control letter}---it tells the @code{printf} statement how to output one item. The format-control letter specifies what @emph{kind} of value to print. The rest of the format specifier is made up of optional @dfn{modifiers} that control @emph{how} to print the value, such as the field width. Here is a list of the format-control letters: @table @code @item %c This prints a number as an ASCII character; thus, @samp{printf "%c", 65} outputs the letter @samp{A}. (The output for a string value is the first character of the string.) @item %d@r{,} %i These are equivalent; they both print a decimal integer. (The @samp{%i} specification is for compatibility with ISO C.) @item %e@r{,} %E These print a number in scientific (exponential) notation; for example: @example printf "%4.3e\n", 1950 @end example @noindent prints @samp{1.950e+03}, with a total of four significant figures, three of which follow the decimal point. (The @samp{4.3} represents two modifiers, discussed in the next @value{SUBSECTION}.) @samp{%E} uses @samp{E} instead of @samp{e} in the output. @item %f This prints a number in floating-point notation. For example: @example printf "%4.3f", 1950 @end example @noindent prints @samp{1950.000}, with a total of four significant figures, three of which follow the decimal point. (The @samp{4.3} represents two modifiers, discussed in the next @value{SUBSECTION}.) @item %g@r{,} %G These print a number in either scientific notation or in floating-point notation, whichever uses fewer characters; if the result is printed in scientific notation, @samp{%G} uses @samp{E} instead of @samp{e}. @item %o This prints an unsigned octal integer. @item %s This prints a string. @item %u This prints an unsigned decimal integer. (This format is of marginal use, because all numbers in @command{awk} are floating-point; it is provided primarily for compatibility with C.) @item %x@r{,} %X These print an unsigned hexadecimal integer; @samp{%X} uses the letters @samp{A} through @samp{F} instead of @samp{a} through @samp{f}. @item %% This isn't a format-control letter, but it does have meaning---the sequence @samp{%%} outputs one @samp{%}; it does not consume an argument and it ignores any modifiers. @end table @cindex dark corner, format-control characters @cindex @command{gawk}, format-control characters @strong{Note:} When using the integer format-control letters for values that are outside the range of the widest C integer type, @command{gawk} switches to the the @samp{%g} format specifier. If @option{--lint} is provided on the command line (@pxref{Options}), @command{gawk} warns about this. Other versions of @command{awk} may print invalid values or do something else entirely. @value{DARKCORNER} @node Format Modifiers @subsection Modifiers for @code{printf} Formats @c STARTOFRANGE pfm @cindex @code{printf} statement, modifiers @c the comma here does NOT start a secondary @cindex modifiers, in format specifiers A format specification can also include @dfn{modifiers} that can control how much of the item's value is printed, as well as how much space it gets. The modifiers come between the @samp{%} and the format-control letter. We will use the bullet symbol ``@bullet{}'' in the following examples to represent spaces in the output. Here are the possible modifiers, in the order in which they may appear: @table @code @cindex differences in @command{awk} and @command{gawk}, @code{print}/@code{printf} statements @cindex @code{printf} statement, positional specifiers @c the command does NOT start a secondary @cindex positional specifiers, @code{printf} statement @item @var{N}$ An integer constant followed by a @samp{$} is a @dfn{positional specifier}. Normally, format specifications are applied to arguments in the order given in the format string. With a positional specifier, the format specification is applied to a specific argument, instead of what would be the next argument in the list. Positional specifiers begin counting with one. Thus: @example printf "%s %s\n", "don't", "panic" printf "%2$s %1$s\n", "panic", "don't" @end example @noindent prints the famous friendly message twice. At first glance, this feature doesn't seem to be of much use. It is in fact a @command{gawk} extension, intended for use in translating messages at runtime. @xref{Printf Ordering}, which describes how and why to use positional specifiers. For now, we will not use them. @item - The minus sign, used before the width modifier (see later on in this table), says to left-justify the argument within its specified width. Normally, the argument is printed right-justified in the specified width. Thus: @example printf "%-4s", "foo" @end example @noindent prints @samp{foo@bullet{}}. @item @var{space} For numeric conversions, prefix positive values with a space and negative values with a minus sign. @item + The plus sign, used before the width modifier (see later on in this table), says to always supply a sign for numeric conversions, even if the data to format is positive. The @samp{+} overrides the space modifier. @item # Use an ``alternate form'' for certain control letters. For @samp{%o}, supply a leading zero. For @samp{%x} and @samp{%X}, supply a leading @samp{0x} or @samp{0X} for a nonzero result. For @samp{%e}, @samp{%E}, and @samp{%f}, the result always contains a decimal point. For @samp{%g} and @samp{%G}, trailing zeros are not removed from the result. @cindex dark corner @item 0 A leading @samp{0} (zero) acts as a flag that indicates that output should be padded with zeros instead of spaces. This applies even to non-numeric output formats. @value{DARKCORNER} This flag only has an effect when the field width is wider than the value to print. @item @var{width} This is a number specifying the desired minimum width of a field. Inserting any number between the @samp{%} sign and the format-control character forces the field to expand to this width. The default way to do this is to pad with spaces on the left. For example: @example printf "%4s", "foo" @end example @noindent prints @samp{@bullet{}foo}. The value of @var{width} is a minimum width, not a maximum. If the item value requires more than @var{width} characters, it can be as wide as necessary. Thus, the following: @example printf "%4s", "foobar" @end example @noindent prints @samp{foobar}. Preceding the @var{width} with a minus sign causes the output to be padded with spaces on the right, instead of on the left. @item .@var{prec} A period followed by an integer constant specifies the precision to use when printing. The meaning of the precision varies by control letter: @table @asis @item @code{%e}, @code{%E}, @code{%f} Number of digits to the right of the decimal point. @item @code{%g}, @code{%G} Maximum number of significant digits. @item @code{%d}, @code{%i}, @code{%o}, @code{%u}, @code{%x}, @code{%X} Minimum number of digits to print. @item @code{%s} Maximum number of characters from the string that should print. @end table Thus, the following: @example printf "%.4s", "foobar" @end example @noindent prints @samp{foob}. @end table The C library @code{printf}'s dynamic @var{width} and @var{prec} capability (for example, @code{"%*.*s"}) is supported. Instead of supplying explicit @var{width} and/or @var{prec} values in the format string, they are passed in the argument list. For example: @example w = 5 p = 3 s = "abcdefg" printf "%*.*s\n", w, p, s @end example @noindent is exactly equivalent to: @example s = "abcdefg" printf "%5.3s\n", s @end example @noindent Both programs output @samp{@w{@bullet{}@bullet{}abc}}. Earlier versions of @command{awk} did not support this capability. If you must use such a version, you may simulate this feature by using concatenation to build up the format string, like so: @example w = 5 p = 3 s = "abcdefg" printf "%" w "." p "s\n", s @end example @noindent This is not particularly easy to read but it does work. @c @cindex lint checks @cindex troubleshooting, fatal errors, @code{printf} format strings @cindex POSIX @command{awk}, @code{printf} format strings and C programmers may be used to supplying additional @samp{l}, @samp{L}, and @samp{h} modifiers in @code{printf} format strings. These are not valid in @command{awk}. Most @command{awk} implementations silently ignore these modifiers. If @option{--lint} is provided on the command line (@pxref{Options}), @command{gawk} warns about their use. If @option{--posix} is supplied, their use is a fatal error. @c ENDOFRANGE pfm @node Printf Examples @subsection Examples Using @code{printf} The following is a simple example of how to use @code{printf} to make an aligned table: @example awk '@{ printf "%-10s %s\n", $1, $2 @}' BBS-list @end example @noindent This command prints the names of the bulletin boards (@code{$1}) in the file @file{BBS-list} as a string of 10 characters that are left-justified. It also prints the phone numbers (@code{$2}) next on the line. This produces an aligned two-column table of names and phone numbers, as shown here: @example $ awk '@{ printf "%-10s %s\n", $1, $2 @}' BBS-list @print{} aardvark 555-5553 @print{} alpo-net 555-3412 @print{} barfly 555-7685 @print{} bites 555-1675 @print{} camelot 555-0542 @print{} core 555-2912 @print{} fooey 555-1234 @print{} foot 555-6699 @print{} macfoo 555-6480 @print{} sdace 555-3430 @print{} sabafoo 555-2127 @end example In this case, the phone numbers had to be printed as strings because the numbers are separated by a dash. Printing the phone numbers as numbers would have produced just the first three digits: @samp{555}. This would have been pretty confusing. It wasn't necessary to specify a width for the phone numbers because they are last on their lines. They don't need to have spaces after them. The table could be made to look even nicer by adding headings to the tops of the columns. This is done using the @code{BEGIN} pattern (@pxref{BEGIN/END}) so that the headers are only printed once, at the beginning of the @command{awk} program: @example awk 'BEGIN @{ print "Name Number" print "---- ------" @} @{ printf "%-10s %s\n", $1, $2 @}' BBS-list @end example The above example mixed @code{print} and @code{printf} statements in the same program. Using just @code{printf} statements can produce the same results: @example awk 'BEGIN @{ printf "%-10s %s\n", "Name", "Number" printf "%-10s %s\n", "----", "------" @} @{ printf "%-10s %s\n", $1, $2 @}' BBS-list @end example @noindent Printing each column heading with the same format specification used for the column elements ensures that the headings are aligned just like the columns. The fact that the same format specification is used three times can be emphasized by storing it in a variable, like this: @example awk 'BEGIN @{ format = "%-10s %s\n" printf format, "Name", "Number" printf format, "----", "------" @} @{ printf format, $1, $2 @}' BBS-list @end example @c !!! exercise At this point, it would be a worthwhile exercise to use the @code{printf} statement to line up the headings and table data for the @file{inventory-shipped} example that was covered earlier in the @value{SECTION} on the @code{print} statement (@pxref{Print}). @c ENDOFRANGE printfs @node Redirection @section Redirecting Output of @code{print} and @code{printf} @cindex output redirection @cindex redirection of output So far, the output from @code{print} and @code{printf} has gone to the standard output, usually the terminal. Both @code{print} and @code{printf} can also send their output to other places. This is called @dfn{redirection}. A redirection appears after the @code{print} or @code{printf} statement. Redirections in @command{awk} are written just like redirections in shell commands, except that they are written inside the @command{awk} program. @c the commas here are part of the see also @cindex @code{print} statement, See Also redirection, of output @cindex @code{printf} statement, See Also redirection, of output There are four forms of output redirection: output to a file, output appended to a file, output through a pipe to another command, and output to a coprocess. They are all shown for the @code{print} statement, but they work identically for @code{printf}: @table @code @cindex @code{>} (right angle bracket), @code{>} operator (I/O) @cindex right angle bracket (@code{>}), @code{>} operator (I/O) @cindex operators, input/output @item print @var{items} > @var{output-file} This type of redirection prints the items into the output file named @var{output-file}. The @value{FN} @var{output-file} can be any expression. Its value is changed to a string and then used as a @value{FN} (@pxref{Expressions}). When this type of redirection is used, the @var{output-file} is erased before the first output is written to it. Subsequent writes to the same @var{output-file} do not erase @var{output-file}, but append to it. (This is different from how you use redirections in shell scripts.) If @var{output-file} does not exist, it is created. For example, here is how an @command{awk} program can write a list of BBS names to one file named @file{name-list}, and a list of phone numbers to another file named @file{phone-list}: @example $ awk '@{ print $2 > "phone-list" > print $1 > "name-list" @}' BBS-list $ cat phone-list @print{} 555-5553 @print{} 555-3412 @dots{} $ cat name-list @print{} aardvark @print{} alpo-net @dots{} @end example @noindent Each output file contains one name or number per line. @cindex @code{>} (right angle bracket), @code{>>} operator (I/O) @cindex right angle bracket (@code{>}), @code{>>} operator (I/O) @item print @var{items} >> @var{output-file} This type of redirection prints the items into the pre-existing output file named @var{output-file}. The difference between this and the single-@samp{>} redirection is that the old contents (if any) of @var{output-file} are not erased. Instead, the @command{awk} output is appended to the file. If @var{output-file} does not exist, then it is created. @cindex @code{|} (vertical bar), @code{|} operator (I/O) @cindex pipes, output @cindex output, pipes @item print @var{items} | @var{command} It is also possible to send output to another program through a pipe instead of into a file. This type of redirection opens a pipe to @var{command}, and writes the values of @var{items} through this pipe to another process created to execute @var{command}. The redirection argument @var{command} is actually an @command{awk} expression. Its value is converted to a string whose contents give the shell command to be run. For example, the following produces two files, one unsorted list of BBS names, and one list sorted in reverse alphabetical order: @ignore 10/2000: This isn't the best style, since COMMAND is assigned for each record. It's done to avoid overfull hboxes in TeX. Leave it alone for now and let's hope no-one notices. @end ignore @example awk '@{ print $1 > "names.unsorted" command = "sort -r > names.sorted" print $1 | command @}' BBS-list @end example The unsorted list is written with an ordinary redirection, while the sorted list is written by piping through the @command{sort} utility. The next example uses redirection to mail a message to the mailing list @samp{bug-system}. This might be useful when trouble is encountered in an @command{awk} script run periodically for system maintenance: @example report = "mail bug-system" print "Awk script failed:", $0 | report m = ("at record number " FNR " of " FILENAME) print m | report close(report) @end example The message is built using string concatenation and saved in the variable @code{m}. It's then sent down the pipeline to the @command{mail} program. (The parentheses group the items to concatenate---see @ref{Concatenation}.) The @code{close} function is called here because it's a good idea to close the pipe as soon as all the intended output has been sent to it. @xref{Close Files And Pipes}, for more information. This example also illustrates the use of a variable to represent a @var{file} or @var{command}---it is not necessary to always use a string constant. Using a variable is generally a good idea, because @command{awk} requires that the string value be spelled identically every time. @cindex coprocesses @cindex @code{|} (vertical bar), @code{|&} operator (I/O) @cindex operators, input/output @cindex differences in @command{awk} and @command{gawk}, input/output operators @item print @var{items} |& @var{command} This type of redirection prints the items to the input of @var{command}. The difference between this and the single-@samp{|} redirection is that the output from @var{command} can be read with @code{getline}. Thus @var{command} is a @dfn{coprocess}, which works together with, but subsidiary to, the @command{awk} program. This feature is a @command{gawk} extension, and is not available in POSIX @command{awk}. @xref{Two-way I/O}, for a more complete discussion. @end table Redirecting output using @samp{>}, @samp{>>}, @samp{|}, or @samp{|&} asks the system to open a file, pipe, or coprocess only if the particular @var{file} or @var{command} you specify has not already been written to by your program or if it has been closed since it was last written to. @cindex troubleshooting, printing It is a common error to use @samp{>} redirection for the first @code{print} to a file, and then to use @samp{>>} for subsequent output: @example # clear the file print "Don't panic" > "guide.txt" @dots{} # append print "Avoid improbability generators" >> "guide.txt" @end example @noindent This is indeed how redirections must be used from the shell. But in @command{awk}, it isn't necessary. In this kind of case, a program should use @samp{>} for all the @code{print} statements, since the output file is only opened once. @cindex differences in @command{awk} and @command{gawk}, implementation limitations @c the comma here does NOT start a secondary @cindex implementation issues, @command{gawk}, limits @cindex @command{awk}, implementation issues, pipes @cindex @command{gawk}, implementation issues, pipes @ifnotinfo As mentioned earlier (@pxref{Getline Notes}), many @end ifnotinfo @ifnottex Many @end ifnottex @command{awk} implementations limit the number of pipelines that an @command{awk} program may have open to just one! In @command{gawk}, there is no such limit. @command{gawk} allows a program to open as many pipelines as the underlying operating system permits. @c fakenode --- for prepinfo @subheading Advanced Notes: Piping into @command{sh} @cindex advanced features, piping into @command{sh} @cindex shells, piping commands into A particularly powerful way to use redirection is to build command lines and pipe them into the shell, @command{sh}. For example, suppose you have a list of files brought over from a system where all the @value{FN}s are stored in uppercase, and you wish to rename them to have names in all lowercase. The following program is both simple and efficient: @c @cindex @command{mv} utility @example @{ printf("mv %s %s\n", $0, tolower($0)) | "sh" @} END @{ close("sh") @} @end example The @code{tolower} function returns its argument string with all uppercase characters converted to lowercase (@pxref{String Functions}). The program builds up a list of command lines, using the @command{mv} utility to rename the files. It then sends the list to the shell for execution. @c ENDOFRANGE outre @c ENDOFRANGE reout @node Special Files @section Special @value{FFN}s in @command{gawk} @c STARTOFRANGE gfn @cindex @command{gawk}, @value{FN}s in @command{gawk} provides a number of special @value{FN}s that it interprets internally. These @value{FN}s provide access to standard file descriptors, process-related information, and TCP/IP networking. @menu * Special FD:: Special files for I/O. * Special Process:: Special files for process information. * Special Network:: Special files for network communications. * Special Caveats:: Things to watch out for. @end menu @node Special FD @subsection Special Files for Standard Descriptors @cindex standard input @cindex input, standard @cindex standard output @cindex output, standard @cindex error output @cindex file descriptors @cindex files, descriptors, See file descriptors Running programs conventionally have three input and output streams already available to them for reading and writing. These are known as the @dfn{standard input}, @dfn{standard output}, and @dfn{standard error output}. These streams are, by default, connected to your terminal, but they are often redirected with the shell, via the @samp{<}, @samp{<<}, @samp{>}, @samp{>>}, @samp{>&}, and @samp{|} operators. Standard error is typically used for writing error messages; the reason there are two separate streams, standard output and standard error, is so that they can be redirected separately. @cindex differences in @command{awk} and @command{gawk}, error messages @cindex error handling In other implementations of @command{awk}, the only way to write an error message to standard error in an @command{awk} program is as follows: @example print "Serious error detected!" | "cat 1>&2" @end example @noindent This works by opening a pipeline to a shell command that can access the standard error stream that it inherits from the @command{awk} process. This is far from elegant, and it is also inefficient, because it requires a separate process. So people writing @command{awk} programs often don't do this. Instead, they send the error messages to the terminal, like this: @example print "Serious error detected!" > "/dev/tty" @end example @noindent This usually has the same effect but not always: although the standard error stream is usually the terminal, it can be redirected; when that happens, writing to the terminal is not correct. In fact, if @command{awk} is run from a background job, it may not have a terminal at all. Then opening @file{/dev/tty} fails. @command{gawk} provides special @value{FN}s for accessing the three standard streams, as well as any other inherited open files. If the @value{FN} matches one of these special names when @command{gawk} redirects input or output, then it directly uses the stream that the @value{FN} stands for. These special @value{FN}s work for all operating systems that @command{gawk} has been ported to, not just those that are POSIX-compliant: @cindex @value{FN}s, standard streams in @command{gawk} @cindex @code{/dev/@dots{}} special files (@command{gawk}) @cindex files, @code{/dev/@dots{}} special files @c @cindex @code{/dev/stdin} special file @c @cindex @code{/dev/stdout} special file @c @cindex @code{/dev/stderr} special file @c @cindex @code{/dev/fd} special files @table @file @item /dev/stdin The standard input (file descriptor 0). @item /dev/stdout The standard output (file descriptor 1). @item /dev/stderr The standard error output (file descriptor 2). @item /dev/fd/@var{N} The file associated with file descriptor @var{N}. Such a file must be opened by the program initiating the @command{awk} execution (typically the shell). Unless special pains are taken in the shell from which @command{gawk} is invoked, only descriptors 0, 1, and 2 are available. @end table The @value{FN}s @file{/dev/stdin}, @file{/dev/stdout}, and @file{/dev/stderr} are aliases for @file{/dev/fd/0}, @file{/dev/fd/1}, and @file{/dev/fd/2}, respectively. However, they are more self-explanatory. The proper way to write an error message in a @command{gawk} program is to use @file{/dev/stderr}, like this: @example print "Serious error detected!" > "/dev/stderr" @end example @cindex troubleshooting, quotes with @value{FN}s Note the use of quotes around the @value{FN}. Like any other redirection, the value must be a string. It is a common error to omit the quotes, which leads to confusing results. @c Exercise: What does it do? :-) @node Special Process @subsection Special Files for Process-Related Information @cindex files, for process information @cindex process information, files for @command{gawk} also provides special @value{FN}s that give access to information about the running @command{gawk} process. Each of these ``files'' provides a single record of information. To read them more than once, they must first be closed with the @code{close} function (@pxref{Close Files And Pipes}). The @value{FN}s are: @c @cindex @code{/dev/pid} special file @c @cindex @code{/dev/pgrpid} special file @c @cindex @code{/dev/ppid} special file @c @cindex @code{/dev/user} special file @table @file @item /dev/pid Reading this file returns the process ID of the current process, in decimal form, terminated with a newline. @item /dev/ppid Reading this file returns the parent process ID of the current process, in decimal form, terminated with a newline. @item /dev/pgrpid Reading this file returns the process group ID of the current process, in decimal form, terminated with a newline. @item /dev/user Reading this file returns a single record terminated with a newline. The fields are separated with spaces. The fields represent the following information: @table @code @item $1 The return value of the @code{getuid} system call (the real user ID number). @item $2 The return value of the @code{geteuid} system call (the effective user ID number). @item $3 The return value of the @code{getgid} system call (the real group ID number). @item $4 The return value of the @code{getegid} system call (the effective group ID number). @end table If there are any additional fields, they are the group IDs returned by the @code{getgroups} system call. (Multiple groups may not be supported on all systems.) @end table These special @value{FN}s may be used on the command line as @value{DF}s, as well as for I/O redirections within an @command{awk} program. They may not be used as source files with the @option{-f} option. @c @cindex automatic warnings @c @cindex warnings, automatic @strong{Note:} The special files that provide process-related information are now considered obsolete and will disappear entirely in the next release of @command{gawk}. @command{gawk} prints a warning message every time you use one of these files. To obtain process-related information, use the @code{PROCINFO} array. @xref{Auto-set}. @node Special Network @subsection Special Files for Network Communications @cindex networks, support for @cindex TCP/IP, support for Starting with @value{PVERSION} 3.1 of @command{gawk}, @command{awk} programs can open a two-way TCP/IP connection, acting as either a client or a server. This is done using a special @value{FN} of the form: @example @file{/inet/@var{protocol}/@var{local-port}/@var{remote-host}/@var{remote-port}} @end example The @var{protocol} is one of @samp{tcp}, @samp{udp}, or @samp{raw}, and the other fields represent the other essential pieces of information for making a networking connection. These @value{FN}s are used with the @samp{|&} operator for communicating with a coprocess (@pxref{Two-way I/O}). This is an advanced feature, mentioned here only for completeness. Full discussion is delayed until @ref{TCP/IP Networking}. @node Special Caveats @subsection Special @value{FFN} Caveats Here is a list of things to bear in mind when using the special @value{FN}s that @command{gawk} provides: @itemize @bullet @cindex compatibility mode (@command{gawk}), @value{FN}s @cindex @value{FN}s, in compatibility mode @item Recognition of these special @value{FN}s is disabled if @command{gawk} is in compatibility mode (@pxref{Options}). @c @cindex automatic warnings @c @cindex warnings, automatic @cindex @code{PROCINFO} array @item @ifnottex The @end ifnottex @ifnotinfo As mentioned earlier, the @end ifnotinfo special files that provide process-related information are now considered obsolete and will disappear entirely in the next release of @command{gawk}. @command{gawk} prints a warning message every time you use one of these files. @ifnottex To obtain process-related information, use the @code{PROCINFO} array. @xref{Built-in Variables}. @end ifnottex @item Starting with @value{PVERSION} 3.1, @command{gawk} @emph{always} interprets these special @value{FN}s.@footnote{Older versions of @command{gawk} would interpret these names internally only if the system did not actually have a @file{/dev/fd} directory or any of the other special files listed earlier. Usually this didn't make a difference, but sometimes it did; thus, it was decided to make @command{gawk}'s behavior consistent on all systems and to have it always interpret the special @value{FN}s itself.} For example, using @samp{/dev/fd/4} for output actually writes on file descriptor 4, and not on a new file descriptor that is @code{dup}'ed from file descriptor 4. Most of the time this does not matter; however, it is important to @emph{not} close any of the files related to file descriptors 0, 1, and 2. Doing so results in unpredictable behavior. @end itemize @c ENDOFRANGE gfn @node Close Files And Pipes @section Closing Input and Output Redirections @cindex files, output, See output files @c STARTOFRANGE ifc @cindex input files, closing @c comma before closing is NOT start of tertiary @c STARTOFRANGE ofc @cindex output, files, closing @c STARTOFRANGE pc @cindex pipes, closing @c STARTOFRANGE cc @cindex coprocesses, closing @c comma before using is NOT start of tertiary @cindex @code{getline} command, coprocesses, using from If the same @value{FN} or the same shell command is used with @code{getline} more than once during the execution of an @command{awk} program (@pxref{Getline}), the file is opened (or the command is executed) the first time only. At that time, the first record of input is read from that file or command. The next time the same file or command is used with @code{getline}, another record is read from it, and so on. Similarly, when a file or pipe is opened for output, the @value{FN} or command associated with it is remembered by @command{awk}, and subsequent writes to the same file or command are appended to the previous writes. The file or pipe stays open until @command{awk} exits. @cindex @code{close} function This implies that special steps are necessary in order to read the same file again from the beginning, or to rerun a shell command (rather than reading more output from the same command). The @code{close} function makes these things possible: @example close(@var{filename}) @end example @noindent or: @example close(@var{command}) @end example The argument @var{filename} or @var{command} can be any expression. Its value must @emph{exactly} match the string that was used to open the file or start the command (spaces and other ``irrelevant'' characters included). For example, if you open a pipe with this: @example "sort -r names" | getline foo @end example @noindent then you must close it with this: @example close("sort -r names") @end example Once this function call is executed, the next @code{getline} from that file or command, or the next @code{print} or @code{printf} to that file or command, reopens the file or reruns the command. Because the expression that you use to close a file or pipeline must exactly match the expression used to open the file or run the command, it is good practice to use a variable to store the @value{FN} or command. The previous example becomes the following: @example sortcom = "sort -r names" sortcom | getline foo @dots{} close(sortcom) @end example @noindent This helps avoid hard-to-find typographical errors in your @command{awk} programs. Here are some of the reasons for closing an output file: @itemize @bullet @item To write a file and read it back later on in the same @command{awk} program. Close the file after writing it, then begin reading it with @code{getline}. @item To write numerous files, successively, in the same @command{awk} program. If the files aren't closed, eventually @command{awk} may exceed a system limit on the number of open files in one process. It is best to close each one when the program has finished writing it. @item To make a command finish. When output is redirected through a pipe, the command reading the pipe normally continues to try to read input as long as the pipe is open. Often this means the command cannot really do its work until the pipe is closed. For example, if output is redirected to the @command{mail} program, the message is not actually sent until the pipe is closed. @item To run the same program a second time, with the same arguments. This is not the same thing as giving more input to the first run! For example, suppose a program pipes output to the @command{mail} program. If it outputs several lines redirected to this pipe without closing it, they make a single message of several lines. By contrast, if the program closes the pipe after each line of output, then each line makes a separate message. @end itemize @cindex differences in @command{awk} and @command{gawk}, @code{close} function @cindex portability, @code{close} function and If you use more files than the system allows you to have open, @command{gawk} attempts to multiplex the available open files among your @value{DF}s. @command{gawk}'s ability to do this depends upon the facilities of your operating system, so it may not always work. It is therefore both good practice and good portability advice to always use @code{close} on your files when you are done with them. In fact, if you are using a lot of pipes, it is essential that you close commands when done. For example, consider something like this: @example @{ @dots{} command = ("grep " $1 " /some/file | my_prog -q " $3) while ((command | getline) > 0) @{ @var{process output of} command @} # need close(command) here @} @end example This example creates a new pipeline based on data in @emph{each} record. Without the call to @code{close} indicated in the comment, @command{awk} creates child processes to run the commands, until it eventually runs out of file descriptors for more pipelines. Even though each command has finished (as indicated by the end-of-file return status from @code{getline}), the child process is not terminated;@footnote{The technical terminology is rather morbid. The finished child is called a ``zombie,'' and cleaning up after it is referred to as ``reaping.''} @c Good old UNIX: give the marketing guys fits, that's the ticket more importantly, the file descriptor for the pipe is not closed and released until @code{close} is called or @command{awk} exits. @code{close} will silently do nothing if given an argument that does not represent a file, pipe or coprocess that was opened with a redirection. Note also that @samp{close(FILENAME)} has no ``magic'' effects on the implicit loop that reads through the files named on the command line. It is, more likely, a close of a file that was never opened, so @command{awk} silently does nothing. @c comma is part of tertiary @cindex @code{|} (vertical bar), @code{|&} operator (I/O), pipes, closing When using the @samp{|&} operator to communicate with a coprocess, it is occasionally useful to be able to close one end of the two-way pipe without closing the other. This is done by supplying a second argument to @code{close}. As in any other call to @code{close}, the first argument is the name of the command or special file used to start the coprocess. The second argument should be a string, with either of the values @code{"to"} or @code{"from"}. Case does not matter. As this is an advanced feature, a more complete discussion is delayed until @ref{Two-way I/O}, which discusses it in more detail and gives an example. @c fakenode --- for prepinfo @subheading Advanced Notes: Using @code{close}'s Return Value @cindex advanced features, @code{close} function @cindex dark corner, @code{close} function @cindex @code{close} function, return values @c comma does NOT start secondary @cindex return values, @code{close} function @cindex differences in @command{awk} and @command{gawk}, @code{close} function @cindex Unix @command{awk}, @code{close} function and In many versions of Unix @command{awk}, the @code{close} function is actually a statement. It is a syntax error to try and use the return value from @code{close}: @value{DARKCORNER} @example command = "@dots{}" command | getline info retval = close(command) # syntax error in most Unix awks @end example @command{gawk} treats @code{close} as a function. The return value is @minus{}1 if the argument names something that was never opened with a redirection, or if there is a system problem closing the file or process. In these cases, @command{gawk} sets the built-in variable @code{ERRNO} to a string describing the problem. In @command{gawk}, when closing a pipe or coprocess, the return value is the exit status of the command.@footnote{ This is a full 16-bit value as returned by the @code{wait} system call. See the system manual pages for information on how to decode this value.} Otherwise, it is the return value from the system's @code{close} or @code{fclose} C functions when closing input or output files, respectively. This value is zero if the close succeeds, or @minus{}1 if it fails. The POSIX standard is very vague; it says that @code{close} returns zero on success and non-zero otherwise. In general, different implementations vary in what they report when closing pipes; thus the return value cannot be used portably. @value{DARKCORNER} @ignore @c 4/27/2003: Commenting this out for now, given the above @c return of 16-bit value The return value for closing a pipeline is particularly useful. It allows you to get the output from a command as well as its exit status. @c 8/21/2002, FIXME: Maybe the code and this doc should be adjusted to @c create values indicating death-by-signal? Sigh. @cindex pipes, closing @c comma does NOT start tertiary @cindex POSIX @command{awk}, pipes, closing For POSIX-compliant systems, if the exit status is a number above 128, then the program was terminated by a signal. Subtract 128 to get the signal number: @example exit_val = close(command) if (exit_val > 128) print command, "died with signal", exit_val - 128 else print command, "exited with code", exit_val @end example Currently, in @command{gawk}, this only works for commands piping into @code{getline}. For commands piped into from @code{print} or @code{printf}, the return value from @code{close} is that of the library's @code{pclose} function. @end ignore @c ENDOFRANGE ifc @c ENDOFRANGE ofc @c ENDOFRANGE pc @c ENDOFRANGE cc @c ENDOFRANGE prnt @node Expressions @chapter Expressions @c STARTOFRANGE exps @cindex expressions Expressions are the basic building blocks of @command{awk} patterns and actions. An expression evaluates to a value that you can print, test, or pass to a function. Additionally, an expression can assign a new value to a variable or a field by using an assignment operator. An expression can serve as a pattern or action statement on its own. Most other kinds of statements contain one or more expressions that specify the data on which to operate. As in other languages, expressions in @command{awk} include variables, array references, constants, and function calls, as well as combinations of these with various operators. @menu * Constants:: String, numeric and regexp constants. * Using Constant Regexps:: When and how to use a regexp constant. * Variables:: Variables give names to values for later use. * Conversion:: The conversion of strings to numbers and vice versa. * Arithmetic Ops:: Arithmetic operations (@samp{+}, @samp{-}, etc.) * Concatenation:: Concatenating strings. * Assignment Ops:: Changing the value of a variable or a field. * Increment Ops:: Incrementing the numeric value of a variable. * Truth Values:: What is ``true'' and what is ``false''. * Typing and Comparison:: How variables acquire types and how this affects comparison of numbers and strings with @samp{<}, etc. * Boolean Ops:: Combining comparison expressions using boolean operators @samp{||} (``or''), @samp{&&} (``and'') and @samp{!} (``not''). * Conditional Exp:: Conditional expressions select between two subexpressions under control of a third subexpression. * Function Calls:: A function call is an expression. * Precedence:: How various operators nest. @end menu @node Constants @section Constant Expressions @cindex constants, types of The simplest type of expression is the @dfn{constant}, which always has the same value. There are three types of constants: numeric, string, and regular expression. Each is used in the appropriate context when you need a data value that isn't going to change. Numeric constants can have different forms, but are stored identically internally. @menu * Scalar Constants:: Numeric and string constants. * Nondecimal-numbers:: What are octal and hex numbers. * Regexp Constants:: Regular Expression constants. @end menu @node Scalar Constants @subsection Numeric and String Constants @cindex numeric, constants A @dfn{numeric constant} stands for a number. This number can be an integer, a decimal fraction, or a number in scientific (exponential) notation.@footnote{The internal representation of all numbers, including integers, uses double-precision floating-point numbers. On most modern systems, these are in IEEE 754 standard format.} Here are some examples of numeric constants that all have the same value: @example 105 1.05e+2 1050e-1 @end example @cindex string constants A string constant consists of a sequence of characters enclosed in double-quotation marks. For example: @example "parrot" @end example @noindent @cindex differences in @command{awk} and @command{gawk}, strings @cindex strings, length of represents the string whose contents are @samp{parrot}. Strings in @command{gawk} can be of any length, and they can contain any of the possible eight-bit ASCII characters including ASCII @sc{nul} (character code zero). Other @command{awk} implementations may have difficulty with some character codes. @node Nondecimal-numbers @subsection Octal and Hexadecimal Numbers @cindex octal numbers @cindex hexadecimal numbers @cindex numbers, octal @cindex numbers, hexadecimal In @command{awk}, all numbers are in decimal; i.e., base 10. Many other programming languages allow you to specify numbers in other bases, often octal (base 8) and hexadecimal (base 16). In octal, the numbers go 0, 1, 2, 3, 4, 5, 6, 7, 10, 11, 12, etc. Just as @samp{11}, in decimal, is 1 times 10 plus 1, so @samp{11}, in octal, is 1 times 8, plus 1. This equals 9 in decimal. In hexadecimal, there are 16 digits. Since the everyday decimal number system only has ten digits (@samp{0}--@samp{9}), the letters @samp{a} through @samp{f} are used to represent the rest. (Case in the letters is usually irrelevant; hexadecimal @samp{a} and @samp{A} have the same value.) Thus, @samp{11}, in hexadecimal, is 1 times 16 plus 1, which equals 17 in decimal. Just by looking at plain @samp{11}, you can't tell what base it's in. So, in C, C++, and other languages derived from C, @c such as PERL, but we won't mention that.... there is a special notation to help signify the base. Octal numbers start with a leading @samp{0}, and hexadecimal numbers start with a leading @samp{0x} or @samp{0X}: @table @code @item 11 Decimal value 11. @item 011 Octal 11, decimal value 9. @item 0x11 Hexadecimal 11, decimal value 17. @end table This example shows the difference: @example $ gawk 'BEGIN @{ printf "%d, %d, %d\n", 011, 11, 0x11 @}' @print{} 9, 11, 17 @end example Being able to use octal and hexadecimal constants in your programs is most useful when working with data that cannot be represented conveniently as characters or as regular numbers, such as binary data of various sorts. @cindex @command{gawk}, octal numbers and @cindex @command{gawk}, hexadecimal numbers and @command{gawk} allows the use of octal and hexadecimal constants in your program text. However, such numbers in the input data are not treated differently; doing so by default would break old programs. (If you really need to do this, use the @option{--non-decimal-data} command-line option; @pxref{Nondecimal Data}.) If you have octal or hexadecimal data, you can use the @code{strtonum} function (@pxref{String Functions}) to convert the data into a number. Most of the time, you will want to use octal or hexadecimal constants when working with the built-in bit manipulation functions; see @ref{Bitwise Functions}, for more information. Unlike some early C implementations, @samp{8} and @samp{9} are not valid in octal constants; e.g., @command{gawk} treats @samp{018} as decimal 18: @example $ gawk 'BEGIN @{ print "021 is", 021 ; print 018 @}' @print{} 021 is 17 @print{} 18 @end example @cindex compatibility mode (@command{gawk}), octal numbers @cindex compatibility mode (@command{gawk}), hexadecimal numbers Octal and hexadecimal source code constants are a @command{gawk} extension. If @command{gawk} is in compatibility mode (@pxref{Options}), they are not available. @c fakenode --- for prepinfo @subheading Advanced Notes: A Constant's Base Does Not Affect Its Value @c comma before values does NOT start tertiary @cindex advanced features, constants, values of Once a numeric constant has been converted internally into a number, @command{gawk} no longer remembers what the original form of the constant was; the internal value is always used. This has particular consequences for conversion of numbers to strings: @example $ gawk 'BEGIN @{ printf "0x11 is <%s>\n", 0x11 @}' @print{} 0x11 is <17> @end example @node Regexp Constants @subsection Regular Expression Constants @c STARTOFRANGE rec @cindex regexp constants @cindex @code{~} (tilde), @code{~} operator @cindex tilde (@code{~}), @code{~} operator @cindex @code{!} (exclamation point), @code{!~} operator @cindex exclamation point (@code{!}), @code{!~} operator A regexp constant is a regular expression description enclosed in slashes, such as @code{@w{/^beginning and end$/}}. Most regexps used in @command{awk} programs are constant, but the @samp{~} and @samp{!~} matching operators can also match computed or ``dynamic'' regexps (which are just ordinary strings or variables that contain a regexp). @c ENDOFRANGE cnst @node Using Constant Regexps @section Using Regular Expression Constants @cindex dark corner, regexp constants When used on the righthand side of the @samp{~} or @samp{!~} operators, a regexp constant merely stands for the regexp that is to be matched. However, regexp constants (such as @code{/foo/}) may be used like simple expressions. When a regexp constant appears by itself, it has the same meaning as if it appeared in a pattern, i.e., @samp{($0 ~ /foo/)} @value{DARKCORNER} @xref{Expression Patterns}. This means that the following two code segments: @example if ($0 ~ /barfly/ || $0 ~ /camelot/) print "found" @end example @noindent and: @example if (/barfly/ || /camelot/) print "found" @end example @noindent are exactly equivalent. One rather bizarre consequence of this rule is that the following Boolean expression is valid, but does not do what the user probably intended: @example # note that /foo/ is on the left of the ~ if (/foo/ ~ $1) print "found foo" @end example @c @cindex automatic warnings @c @cindex warnings, automatic @cindex @command{gawk}, regexp constants and @cindex regexp constants, in @command{gawk} @noindent This code is ``obviously'' testing @code{$1} for a match against the regexp @code{/foo/}. But in fact, the expression @samp{/foo/ ~ $1} actually means @samp{($0 ~ /foo/) ~ $1}. In other words, first match the input record against the regexp @code{/foo/}. The result is either zero or one, depending upon the success or failure of the match. That result is then matched against the first field in the record. Because it is unlikely that you would ever really want to make this kind of test, @command{gawk} issues a warning when it sees this construct in a program. Another consequence of this rule is that the assignment statement: @example matches = /foo/ @end example @noindent assigns either zero or one to the variable @code{matches}, depending upon the contents of the current input record. This feature of the language has never been well documented until the POSIX specification. @cindex differences in @command{awk} and @command{gawk}, regexp constants @cindex dark corner, regexp constants, as arguments to user-defined functions @cindex @code{gensub} function (@command{gawk}) @cindex @code{sub} function @cindex @code{gsub} function Constant regular expressions are also used as the first argument for the @code{gensub}, @code{sub}, and @code{gsub} functions, and as the second argument of the @code{match} function (@pxref{String Functions}). Modern implementations of @command{awk}, including @command{gawk}, allow the third argument of @code{split} to be a regexp constant, but some older implementations do not. @value{DARKCORNER} This can lead to confusion when attempting to use regexp constants as arguments to user-defined functions (@pxref{User-defined}). For example: @example function mysub(pat, repl, str, global) @{ if (global) gsub(pat, repl, str) else sub(pat, repl, str) return str @} @{ @dots{} text = "hi! hi yourself!" mysub(/hi/, "howdy", text, 1) @dots{} @} @end example @c @cindex automatic warnings @c @cindex warnings, automatic In this example, the programmer wants to pass a regexp constant to the user-defined function @code{mysub}, which in turn passes it on to either @code{sub} or @code{gsub}. However, what really happens is that the @code{pat} parameter is either one or zero, depending upon whether or not @code{$0} matches @code{/hi/}. @command{gawk} issues a warning when it sees a regexp constant used as a parameter to a user-defined function, since passing a truth value in this way is probably not what was intended. @c ENDOFRANGE rec @node Variables @section Variables @cindex variables, user-defined @cindex user-defined, variables Variables are ways of storing values at one point in your program for use later in another part of your program. They can be manipulated entirely within the program text, and they can also be assigned values on the @command{awk} command line. @menu * Using Variables:: Using variables in your programs. * Assignment Options:: Setting variables on the command-line and a summary of command-line syntax. This is an advanced method of input. @end menu @node Using Variables @subsection Using Variables in a Program Variables let you give names to values and refer to them later. Variables have already been used in many of the examples. The name of a variable must be a sequence of letters, digits, or underscores, and it may not begin with a digit. Case is significant in variable names; @code{a} and @code{A} are distinct variables. A variable name is a valid expression by itself; it represents the variable's current value. Variables are given new values with @dfn{assignment operators}, @dfn{increment operators}, and @dfn{decrement operators}. @xref{Assignment Ops}. @c NEXT ED: Can also be changed by sub, gsub, split @cindex variables, built-in @cindex variables, initializing A few variables have special built-in meanings, such as @code{FS} (the field separator), and @code{NF} (the number of fields in the current input record). @xref{Built-in Variables}, for a list of the built-in variables. These built-in variables can be used and assigned just like all other variables, but their values are also used or changed automatically by @command{awk}. All built-in variables' names are entirely uppercase. Variables in @command{awk} can be assigned either numeric or string values. The kind of value a variable holds can change over the life of a program. By default, variables are initialized to the empty string, which is zero if converted to a number. There is no need to ``initialize'' each variable explicitly in @command{awk}, which is what you would do in C and in most other traditional languages. @node Assignment Options @subsection Assigning Variables on the Command Line @cindex variables, assigning on command line @c comma before assigning does NOT start tertiary @cindex command line, variables, assigning on Any @command{awk} variable can be set by including a @dfn{variable assignment} among the arguments on the command line when @command{awk} is invoked (@pxref{Other Arguments}). Such an assignment has the following form: @example @var{variable}=@var{text} @end example @c comma before assigning does NOT start tertiary @cindex @code{-v} option, variables, assigning @noindent With it, a variable is set either at the beginning of the @command{awk} run or in between input files. When the assignment is preceded with the @option{-v} option, as in the following: @example -v @var{variable}=@var{text} @end example @noindent the variable is set at the very beginning, even before the @code{BEGIN} rules are run. The @option{-v} option and its assignment must precede all the @value{FN} arguments, as well as the program text. (@xref{Options}, for more information about the @option{-v} option.) Otherwise, the variable assignment is performed at a time determined by its position among the input file arguments---after the processing of the preceding input file argument. For example: @example awk '@{ print $n @}' n=4 inventory-shipped n=2 BBS-list @end example @noindent prints the value of field number @code{n} for all input records. Before the first file is read, the command line sets the variable @code{n} equal to four. This causes the fourth field to be printed in lines from the file @file{inventory-shipped}. After the first file has finished, but before the second file is started, @code{n} is set to two, so that the second field is printed in lines from @file{BBS-list}: @example $ awk '@{ print $n @}' n=4 inventory-shipped n=2 BBS-list @print{} 15 @print{} 24 @dots{} @print{} 555-5553 @print{} 555-3412 @dots{} @end example @cindex dark corner, command-line arguments Command-line arguments are made available for explicit examination by the @command{awk} program in the @code{ARGV} array (@pxref{ARGC and ARGV}). @command{awk} processes the values of command-line assignments for escape sequences (@pxref{Escape Sequences}). @value{DARKCORNER} @node Conversion @section Conversion of Strings and Numbers @cindex converting, strings to numbers @cindex strings, converting @cindex numbers, converting @cindex converting, numbers Strings are converted to numbers and numbers are converted to strings, if the context of the @command{awk} program demands it. For example, if the value of either @code{foo} or @code{bar} in the expression @samp{foo + bar} happens to be a string, it is converted to a number before the addition is performed. If numeric values appear in string concatenation, they are converted to strings. Consider the following: @example two = 2; three = 3 print (two three) + 4 @end example @noindent This prints the (numeric) value 27. The numeric values of the variables @code{two} and @code{three} are converted to strings and concatenated together. The resulting string is converted back to the number 23, to which 4 is then added. @cindex null strings, converting numbers to strings @cindex type conversion If, for some reason, you need to force a number to be converted to a string, concatenate the empty string, @code{""}, with that number. To force a string to be converted to a number, add zero to that string. A string is converted to a number by interpreting any numeric prefix of the string as numerals: @code{"2.5"} converts to 2.5, @code{"1e3"} converts to 1000, and @code{"25fix"} has a numeric value of 25. Strings that can't be interpreted as valid numbers convert to zero. @cindex @code{CONVFMT} variable The exact manner in which numbers are converted into strings is controlled by the @command{awk} built-in variable @code{CONVFMT} (@pxref{Built-in Variables}). Numbers are converted using the @code{sprintf} function with @code{CONVFMT} as the format specifier (@pxref{String Functions}). @code{CONVFMT}'s default value is @code{"%.6g"}, which prints a value with at least six significant digits. For some applications, you might want to change it to specify more precision. On most modern machines, 17 digits is enough to capture a floating-point number's value exactly, most of the time.@footnote{Pathological cases can require up to 752 digits (!), but we doubt that you need to worry about this.} @cindex dark corner, @code{CONVFMT} variable Strange results can occur if you set @code{CONVFMT} to a string that doesn't tell @code{sprintf} how to format floating-point numbers in a useful way. For example, if you forget the @samp{%} in the format, @command{awk} converts all numbers to the same constant string. As a special case, if a number is an integer, then the result of converting it to a string is @emph{always} an integer, no matter what the value of @code{CONVFMT} may be. Given the following code fragment: @example CONVFMT = "%2.2f" a = 12 b = a "" @end example @noindent @code{b} has the value @code{"12"}, not @code{"12.00"}. @value{DARKCORNER} @cindex POSIX @command{awk}, @code{OFMT} variable and @cindex @code{OFMT} variable @cindex portability, new @command{awk} vs. old @command{awk} @cindex @command{awk}, new vs. old, @code{OFMT} variable Prior to the POSIX standard, @command{awk} used the value of @code{OFMT} for converting numbers to strings. @code{OFMT} specifies the output format to use when printing numbers with @code{print}. @code{CONVFMT} was introduced in order to separate the semantics of conversion from the semantics of printing. Both @code{CONVFMT} and @code{OFMT} have the same default value: @code{"%.6g"}. In the vast majority of cases, old @command{awk} programs do not change their behavior. However, these semantics for @code{OFMT} are something to keep in mind if you must port your new style program to older implementations of @command{awk}. We recommend that instead of changing your programs, just port @command{gawk} itself. @xref{Print}, for more information on the @code{print} statement. Finally, once again, where you are can matter when it comes to converting between numbers and strings. In @ref{Locales}, we mentioned that the local character set and language (the locale) can affect how @command{gawk} matches characters. The locale also affects numeric formats. In particular, for @command{awk} programs, it affects the decimal point character. The @code{"C"} locale, and most English-language locales, use the period character (@samp{.}) as the decimal point. However, many (if not most) European and non-English locales use the comma (@samp{,}) as the decimal point character. The POSIX standard says that @command{awk} always uses the period as the decimal point when reading the @command{awk} program source code, and for command-line variable assignments (@pxref{Other Arguments}). However, when interpreting input data, for @code{print} and @code{printf} output, and for number to string conversion, the local decimal point character is used. As of @value{PVERSION} 3.1.3, @command{gawk} fully complies with this aspect of the standard. Here are some examples indicating the difference in behavior, on a GNU/Linux system: @example $ gawk 'BEGIN @{ printf "%g\n", 3.1415927 @}' @print{} 3.14159 $ LC_ALL=en_DK gawk 'BEGIN @{ printf "%g\n", 3.1415927 @}' @print{} 3,14159 $ echo 4,321 | gawk '@{ print $1 + 1 @}' @print{} 5 $ echo 4,321 | LC_ALL=en_DK gawk '@{ print $1 + 1 @}' @print{} 5,321 @end example @noindent The @samp{en_DK} locale is for English in Denmark, where the comma acts as the decimal point separator. In the normal @code{"C"} locale, @command{gawk} treats @samp{4,321} as @samp{4}, while in the Danish locale, it's treated as the full number, @samp{4.321}. @node Arithmetic Ops @section Arithmetic Operators @cindex arithmetic operators @cindex operators, arithmetic @c @cindex addition @c @cindex subtraction @c @cindex multiplication @c @cindex division @c @cindex remainder @c @cindex quotient @c @cindex exponentiation The @command{awk} language uses the common arithmetic operators when evaluating expressions. All of these arithmetic operators follow normal precedence rules and work as you would expect them to. The following example uses a file named @file{grades}, which contains a list of student names as well as three test scores per student (it's a small class): @example Pat 100 97 58 Sandy 84 72 93 Chris 72 92 89 @end example @noindent This programs takes the file @file{grades} and prints the average of the scores: @example $ awk '@{ sum = $2 + $3 + $4 ; avg = sum / 3 > print $1, avg @}' grades @print{} Pat 85 @print{} Sandy 83 @print{} Chris 84.3333 @end example The following list provides the arithmetic operators in @command{awk}, in order from the highest precedence to the lowest: @table @code @item - @var{x} Negation. @item + @var{x} Unary plus; the expression is converted to a number. @cindex POSIX @command{awk}, arithmetic operators and @item @var{x} ^ @var{y} @itemx @var{x} ** @var{y} Exponentiation; @var{x} raised to the @var{y} power. @samp{2 ^ 3} has the value eight; the character sequence @samp{**} is equivalent to @samp{^}. @item @var{x} * @var{y} Multiplication. @cindex troubleshooting, division @cindex division @item @var{x} / @var{y} Division; because all numbers in @command{awk} are floating-point numbers, the result is @emph{not} rounded to an integer---@samp{3 / 4} has the value 0.75. (It is a common mistake, especially for C programmers, to forget that @emph{all} numbers in @command{awk} are floating-point, and that division of integer-looking constants produces a real number, not an integer.) @item @var{x} % @var{y} Remainder; further discussion is provided in the text, just after this list. @item @var{x} + @var{y} Addition. @item @var{x} - @var{y} Subtraction. @end table Unary plus and minus have the same precedence, the multiplication operators all have the same precedence, and addition and subtraction have the same precedence. @cindex differences in @command{awk} and @command{gawk}, trunc-mod operation @cindex trunc-mod operation When computing the remainder of @code{@var{x} % @var{y}}, the quotient is rounded toward zero to an integer and multiplied by @var{y}. This result is subtracted from @var{x}; this operation is sometimes known as ``trunc-mod.'' The following relation always holds: @example b * int(a / b) + (a % b) == a @end example One possibly undesirable effect of this definition of remainder is that @code{@var{x} % @var{y}} is negative if @var{x} is negative. Thus: @example -17 % 8 = -1 @end example In other @command{awk} implementations, the signedness of the remainder may be machine-dependent. @c !!! what does posix say? @cindex portability, @code{**} operator and @cindex @code{*} (asterisk), @code{**} operator @cindex asterisk (@code{*}), @code{**} operator @strong{Note:} The POSIX standard only specifies the use of @samp{^} for exponentiation. For maximum portability, do not use the @samp{**} operator. @node Concatenation @section String Concatenation @cindex Kernighan, Brian @quotation @i{It seemed like a good idea at the time.}@* Brian Kernighan @end quotation @cindex string operators @cindex operators, string @cindex concatenating There is only one string operation: concatenation. It does not have a specific operator to represent it. Instead, concatenation is performed by writing expressions next to one another, with no operator. For example: @example $ awk '@{ print "Field number one: " $1 @}' BBS-list @print{} Field number one: aardvark @print{} Field number one: alpo-net @dots{} @end example Without the space in the string constant after the @samp{:}, the line runs together. For example: @example $ awk '@{ print "Field number one:" $1 @}' BBS-list @print{} Field number one:aardvark @print{} Field number one:alpo-net @dots{} @end example @cindex troubleshooting, string concatenation Because string concatenation does not have an explicit operator, it is often necessary to insure that it happens at the right time by using parentheses to enclose the items to concatenate. For example, the following code fragment does not concatenate @code{file} and @code{name} as you might expect: @example file = "file" name = "name" print "something meaningful" > file name @end example @noindent It is necessary to use the following: @example print "something meaningful" > (file name) @end example @cindex order of evaluation, concatenation @cindex evaluation order, concatenation @cindex side effects Parentheses should be used around concatenation in all but the most common contexts, such as on the righthand side of @samp{=}. Be careful about the kinds of expressions used in string concatenation. In particular, the order of evaluation of expressions used for concatenation is undefined in the @command{awk} language. Consider this example: @example BEGIN @{ a = "don't" print (a " " (a = "panic")) @} @end example @noindent It is not defined whether the assignment to @code{a} happens before or after the value of @code{a} is retrieved for producing the concatenated value. The result could be either @samp{don't panic}, or @samp{panic panic}. @c see test/nasty.awk for a worse example The precedence of concatenation, when mixed with other operators, is often counter-intuitive. Consider this example: @ignore > To: bug-gnu-utils@@gnu.org > CC: arnold@gnu.org > Subject: gawk 3.0.4 bug with {print -12 " " -24} > From: Russell Schulz > Date: Tue, 8 Feb 2000 19:56:08 -0700 > > gawk 3.0.4 on NT gives me: > > prompt> cat bad.awk > BEGIN { print -12 " " -24; } > > prompt> gawk -f bad.awk > -12-24 > > when I would expect > > -12 -24 > > I have not investigated the source, or other implementations. The > bug is there on my NT and DOS versions 2.15.6 . @end ignore @example $ awk 'BEGIN @{ print -12 " " -24 @}' @print{} -12-24 @end example This ``obviously'' is concatenating @minus{}12, a space, and @minus{}24. But where did the space disappear to? The answer lies in the combination of operator precedences and @command{awk}'s automatic conversion rules. To get the desired result, write the program in the following manner: @example $ awk 'BEGIN @{ print -12 " " (-24) @}' @print{} -12 -24 @end example This forces @command{awk} to treat the @samp{-} on the @samp{-24} as unary. Otherwise, it's parsed as follows: @display @minus{}12 (@code{"@ "} @minus{} 24) @result{} @minus{}12 (0 @minus{} 24) @result{} @minus{}12 (@minus{}24) @result{} @minus{}12@minus{}24 @end display As mentioned earlier, when doing concatenation, @emph{parenthesize}. Otherwise, you're never quite sure what you'll get. @node Assignment Ops @section Assignment Expressions @c STARTOFRANGE asop @cindex assignment operators @c STARTOFRANGE opas @cindex operators, assignment @c STARTOFRANGE exas @cindex expressions, assignment @cindex @code{=} (equals sign), @code{=} operator @cindex equals sign (@code{=}), @code{=} operator An @dfn{assignment} is an expression that stores a (usually different) value into a variable. For example, let's assign the value one to the variable @code{z}: @example z = 1 @end example After this expression is executed, the variable @code{z} has the value one. Whatever old value @code{z} had before the assignment is forgotten. Assignments can also store string values. For example, the following stores the value @code{"this food is good"} in the variable @code{message}: @example thing = "food" predicate = "good" message = "this " thing " is " predicate @end example @noindent @cindex side effects, assignment expressions This also illustrates string concatenation. The @samp{=} sign is called an @dfn{assignment operator}. It is the simplest assignment operator because the value of the righthand operand is stored unchanged. Most operators (addition, concatenation, and so on) have no effect except to compute a value. If the value isn't used, there's no reason to use the operator. An assignment operator is different; it does produce a value, but even if you ignore it, the assignment still makes itself felt through the alteration of the variable. We call this a @dfn{side effect}. @cindex lvalues/rvalues @cindex rvalues/lvalues @cindex assignment operators, lvalues/rvalues @cindex operators, assignment The lefthand operand of an assignment need not be a variable (@pxref{Variables}); it can also be a field (@pxref{Changing Fields}) or an array element (@pxref{Arrays}). These are all called @dfn{lvalues}, which means they can appear on the lefthand side of an assignment operator. The righthand operand may be any expression; it produces the new value that the assignment stores in the specified variable, field, or array element. (Such values are called @dfn{rvalues}.) @cindex variables, types of It is important to note that variables do @emph{not} have permanent types. A variable's type is simply the type of whatever value it happens to hold at the moment. In the following program fragment, the variable @code{foo} has a numeric value at first, and a string value later on: @example foo = 1 print foo foo = "bar" print foo @end example @noindent When the second assignment gives @code{foo} a string value, the fact that it previously had a numeric value is forgotten. String values that do not begin with a digit have a numeric value of zero. After executing the following code, the value of @code{foo} is five: @example foo = "a string" foo = foo + 5 @end example @noindent @strong{Note:} Using a variable as a number and then later as a string can be confusing and is poor programming style. The previous two examples illustrate how @command{awk} works, @emph{not} how you should write your programs! An assignment is an expression, so it has a value---the same value that is assigned. Thus, @samp{z = 1} is an expression with the value one. One consequence of this is that you can write multiple assignments together, such as: @example x = y = z = 5 @end example @noindent This example stores the value five in all three variables (@code{x}, @code{y}, and @code{z}). It does so because the value of @samp{z = 5}, which is five, is stored into @code{y} and then the value of @samp{y = z = 5}, which is five, is stored into @code{x}. Assignments may be used anywhere an expression is called for. For example, it is valid to write @samp{x != (y = 1)} to set @code{y} to one, and then test whether @code{x} equals one. But this style tends to make programs hard to read; such nesting of assignments should be avoided, except perhaps in a one-shot program. @cindex @code{+} (plus sign), @code{+=} operator @cindex plus sign (@code{+}), @code{+=} operator Aside from @samp{=}, there are several other assignment operators that do arithmetic with the old value of the variable. For example, the operator @samp{+=} computes a new value by adding the righthand value to the old value of the variable. Thus, the following assignment adds five to the value of @code{foo}: @example foo += 5 @end example @noindent This is equivalent to the following: @example foo = foo + 5 @end example @noindent Use whichever makes the meaning of your program clearer. There are situations where using @samp{+=} (or any assignment operator) is @emph{not} the same as simply repeating the lefthand operand in the righthand expression. For example: @cindex Rankin, Pat @example # Thanks to Pat Rankin for this example BEGIN @{ foo[rand()] += 5 for (x in foo) print x, foo[x] bar[rand()] = bar[rand()] + 5 for (x in bar) print x, bar[x] @} @end example @cindex operators, assignment, evaluation order @cindex assignment operators, evaluation order @noindent The indices of @code{bar} are practically guaranteed to be different, because @code{rand} returns different values each time it is called. (Arrays and the @code{rand} function haven't been covered yet. @xref{Arrays}, and see @ref{Numeric Functions}, for more information). This example illustrates an important fact about assignment operators: the lefthand expression is only evaluated @emph{once}. It is up to the implementation as to which expression is evaluated first, the lefthand or the righthand. Consider this example: @example i = 1 a[i += 2] = i + 1 @end example @noindent The value of @code{a[3]} could be either two or four. Here is a table of the arithmetic assignment operators. In each case, the righthand operand is an expression whose value is converted to a number. @ignore @table @code @item @var{lvalue} += @var{increment} Adds @var{increment} to the value of @var{lvalue}. @item @var{lvalue} -= @var{decrement} Subtracts @var{decrement} from the value of @var{lvalue}. @item @var{lvalue} *= @var{coefficient} Multiplies the value of @var{lvalue} by @var{coefficient}. @item @var{lvalue} /= @var{divisor} Divides the value of @var{lvalue} by @var{divisor}. @item @var{lvalue} %= @var{modulus} Sets @var{lvalue} to its remainder by @var{modulus}. @cindex @command{awk} language, POSIX version @cindex POSIX @command{awk} @item @var{lvalue} ^= @var{power} @itemx @var{lvalue} **= @var{power} Raises @var{lvalue} to the power @var{power}. (Only the @samp{^=} operator is specified by POSIX.) @end table @end ignore @cindex @code{-} (hyphen), @code{-=} operator @cindex hyphen (@code{-}), @code{-=} operator @cindex @code{*} (asterisk), @code{*=} operator @cindex asterisk (@code{*}), @code{*=} operator @cindex @code{/} (forward slash), @code{/=} operator @cindex forward slash (@code{/}), @code{/=} operator @cindex @code{%} (percent sign), @code{%=} operator @cindex percent sign (@code{%}), @code{%=} operator @cindex @code{^} (caret), @code{^=} operator @cindex caret (@code{^}), @code{^=} operator @cindex @code{*} (asterisk), @code{**=} operator @cindex asterisk (@code{*}), @code{**=} operator @multitable {@var{lvalue} *= @var{coefficient}} {Subtracts @var{decrement} from the value of @var{lvalue}.} @item @var{lvalue} @code{+=} @var{increment} @tab Adds @var{increment} to the value of @var{lvalue}. @item @var{lvalue} @code{-=} @var{decrement} @tab Subtracts @var{decrement} from the value of @var{lvalue}. @item @var{lvalue} @code{*=} @var{coefficient} @tab Multiplies the value of @var{lvalue} by @var{coefficient}. @item @var{lvalue} @code{/=} @var{divisor} @tab Divides the value of @var{lvalue} by @var{divisor}. @item @var{lvalue} @code{%=} @var{modulus} @tab Sets @var{lvalue} to its remainder by @var{modulus}. @cindex @command{awk} language, POSIX version @cindex POSIX @command{awk} @item @var{lvalue} @code{^=} @var{power} @tab @item @var{lvalue} @code{**=} @var{power} @tab Raises @var{lvalue} to the power @var{power}. @end multitable @cindex POSIX @command{awk}, @code{**=} operator and @cindex portability, @code{**=} operator and @strong{Note:} Only the @samp{^=} operator is specified by POSIX. For maximum portability, do not use the @samp{**=} operator. @c fakenode --- for prepinfo @subheading Advanced Notes: Syntactic Ambiguities Between @samp{/=} and Regular Expressions @cindex advanced features, regexp constants @cindex dark corner, regexp constants, @code{/=} operator and @cindex @code{/} (forward slash), @code{/=} operator, vs. @code{/=@dots{}/} regexp constant @cindex forward slash (@code{/}), @code{/=} operator, vs. @code{/=@dots{}/} regexp constant @cindex regexp constants, @code{/=@dots{}/}, @code{/=} operator and @c derived from email from "Nelson H. F. Beebe" @c Date: Mon, 1 Sep 1997 13:38:35 -0600 (MDT) @cindex dark corner @cindex ambiguity, syntactic: @code{/=} operator vs. @code{/=@dots{}/} regexp constant @cindex syntactic ambiguity: @code{/=} operator vs. @code{/=@dots{}/} regexp constant @cindex @code{/=} operator vs. @code{/=@dots{}/} regexp constant There is a syntactic ambiguity between the @samp{/=} assignment operator and regexp constants whose first character is an @samp{=}. @value{DARKCORNER} This is most notable in commercial @command{awk} versions. For example: @example $ awk /==/ /dev/null @error{} awk: syntax error at source line 1 @error{} context is @error{} >>> /= <<< @error{} awk: bailing out at source line 1 @end example @noindent A workaround is: @example awk '/[=]=/' /dev/null @end example @command{gawk} does not have this problem, nor do the other freely available versions described in @ref{Other Versions}. @c ENDOFRANGE exas @c ENDOFRANGE opas @c ENDOFRANGE asop @node Increment Ops @section Increment and Decrement Operators @c STARTOFRANGE inop @cindex increment operators @c STARTOFRANGE opde @cindex operators, decrement/increment @dfn{Increment} and @dfn{decrement operators} increase or decrease the value of a variable by one. An assignment operator can do the same thing, so the increment operators add no power to the @command{awk} language; however, they are convenient abbreviations for very common operations. @cindex side effects @cindex @code{+} (plus sign), decrement/increment operators @cindex plus sign (@code{+}), decrement/increment operators @cindex side effects, decrement/increment operators The operator used for adding one is written @samp{++}. It can be used to increment a variable either before or after taking its value. To pre-increment a variable @code{v}, write @samp{++v}. This adds one to the value of @code{v}---that new value is also the value of the expression. (The assignment expression @samp{v += 1} is completely equivalent.) Writing the @samp{++} after the variable specifies post-increment. This increments the variable value just the same; the difference is that the value of the increment expression itself is the variable's @emph{old} value. Thus, if @code{foo} has the value four, then the expression @samp{foo++} has the value four, but it changes the value of @code{foo} to five. In other words, the operator returns the old value of the variable, but with the side effect of incrementing it. The post-increment @samp{foo++} is nearly the same as writing @samp{(foo += 1) - 1}. It is not perfectly equivalent because all numbers in @command{awk} are floating-point---in floating-point, @samp{foo + 1 - 1} does not necessarily equal @code{foo}. But the difference is minute as long as you stick to numbers that are fairly small (less than 10e12). @cindex @code{$} (dollar sign), incrementing fields and arrays @cindex dollar sign (@code{$}), incrementing fields and arrays Fields and array elements are incremented just like variables. (Use @samp{$(i++)} when you want to do a field reference and a variable increment at the same time. The parentheses are necessary because of the precedence of the field reference operator @samp{$}.) @cindex decrement operators The decrement operator @samp{--} works just like @samp{++}, except that it subtracts one instead of adding it. As with @samp{++}, it can be used before the lvalue to pre-decrement or after it to post-decrement. Following is a summary of increment and decrement expressions: @table @code @cindex @code{+} (plus sign), @code{++} operator @cindex plus sign (@code{+}), @code{++} operator @item ++@var{lvalue} This expression increments @var{lvalue}, and the new value becomes the value of the expression. @item @var{lvalue}++ This expression increments @var{lvalue}, but the value of the expression is the @emph{old} value of @var{lvalue}. @cindex @code{-} (hyphen), @code{--} operator @cindex hyphen (@code{-}), @code{--} operator @item --@var{lvalue} This expression is like @samp{++@var{lvalue}}, but instead of adding, it subtracts. It decrements @var{lvalue} and delivers the value that is the result. @item @var{lvalue}-- This expression is like @samp{@var{lvalue}++}, but instead of adding, it subtracts. It decrements @var{lvalue}. The value of the expression is the @emph{old} value of @var{lvalue}. @end table @c fakenode --- for prepinfo @subheading Advanced Notes: Operator Evaluation Order @c comma before precedence does NOT start tertiary @cindex advanced features, operators, precedence @cindex precedence @cindex operators, precedence @cindex portability, operators @cindex evaluation order @cindex Marx, Groucho @quotation @i{Doctor, doctor! It hurts when I do this!@* So don't do that!}@* Groucho Marx @end quotation @noindent What happens for something like the following? @example b = 6 print b += b++ @end example @noindent Or something even stranger? @example b = 6 b += ++b + b++ print b @end example @cindex side effects In other words, when do the various side effects prescribed by the postfix operators (@samp{b++}) take effect? When side effects happen is @dfn{implementation defined}. In other words, it is up to the particular version of @command{awk}. The result for the first example may be 12 or 13, and for the second, it may be 22 or 23. In short, doing things like this is not recommended and definitely not anything that you can rely upon for portability. You should avoid such things in your own programs. @c You'll sleep better at night and be able to look at yourself @c in the mirror in the morning. @c ENDOFRANGE inop @c ENDOFRANGE opde @c ENDOFRANGE deop @node Truth Values @section True and False in @command{awk} @cindex truth values @cindex logical false/true @cindex false, logical @cindex true, logical @cindex null strings Many programming languages have a special representation for the concepts of ``true'' and ``false.'' Such languages usually use the special constants @code{true} and @code{false}, or perhaps their uppercase equivalents. However, @command{awk} is different. It borrows a very simple concept of true and false from C. In @command{awk}, any nonzero numeric value @emph{or} any nonempty string value is true. Any other value (zero or the null string @code{""}) is false. The following program prints @samp{A strange truth value} three times: @example BEGIN @{ if (3.1415927) print "A strange truth value" if ("Four Score And Seven Years Ago") print "A strange truth value" if (j = 57) print "A strange truth value" @} @end example @cindex dark corner There is a surprising consequence of the ``nonzero or non-null'' rule: the string constant @code{"0"} is actually true, because it is non-null. @value{DARKCORNER} @node Typing and Comparison @section Variable Typing and Comparison Expressions @quotation @i{The Guide is definitive. Reality is frequently inaccurate.}@* The Hitchhiker's Guide to the Galaxy @end quotation @c STARTOFRANGE comex @cindex comparison expressions @c STARTOFRANGE excom @cindex expressions, comparison @cindex expressions, matching, See comparison expressions @cindex matching, expressions, See comparison expressions @cindex relational operators, See comparison operators @c comma is part of See @cindex operators, relational, See operators, comparison @c STARTOFRANGE varting @cindex variable typing @c STARTOFRANGE vartypc @cindex variables, types of, comparison expressions and Unlike other programming languages, @command{awk} variables do not have a fixed type. Instead, they can be either a number or a string, depending upon the value that is assigned to them. @cindex numeric, strings @cindex strings, numeric @cindex POSIX @command{awk}, numeric strings and The 1992 POSIX standard introduced the concept of a @dfn{numeric string}, which is simply a string that looks like a number---for example, @code{@w{" +2"}}. This concept is used for determining the type of a variable. The type of the variable is important because the types of two variables determine how they are compared. In @command{gawk}, variable typing follows these rules: @itemize @bullet @item A numeric constant or the result of a numeric operation has the @var{numeric} attribute. @item A string constant or the result of a string operation has the @var{string} attribute. @item Fields, @code{getline} input, @code{FILENAME}, @code{ARGV} elements, @code{ENVIRON} elements, and the elements of an array created by @code{split} that are numeric strings have the @var{strnum} attribute. Otherwise, they have the @var{string} attribute. Uninitialized variables also have the @var{strnum} attribute. @item Attributes propagate across assignments but are not changed by any use. @c (Although a use may cause the entity to acquire an additional @c value such that it has both a numeric and string value, this leaves the @c attribute unchanged.) @c This is important but not relevant @end itemize The last rule is particularly important. In the following program, @code{a} has numeric type, even though it is later used in a string operation: @example BEGIN @{ a = 12.345 b = a " is a cute number" print b @} @end example When two operands are compared, either string comparison or numeric comparison may be used. This depends upon the attributes of the operands, according to the following symmetric matrix: @c thanks to Karl Berry, kb@cs.umb.edu, for major help with TeX tables @tex \centerline{ \vbox{\bigskip % space above the table (about 1 linespace) % Because we have vertical rules, we can't let TeX insert interline space % in its usual way. \offinterlineskip % % Define the table template. & separates columns, and \cr ends the % template (and each row). # is replaced by the text of that entry on % each row. The template for the first column breaks down like this: % \strut -- a way to make each line have the height and depth % of a normal line of type, since we turned off interline spacing. % \hfil -- infinite glue; has the effect of right-justifying in this case. % # -- replaced by the text (for instance, `STRNUM', in the last row). % \quad -- about the width of an `M'. Just separates the columns. % % The second column (\vrule#) is what generates the vertical rule that % spans table rows. % % The doubled && before the next entry means `repeat the following % template as many times as necessary on each line' -- in our case, twice. % % The template itself, \quad#\hfil, left-justifies with a little space before. % \halign{\strut\hfil#\quad&\vrule#&&\quad#\hfil\cr &&STRING &NUMERIC &STRNUM\cr % The \omit tells TeX to skip inserting the template for this column on % this particular row. In this case, we only want a little extra space % to separate the heading row from the rule below it. the depth 2pt -- % `\vrule depth 2pt' is that little space. \omit &depth 2pt\cr % This is the horizontal rule below the heading. Since it has nothing to % do with the columns of the table, we use \noalign to get it in there. \noalign{\hrule} % Like above, this time a little more space. \omit &depth 4pt\cr % The remaining rows have nothing special about them. STRING &&string &string &string\cr NUMERIC &&string &numeric &numeric\cr STRNUM &&string &numeric &numeric\cr }}} @end tex @ifnottex @display +---------------------------------------------- | STRING NUMERIC STRNUM --------+---------------------------------------------- | STRING | string string string | NUMERIC | string numeric numeric | STRNUM | string numeric numeric --------+---------------------------------------------- @end display @end ifnottex The basic idea is that user input that looks numeric---and @emph{only} user input---should be treated as numeric, even though it is actually made of characters and is therefore also a string. Thus, for example, the string constant @w{@code{" +3.14"}} is a string, even though it looks numeric, and is @emph{never} treated as number for comparison purposes. In short, when one operand is a ``pure'' string, such as a string constant, then a string comparison is performed. Otherwise, a numeric comparison is performed.@footnote{The POSIX standard is under revision. The revised standard's rules for typing and comparison are the same as just described for @command{gawk}.} @dfn{Comparison expressions} compare strings or numbers for relationships such as equality. They are written using @dfn{relational operators}, which are a superset of those in C. Here is a table of them: @cindex @code{<} (left angle bracket), @code{<} operator @cindex left angle bracket (@code{<}), @code{<} operator @cindex @code{<} (left angle bracket), @code{<=} operator @cindex left angle bracket (@code{<}), @code{<=} operator @cindex @code{>} (right angle bracket), @code{>=} operator @cindex right angle bracket (@code{>}), @code{>=} operator @cindex @code{>} (right angle bracket), @code{>} operator @cindex right angle bracket (@code{>}), @code{>} operator @cindex @code{=} (equals sign), @code{==} operator @cindex equals sign (@code{=}), @code{==} operator @cindex @code{!} (exclamation point), @code{!=} operator @cindex exclamation point (@code{!}), @code{!=} operator @cindex @code{~} (tilde), @code{~} operator @cindex tilde (@code{~}), @code{~} operator @cindex @code{!} (exclamation point), @code{!~} operator @cindex exclamation point (@code{!}), @code{!~} operator @cindex @code{in} operator @table @code @item @var{x} < @var{y} True if @var{x} is less than @var{y}. @item @var{x} <= @var{y} True if @var{x} is less than or equal to @var{y}. @item @var{x} > @var{y} True if @var{x} is greater than @var{y}. @item @var{x} >= @var{y} True if @var{x} is greater than or equal to @var{y}. @item @var{x} == @var{y} True if @var{x} is equal to @var{y}. @item @var{x} != @var{y} True if @var{x} is not equal to @var{y}. @item @var{x} ~ @var{y} True if the string @var{x} matches the regexp denoted by @var{y}. @item @var{x} !~ @var{y} True if the string @var{x} does not match the regexp denoted by @var{y}. @item @var{subscript} in @var{array} True if the array @var{array} has an element with the subscript @var{subscript}. @end table Comparison expressions have the value one if true and zero if false. When comparing operands of mixed types, numeric operands are converted to strings using the value of @code{CONVFMT} (@pxref{Conversion}). Strings are compared by comparing the first character of each, then the second character of each, and so on. Thus, @code{"10"} is less than @code{"9"}. If there are two strings where one is a prefix of the other, the shorter string is less than the longer one. Thus, @code{"abc"} is less than @code{"abcd"}. @cindex troubleshooting, @code{==} operator It is very easy to accidentally mistype the @samp{==} operator and leave off one of the @samp{=} characters. The result is still valid @command{awk} code, but the program does not do what is intended: @example if (a = b) # oops! should be a == b @dots{} else @dots{} @end example @noindent Unless @code{b} happens to be zero or the null string, the @code{if} part of the test always succeeds. Because the operators are so similar, this kind of error is very difficult to spot when scanning the source code. @cindex @command{gawk}, comparison operators and The following table of expressions illustrates the kind of comparison @command{gawk} performs, as well as what the result of the comparison is: @table @code @item 1.5 <= 2.0 numeric comparison (true) @item "abc" >= "xyz" string comparison (false) @item 1.5 != " +2" string comparison (true) @item "1e2" < "3" string comparison (true) @item a = 2; b = "2" @itemx a == b string comparison (true) @item a = 2; b = " +2" @item a == b string comparison (false) @end table In the next example: @example $ echo 1e2 3 | awk '@{ print ($1 < $2) ? "true" : "false" @}' @print{} false @end example @cindex comparison expressions, string vs. regexp @c @cindex string comparison vs. regexp comparison @c @cindex regexp comparison vs. string comparison @noindent the result is @samp{false} because both @code{$1} and @code{$2} are user input. They are numeric strings---therefore both have the @var{strnum} attribute, dictating a numeric comparison. The purpose of the comparison rules and the use of numeric strings is to attempt to produce the behavior that is ``least surprising,'' while still ``doing the right thing.'' String comparisons and regular expression comparisons are very different. For example: @example x == "foo" @end example @noindent has the value one, or is true if the variable @code{x} is precisely @samp{foo}. By contrast: @example x ~ /foo/ @end example @noindent has the value one if @code{x} contains @samp{foo}, such as @code{"Oh, what a fool am I!"}. @cindex @code{~} (tilde), @code{~} operator @cindex tilde (@code{~}), @code{~} operator @cindex @code{!} (exclamation point), @code{!~} operator @cindex exclamation point (@code{!}), @code{!~} operator The righthand operand of the @samp{~} and @samp{!~} operators may be either a regexp constant (@code{/@dots{}/}) or an ordinary expression. In the latter case, the value of the expression as a string is used as a dynamic regexp (@pxref{Regexp Usage}; also @pxref{Computed Regexps}). @cindex @command{awk}, regexp constants and @cindex regexp constants In modern implementations of @command{awk}, a constant regular expression in slashes by itself is also an expression. The regexp @code{/@var{regexp}/} is an abbreviation for the following comparison expression: @example $0 ~ /@var{regexp}/ @end example One special place where @code{/foo/} is @emph{not} an abbreviation for @samp{$0 ~ /foo/} is when it is the righthand operand of @samp{~} or @samp{!~}. @xref{Using Constant Regexps}, where this is discussed in more detail. @c ENDOFRANGE comex @c ENDOFRANGE excom @c ENDOFRANGE vartypc @c ENDOFRANGE varting @node Boolean Ops @section Boolean Expressions @cindex and Boolean-logic operator @cindex or Boolean-logic operator @cindex not Boolean-logic operator @c STARTOFRANGE exbo @cindex expressions, Boolean @c STARTOFRANGE boex @cindex Boolean expressions @cindex operators, Boolean, See Boolean expressions @cindex Boolean operators, See Boolean expressions @cindex logical operators, See Boolean expressions @cindex operators, logical, See Boolean expressions A @dfn{Boolean expression} is a combination of comparison expressions or matching expressions, using the Boolean operators ``or'' (@samp{||}), ``and'' (@samp{&&}), and ``not'' (@samp{!}), along with parentheses to control nesting. The truth value of the Boolean expression is computed by combining the truth values of the component expressions. Boolean expressions are also referred to as @dfn{logical expressions}. The terms are equivalent. Boolean expressions can be used wherever comparison and matching expressions can be used. They can be used in @code{if}, @code{while}, @code{do}, and @code{for} statements (@pxref{Statements}). They have numeric values (one if true, zero if false) that come into play if the result of the Boolean expression is stored in a variable or used in arithmetic. In addition, every Boolean expression is also a valid pattern, so you can use one as a pattern to control the execution of rules. The Boolean operators are: @table @code @item @var{boolean1} && @var{boolean2} True if both @var{boolean1} and @var{boolean2} are true. For example, the following statement prints the current input record if it contains both @samp{2400} and @samp{foo}: @example if ($0 ~ /2400/ && $0 ~ /foo/) print @end example @cindex side effects, Boolean operators The subexpression @var{boolean2} is evaluated only if @var{boolean1} is true. This can make a difference when @var{boolean2} contains expressions that have side effects. In the case of @samp{$0 ~ /foo/ && ($2 == bar++)}, the variable @code{bar} is not incremented if there is no substring @samp{foo} in the record. @item @var{boolean1} || @var{boolean2} True if at least one of @var{boolean1} or @var{boolean2} is true. For example, the following statement prints all records in the input that contain @emph{either} @samp{2400} or @samp{foo} or both: @example if ($0 ~ /2400/ || $0 ~ /foo/) print @end example The subexpression @var{boolean2} is evaluated only if @var{boolean1} is false. This can make a difference when @var{boolean2} contains expressions that have side effects. @item ! @var{boolean} True if @var{boolean} is false. For example, the following program prints @samp{no home!} in the unusual event that the @env{HOME} environment variable is not defined: @example BEGIN @{ if (! ("HOME" in ENVIRON)) print "no home!" @} @end example (The @code{in} operator is described in @ref{Reference to Elements}.) @end table @cindex short-circuit operators @cindex operators, short-circuit @cindex @code{&} (ampersand), @code{&&} operator @cindex ampersand (@code{&}), @code{&&} operator @cindex @code{|} (vertical bar), @code{||} operator @cindex vertical bar (@code{|}), @code{||} operator The @samp{&&} and @samp{||} operators are called @dfn{short-circuit} operators because of the way they work. Evaluation of the full expression is ``short-circuited'' if the result can be determined part way through its evaluation. @cindex line continuations Statements that use @samp{&&} or @samp{||} can be continued simply by putting a newline after them. But you cannot put a newline in front of either of these operators without using backslash continuation (@pxref{Statements/Lines}). @cindex @code{!} (exclamation point), @code{!} operator @cindex exclamation point (@code{!}), @code{!} operator @cindex newlines @cindex variables, flag @cindex flag variables The actual value of an expression using the @samp{!} operator is either one or zero, depending upon the truth value of the expression it is applied to. The @samp{!} operator is often useful for changing the sense of a flag variable from false to true and back again. For example, the following program is one way to print lines in between special bracketing lines: @example $1 == "START" @{ interested = ! interested; next @} interested == 1 @{ print @} $1 == "END" @{ interested = ! interested; next @} @end example @noindent The variable @code{interested}, as with all @command{awk} variables, starts out initialized to zero, which is also false. When a line is seen whose first field is @samp{START}, the value of @code{interested} is toggled to true, using @samp{!}. The next rule prints lines as long as @code{interested} is true. When a line is seen whose first field is @samp{END}, @code{interested} is toggled back to false. @ignore Scott Deifik points out that this program isn't robust against bogus input data, but the point is to illustrate the use of `!', so we'll leave well enough alone. @end ignore @cindex @code{next} statement @strong{Note:} The @code{next} statement is discussed in @ref{Next Statement}. @code{next} tells @command{awk} to skip the rest of the rules, get the next record, and start processing the rules over again at the top. The reason it's there is to avoid printing the bracketing @samp{START} and @samp{END} lines. @c ENDOFRANGE exbo @c ENDOFRANGE boex @node Conditional Exp @section Conditional Expressions @cindex conditional expressions @cindex expressions, conditional @cindex expressions, selecting A @dfn{conditional expression} is a special kind of expression that has three operands. It allows you to use one expression's value to select one of two other expressions. The conditional expression is the same as in the C language, as shown here: @example @var{selector} ? @var{if-true-exp} : @var{if-false-exp} @end example @noindent There are three subexpressions. The first, @var{selector}, is always computed first. If it is ``true'' (not zero or not null), then @var{if-true-exp} is computed next and its value becomes the value of the whole expression. Otherwise, @var{if-false-exp} is computed next and its value becomes the value of the whole expression. For example, the following expression produces the absolute value of @code{x}: @example x >= 0 ? x : -x @end example @cindex side effects, conditional expressions Each time the conditional expression is computed, only one of @var{if-true-exp} and @var{if-false-exp} is used; the other is ignored. This is important when the expressions have side effects. For example, this conditional expression examines element @code{i} of either array @code{a} or array @code{b}, and increments @code{i}: @example x == y ? a[i++] : b[i++] @end example @noindent This is guaranteed to increment @code{i} exactly once, because each time only one of the two increment expressions is executed and the other is not. @xref{Arrays}, for more information about arrays. @cindex differences in @command{awk} and @command{gawk}, line continuations @cindex line continuations, @command{gawk} @cindex @command{gawk}, line continuation in As a minor @command{gawk} extension, a statement that uses @samp{?:} can be continued simply by putting a newline after either character. However, putting a newline in front of either character does not work without using backslash continuation (@pxref{Statements/Lines}). If @option{--posix} is specified (@pxref{Options}), then this extension is disabled. @node Function Calls @section Function Calls @cindex function calls A @dfn{function} is a name for a particular calculation. This enables you to ask for it by name at any point in the program. For example, the function @code{sqrt} computes the square root of a number. @cindex functions, built-in A fixed set of functions are @dfn{built-in}, which means they are available in every @command{awk} program. The @code{sqrt} function is one of these. @xref{Built-in}, for a list of built-in functions and their descriptions. In addition, you can define functions for use in your program. @xref{User-defined}, for instructions on how to do this. @cindex arguments, in function calls The way to use a function is with a @dfn{function call} expression, which consists of the function name followed immediately by a list of @dfn{arguments} in parentheses. The arguments are expressions that provide the raw materials for the function's calculations. When there is more than one argument, they are separated by commas. If there are no arguments, just write @samp{()} after the function name. The following examples show function calls with and without arguments: @example sqrt(x^2 + y^2) @i{one argument} atan2(y, x) @i{two arguments} rand() @i{no arguments} @end example @cindex troubleshooting, function call syntax @strong{Caution:} Do not put any space between the function name and the open-parenthesis! A user-defined function name looks just like the name of a variable---a space would make the expression look like concatenation of a variable with an expression inside parentheses. With built-in functions, space before the parenthesis is harmless, but it is best not to get into the habit of using space to avoid mistakes with user-defined functions. Each function expects a particular number of arguments. For example, the @code{sqrt} function must be called with a single argument, the number of which to take the square root: @example sqrt(@var{argument}) @end example Some of the built-in functions have one or more optional arguments. If those arguments are not supplied, the functions use a reasonable default value. @xref{Built-in}, for full details. If arguments are omitted in calls to user-defined functions, then those arguments are treated as local variables and initialized to the empty string (@pxref{User-defined}). @cindex side effects, function calls Like every other expression, the function call has a value, which is computed by the function based on the arguments you give it. In this example, the value of @samp{sqrt(@var{argument})} is the square root of @var{argument}. A function can also have side effects, such as assigning values to certain variables or doing I/O. The following program reads numbers, one number per line, and prints the square root of each one: @example $ awk '@{ print "The square root of", $1, "is", sqrt($1) @}' 1 @print{} The square root of 1 is 1 3 @print{} The square root of 3 is 1.73205 5 @print{} The square root of 5 is 2.23607 @kbd{@value{CTL}-d} @end example @node Precedence @section Operator Precedence (How Operators Nest) @c STARTOFRANGE prec @cindex precedence @c STARTOFRANGE oppr @cindex operators, precedence @dfn{Operator precedence} determines how operators are grouped when different operators appear close by in one expression. For example, @samp{*} has higher precedence than @samp{+}; thus, @samp{a + b * c} means to multiply @code{b} and @code{c}, and then add @code{a} to the product (i.e., @samp{a + (b * c)}). The normal precedence of the operators can be overruled by using parentheses. Think of the precedence rules as saying where the parentheses are assumed to be. In fact, it is wise to always use parentheses whenever there is an unusual combination of operators, because other people who read the program may not remember what the precedence is in this case. Even experienced programmers occasionally forget the exact rules, which leads to mistakes. Explicit parentheses help prevent any such mistakes. When operators of equal precedence are used together, the leftmost operator groups first, except for the assignment, conditional, and exponentiation operators, which group in the opposite order. Thus, @samp{a - b + c} groups as @samp{(a - b) + c} and @samp{a = b = c} groups as @samp{a = (b = c)}. The precedence of prefix unary operators does not matter as long as only unary operators are involved, because there is only one way to interpret them: innermost first. Thus, @samp{$++i} means @samp{$(++i)} and @samp{++$x} means @samp{++($x)}. However, when another operator follows the operand, then the precedence of the unary operators can matter. @samp{$x^2} means @samp{($x)^2}, but @samp{-x^2} means @samp{-(x^2)}, because @samp{-} has lower precedence than @samp{^}, whereas @samp{$} has higher precedence. This table presents @command{awk}'s operators, in order of highest to lowest precedence: @c use @code in the items, looks better in TeX w/o all the quotes @table @code @item (@dots{}) Grouping. @cindex @code{$} (dollar sign), @code{$} field operator @cindex dollar sign (@code{$}), @code{$} field operator @item $ Field. @cindex @code{+} (plus sign), @code{++} operator @cindex plus sign (@code{+}), @code{++} operator @cindex @code{-} (hyphen), @code{--} (decrement/increment) operator @cindex hyphen (@code{-}), @code{--} (decrement/increment) operators @item ++ -- Increment, decrement. @cindex @code{^} (caret), @code{^} operator @cindex caret (@code{^}), @code{^} operator @cindex @code{*} (asterisk), @code{**} operator @cindex asterisk (@code{*}), @code{**} operator @item ^ ** Exponentiation. These operators group right-to-left. @cindex @code{+} (plus sign), @code{+} operator @cindex plus sign (@code{+}), @code{+} operator @cindex @code{-} (hyphen), @code{-} operator @cindex hyphen (@code{-}), @code{-} operator @cindex @code{!} (exclamation point), @code{!} operator @cindex exclamation point (@code{!}), @code{!} operator @item + - ! Unary plus, minus, logical ``not.'' @cindex @code{*} (asterisk), @code{*} operator, as multiplication operator @cindex asterisk (@code{*}), @code{*} operator, as multiplication operator @cindex @code{/} (forward slash), @code{/} operator @cindex forward slash (@code{/}), @code{/} operator @cindex @code{%} (percent sign), @code{%} operator @cindex percent sign (@code{%}), @code{%} operator @item * / % Multiplication, division, modulus. @cindex @code{+} (plus sign), @code{+} operator @cindex plus sign (@code{+}), @code{+} operator @cindex @code{-} (hyphen), @code{-} operator @cindex hyphen (@code{-}), @code{-} operator @item + - Addition, subtraction. @item @r{String Concatenation} No special symbol is used to indicate concatenation. The operands are simply written side by side (@pxref{Concatenation}). @cindex @code{<} (left angle bracket), @code{<} operator @cindex left angle bracket (@code{<}), @code{<} operator @cindex @code{<} (left angle bracket), @code{<=} operator @cindex left angle bracket (@code{<}), @code{<=} operator @cindex @code{>} (right angle bracket), @code{>=} operator @cindex right angle bracket (@code{>}), @code{>=} operator @cindex @code{>} (right angle bracket), @code{>} operator @cindex right angle bracket (@code{>}), @code{>} operator @cindex @code{=} (equals sign), @code{==} operator @cindex equals sign (@code{=}), @code{==} operator @cindex @code{!} (exclamation point), @code{!=} operator @cindex exclamation point (@code{!}), @code{!=} operator @cindex @code{>} (right angle bracket), @code{>>} operator (I/O) @cindex right angle bracket (@code{>}), @code{>>} operator (I/O) @cindex operators, input/output @cindex @code{|} (vertical bar), @code{|} operator (I/O) @cindex vertical bar (@code{|}), @code{|} operator (I/O) @cindex operators, input/output @cindex @code{|} (vertical bar), @code{|&} operator (I/O) @cindex vertical bar (@code{|}), @code{|&} operator (I/O) @cindex operators, input/output @item < <= == != @itemx > >= >> | |& Relational and redirection. The relational operators and the redirections have the same precedence level. Characters such as @samp{>} serve both as relationals and as redirections; the context distinguishes between the two meanings. @cindex @code{print} statement, I/O operators in @cindex @code{printf} statement, I/O operators in Note that the I/O redirection operators in @code{print} and @code{printf} statements belong to the statement level, not to expressions. The redirection does not produce an expression that could be the operand of another operator. As a result, it does not make sense to use a redirection operator near another operator of lower precedence without parentheses. Such combinations (for example, @samp{print foo > a ? b : c}), result in syntax errors. The correct way to write this statement is @samp{print foo > (a ? b : c)}. @cindex @code{~} (tilde), @code{~} operator @cindex tilde (@code{~}), @code{~} operator @cindex @code{!} (exclamation point), @code{!~} operator @cindex exclamation point (@code{!}), @code{!~} operator @item ~ !~ Matching, nonmatching. @cindex @code{in} operator @item in Array membership. @cindex @code{&} (ampersand), @code{&&} operator @cindex ampersand (@code{&}), @code{&&}operator @item && Logical ``and''. @cindex @code{|} (vertical bar), @code{||} operator @cindex vertical bar (@code{|}), @code{||} operator @item || Logical ``or''. @cindex @code{?} (question mark), @code{?:} operator @cindex question mark (@code{?}), @code{?:} operator @item ?: Conditional. This operator groups right-to-left. @cindex @code{+} (plus sign), @code{+=} operator @cindex plus sign (@code{+}), @code{+=} operator @cindex @code{-} (hyphen), @code{-=} operator @cindex hyphen (@code{-}), @code{-=} operator @cindex @code{*} (asterisk), @code{*=} operator @cindex asterisk (@code{*}), @code{*=} operator @cindex @code{*} (asterisk), @code{**=} operator @cindex asterisk (@code{*}), @code{**=} operator @cindex @code{/} (forward slash), @code{/=} operator @cindex forward slash (@code{/}), @code{/=} operator @cindex @code{%} (percent sign), @code{%=} operator @cindex percent sign (@code{%}), @code{%=} operator @cindex @code{^} (caret), @code{^=} operator @cindex caret (@code{^}), @code{^=} operator @item = += -= *= @itemx /= %= ^= **= Assignment. These operators group right to left. @end table @cindex portability, operators, not in POSIX @command{awk} @strong{Note:} The @samp{|&}, @samp{**}, and @samp{**=} operators are not specified by POSIX. For maximum portability, do not use them. @c ENDOFRANGE prec @c ENDOFRANGE oppr @c ENDOFRANGE exps @node Patterns and Actions @chapter Patterns, Actions, and Variables @c STARTOFRANGE pat @cindex patterns As you have already seen, each @command{awk} statement consists of a pattern with an associated action. This @value{CHAPTER} describes how you build patterns and actions, what kinds of things you can do within actions, and @command{awk}'s built-in variables. The pattern-action rules and the statements available for use within actions form the core of @command{awk} programming. In a sense, everything covered up to here has been the foundation that programs are built on top of. Now it's time to start building something useful. @menu * Pattern Overview:: What goes into a pattern. * Using Shell Variables:: How to use shell variables with @command{awk}. * Action Overview:: What goes into an action. * Statements:: Describes the various control statements in detail. * Built-in Variables:: Summarizes the built-in variables. @end menu @node Pattern Overview @section Pattern Elements @menu * Regexp Patterns:: Using regexps as patterns. * Expression Patterns:: Any expression can be used as a pattern. * Ranges:: Pairs of patterns specify record ranges. * BEGIN/END:: Specifying initialization and cleanup rules. * Empty:: The empty pattern, which matches every record. @end menu @cindex patterns, types of Patterns in @command{awk} control the execution of rules---a rule is executed when its pattern matches the current input record. The following is a summary of the types of @command{awk} patterns: @table @code @item /@var{regular expression}/ A regular expression. It matches when the text of the input record fits the regular expression. (@xref{Regexp}.) @item @var{expression} A single expression. It matches when its value is nonzero (if a number) or non-null (if a string). (@xref{Expression Patterns}.) @item @var{pat1}, @var{pat2} A pair of patterns separated by a comma, specifying a range of records. The range includes both the initial record that matches @var{pat1} and the final record that matches @var{pat2}. (@xref{Ranges}.) @item BEGIN @itemx END Special patterns for you to supply startup or cleanup actions for your @command{awk} program. (@xref{BEGIN/END}.) @item @var{empty} The empty pattern matches every input record. (@xref{Empty}.) @end table @node Regexp Patterns @subsection Regular Expressions as Patterns @cindex patterns, expressions as @cindex regular expressions, as patterns Regular expressions are one of the first kinds of patterns presented in this book. This kind of pattern is simply a regexp constant in the pattern part of a rule. Its meaning is @samp{$0 ~ /@var{pattern}/}. The pattern matches when the input record matches the regexp. For example: @example /foo|bar|baz/ @{ buzzwords++ @} END @{ print buzzwords, "buzzwords seen" @} @end example @node Expression Patterns @subsection Expressions as Patterns @cindex expressions, as patterns Any @command{awk} expression is valid as an @command{awk} pattern. The pattern matches if the expression's value is nonzero (if a number) or non-null (if a string). The expression is reevaluated each time the rule is tested against a new input record. If the expression uses fields such as @code{$1}, the value depends directly on the new input record's text; otherwise, it depends on only what has happened so far in the execution of the @command{awk} program. @cindex comparison expressions, as patterns @cindex patterns, comparison expressions as Comparison expressions, using the comparison operators described in @ref{Typing and Comparison}, are a very common kind of pattern. Regexp matching and nonmatching are also very common expressions. The left operand of the @samp{~} and @samp{!~} operators is a string. The right operand is either a constant regular expression enclosed in slashes (@code{/@var{regexp}/}), or any expression whose string value is used as a dynamic regular expression (@pxref{Computed Regexps}). The following example prints the second field of each input record whose first field is precisely @samp{foo}: @cindex @code{/} (forward slash), patterns and @cindex forward slash (@code{/}), patterns and @cindex @code{~} (tilde), @code{~} operator @cindex tilde (@code{~}), @code{~} operator @cindex @code{!} (exclamation point), @code{!~} operator @cindex exclamation point (@code{!}), @code{!~} operator @example $ awk '$1 == "foo" @{ print $2 @}' BBS-list @end example @noindent (There is no output, because there is no BBS site with the exact name @samp{foo}.) Contrast this with the following regular expression match, which accepts any record with a first field that contains @samp{foo}: @example $ awk '$1 ~ /foo/ @{ print $2 @}' BBS-list @print{} 555-1234 @print{} 555-6699 @print{} 555-6480 @print{} 555-2127 @end example @cindex regexp constants, as patterns @cindex patterns, regexp constants as A regexp constant as a pattern is also a special case of an expression pattern. The expression @code{/foo/} has the value one if @samp{foo} appears in the current input record. Thus, as a pattern, @code{/foo/} matches any record containing @samp{foo}. @cindex Boolean expressions, as patterns Boolean expressions are also commonly used as patterns. Whether the pattern matches an input record depends on whether its subexpressions match. For example, the following command prints all the records in @file{BBS-list} that contain both @samp{2400} and @samp{foo}: @example $ awk '/2400/ && /foo/' BBS-list @print{} fooey 555-1234 2400/1200/300 B @end example The following command prints all records in @file{BBS-list} that contain @emph{either} @samp{2400} or @samp{foo} (or both, of course): @example $ awk '/2400/ || /foo/' BBS-list @print{} alpo-net 555-3412 2400/1200/300 A @print{} bites 555-1675 2400/1200/300 A @print{} fooey 555-1234 2400/1200/300 B @print{} foot 555-6699 1200/300 B @print{} macfoo 555-6480 1200/300 A @print{} sdace 555-3430 2400/1200/300 A @print{} sabafoo 555-2127 1200/300 C @end example The following command prints all records in @file{BBS-list} that do @emph{not} contain the string @samp{foo}: @example $ awk '! /foo/' BBS-list @print{} aardvark 555-5553 1200/300 B @print{} alpo-net 555-3412 2400/1200/300 A @print{} barfly 555-7685 1200/300 A @print{} bites 555-1675 2400/1200/300 A @print{} camelot 555-0542 300 C @print{} core 555-2912 1200/300 C @print{} sdace 555-3430 2400/1200/300 A @end example @cindex @code{BEGIN} pattern, Boolean patterns and @cindex @code{END} pattern, Boolean patterns and The subexpressions of a Boolean operator in a pattern can be constant regular expressions, comparisons, or any other @command{awk} expressions. Range patterns are not expressions, so they cannot appear inside Boolean patterns. Likewise, the special patterns @code{BEGIN} and @code{END}, which never match any input record, are not expressions and cannot appear inside Boolean patterns. @node Ranges @subsection Specifying Record Ranges with Patterns @cindex range patterns @cindex patterns, ranges in @cindex lines, matching ranges of @cindex @code{,} (comma), in range patterns @cindex comma (@code{,}), in range patterns A @dfn{range pattern} is made of two patterns separated by a comma, in the form @samp{@var{begpat}, @var{endpat}}. It is used to match ranges of consecutive input records. The first pattern, @var{begpat}, controls where the range begins, while @var{endpat} controls where the pattern ends. For example, the following: @example awk '$1 == "on", $1 == "off"' myfile @end example @noindent prints every record in @file{myfile} between @samp{on}/@samp{off} pairs, inclusive. A range pattern starts out by matching @var{begpat} against every input record. When a record matches @var{begpat}, the range pattern is @dfn{turned on} and the range pattern matches this record as well. As long as the range pattern stays turned on, it automatically matches every input record read. The range pattern also matches @var{endpat} against every input record; when this succeeds, the range pattern is turned off again for the following record. Then the range pattern goes back to checking @var{begpat} against each record. @c last comma does NOT start a tertiary @cindex @code{if} statement, actions, changing The record that turns on the range pattern and the one that turns it off both match the range pattern. If you don't want to operate on these records, you can write @code{if} statements in the rule's action to distinguish them from the records you are interested in. It is possible for a pattern to be turned on and off by the same record. If the record satisfies both conditions, then the action is executed for just that record. For example, suppose there is text between two identical markers (e.g., the @samp{%} symbol), each on its own line, that should be ignored. A first attempt would be to combine a range pattern that describes the delimited text with the @code{next} statement (not discussed yet, @pxref{Next Statement}). This causes @command{awk} to skip any further processing of the current record and start over again with the next input record. Such a program looks like this: @example /^%$/,/^%$/ @{ next @} @{ print @} @end example @noindent @cindex lines, skipping between markers @c @cindex flag variables This program fails because the range pattern is both turned on and turned off by the first line, which just has a @samp{%} on it. To accomplish this task, write the program in the following manner, using a flag: @cindex @code{!} operator @example /^%$/ @{ skip = ! skip; next @} skip == 1 @{ next @} # skip lines with `skip' set @end example In a range pattern, the comma (@samp{,}) has the lowest precedence of all the operators (i.e., it is evaluated last). Thus, the following program attempts to combine a range pattern with another, simpler test: @example echo Yes | awk '/1/,/2/ || /Yes/' @end example The intent of this program is @samp{(/1/,/2/) || /Yes/}. However, @command{awk} interprets this as @samp{/1/, (/2/ || /Yes/)}. This cannot be changed or worked around; range patterns do not combine with other patterns: @example $ echo Yes | gawk '(/1/,/2/) || /Yes/' @error{} gawk: cmd. line:1: (/1/,/2/) || /Yes/ @error{} gawk: cmd. line:1: ^ parse error @error{} gawk: cmd. line:2: (/1/,/2/) || /Yes/ @error{} gawk: cmd. line:2: ^ unexpected newline @end example @node BEGIN/END @subsection The @code{BEGIN} and @code{END} Special Patterns @c STARTOFRANGE beg @cindex @code{BEGIN} pattern @c STARTOFRANGE end @cindex @code{END} pattern All the patterns described so far are for matching input records. The @code{BEGIN} and @code{END} special patterns are different. They supply startup and cleanup actions for @command{awk} programs. @code{BEGIN} and @code{END} rules must have actions; there is no default action for these rules because there is no current record when they run. @code{BEGIN} and @code{END} rules are often referred to as ``@code{BEGIN} and @code{END} blocks'' by long-time @command{awk} programmers. @menu * Using BEGIN/END:: How and why to use BEGIN/END rules. * I/O And BEGIN/END:: I/O issues in BEGIN/END rules. @end menu @node Using BEGIN/END @subsubsection Startup and Cleanup Actions A @code{BEGIN} rule is executed once only, before the first input record is read. Likewise, an @code{END} rule is executed once only, after all the input is read. For example: @example $ awk ' > BEGIN @{ print "Analysis of \"foo\"" @} > /foo/ @{ ++n @} > END @{ print "\"foo\" appears", n, "times." @}' BBS-list @print{} Analysis of "foo" @print{} "foo" appears 4 times. @end example @cindex @code{BEGIN} pattern, operators and @cindex @code{END} pattern, operators and This program finds the number of records in the input file @file{BBS-list} that contain the string @samp{foo}. The @code{BEGIN} rule prints a title for the report. There is no need to use the @code{BEGIN} rule to initialize the counter @code{n} to zero, since @command{awk} does this automatically (@pxref{Variables}). The second rule increments the variable @code{n} every time a record containing the pattern @samp{foo} is read. The @code{END} rule prints the value of @code{n} at the end of the run. The special patterns @code{BEGIN} and @code{END} cannot be used in ranges or with Boolean operators (indeed, they cannot be used with any operators). An @command{awk} program may have multiple @code{BEGIN} and/or @code{END} rules. They are executed in the order in which they appear: all the @code{BEGIN} rules at startup and all the @code{END} rules at termination. @code{BEGIN} and @code{END} rules may be intermixed with other rules. This feature was added in the 1987 version of @command{awk} and is included in the POSIX standard. The original (1978) version of @command{awk} required the @code{BEGIN} rule to be placed at the beginning of the program, the @code{END} rule to be placed at the end, and only allowed one of each. This is no longer required, but it is a good idea to follow this template in terms of program organization and readability. Multiple @code{BEGIN} and @code{END} rules are useful for writing library functions, because each library file can have its own @code{BEGIN} and/or @code{END} rule to do its own initialization and/or cleanup. The order in which library functions are named on the command line controls the order in which their @code{BEGIN} and @code{END} rules are executed. Therefore, you have to be careful when writing such rules in library files so that the order in which they are executed doesn't matter. @xref{Options}, for more information on using library functions. @xref{Library Functions}, for a number of useful library functions. If an @command{awk} program has only a @code{BEGIN} rule and no other rules, then the program exits after the @code{BEGIN} rule is run.@footnote{The original version of @command{awk} used to keep reading and ignoring input until the end of the file was seen.} However, if an @code{END} rule exists, then the input is read, even if there are no other rules in the program. This is necessary in case the @code{END} rule checks the @code{FNR} and @code{NR} variables. @node I/O And BEGIN/END @subsubsection Input/Output from @code{BEGIN} and @code{END} Rules @cindex input/output, from @code{BEGIN} and @code{END} There are several (sometimes subtle) points to remember when doing I/O from a @code{BEGIN} or @code{END} rule. The first has to do with the value of @code{$0} in a @code{BEGIN} rule. Because @code{BEGIN} rules are executed before any input is read, there simply is no input record, and therefore no fields, when executing @code{BEGIN} rules. References to @code{$0} and the fields yield a null string or zero, depending upon the context. One way to give @code{$0} a real value is to execute a @code{getline} command without a variable (@pxref{Getline}). Another way is simply to assign a value to @code{$0}. @cindex differences in @command{awk} and @command{gawk}, @code{BEGIN}/@code{END} patterns @cindex POSIX @command{awk}, @code{BEGIN}/@code{END} patterns @cindex @code{print} statement, @code{BEGIN}/@code{END} patterns and @cindex @code{BEGIN} pattern, @code{print} statement and @cindex @code{END} pattern, @code{print} statement and The second point is similar to the first but from the other direction. Traditionally, due largely to implementation issues, @code{$0} and @code{NF} were @emph{undefined} inside an @code{END} rule. The POSIX standard specifies that @code{NF} is available in an @code{END} rule. It contains the number of fields from the last input record. Most probably due to an oversight, the standard does not say that @code{$0} is also preserved, although logically one would think that it should be. In fact, @command{gawk} does preserve the value of @code{$0} for use in @code{END} rules. Be aware, however, that Unix @command{awk}, and possibly other implementations, do not. The third point follows from the first two. The meaning of @samp{print} inside a @code{BEGIN} or @code{END} rule is the same as always: @samp{print $0}. If @code{$0} is the null string, then this prints an empty line. Many long time @command{awk} programmers use an unadorned @samp{print} in @code{BEGIN} and @code{END} rules, to mean @samp{@w{print ""}}, relying on @code{$0} being null. Although one might generally get away with this in @code{BEGIN} rules, it is a very bad idea in @code{END} rules, at least in @command{gawk}. It is also poor style, since if an empty line is needed in the output, the program should print one explicitly. @cindex @code{next} statement, @code{BEGIN}/@code{END} patterns and @cindex @code{nextfile} statement, @code{BEGIN}/@code{END} patterns and @cindex @code{BEGIN} pattern, @code{next}/@code{nextfile} statements and @cindex @code{END} pattern, @code{next}/@code{nextfile} statements and Finally, the @code{next} and @code{nextfile} statements are not allowed in a @code{BEGIN} rule, because the implicit read-a-record-and-match-against-the-rules loop has not started yet. Similarly, those statements are not valid in an @code{END} rule, since all the input has been read. (@xref{Next Statement}, and see @ref{Nextfile Statement}.) @c ENDOFRANGE beg @c ENDOFRANGE end @node Empty @subsection The Empty Pattern @cindex empty pattern @cindex patterns, empty An empty (i.e., nonexistent) pattern is considered to match @emph{every} input record. For example, the program: @example awk '@{ print $1 @}' BBS-list @end example @noindent prints the first field of every record. @c ENDOFRANGE pat @node Using Shell Variables @section Using Shell Variables in Programs @cindex shells, variables @cindex @command{awk} programs, shell variables in @c @cindex shell and @command{awk} interaction @command{awk} programs are often used as components in larger programs written in shell. For example, it is very common to use a shell variable to hold a pattern that the @command{awk} program searches for. There are two ways to get the value of the shell variable into the body of the @command{awk} program. @cindex shells, quoting The most common method is to use shell quoting to substitute the variable's value into the program inside the script. For example, in the following program: @example echo -n "Enter search pattern: " read pattern awk "/$pattern/ "'@{ nmatches++ @} END @{ print nmatches, "found" @}' /path/to/data @end example @noindent the @command{awk} program consists of two pieces of quoted text that are concatenated together to form the program. The first part is double-quoted, which allows substitution of the @code{pattern} variable inside the quotes. The second part is single-quoted. Variable substitution via quoting works, but can be potentially messy. It requires a good understanding of the shell's quoting rules (@pxref{Quoting}), and it's often difficult to correctly match up the quotes when reading the program. A better method is to use @command{awk}'s variable assignment feature (@pxref{Assignment Options}) to assign the shell variable's value to an @command{awk} variable's value. Then use dynamic regexps to match the pattern (@pxref{Computed Regexps}). The following shows how to redo the previous example using this technique: @example echo -n "Enter search pattern: " read pattern awk -v pat="$pattern" '$0 ~ pat @{ nmatches++ @} END @{ print nmatches, "found" @}' /path/to/data @end example @noindent Now, the @command{awk} program is just one single-quoted string. The assignment @samp{-v pat="$pattern"} still requires double quotes, in case there is whitespace in the value of @code{$pattern}. The @command{awk} variable @code{pat} could be named @code{pattern} too, but that would be more confusing. Using a variable also provides more flexibility, since the variable can be used anywhere inside the program---for printing, as an array subscript, or for any other use---without requiring the quoting tricks at every point in the program. @node Action Overview @section Actions @c @cindex action, definition of @c @cindex curly braces @c @cindex action, curly braces @c @cindex action, separating statements @cindex actions An @command{awk} program or script consists of a series of rules and function definitions interspersed. (Functions are described later. @xref{User-defined}.) A rule contains a pattern and an action, either of which (but not both) may be omitted. The purpose of the @dfn{action} is to tell @command{awk} what to do once a match for the pattern is found. Thus, in outline, an @command{awk} program generally looks like this: @example @r{[}@var{pattern}@r{]} @r{[}@{ @var{action} @}@r{]} @r{[}@var{pattern}@r{]} @r{[}@{ @var{action} @}@r{]} @dots{} function @var{name}(@var{args}) @{ @dots{} @} @dots{} @end example @cindex @code{@{@}} (braces), actions and @cindex braces (@code{@{@}}), actions and @cindex separators, for statements in actions @cindex newlines, separating statements in actions @cindex @code{;} (semicolon), separating statements in actions @cindex semicolon (@code{;}), separating statements in actions An action consists of one or more @command{awk} @dfn{statements}, enclosed in curly braces (@samp{@{@dots{}@}}). Each statement specifies one thing to do. The statements are separated by newlines or semicolons. The curly braces around an action must be used even if the action contains only one statement, or if it contains no statements at all. However, if you omit the action entirely, omit the curly braces as well. An omitted action is equivalent to @samp{@{ print $0 @}}: @example /foo/ @{ @} @i{match @code{foo}, do nothing --- empty action} /foo/ @i{match @code{foo}, print the record --- omitted action} @end example The following types of statements are supported in @command{awk}: @table @asis @cindex side effects, statements @item Expressions Call functions or assign values to variables (@pxref{Expressions}). Executing this kind of statement simply computes the value of the expression. This is useful when the expression has side effects (@pxref{Assignment Ops}). @item Control statements Specify the control flow of @command{awk} programs. The @command{awk} language gives you C-like constructs (@code{if}, @code{for}, @code{while}, and @code{do}) as well as a few special ones (@pxref{Statements}). @item Compound statements Consist of one or more statements enclosed in curly braces. A compound statement is used in order to put several statements together in the body of an @code{if}, @code{while}, @code{do}, or @code{for} statement. @item Input statements Use the @code{getline} command (@pxref{Getline}). Also supplied in @command{awk} are the @code{next} statement (@pxref{Next Statement}), and the @code{nextfile} statement (@pxref{Nextfile Statement}). @item Output statements Such as @code{print} and @code{printf}. @xref{Printing}. @item Deletion statements For deleting array elements. @xref{Delete}. @end table @node Statements @section Control Statements in Actions @c STARTOFRANGE csta @cindex control statements @c STARTOFRANGE acs @cindex statements, control, in actions @c STARTOFRANGE accs @cindex actions, control statements in @dfn{Control statements}, such as @code{if}, @code{while}, and so on, control the flow of execution in @command{awk} programs. Most of the control statements in @command{awk} are patterned on similar statements in C. @c the comma here does NOT start a secondary @cindex compound statements, control statements and @c the second comma here does NOT start a tertiary @cindex statements, compound, control statements and @cindex body, in actions @cindex @code{@{@}} (braces), statements, grouping @cindex braces (@code{@{@}}), statements, grouping @cindex newlines, separating statements in actions @cindex @code{;} (semicolon), separating statements in actions @cindex semicolon (@code{;}), separating statements in actions All the control statements start with special keywords, such as @code{if} and @code{while}, to distinguish them from simple expressions. Many control statements contain other statements. For example, the @code{if} statement contains another statement that may or may not be executed. The contained statement is called the @dfn{body}. To include more than one statement in the body, group them into a single @dfn{compound statement} with curly braces, separating them with newlines or semicolons. @menu * If Statement:: Conditionally execute some @command{awk} statements. * While Statement:: Loop until some condition is satisfied. * Do Statement:: Do specified action while looping until some condition is satisfied. * For Statement:: Another looping statement, that provides initialization and increment clauses. * Switch Statement:: Switch/case evaluation for conditional execution of statements based on a value. * Break Statement:: Immediately exit the innermost enclosing loop. * Continue Statement:: Skip to the end of the innermost enclosing loop. * Next Statement:: Stop processing the current input record. * Nextfile Statement:: Stop processing the current file. * Exit Statement:: Stop execution of @command{awk}. @end menu @node If Statement @subsection The @code{if}-@code{else} Statement @cindex @code{if} statement The @code{if}-@code{else} statement is @command{awk}'s decision-making statement. It looks like this: @example if (@var{condition}) @var{then-body} @r{[}else @var{else-body}@r{]} @end example @noindent The @var{condition} is an expression that controls what the rest of the statement does. If the @var{condition} is true, @var{then-body} is executed; otherwise, @var{else-body} is executed. The @code{else} part of the statement is optional. The condition is considered false if its value is zero or the null string; otherwise, the condition is true. Refer to the following: @example if (x % 2 == 0) print "x is even" else print "x is odd" @end example In this example, if the expression @samp{x % 2 == 0} is true (that is, if the value of @code{x} is evenly divisible by two), then the first @code{print} statement is executed; otherwise, the second @code{print} statement is executed. If the @code{else} keyword appears on the same line as @var{then-body} and @var{then-body} is not a compound statement (i.e., not surrounded by curly braces), then a semicolon must separate @var{then-body} from the @code{else}. To illustrate this, the previous example can be rewritten as: @example if (x % 2 == 0) print "x is even"; else print "x is odd" @end example @noindent If the @samp{;} is left out, @command{awk} can't interpret the statement and it produces a syntax error. Don't actually write programs this way, because a human reader might fail to see the @code{else} if it is not the first thing on its line. @node While Statement @subsection The @code{while} Statement @cindex @code{while} statement @cindex loops @cindex loops, See Also @code{while} statement In programming, a @dfn{loop} is a part of a program that can be executed two or more times in succession. The @code{while} statement is the simplest looping statement in @command{awk}. It repeatedly executes a statement as long as a condition is true. For example: @example while (@var{condition}) @var{body} @end example @cindex body, in loops @noindent @var{body} is a statement called the @dfn{body} of the loop, and @var{condition} is an expression that controls how long the loop keeps running. The first thing the @code{while} statement does is test the @var{condition}. If the @var{condition} is true, it executes the statement @var{body}. @ifinfo (The @var{condition} is true when the value is not zero and not a null string.) @end ifinfo After @var{body} has been executed, @var{condition} is tested again, and if it is still true, @var{body} is executed again. This process repeats until the @var{condition} is no longer true. If the @var{condition} is initially false, the body of the loop is never executed and @command{awk} continues with the statement following the loop. This example prints the first three fields of each record, one per line: @example awk '@{ i = 1 while (i <= 3) @{ print $i i++ @} @}' inventory-shipped @end example @noindent The body of this loop is a compound statement enclosed in braces, containing two statements. The loop works in the following manner: first, the value of @code{i} is set to one. Then, the @code{while} statement tests whether @code{i} is less than or equal to three. This is true when @code{i} equals one, so the @code{i}-th field is printed. Then the @samp{i++} increments the value of @code{i} and the loop repeats. The loop terminates when @code{i} reaches four. A newline is not required between the condition and the body; however using one makes the program clearer unless the body is a compound statement or else is very simple. The newline after the open-brace that begins the compound statement is not required either, but the program is harder to read without it. @node Do Statement @subsection The @code{do}-@code{while} Statement @cindex @code{do}-@code{while} statement The @code{do} loop is a variation of the @code{while} looping statement. The @code{do} loop executes the @var{body} once and then repeats the @var{body} as long as the @var{condition} is true. It looks like this: @example do @var{body} while (@var{condition}) @end example Even if the @var{condition} is false at the start, the @var{body} is executed at least once (and only once, unless executing @var{body} makes @var{condition} true). Contrast this with the corresponding @code{while} statement: @example while (@var{condition}) @var{body} @end example @noindent This statement does not execute @var{body} even once if the @var{condition} is false to begin with. The following is an example of a @code{do} statement: @example @{ i = 1 do @{ print $0 i++ @} while (i <= 10) @} @end example @noindent This program prints each input record 10 times. However, it isn't a very realistic example, since in this case an ordinary @code{while} would do just as well. This situation reflects actual experience; only occasionally is there a real use for a @code{do} statement. @node For Statement @subsection The @code{for} Statement @cindex @code{for} statement The @code{for} statement makes it more convenient to count iterations of a loop. The general form of the @code{for} statement looks like this: @example for (@var{initialization}; @var{condition}; @var{increment}) @var{body} @end example @noindent The @var{initialization}, @var{condition}, and @var{increment} parts are arbitrary @command{awk} expressions, and @var{body} stands for any @command{awk} statement. The @code{for} statement starts by executing @var{initialization}. Then, as long as the @var{condition} is true, it repeatedly executes @var{body} and then @var{increment}. Typically, @var{initialization} sets a variable to either zero or one, @var{increment} adds one to it, and @var{condition} compares it against the desired number of iterations. For example: @example awk '@{ for (i = 1; i <= 3; i++) print $i @}' inventory-shipped @end example @noindent This prints the first three fields of each input record, with one field per line. It isn't possible to set more than one variable in the @var{initialization} part without using a multiple assignment statement such as @samp{x = y = 0}. This makes sense only if all the initial values are equal. (But it is possible to initialize additional variables by writing their assignments as separate statements preceding the @code{for} loop.) @c @cindex comma operator, not supported The same is true of the @var{increment} part. Incrementing additional variables requires separate statements at the end of the loop. The C compound expression, using C's comma operator, is useful in this context but it is not supported in @command{awk}. Most often, @var{increment} is an increment expression, as in the previous example. But this is not required; it can be any expression whatsoever. For example, the following statement prints all the powers of two between 1 and 100: @example for (i = 1; i <= 100; i *= 2) print i @end example If there is nothing to be done, any of the three expressions in the parentheses following the @code{for} keyword may be omitted. Thus, @w{@samp{for (; x > 0;)}} is equivalent to @w{@samp{while (x > 0)}}. If the @var{condition} is omitted, it is treated as true, effectively yielding an @dfn{infinite loop} (i.e., a loop that never terminates). In most cases, a @code{for} loop is an abbreviation for a @code{while} loop, as shown here: @example @var{initialization} while (@var{condition}) @{ @var{body} @var{increment} @} @end example @cindex loops, @code{continue} statements and @noindent The only exception is when the @code{continue} statement (@pxref{Continue Statement}) is used inside the loop. Changing a @code{for} statement to a @code{while} statement in this way can change the effect of the @code{continue} statement inside the loop. The @command{awk} language has a @code{for} statement in addition to a @code{while} statement because a @code{for} loop is often both less work to type and more natural to think of. Counting the number of iterations is very common in loops. It can be easier to think of this counting as part of looping rather than as something to do inside the loop. @ifinfo @cindex @code{in} operator There is an alternate version of the @code{for} loop, for iterating over all the indices of an array: @example for (i in array) @var{do something with} array[i] @end example @noindent @xref{Scanning an Array}, for more information on this version of the @code{for} loop. @end ifinfo @node Switch Statement @subsection The @code{switch} Statement @cindex @code{switch} statement @cindex @code{case} keyword @cindex @code{default} keyword @strong{NOTE:} This @value{SUBSECTION} describes an experimental feature added in @command{gawk} 3.1.3. It is @emph{not} enabled by default. To enable it, use the @option{--enable-switch} option to @command{configure} when @command{gawk} is being configured and built. @xref{Additional Configuration Options}, for more information. The @code{switch} statement allows the evaluation of an expression and the execution of statements based on a @code{case} match. Case statements are checked for a match in the order they are defined. If no suitable @code{case} is found, the @code{default} section is executed, if supplied. The general form of the @code{switch} statement looks like this: @example switch (@var{expression}) @{ case @var{value or regular expression}: @var{case-body} default: @var{default-body} @} @end example The @code{switch} statement works as it does in C. Once a match to a given case is made, case statement bodies are executed until a @code{break}, @code{continue}, @code{next}, @code{nextfile} or @code{exit} is encountered, or the end of the @code{switch} statement itself. For example: @example switch (NR * 2 + 1) @{ case 3: case "11": print NR - 1 break case /2[[:digit:]]+/: print NR default: print NR + 1 case -1: print NR * -1 @} @end example Note that if none of the statements specified above halt execution of a matched @code{case} statement, execution falls through to the next @code{case} until execution halts. In the above example, for any case value starting with @samp{2} followed by one or more digits, the @code{print} statement is executed and then falls through into the @code{default} section, executing its @code{print} statement. In turn, the @minus{}1 case will also be executed since the @code{default} does not halt execution. @node Break Statement @subsection The @code{break} Statement @cindex @code{break} statement @cindex loops, exiting The @code{break} statement jumps out of the innermost @code{for}, @code{while}, or @code{do} loop that encloses it. The following example finds the smallest divisor of any integer, and also identifies prime numbers: @example # find smallest divisor of num @{ num = $1 for (div = 2; div*div <= num; div++) if (num % div == 0) break if (num % div == 0) printf "Smallest divisor of %d is %d\n", num, div else printf "%d is prime\n", num @} @end example When the remainder is zero in the first @code{if} statement, @command{awk} immediately @dfn{breaks out} of the containing @code{for} loop. This means that @command{awk} proceeds immediately to the statement following the loop and continues processing. (This is very different from the @code{exit} statement, which stops the entire @command{awk} program. @xref{Exit Statement}.) Th following program illustrates how the @var{condition} of a @code{for} or @code{while} statement could be replaced with a @code{break} inside an @code{if}: @example # find smallest divisor of num @{ num = $1 for (div = 2; ; div++) @{ if (num % div == 0) @{ printf "Smallest divisor of %d is %d\n", num, div break @} if (div*div > num) @{ printf "%d is prime\n", num break @} @} @} @end example @c @cindex @code{break}, outside of loops @c @cindex historical features @c @cindex @command{awk} language, POSIX version @cindex POSIX @command{awk}, @code{break} statement and @cindex dark corner, @code{break} statement @cindex @command{gawk}, @code{break} statement in The @code{break} statement has no meaning when used outside the body of a loop. However, although it was never documented, historical implementations of @command{awk} treated the @code{break} statement outside of a loop as if it were a @code{next} statement (@pxref{Next Statement}). Recent versions of Unix @command{awk} no longer allow this usage. @command{gawk} supports this use of @code{break} only if @option{--traditional} has been specified on the command line (@pxref{Options}). Otherwise, it is treated as an error, since the POSIX standard specifies that @code{break} should only be used inside the body of a loop. @value{DARKCORNER} @node Continue Statement @subsection The @code{continue} Statement @cindex @code{continue} statement As with @code{break}, the @code{continue} statement is used only inside @code{for}, @code{while}, and @code{do} loops. It skips over the rest of the loop body, causing the next cycle around the loop to begin immediately. Contrast this with @code{break}, which jumps out of the loop altogether. The @code{continue} statement in a @code{for} loop directs @command{awk} to skip the rest of the body of the loop and resume execution with the increment-expression of the @code{for} statement. The following program illustrates this fact: @example BEGIN @{ for (x = 0; x <= 20; x++) @{ if (x == 5) continue printf "%d ", x @} print "" @} @end example @noindent This program prints all the numbers from 0 to 20---except for 5, for which the @code{printf} is skipped. Because the increment @samp{x++} is not skipped, @code{x} does not remain stuck at 5. Contrast the @code{for} loop from the previous example with the following @code{while} loop: @example BEGIN @{ x = 0 while (x <= 20) @{ if (x == 5) continue printf "%d ", x x++ @} print "" @} @end example @noindent This program loops forever once @code{x} reaches 5. @c @cindex @code{continue}, outside of loops @c @cindex historical features @c @cindex @command{awk} language, POSIX version @cindex POSIX @command{awk}, @code{continue} statement and @cindex dark corner, @code{continue} statement @cindex @command{gawk}, @code{continue} statement in The @code{continue} statement has no meaning when used outside the body of a loop. Historical versions of @command{awk} treated a @code{continue} statement outside a loop the same way they treated a @code{break} statement outside a loop: as if it were a @code{next} statement (@pxref{Next Statement}). Recent versions of Unix @command{awk} no longer work this way, and @command{gawk} allows it only if @option{--traditional} is specified on the command line (@pxref{Options}). Just like the @code{break} statement, the POSIX standard specifies that @code{continue} should only be used inside the body of a loop. @value{DARKCORNER} @node Next Statement @subsection The @code{next} Statement @cindex @code{next} statement The @code{next} statement forces @command{awk} to immediately stop processing the current record and go on to the next record. This means that no further rules are executed for the current record, and the rest of the current rule's action isn't executed. Contrast this with the effect of the @code{getline} function (@pxref{Getline}). That also causes @command{awk} to read the next record immediately, but it does not alter the flow of control in any way (i.e., the rest of the current action executes with a new input record). @cindex @command{awk} programs, execution of At the highest level, @command{awk} program execution is a loop that reads an input record and then tests each rule's pattern against it. If you think of this loop as a @code{for} statement whose body contains the rules, then the @code{next} statement is analogous to a @code{continue} statement. It skips to the end of the body of this implicit loop and executes the increment (which reads another record). For example, suppose an @command{awk} program works only on records with four fields, and it shouldn't fail when given bad input. To avoid complicating the rest of the program, write a ``weed out'' rule near the beginning, in the following manner: @example NF != 4 @{ err = sprintf("%s:%d: skipped: NF != 4\n", FILENAME, FNR) print err > "/dev/stderr" next @} @end example @noindent Because of the @code{next} statement, the program's subsequent rules won't see the bad record. The error message is redirected to the standard error output stream, as error messages should be. For more detail see @ref{Special Files}. @c @cindex @command{awk} language, POSIX version @c @cindex @code{next}, inside a user-defined function @cindex @code{BEGIN} pattern, @code{next}/@code{nextfile} statements and @cindex @code{END} pattern, @code{next}/@code{nextfile} statements and @cindex POSIX @command{awk}, @code{next}/@code{nextfile} statements and @cindex @code{next} statement, user-defined functions and @cindex functions, user-defined, @code{next}/@code{nextfile} statements and According to the POSIX standard, the behavior is undefined if the @code{next} statement is used in a @code{BEGIN} or @code{END} rule. @command{gawk} treats it as a syntax error. Although POSIX permits it, some other @command{awk} implementations don't allow the @code{next} statement inside function bodies (@pxref{User-defined}). Just as with any other @code{next} statement, a @code{next} statement inside a function body reads the next record and starts processing it with the first rule in the program. If the @code{next} statement causes the end of the input to be reached, then the code in any @code{END} rules is executed. @xref{BEGIN/END}. @node Nextfile Statement @subsection Using @command{gawk}'s @code{nextfile} Statement @cindex @code{nextfile} statement @cindex differences in @command{awk} and @command{gawk}, @code{next}/@code{nextfile} statements @command{gawk} provides the @code{nextfile} statement, which is similar to the @code{next} statement. However, instead of abandoning processing of the current record, the @code{nextfile} statement instructs @command{gawk} to stop processing the current @value{DF}. The @code{nextfile} statement is a @command{gawk} extension. In most other @command{awk} implementations, or if @command{gawk} is in compatibility mode (@pxref{Options}), @code{nextfile} is not special. Upon execution of the @code{nextfile} statement, @code{FILENAME} is updated to the name of the next @value{DF} listed on the command line, @code{FNR} is reset to one, @code{ARGIND} is incremented, and processing starts over with the first rule in the program. (@code{ARGIND} hasn't been introduced yet. @xref{Built-in Variables}.) If the @code{nextfile} statement causes the end of the input to be reached, then the code in any @code{END} rules is executed. @xref{BEGIN/END}. The @code{nextfile} statement is useful when there are many @value{DF}s to process but it isn't necessary to process every record in every file. Normally, in order to move on to the next @value{DF}, a program has to continue scanning the unwanted records. The @code{nextfile} statement accomplishes this much more efficiently. While one might think that @samp{close(FILENAME)} would accomplish the same as @code{nextfile}, this isn't true. @code{close} is reserved for closing files, pipes, and coprocesses that are opened with redirections. It is not related to the main processing that @command{awk} does with the files listed in @code{ARGV}. If it's necessary to use an @command{awk} version that doesn't support @code{nextfile}, see @ref{Nextfile Function}, for a user-defined function that simulates the @code{nextfile} statement. @cindex functions, user-defined, @code{next}/@code{nextfile} statements and @cindex @code{nextfile} statement, user-defined functions and The current version of the Bell Laboratories @command{awk} (@pxref{Other Versions}) also supports @code{nextfile}. However, it doesn't allow the @code{nextfile} statement inside function bodies (@pxref{User-defined}). @command{gawk} does; a @code{nextfile} inside a function body reads the next record and starts processing it with the first rule in the program, just as any other @code{nextfile} statement. @cindex @code{next file} statement, in @command{gawk} @cindex @command{gawk}, @code{next file} statement in @cindex @code{nextfile} statement, in @command{gawk} @cindex @command{gawk}, @code{nextfile} statement in @strong{Caution:} Versions of @command{gawk} prior to 3.0 used two words (@samp{next file}) for the @code{nextfile} statement. In @value{PVERSION} 3.0, this was changed to one word, because the treatment of @samp{file} was inconsistent. When it appeared after @code{next}, @samp{file} was a keyword; otherwise, it was a regular identifier. The old usage is no longer accepted; @samp{next file} generates a syntax error. @node Exit Statement @subsection The @code{exit} Statement @cindex @code{exit} statement The @code{exit} statement causes @command{awk} to immediately stop executing the current rule and to stop processing input; any remaining input is ignored. The @code{exit} statement is written as follows: @example exit @r{[}@var{return code}@r{]} @end example @cindex @code{BEGIN} pattern, @code{exit} statement and @cindex @code{END} pattern, @code{exit} statement and When an @code{exit} statement is executed from a @code{BEGIN} rule, the program stops processing everything immediately. No input records are read. However, if an @code{END} rule is present, as part of executing the @code{exit} statement, the @code{END} rule is executed (@pxref{BEGIN/END}). If @code{exit} is used as part of an @code{END} rule, it causes the program to stop immediately. An @code{exit} statement that is not part of a @code{BEGIN} or @code{END} rule stops the execution of any further automatic rules for the current record, skips reading any remaining input records, and executes the @code{END} rule if there is one. In such a case, if you don't want the @code{END} rule to do its job, set a variable to nonzero before the @code{exit} statement and check that variable in the @code{END} rule. @xref{Assert Function}, for an example that does this. @cindex dark corner, @code{exit} statement If an argument is supplied to @code{exit}, its value is used as the exit status code for the @command{awk} process. If no argument is supplied, @code{exit} returns status zero (success). In the case where an argument is supplied to a first @code{exit} statement, and then @code{exit} is called a second time from an @code{END} rule with no argument, @command{awk} uses the previously supplied exit value. @value{DARKCORNER} @cindex programming conventions, @code{exit} statement For example, suppose an error condition occurs that is difficult or impossible to handle. Conventionally, programs report this by exiting with a nonzero status. An @command{awk} program can do this using an @code{exit} statement with a nonzero argument, as shown in the following example: @example BEGIN @{ if (("date" | getline date_now) <= 0) @{ print "Can't get system date" > "/dev/stderr" exit 1 @} print "current date is", date_now close("date") @} @end example @c ENDOFRANGE csta @c ENDOFRANGE acs @c ENDOFRANGE accs @node Built-in Variables @section Built-in Variables @c STARTOFRANGE bvar @cindex built-in variables @c STARTOFRANGE varb @cindex variables, built-in Most @command{awk} variables are available to use for your own purposes; they never change unless your program assigns values to them, and they never affect anything unless your program examines them. However, a few variables in @command{awk} have special built-in meanings. @command{awk} examines some of these automatically, so that they enable you to tell @command{awk} how to do certain things. Others are set automatically by @command{awk}, so that they carry information from the internal workings of @command{awk} to your program. @cindex @command{gawk}, built-in variables and This @value{SECTION} documents all the built-in variables of @command{gawk}, most of which are also documented in the chapters describing their areas of activity. @menu * User-modified:: Built-in variables that you change to control @command{awk}. * Auto-set:: Built-in variables where @command{awk} gives you information. * ARGC and ARGV:: Ways to use @code{ARGC} and @code{ARGV}. @end menu @node User-modified @subsection Built-in Variables That Control @command{awk} @c STARTOFRANGE bvaru @cindex built-in variables, user-modifiable @c STARTOFRANGE nmbv @cindex user-modifiable variables The following is an alphabetical list of variables that you can change to control how @command{awk} does certain things. The variables that are specific to @command{gawk} are marked with a pound sign@w{ (@samp{#}).} @table @code @cindex @code{BINMODE} variable @cindex binary input/output @cindex input/output, binary @item BINMODE # On non-POSIX systems, this variable specifies use of binary mode for all I/O. Numeric values of one, two, or three specify that input files, output files, or all files, respectively, should use binary I/O. Alternatively, string values of @code{"r"} or @code{"w"} specify that input files and output files, respectively, should use binary I/O. A string value of @code{"rw"} or @code{"wr"} indicates that all files should use binary I/O. Any other string value is equivalent to @code{"rw"}, but @command{gawk} generates a warning message. @code{BINMODE} is described in more detail in @ref{PC Using}. @cindex differences in @command{awk} and @command{gawk}, @code{BINMODE} variable This variable is a @command{gawk} extension. In other @command{awk} implementations (except @command{mawk}, @pxref{Other Versions}), or if @command{gawk} is in compatibility mode (@pxref{Options}), it is not special. @cindex @code{CONVFMT} variable @cindex POSIX @command{awk}, @code{CONVFMT} variable and @cindex numbers, converting, to strings @cindex strings, converting, numbers to @item CONVFMT This string controls conversion of numbers to strings (@pxref{Conversion}). It works by being passed, in effect, as the first argument to the @code{sprintf} function (@pxref{String Functions}). Its default value is @code{"%.6g"}. @code{CONVFMT} was introduced by the POSIX standard. @cindex @code{FIELDWIDTHS} variable @cindex differences in @command{awk} and @command{gawk}, @code{FIELDWIDTHS} variable @cindex field separators, @code{FIELDWIDTHS} variable and @cindex separators, field, @code{FIELDWIDTHS} variable and @item FIELDWIDTHS # This is a space-separated list of columns that tells @command{gawk} how to split input with fixed columnar boundaries. Assigning a value to @code{FIELDWIDTHS} overrides the use of @code{FS} for field splitting. @xref{Constant Size}, for more information. @cindex @command{gawk}, @code{FIELDWIDTHS} variable in If @command{gawk} is in compatibility mode (@pxref{Options}), then @code{FIELDWIDTHS} has no special meaning, and field-splitting operations occur based exclusively on the value of @code{FS}. @cindex @code{FS} variable @cindex separators, field @cindex field separators @item FS This is the input field separator (@pxref{Field Separators}). The value is a single-character string or a multi-character regular expression that matches the separations between fields in an input record. If the value is the null string (@code{""}), then each character in the record becomes a separate field. (This behavior is a @command{gawk} extension. POSIX @command{awk} does not specify the behavior when @code{FS} is the null string.) @c NEXT ED: Mark as common extension @cindex POSIX @command{awk}, @code{FS} variable and The default value is @w{@code{" "}}, a string consisting of a single space. As a special exception, this value means that any sequence of spaces, tabs, and/or newlines is a single separator.@footnote{In POSIX @command{awk}, newline does not count as whitespace.} It also causes spaces, tabs, and newlines at the beginning and end of a record to be ignored. You can set the value of @code{FS} on the command line using the @option{-F} option: @example awk -F, '@var{program}' @var{input-files} @end example @cindex @command{gawk}, field separators and If @command{gawk} is using @code{FIELDWIDTHS} for field splitting, assigning a value to @code{FS} causes @command{gawk} to return to the normal, @code{FS}-based field splitting. An easy way to do this is to simply say @samp{FS = FS}, perhaps with an explanatory comment. @cindex @code{IGNORECASE} variable @cindex differences in @command{awk} and @command{gawk}, @code{IGNORECASE} variable @cindex case sensitivity, string comparisons and @cindex case sensitivity, regexps and @cindex regular expressions, case sensitivity @item IGNORECASE # If @code{IGNORECASE} is nonzero or non-null, then all string comparisons and all regular expression matching are case independent. Thus, regexp matching with @samp{~} and @samp{!~}, as well as the @code{gensub}, @code{gsub}, @code{index}, @code{match}, @code{split}, and @code{sub} functions, record termination with @code{RS}, and field splitting with @code{FS}, all ignore case when doing their particular regexp operations. However, the value of @code{IGNORECASE} does @emph{not} affect array subscripting and it does not affect field splitting when using a single-character field separator. @xref{Case-sensitivity}. @cindex @command{gawk}, @code{IGNORECASE} variable in If @command{gawk} is in compatibility mode (@pxref{Options}), then @code{IGNORECASE} has no special meaning. Thus, string and regexp operations are always case-sensitive. @cindex @code{LINT} variable @cindex differences in @command{awk} and @command{gawk}, @code{LINT} variable @cindex lint checking @item LINT # When this variable is true (nonzero or non-null), @command{gawk} behaves as if the @option{--lint} command-line option is in effect. (@pxref{Options}). With a value of @code{"fatal"}, lint warnings become fatal errors. With a value of @code{"invalid"}, only warnings about things that are actually invalid are issued. (This is not fully implemented yet.) Any other true value prints nonfatal warnings. Assigning a false value to @code{LINT} turns off the lint warnings. @cindex @command{gawk}, @code{LINT} variable in This variable is a @command{gawk} extension. It is not special in other @command{awk} implementations. Unlike the other special variables, changing @code{LINT} does affect the production of lint warnings, even if @command{gawk} is in compatibility mode. Much as the @option{--lint} and @option{--traditional} options independently control different aspects of @command{gawk}'s behavior, the control of lint warnings during program execution is independent of the flavor of @command{awk} being executed. @cindex @code{OFMT} variable @cindex numbers, converting, to strings @cindex strings, converting, numbers to @item OFMT This string controls conversion of numbers to strings (@pxref{Conversion}) for printing with the @code{print} statement. It works by being passed as the first argument to the @code{sprintf} function (@pxref{String Functions}). Its default value is @code{"%.6g"}. Earlier versions of @command{awk} also used @code{OFMT} to specify the format for converting numbers to strings in general expressions; this is now done by @code{CONVFMT}. @cindex @code{sprintf} function, @code{OFMT} variable and @cindex @code{print} statement, @code{OFMT} variable and @cindex @code{OFS} variable @cindex separators, field @cindex field separators @item OFS This is the output field separator (@pxref{Output Separators}). It is output between the fields printed by a @code{print} statement. Its default value is @w{@code{" "}}, a string consisting of a single space. @cindex @code{ORS} variable @item ORS This is the output record separator. It is output at the end of every @code{print} statement. Its default value is @code{"\n"}, the newline character. (@xref{Output Separators}.) @cindex @code{RS} variable @cindex separators, record @cindex record separators @item RS This is @command{awk}'s input record separator. Its default value is a string containing a single newline character, which means that an input record consists of a single line of text. It can also be the null string, in which case records are separated by runs of blank lines. If it is a regexp, records are separated by matches of the regexp in the input text. (@xref{Records}.) The ability for @code{RS} to be a regular expression is a @command{gawk} extension. In most other @command{awk} implementations, or if @command{gawk} is in compatibility mode (@pxref{Options}), just the first character of @code{RS}'s value is used. @cindex @code{SUBSEP} variable @cindex separators, subscript @cindex subscript separators @item SUBSEP This is the subscript separator. It has the default value of @code{"\034"} and is used to separate the parts of the indices of a multidimensional array. Thus, the expression @code{@w{foo["A", "B"]}} really accesses @code{foo["A\034B"]} (@pxref{Multi-dimensional}). @cindex @code{TEXTDOMAIN} variable @cindex differences in @command{awk} and @command{gawk}, @code{TEXTDOMAIN} variable @cindex internationalization, localization @item TEXTDOMAIN # This variable is used for internationalization of programs at the @command{awk} level. It sets the default text domain for specially marked string constants in the source text, as well as for the @code{dcgettext}, @code{dcngettext} and @code{bindtextdomain} functions (@pxref{Internationalization}). The default value of @code{TEXTDOMAIN} is @code{"messages"}. This variable is a @command{gawk} extension. In other @command{awk} implementations, or if @command{gawk} is in compatibility mode (@pxref{Options}), it is not special. @end table @c ENDOFRANGE bvar @c ENDOFRANGE varb @c ENDOFRANGE bvaru @c ENDOFRANGE nmbv @node Auto-set @subsection Built-in Variables That Convey Information @c STARTOFRANGE bvconi @cindex built-in variables, conveying information @c STARTOFRANGE vbconi @cindex variables, built-in, conveying information The following is an alphabetical list of variables that @command{awk} sets automatically on certain occasions in order to provide information to your program. The variables that are specific to @command{gawk} are marked with a pound sign@w{ (@samp{#}).} @table @code @cindex @code{ARGC}/@code{ARGV} variables @cindex arguments, command-line @cindex command line, arguments @item ARGC@r{,} ARGV The command-line arguments available to @command{awk} programs are stored in an array called @code{ARGV}. @code{ARGC} is the number of command-line arguments present. @xref{Other Arguments}. Unlike most @command{awk} arrays, @code{ARGV} is indexed from 0 to @code{ARGC} @minus{} 1. In the following example: @example $ awk 'BEGIN @{ > for (i = 0; i < ARGC; i++) > print ARGV[i] > @}' inventory-shipped BBS-list @print{} awk @print{} inventory-shipped @print{} BBS-list @end example @noindent @code{ARGV[0]} contains @code{"awk"}, @code{ARGV[1]} contains @code{"inventory-shipped"}, and @code{ARGV[2]} contains @code{"BBS-list"}. The value of @code{ARGC} is three, one more than the index of the last element in @code{ARGV}, because the elements are numbered from zero. @cindex programming conventions, @code{ARGC}/@code{ARGV} variables The names @code{ARGC} and @code{ARGV}, as well as the convention of indexing the array from 0 to @code{ARGC} @minus{} 1, are derived from the C language's method of accessing command-line arguments. The value of @code{ARGV[0]} can vary from system to system. Also, you should note that the program text is @emph{not} included in @code{ARGV}, nor are any of @command{awk}'s command-line options. @xref{ARGC and ARGV}, for information about how @command{awk} uses these variables. @cindex @code{ARGIND} variable @cindex differences in @command{awk} and @command{gawk}, @code{ARGIND} variable @item ARGIND # The index in @code{ARGV} of the current file being processed. Every time @command{gawk} opens a new @value{DF} for processing, it sets @code{ARGIND} to the index in @code{ARGV} of the @value{FN}. When @command{gawk} is processing the input files, @samp{FILENAME == ARGV[ARGIND]} is always true. @c comma before ARGIND does NOT mark a tertiary @cindex files, processing, @code{ARGIND} variable and This variable is useful in file processing; it allows you to tell how far along you are in the list of @value{DF}s as well as to distinguish between successive instances of the same @value{FN} on the command line. @cindex @value{FN}s, distinguishing While you can change the value of @code{ARGIND} within your @command{awk} program, @command{gawk} automatically sets it to a new value when the next file is opened. This variable is a @command{gawk} extension. In other @command{awk} implementations, or if @command{gawk} is in compatibility mode (@pxref{Options}), it is not special. @cindex @code{ENVIRON} variable @cindex environment variables @item ENVIRON An associative array that contains the values of the environment. The array indices are the environment variable names; the elements are the values of the particular environment variables. For example, @code{ENVIRON["HOME"]} might be @file{/home/arnold}. Changing this array does not affect the environment passed on to any programs that @command{awk} may spawn via redirection or the @code{system} function. @c (In a future version of @command{gawk}, it may do so.) Some operating systems may not have environment variables. On such systems, the @code{ENVIRON} array is empty (except for @w{@code{ENVIRON["AWKPATH"]}}, @pxref{AWKPATH Variable}). @cindex @code{ERRNO} variable @cindex differences in @command{awk} and @command{gawk}, @code{ERRNO} variable @cindex error handling, @code{ERRNO} variable and @item ERRNO # If a system error occurs during a redirection for @code{getline}, during a read for @code{getline}, or during a @code{close} operation, then @code{ERRNO} contains a string describing the error. This variable is a @command{gawk} extension. In other @command{awk} implementations, or if @command{gawk} is in compatibility mode (@pxref{Options}), it is not special. @cindex @code{FILENAME} variable @cindex dark corner, @code{FILENAME} variable @item FILENAME The name of the file that @command{awk} is currently reading. When no @value{DF}s are listed on the command line, @command{awk} reads from the standard input and @code{FILENAME} is set to @code{"-"}. @code{FILENAME} is changed each time a new file is read (@pxref{Reading Files}). Inside a @code{BEGIN} rule, the value of @code{FILENAME} is @code{""}, since there are no input files being processed yet.@footnote{Some early implementations of Unix @command{awk} initialized @code{FILENAME} to @code{"-"}, even if there were @value{DF}s to be processed. This behavior was incorrect and should not be relied upon in your programs.} @value{DARKCORNER} Note, though, that using @code{getline} (@pxref{Getline}) inside a @code{BEGIN} rule can give @code{FILENAME} a value. @cindex @code{FNR} variable @item FNR The current record number in the current file. @code{FNR} is incremented each time a new record is read (@pxref{Getline}). It is reinitialized to zero each time a new input file is started. @cindex @code{NF} variable @item NF The number of fields in the current input record. @code{NF} is set each time a new record is read, when a new field is created or when @code{$0} changes (@pxref{Fields}). Unlike most of the variables described in this @ifnotinfo section, @end ifnotinfo @ifinfo node, @end ifinfo assigning a value to @code{NF} has the potential to affect @command{awk}'s internal workings. In particular, assignments to @code{NF} can be used to create or remove fields from the current record: @xref{Changing Fields}. @cindex @code{NR} variable @item NR The number of input records @command{awk} has processed since the beginning of the program's execution (@pxref{Records}). @code{NR} is incremented each time a new record is read. @cindex @code{PROCINFO} array @cindex differences in @command{awk} and @command{gawk}, @code{PROCINFO} array @item PROCINFO # The elements of this array provide access to information about the running @command{awk} program. The following elements (listed alphabetically) are guaranteed to be available: @table @code @item PROCINFO["egid"] The value of the @code{getegid} system call. @item PROCINFO["euid"] The value of the @code{geteuid} system call. @item PROCINFO["FS"] This is @code{"FS"} if field splitting with @code{FS} is in effect, or it is @code{"FIELDWIDTHS"} if field splitting with @code{FIELDWIDTHS} is in effect. @item PROCINFO["gid"] The value of the @code{getgid} system call. @item PROCINFO["pgrpid"] The process group ID of the current process. @item PROCINFO["pid"] The process ID of the current process. @item PROCINFO["ppid"] The parent process ID of the current process. @item PROCINFO["uid"] The value of the @code{getuid} system call. @end table On some systems, there may be elements in the array, @code{"group1"} through @code{"group@var{N}"} for some @var{N}. @var{N} is the number of supplementary groups that the process has. Use the @code{in} operator to test for these elements (@pxref{Reference to Elements}). This array is a @command{gawk} extension. In other @command{awk} implementations, or if @command{gawk} is in compatibility mode (@pxref{Options}), it is not special. @cindex @code{RLENGTH} variable @item RLENGTH The length of the substring matched by the @code{match} function (@pxref{String Functions}). @code{RLENGTH} is set by invoking the @code{match} function. Its value is the length of the matched string, or @minus{}1 if no match is found. @cindex @code{RSTART} variable @item RSTART The start-index in characters of the substring that is matched by the @code{match} function (@pxref{String Functions}). @code{RSTART} is set by invoking the @code{match} function. Its value is the position of the string where the matched substring starts, or zero if no match was found. @cindex @code{RT} variable @cindex differences in @command{awk} and @command{gawk}, @code{RT} variable @item RT # This is set each time a record is read. It contains the input text that matched the text denoted by @code{RS}, the record separator. This variable is a @command{gawk} extension. In other @command{awk} implementations, or if @command{gawk} is in compatibility mode (@pxref{Options}), it is not special. @end table @c ENDOFRANGE bvconi @c ENDOFRANGE vbconi @c fakenode --- for prepinfo @subheading Advanced Notes: Changing @code{NR} and @code{FNR} @cindex @code{NR} variable, changing @cindex @code{FNR} variable, changing @cindex advanced features, @code{FNR}/@code{NR} variables @cindex dark corner, @code{FNR}/@code{NR} variables @command{awk} increments @code{NR} and @code{FNR} each time it reads a record, instead of setting them to the absolute value of the number of records read. This means that a program can change these variables and their new values are incremented for each record. @value{DARKCORNER} This is demonstrated in the following example: @example $ echo '1 > 2 > 3 > 4' | awk 'NR == 2 @{ NR = 17 @} > @{ print NR @}' @print{} 1 @print{} 17 @print{} 18 @print{} 19 @end example @noindent Before @code{FNR} was added to the @command{awk} language (@pxref{V7/SVR3.1}), many @command{awk} programs used this feature to track the number of records in a file by resetting @code{NR} to zero when @code{FILENAME} changed. @node ARGC and ARGV @subsection Using @code{ARGC} and @code{ARGV} @cindex @code{ARGC}/@code{ARGV} variables @cindex arguments, command-line @cindex command line, arguments @ref{Auto-set}, presented the following program describing the information contained in @code{ARGC} and @code{ARGV}: @example $ awk 'BEGIN @{ > for (i = 0; i < ARGC; i++) > print ARGV[i] > @}' inventory-shipped BBS-list @print{} awk @print{} inventory-shipped @print{} BBS-list @end example @noindent In this example, @code{ARGV[0]} contains @samp{awk}, @code{ARGV[1]} contains @samp{inventory-shipped}, and @code{ARGV[2]} contains @samp{BBS-list}. Notice that the @command{awk} program is not entered in @code{ARGV}. The other special command-line options, with their arguments, are also not entered. This includes variable assignments done with the @option{-v} option (@pxref{Options}). Normal variable assignments on the command line @emph{are} treated as arguments and do show up in the @code{ARGV} array: @example $ cat showargs.awk @print{} BEGIN @{ @print{} printf "A=%d, B=%d\n", A, B @print{} for (i = 0; i < ARGC; i++) @print{} printf "\tARGV[%d] = %s\n", i, ARGV[i] @print{} @} @print{} END @{ printf "A=%d, B=%d\n", A, B @} $ awk -v A=1 -f showargs.awk B=2 /dev/null @print{} A=1, B=0 @print{} ARGV[0] = awk @print{} ARGV[1] = B=2 @print{} ARGV[2] = /dev/null @print{} A=1, B=2 @end example A program can alter @code{ARGC} and the elements of @code{ARGV}. Each time @command{awk} reaches the end of an input file, it uses the next element of @code{ARGV} as the name of the next input file. By storing a different string there, a program can change which files are read. Use @code{"-"} to represent the standard input. Storing additional elements and incrementing @code{ARGC} causes additional files to be read. If the value of @code{ARGC} is decreased, that eliminates input files from the end of the list. By recording the old value of @code{ARGC} elsewhere, a program can treat the eliminated arguments as something other than @value{FN}s. To eliminate a file from the middle of the list, store the null string (@code{""}) into @code{ARGV} in place of the file's name. As a special feature, @command{awk} ignores @value{FN}s that have been replaced with the null string. Another option is to use the @code{delete} statement to remove elements from @code{ARGV} (@pxref{Delete}). All of these actions are typically done in the @code{BEGIN} rule, before actual processing of the input begins. @xref{Split Program}, and see @ref{Tee Program}, for examples of each way of removing elements from @code{ARGV}. The following fragment processes @code{ARGV} in order to examine, and then remove, command-line options: @c NEXT ED: Add xref to rewind() function @example BEGIN @{ for (i = 1; i < ARGC; i++) @{ if (ARGV[i] == "-v") verbose = 1 else if (ARGV[i] == "-d") debug = 1 else if (ARGV[i] ~ /^-?/) @{ e = sprintf("%s: unrecognized option -- %c", ARGV[0], substr(ARGV[i], 1, ,1)) print e > "/dev/stderr" @} else break delete ARGV[i] @} @} @end example To actually get the options into the @command{awk} program, end the @command{awk} options with @option{--} and then supply the @command{awk} program's options, in the following manner: @example awk -f myprog -- -v -d file1 file2 @dots{} @end example @cindex differences in @command{awk} and @command{gawk}, @code{ARGC}/@code{ARGV} variables This is not necessary in @command{gawk}. Unless @option{--posix} has been specified, @command{gawk} silently puts any unrecognized options into @code{ARGV} for the @command{awk} program to deal with. As soon as it sees an unknown option, @command{gawk} stops looking for other options that it might otherwise recognize. The previous example with @command{gawk} would be: @example gawk -f myprog -d -v file1 file2 @dots{} @end example @noindent Because @option{-d} is not a valid @command{gawk} option, it and the following @option{-v} are passed on to the @command{awk} program. @node Arrays @chapter Arrays in @command{awk} @c STARTOFRANGE arrs @cindex arrays An @dfn{array} is a table of values called @dfn{elements}. The elements of an array are distinguished by their indices. @dfn{Indices} may be either numbers or strings. This @value{CHAPTER} describes how arrays work in @command{awk}, how to use array elements, how to scan through every element in an array, and how to remove array elements. It also describes how @command{awk} simulates multidimensional arrays, as well as some of the less obvious points about array usage. The @value{CHAPTER} finishes with a discussion of @command{gawk}'s facility for sorting an array based on its indices. @cindex variables, names of @cindex functions, names of @cindex arrays, names of @cindex names, arrays/variables @cindex namespace issues @command{awk} maintains a single set of names that may be used for naming variables, arrays, and functions (@pxref{User-defined}). Thus, you cannot have a variable and an array with the same name in the same @command{awk} program. @menu * Array Intro:: Introduction to Arrays * Reference to Elements:: How to examine one element of an array. * Assigning Elements:: How to change an element of an array. * Array Example:: Basic Example of an Array * Scanning an Array:: A variation of the @code{for} statement. It loops through the indices of an array's existing elements. * Delete:: The @code{delete} statement removes an element from an array. * Numeric Array Subscripts:: How to use numbers as subscripts in @command{awk}. * Uninitialized Subscripts:: Using Uninitialized variables as subscripts. * Multi-dimensional:: Emulating multidimensional arrays in @command{awk}. * Multi-scanning:: Scanning multidimensional arrays. * Array Sorting:: Sorting array values and indices. @end menu @node Array Intro @section Introduction to Arrays The @command{awk} language provides one-dimensional arrays for storing groups of related strings or numbers. Every @command{awk} array must have a name. Array names have the same syntax as variable names; any valid variable name would also be a valid array name. But one name cannot be used in both ways (as an array and as a variable) in the same @command{awk} program. Arrays in @command{awk} superficially resemble arrays in other programming languages, but there are fundamental differences. In @command{awk}, it isn't necessary to specify the size of an array before starting to use it. Additionally, any number or string in @command{awk}, not just consecutive integers, may be used as an array index. In most other languages, arrays must be @dfn{declared} before use, including a specification of how many elements or components they contain. In such languages, the declaration causes a contiguous block of memory to be allocated for that many elements. Usually, an index in the array must be a positive integer. For example, the index zero specifies the first element in the array, which is actually stored at the beginning of the block of memory. Index one specifies the second element, which is stored in memory right after the first element, and so on. It is impossible to add more elements to the array, because it has room only for as many elements as given in the declaration. (Some languages allow arbitrary starting and ending indices---e.g., @samp{15 .. 27}---but the size of the array is still fixed when the array is declared.) A contiguous array of four elements might look like the following example, conceptually, if the element values are 8, @code{"foo"}, @code{""}, and 30: @c NEXT ED: Use real images here @iftex @c from Karl Berry, much thanks for the help. @tex \bigskip % space above the table (about 1 linespace) \offinterlineskip \newdimen\width \width = 1.5cm \newdimen\hwidth \hwidth = 4\width \advance\hwidth by 2pt % 5 * 0.4pt \centerline{\vbox{ \halign{\strut\hfil\ignorespaces#&&\vrule#&\hbox to\width{\hfil#\unskip\hfil}\cr \noalign{\hrule width\hwidth} &&{\tt 8} &&{\tt "foo"} &&{\tt ""} &&{\tt 30} &&\quad Value\cr \noalign{\hrule width\hwidth} \noalign{\smallskip} &\omit&0&\omit &1 &\omit&2 &\omit&3 &\omit&\quad Index\cr } }} @end tex @end iftex @ifinfo @example +---------+---------+--------+---------+ | 8 | "foo" | "" | 30 | @r{Value} +---------+---------+--------+---------+ 0 1 2 3 @r{Index} @end example @end ifinfo @ifxml @example +---------+---------+--------+---------+ | 8 | "foo" | "" | 30 | @r{Value} +---------+---------+--------+---------+ 0 1 2 3 @r{Index} @end example @end ifxml @noindent Only the values are stored; the indices are implicit from the order of the values. Here, 8 is the value at index zero, because 8 appears in the position with zero elements before it. @c STARTOFRANGE arrin @cindex arrays, indexing @c STARTOFRANGE inarr @cindex indexing arrays @cindex associative arrays @cindex arrays, associative Arrays in @command{awk} are different---they are @dfn{associative}. This means that each array is a collection of pairs: an index and its corresponding array element value: @example @r{Element} 3 @r{Value} 30 @r{Element} 1 @r{Value} "foo" @r{Element} 0 @r{Value} 8 @r{Element} 2 @r{Value} "" @end example @noindent The pairs are shown in jumbled order because their order is irrelevant. One advantage of associative arrays is that new pairs can be added at any time. For example, suppose a tenth element is added to the array whose value is @w{@code{"number ten"}}. The result is: @example @r{Element} 10 @r{Value} "number ten" @r{Element} 3 @r{Value} 30 @r{Element} 1 @r{Value} "foo" @r{Element} 0 @r{Value} 8 @r{Element} 2 @r{Value} "" @end example @noindent @cindex sparse arrays @cindex arrays, sparse Now the array is @dfn{sparse}, which just means some indices are missing. It has elements 0--3 and 10, but doesn't have elements 4, 5, 6, 7, 8, or 9. Another consequence of associative arrays is that the indices don't have to be positive integers. Any number, or even a string, can be an index. For example, the following is an array that translates words from English to French: @example @r{Element} "dog" @r{Value} "chien" @r{Element} "cat" @r{Value} "chat" @r{Element} "one" @r{Value} "un" @r{Element} 1 @r{Value} "un" @end example @noindent Here we decided to translate the number one in both spelled-out and numeric form---thus illustrating that a single array can have both numbers and strings as indices. In fact, array subscripts are always strings; this is discussed in more detail in @ref{Numeric Array Subscripts}. Here, the number @code{1} isn't double-quoted, since @command{awk} automatically converts it to a string. @cindex case sensitivity, array indices and @cindex arrays, @code{IGNORECASE} variable and @cindex @code{IGNORECASE} variable, array subscripts and The value of @code{IGNORECASE} has no effect upon array subscripting. The identical string value used to store an array element must be used to retrieve it. When @command{awk} creates an array (e.g., with the @code{split} built-in function), that array's indices are consecutive integers starting at one. (@xref{String Functions}.) @command{awk}'s arrays are efficient---the time to access an element is independent of the number of elements in the array. @c ENDOFRANGE arrin @c ENDOFRANGE inarr @node Reference to Elements @section Referring to an Array Element @cindex arrays, elements, referencing @cindex elements in arrays The principal way to use an array is to refer to one of its elements. An array reference is an expression as follows: @example @var{array}[@var{index}] @end example @noindent Here, @var{array} is the name of an array. The expression @var{index} is the index of the desired element of the array. The value of the array reference is the current value of that array element. For example, @code{foo[4.3]} is an expression for the element of array @code{foo} at index @samp{4.3}. A reference to an array element that has no recorded value yields a value of @code{""}, the null string. This includes elements that have not been assigned any value as well as elements that have been deleted (@pxref{Delete}). Such a reference automatically creates that array element, with the null string as its value. (In some cases, this is unfortunate, because it might waste memory inside @command{awk}.) @c @cindex arrays, @code{in} operator and @cindex @code{in} operator, arrays and To determine whether an element exists in an array at a certain index, use the following expression: @example @var{index} in @var{array} @end example @cindex side effects, array indexing @noindent This expression tests whether the particular index exists, without the side effect of creating that element if it is not present. The expression has the value one (true) if @code{@var{array}[@var{index}]} exists and zero (false) if it does not exist. For example, this statement tests whether the array @code{frequencies} contains the index @samp{2}: @example if (2 in frequencies) print "Subscript 2 is present." @end example Note that this is @emph{not} a test of whether the array @code{frequencies} contains an element whose @emph{value} is two. There is no way to do that except to scan all the elements. Also, this @emph{does not} create @code{frequencies[2]}, while the following (incorrect) alternative does: @example if (frequencies[2] != "") print "Subscript 2 is present." @end example @node Assigning Elements @section Assigning Array Elements @cindex arrays, elements, assigning @cindex elements in arrays, assigning Array elements can be assigned values just like @command{awk} variables: @example @var{array}[@var{subscript}] = @var{value} @end example @noindent @var{array} is the name of an array. The expression @var{subscript} is the index of the element of the array that is assigned a value. The expression @var{value} is the value to assign to that element of the array. @node Array Example @section Basic Array Example The following program takes a list of lines, each beginning with a line number, and prints them out in order of line number. The line numbers are not in order when they are first read---instead they are scrambled. This program sorts the lines by making an array using the line numbers as subscripts. The program then prints out the lines in sorted order of their numbers. It is a very simple program and gets confused upon encountering repeated numbers, gaps, or lines that don't begin with a number: @example @c file eg/misc/arraymax.awk @{ if ($1 > max) max = $1 arr[$1] = $0 @} END @{ for (x = 1; x <= max; x++) print arr[x] @} @c endfile @end example The first rule keeps track of the largest line number seen so far; it also stores each line into the array @code{arr}, at an index that is the line's number. The second rule runs after all the input has been read, to print out all the lines. When this program is run with the following input: @example @c file eg/misc/arraymax.data 5 I am the Five man 2 Who are you? The new number two! 4 . . . And four on the floor 1 Who is number one? 3 I three you. @c endfile @end example @noindent Its output is: @example 1 Who is number one? 2 Who are you? The new number two! 3 I three you. 4 . . . And four on the floor 5 I am the Five man @end example If a line number is repeated, the last line with a given number overrides the others. Gaps in the line numbers can be handled with an easy improvement to the program's @code{END} rule, as follows: @example END @{ for (x = 1; x <= max; x++) if (x in arr) print arr[x] @} @end example @node Scanning an Array @section Scanning All Elements of an Array @cindex elements in arrays, scanning @cindex arrays, scanning In programs that use arrays, it is often necessary to use a loop that executes once for each element of an array. In other languages, where arrays are contiguous and indices are limited to positive integers, this is easy: all the valid indices can be found by counting from the lowest index up to the highest. This technique won't do the job in @command{awk}, because any number or string can be an array index. So @command{awk} has a special kind of @code{for} statement for scanning an array: @example for (@var{var} in @var{array}) @var{body} @end example @noindent @cindex @code{in} operator, arrays and This loop executes @var{body} once for each index in @var{array} that the program has previously used, with the variable @var{var} set to that index. @cindex arrays, @code{for} statement and @cindex @code{for} statement, in arrays The following program uses this form of the @code{for} statement. The first rule scans the input records and notes which words appear (at least once) in the input, by storing a one into the array @code{used} with the word as index. The second rule scans the elements of @code{used} to find all the distinct words that appear in the input. It prints each word that is more than 10 characters long and also prints the number of such words. @xref{String Functions}, for more information on the built-in function @code{length}. @example # Record a 1 for each word that is used at least once @{ for (i = 1; i <= NF; i++) used[$i] = 1 @} # Find number of distinct words more than 10 characters long END @{ for (x in used) if (length(x) > 10) @{ ++num_long_words print x @} print num_long_words, "words longer than 10 characters" @} @end example @noindent @xref{Word Sorting}, for a more detailed example of this type. @cindex arrays, elements, order of @cindex elements in arrays, order of The order in which elements of the array are accessed by this statement is determined by the internal arrangement of the array elements within @command{awk} and cannot be controlled or changed. This can lead to problems if new elements are added to @var{array} by statements in the loop body; it is not predictable whether the @code{for} loop will reach them. Similarly, changing @var{var} inside the loop may produce strange results. It is best to avoid such things. @node Delete @section The @code{delete} Statement @cindex @code{delete} statement @cindex deleting elements in arrays @cindex arrays, elements, deleting @cindex elements in arrays, deleting To remove an individual element of an array, use the @code{delete} statement: @example delete @var{array}[@var{index}] @end example Once an array element has been deleted, any value the element once had is no longer available. It is as if the element had never been referred to or had been given a value. The following is an example of deleting elements in an array: @example for (i in frequencies) delete frequencies[i] @end example @noindent This example removes all the elements from the array @code{frequencies}. Once an element is deleted, a subsequent @code{for} statement to scan the array does not report that element and the @code{in} operator to check for the presence of that element returns zero (i.e., false): @example delete foo[4] if (4 in foo) print "This will never be printed" @end example @cindex null strings, array elements and It is important to note that deleting an element is @emph{not} the same as assigning it a null value (the empty string, @code{""}). For example: @example foo[4] = "" if (4 in foo) print "This is printed, even though foo[4] is empty" @end example @cindex lint checking, array elements It is not an error to delete an element that does not exist. If @option{--lint} is provided on the command line (@pxref{Options}), @command{gawk} issues a warning message when an element that is not in the array is deleted. @cindex arrays, deleting entire contents @cindex deleting entire arrays @cindex differences in @command{awk} and @command{gawk}, array elements, deleting All the elements of an array may be deleted with a single statement by leaving off the subscript in the @code{delete} statement, as follows: @example delete @var{array} @end example This ability is a @command{gawk} extension; it is not available in compatibility mode (@pxref{Options}). Using this version of the @code{delete} statement is about three times more efficient than the equivalent loop that deletes each element one at a time. @cindex portability, deleting array elements @cindex Brennan, Michael The following statement provides a portable but nonobvious way to clear out an array:@footnote{Thanks to Michael Brennan for pointing this out.} @example split("", array) @end example @c comma before deleting does NOT start a tertiary @cindex @code{split} function, array elements, deleting The @code{split} function (@pxref{String Functions}) clears out the target array first. This call asks it to split apart the null string. Because there is no data to split out, the function simply clears the array and then returns. @strong{Caution:} Deleting an array does not change its type; you cannot delete an array and then use the array's name as a scalar (i.e., a regular variable). For example, the following does not work: @example a[1] = 3; delete a; a = 3 @end example @node Numeric Array Subscripts @section Using Numbers to Subscript Arrays @cindex numbers, as array subscripts @cindex arrays, subscripts @cindex subscripts in arrays, numbers as @cindex @code{CONVFMT} variable, array subscripts and An important aspect about arrays to remember is that @emph{array subscripts are always strings}. When a numeric value is used as a subscript, it is converted to a string value before being used for subscripting (@pxref{Conversion}). This means that the value of the built-in variable @code{CONVFMT} can affect how your program accesses elements of an array. For example: @example xyz = 12.153 data[xyz] = 1 CONVFMT = "%2.2f" if (xyz in data) printf "%s is in data\n", xyz else printf "%s is not in data\n", xyz @end example @noindent This prints @samp{12.15 is not in data}. The first statement gives @code{xyz} a numeric value. Assigning to @code{data[xyz]} subscripts @code{data} with the string value @code{"12.153"} (using the default conversion value of @code{CONVFMT}, @code{"%.6g"}). Thus, the array element @code{data["12.153"]} is assigned the value one. The program then changes the value of @code{CONVFMT}. The test @samp{(xyz in data)} generates a new string value from @code{xyz}---this time @code{"12.15"}---because the value of @code{CONVFMT} only allows two significant digits. This test fails, since @code{"12.15"} is a different string from @code{"12.153"}. @cindex converting, during subscripting According to the rules for conversions (@pxref{Conversion}), integer values are always converted to strings as integers, no matter what the value of @code{CONVFMT} may happen to be. So the usual case of the following works: @example for (i = 1; i <= maxsub; i++) @i{do something with} array[i] @end example The ``integer values always convert to strings as integers'' rule has an additional consequence for array indexing. Octal and hexadecimal constants (@pxref{Nondecimal-numbers}) are converted internally into numbers, and their original form is forgotten. This means, for example, that @code{array[17]}, @code{array[021]}, and @code{array[0x11]} all refer to the same element! As with many things in @command{awk}, the majority of the time things work as one would expect them to. But it is useful to have a precise knowledge of the actual rules which sometimes can have a subtle effect on your programs. @node Uninitialized Subscripts @section Using Uninitialized Variables as Subscripts @c last comma does NOT start a tertiary @cindex variables, uninitialized, as array subscripts @cindex uninitialized variables, as array subscripts @cindex subscripts in arrays, uninitialized variables as @cindex arrays, subscripts, uninitialized variables as Suppose it's necessary to write a program to print the input data in reverse order. A reasonable attempt to do so (with some test data) might look like this: @example $ echo 'line 1 > line 2 > line 3' | awk '@{ l[lines] = $0; ++lines @} > END @{ > for (i = lines-1; i >= 0; --i) > print l[i] > @}' @print{} line 3 @print{} line 2 @end example Unfortunately, the very first line of input data did not come out in the output! At first glance, this program should have worked. The variable @code{lines} is uninitialized, and uninitialized variables have the numeric value zero. So, @command{awk} should have printed the value of @code{l[0]}. The issue here is that subscripts for @command{awk} arrays are @emph{always} strings. Uninitialized variables, when used as strings, have the value @code{""}, not zero. Thus, @samp{line 1} ends up stored in @code{l[""]}. The following version of the program works correctly: @example @{ l[lines++] = $0 @} END @{ for (i = lines - 1; i >= 0; --i) print l[i] @} @end example Here, the @samp{++} forces @code{lines} to be numeric, thus making the ``old value'' numeric zero. This is then converted to @code{"0"} as the array subscript. @cindex null strings, as array subscripts @cindex dark corner, array subscripts @cindex lint checking, array subscripts Even though it is somewhat unusual, the null string (@code{""}) is a valid array subscript. @value{DARKCORNER} @command{gawk} warns about the use of the null string as a subscript if @option{--lint} is provided on the command line (@pxref{Options}). @node Multi-dimensional @section Multidimensional Arrays @cindex subscripts in arrays, multidimensional @cindex arrays, multidimensional A multidimensional array is an array in which an element is identified by a sequence of indices instead of a single index. For example, a two-dimensional array requires two indices. The usual way (in most languages, including @command{awk}) to refer to an element of a two-dimensional array named @code{grid} is with @code{grid[@var{x},@var{y}]}. @cindex @code{SUBSEP} variable, multidimensional arrays Multidimensional arrays are supported in @command{awk} through concatenation of indices into one string. @command{awk} converts the indices into strings (@pxref{Conversion}) and concatenates them together, with a separator between them. This creates a single string that describes the values of the separate indices. The combined string is used as a single index into an ordinary, one-dimensional array. The separator used is the value of the built-in variable @code{SUBSEP}. For example, suppose we evaluate the expression @samp{foo[5,12] = "value"} when the value of @code{SUBSEP} is @code{"@@"}. The numbers 5 and 12 are converted to strings and concatenated with an @samp{@@} between them, yielding @code{"5@@12"}; thus, the array element @code{foo["5@@12"]} is set to @code{"value"}. Once the element's value is stored, @command{awk} has no record of whether it was stored with a single index or a sequence of indices. The two expressions @samp{foo[5,12]} and @w{@samp{foo[5 SUBSEP 12]}} are always equivalent. The default value of @code{SUBSEP} is the string @code{"\034"}, which contains a nonprinting character that is unlikely to appear in an @command{awk} program or in most input data. The usefulness of choosing an unlikely character comes from the fact that index values that contain a string matching @code{SUBSEP} can lead to combined strings that are ambiguous. Suppose that @code{SUBSEP} is @code{"@@"}; then @w{@samp{foo["a@@b", "c"]}} and @w{@samp{foo["a", "b@@c"]}} are indistinguishable because both are actually stored as @samp{foo["a@@b@@c"]}. To test whether a particular index sequence exists in a multidimensional array, use the same operator (@samp{in}) that is used for single dimensional arrays. Write the whole sequence of indices in parentheses, separated by commas, as the left operand: @example (@var{subscript1}, @var{subscript2}, @dots{}) in @var{array} @end example The following example treats its input as a two-dimensional array of fields; it rotates this array 90 degrees clockwise and prints the result. It assumes that all lines have the same number of elements: @example @{ if (max_nf < NF) max_nf = NF max_nr = NR for (x = 1; x <= NF; x++) vector[x, NR] = $x @} END @{ for (x = 1; x <= max_nf; x++) @{ for (y = max_nr; y >= 1; --y) printf("%s ", vector[x, y]) printf("\n") @} @} @end example @noindent When given the input: @example 1 2 3 4 5 6 2 3 4 5 6 1 3 4 5 6 1 2 4 5 6 1 2 3 @end example @noindent the program produces the following output: @example 4 3 2 1 5 4 3 2 6 5 4 3 1 6 5 4 2 1 6 5 3 2 1 6 @end example @node Multi-scanning @section Scanning Multidimensional Arrays There is no special @code{for} statement for scanning a ``multidimensional'' array. There cannot be one, because, in truth, there are no multidimensional arrays or elements---there is only a multidimensional @emph{way of accessing} an array. @cindex subscripts in arrays, multidimensional, scanning @cindex arrays, multidimensional, scanning However, if your program has an array that is always accessed as multidimensional, you can get the effect of scanning it by combining the scanning @code{for} statement (@pxref{Scanning an Array}) with the built-in @code{split} function (@pxref{String Functions}). It works in the following manner: @example for (combined in array) @{ split(combined, separate, SUBSEP) @dots{} @} @end example @noindent This sets the variable @code{combined} to each concatenated combined index in the array, and splits it into the individual indices by breaking it apart where the value of @code{SUBSEP} appears. The individual indices then become the elements of the array @code{separate}. Thus, if a value is previously stored in @code{array[1, "foo"]}; then an element with index @code{"1\034foo"} exists in @code{array}. (Recall that the default value of @code{SUBSEP} is the character with code 034.) Sooner or later, the @code{for} statement finds that index and does an iteration with the variable @code{combined} set to @code{"1\034foo"}. Then the @code{split} function is called as follows: @example split("1\034foo", separate, "\034") @end example @noindent The result is to set @code{separate[1]} to @code{"1"} and @code{separate[2]} to @code{"foo"}. Presto! The original sequence of separate indices is recovered. @node Array Sorting @section Sorting Array Values and Indices with @command{gawk} @cindex arrays, sorting @cindex @code{asort} function (@command{gawk}) @c last comma does NOT start a tertiary @cindex @code{asort} function (@command{gawk}), arrays, sorting @cindex sort function, arrays, sorting The order in which an array is scanned with a @samp{for (i in array)} loop is essentially arbitrary. In most @command{awk} implementations, sorting an array requires writing a @code{sort} function. While this can be educational for exploring different sorting algorithms, usually that's not the point of the program. @command{gawk} provides the built-in @code{asort} and @code{asorti} functions (@pxref{String Functions}) for sorting arrays. For example: @example @var{populate the array} data n = asort(data) for (i = 1; i <= n; i++) @var{do something with} data[i] @end example After the call to @code{asort}, the array @code{data} is indexed from 1 to some number @var{n}, the total number of elements in @code{data}. (This count is @code{asort}'s return value.) @code{data[1]} @value{LEQ} @code{data[2]} @value{LEQ} @code{data[3]}, and so on. The comparison of array elements is done using @command{gawk}'s usual comparison rules (@pxref{Typing and Comparison}). @cindex side effects, @code{asort} function An important side effect of calling @code{asort} is that @emph{the array's original indices are irrevocably lost}. As this isn't always desirable, @code{asort} accepts a second argument: @example @var{populate the array} source n = asort(source, dest) for (i = 1; i <= n; i++) @var{do something with} dest[i] @end example In this case, @command{gawk} copies the @code{source} array into the @code{dest} array and then sorts @code{dest}, destroying its indices. However, the @code{source} array is not affected. Often, what's needed is to sort on the values of the @emph{indices} instead of the values of the elements. To do that, starting with @command{gawk} 3.1.2, use the @code{asorti} function. The interface is identical to that of @code{asort}, except that the index values are used for sorting, and become the values of the result array: @example @{ source[$0] = some_func($0) @} END @{ n = asorti(source, dest) for (i = 1; i <= n; i++) @var{do something with} dest[i] @} @end example If your version of @command{gawk} is 3.1.0 or 3.1.1, you don't have @code{asorti}. Instead, use a helper array to hold the sorted index values, and then access the original array's elements. It works in the following way: @example @var{populate the array} data # copy indices j = 1 for (i in data) @{ ind[j] = i # index value becomes element value j++ @} n = asort(ind) # index values are now sorted for (i = 1; i <= n; i++) @var{do something with} data[ind[i]] @end example Sorting the array by replacing the indices provides maximal flexibility. To traverse the elements in decreasing order, use a loop that goes from @var{n} down to 1, either over the elements or over the indices. @cindex reference counting, sorting arrays Copying array indices and elements isn't expensive in terms of memory. Internally, @command{gawk} maintains @dfn{reference counts} to data. For example, when @code{asort} copies the first array to the second one, there is only one copy of the original array elements' data, even though both arrays use the values. Similarly, when copying the indices from @code{data} to @code{ind}, there is only one copy of the actual index strings. @c Document It And Call It A Feature. Sigh. @cindex arrays, sorting, @code{IGNORECASE} variable and @cindex @code{IGNORECASE} variable, array sorting and We said previously that comparisons are done using @command{gawk}'s ``usual comparison rules.'' Because @code{IGNORECASE} affects string comparisons, the value of @code{IGNORECASE} also affects sorting for both @code{asort} and @code{asorti}. Caveat Emptor. @c ENDOFRANGE arrs @node Functions @chapter Functions @c STARTOFRANGE funcbi @cindex functions, built-in @c STARTOFRANGE bifunc @cindex built-in functions This @value{CHAPTER} describes @command{awk}'s built-in functions, which fall into three categories: numeric, string, and I/O. @command{gawk} provides additional groups of functions to work with values that represent time, do bit manipulation, and internationalize and localize programs. Besides the built-in functions, @command{awk} has provisions for writing new functions that the rest of a program can use. The second half of this @value{CHAPTER} describes these @dfn{user-defined} functions. @menu * Built-in:: Summarizes the built-in functions. * User-defined:: Describes User-defined functions in detail. @end menu @node Built-in @section Built-in Functions @c 2e: USE TEXINFO-2 FUNCTION DEFINITION STUFF!!!!!!!!!!!!! @dfn{Built-in} functions are always available for your @command{awk} program to call. This @value{SECTION} defines all the built-in functions in @command{awk}; some of these are mentioned in other sections but are summarized here for your convenience. @menu * Calling Built-in:: How to call built-in functions. * Numeric Functions:: Functions that work with numbers, including @code{int}, @code{sin} and @code{rand}. * String Functions:: Functions for string manipulation, such as @code{split}, @code{match} and @code{sprintf}. * I/O Functions:: Functions for files and shell commands. * Time Functions:: Functions for dealing with timestamps. * Bitwise Functions:: Functions for bitwise operations. * I18N Functions:: Functions for string translation. @end menu @node Calling Built-in @subsection Calling Built-in Functions To call one of @command{awk}'s built-in functions, write the name of the function followed by arguments in parentheses. For example, @samp{atan2(y + z, 1)} is a call to the function @code{atan2} and has two arguments. @cindex programming conventions, functions, calling @c last comma does NOT start a tertiary @cindex whitespace, functions, calling Whitespace is ignored between the built-in function name and the open parenthesis, and it is good practice to avoid using whitespace there. User-defined functions do not permit whitespace in this way, and it is easier to avoid mistakes by following a simple convention that always works---no whitespace after a function name. @c last comma is part of tertiary @cindex troubleshooting, @command{gawk}, fatal errors, function arguments @cindex @command{gawk}, function arguments and @cindex differences in @command{awk} and @command{gawk}, function arguments (@command{gawk}) Each built-in function accepts a certain number of arguments. In some cases, arguments can be omitted. The defaults for omitted arguments vary from function to function and are described under the individual functions. In some @command{awk} implementations, extra arguments given to built-in functions are ignored. However, in @command{gawk}, it is a fatal error to give extra arguments to a built-in function. When a function is called, expressions that create the function's actual parameters are evaluated completely before the call is performed. For example, in the following code fragment: @example i = 4 j = sqrt(i++) @end example @cindex evaluation order, functions @cindex functions, built-in, evaluation order @cindex built-in functions, evaluation order @noindent the variable @code{i} is incremented to the value five before @code{sqrt} is called with a value of four for its actual parameter. The order of evaluation of the expressions used for the function's parameters is undefined. Thus, avoid writing programs that assume that parameters are evaluated from left to right or from right to left. For example: @example i = 5 j = atan2(i++, i *= 2) @end example If the order of evaluation is left to right, then @code{i} first becomes 6, and then 12, and @code{atan2} is called with the two arguments 6 and 12. But if the order of evaluation is right to left, @code{i} first becomes 10, then 11, and @code{atan2} is called with the two arguments 11 and 10. @node Numeric Functions @subsection Numeric Functions The following list describes all of the built-in functions that work with numbers. Optional parameters are enclosed in square brackets@w{ ([ ]):} @table @code @item int(@var{x}) @cindex @code{int} function This returns the nearest integer to @var{x}, located between @var{x} and zero and truncated toward zero. For example, @code{int(3)} is 3, @code{int(3.9)} is 3, @code{int(-3.9)} is @minus{}3, and @code{int(-3)} is @minus{}3 as well. @item sqrt(@var{x}) @cindex @code{sqrt} function This returns the positive square root of @var{x}. @command{gawk} reports an error if @var{x} is negative. Thus, @code{sqrt(4)} is 2. @item exp(@var{x}) @cindex @code{exp} function This returns the exponential of @var{x} (@code{e ^ @var{x}}) or reports an error if @var{x} is out of range. The range of values @var{x} can have depends on your machine's floating-point representation. @item log(@var{x}) @cindex @code{log} function This returns the natural logarithm of @var{x}, if @var{x} is positive; otherwise, it reports an error. @item sin(@var{x}) @cindex @code{sin} function This returns the sine of @var{x}, with @var{x} in radians. @item cos(@var{x}) @cindex @code{cos} function This returns the cosine of @var{x}, with @var{x} in radians. @item atan2(@var{y}, @var{x}) @cindex @code{atan2} function This returns the arctangent of @code{@var{y} / @var{x}} in radians. @item rand() @cindex @code{rand} function @cindex random numbers, @code{rand}/@code{srand} functions This returns a random number. The values of @code{rand} are uniformly distributed between zero and one. The value could be zero but is never one.@footnote{The C version of @code{rand} is known to produce fairly poor sequences of random numbers. However, nothing requires that an @command{awk} implementation use the C @code{rand} to implement the @command{awk} version of @code{rand}. In fact, @command{gawk} uses the BSD @code{random} function, which is considerably better than @code{rand}, to produce random numbers.} Often random integers are needed instead. Following is a user-defined function that can be used to obtain a random non-negative integer less than @var{n}: @example function randint(n) @{ return int(n * rand()) @} @end example @noindent The multiplication produces a random number greater than zero and less than @code{n}. Using @code{int}, this result is made into an integer between zero and @code{n} @minus{} 1, inclusive. The following example uses a similar function to produce random integers between one and @var{n}. This program prints a new random number for each input record: @example # Function to roll a simulated die. function roll(n) @{ return 1 + int(rand() * n) @} # Roll 3 six-sided dice and # print total number of points. @{ printf("%d points\n", roll(6)+roll(6)+roll(6)) @} @end example @cindex numbers, random @cindex random numbers, seed of @c MAWK uses a different seed each time. @strong{Caution:} In most @command{awk} implementations, including @command{gawk}, @code{rand} starts generating numbers from the same starting number, or @dfn{seed}, each time you run @command{awk}. Thus, a program generates the same results each time you run it. The numbers are random within one @command{awk} run but predictable from run to run. This is convenient for debugging, but if you want a program to do different things each time it is used, you must change the seed to a value that is different in each run. To do this, use @code{srand}. @item srand(@r{[}@var{x}@r{]}) @cindex @code{srand} function The function @code{srand} sets the starting point, or seed, for generating random numbers to the value @var{x}. Each seed value leads to a particular sequence of random numbers.@footnote{Computer-generated random numbers really are not truly random. They are technically known as ``pseudorandom.'' This means that while the numbers in a sequence appear to be random, you can in fact generate the same sequence of random numbers over and over again.} Thus, if the seed is set to the same value a second time, the same sequence of random numbers is produced again. Different @command{awk} implementations use different random-number generators internally. Don't expect the same @command{awk} program to produce the same series of random numbers when executed by different versions of @command{awk}. If the argument @var{x} is omitted, as in @samp{srand()}, then the current date and time of day are used for a seed. This is the way to get random numbers that are truly unpredictable. The return value of @code{srand} is the previous seed. This makes it easy to keep track of the seeds in case you need to consistently reproduce sequences of random numbers. @end table @node String Functions @subsection String-Manipulation Functions The functions in this @value{SECTION} look at or change the text of one or more strings. Optional parameters are enclosed in square brackets@w{ ([ ]).} Those functions that are specific to @command{gawk} are marked with a pound sign@w{ (@samp{#}):} @menu * Gory Details:: More than you want to know about @samp{\} and @samp{&} with @code{sub}, @code{gsub}, and @code{gensub}. @end menu @table @code @item asort(@var{source} @r{[}, @var{dest}@r{]}) # @cindex arrays, elements, retrieving number of @cindex @code{asort} function (@command{gawk}) @code{asort} is a @command{gawk}-specific extension, returning the number of elements in the array @var{source}. The contents of @var{source} are sorted using @command{gawk}'s normal rules for comparing values (in particular, @code{IGNORECASE} affects the sorting) and the indices of the sorted values of @var{source} are replaced with sequential integers starting with one. If the optional array @var{dest} is specified, then @var{source} is duplicated into @var{dest}. @var{dest} is then sorted, leaving the indices of @var{source} unchanged. For example, if the contents of @code{a} are as follows: @example a["last"] = "de" a["first"] = "sac" a["middle"] = "cul" @end example @noindent A call to @code{asort}: @example asort(a) @end example @noindent results in the following contents of @code{a}: @example a[1] = "cul" a[2] = "de" a[3] = "sac" @end example The @code{asort} function is described in more detail in @ref{Array Sorting}. @code{asort} is a @command{gawk} extension; it is not available in compatibility mode (@pxref{Options}). @item asorti(@var{source} @r{[}, @var{dest}@r{]}) # @cindex @code{asorti} function (@command{gawk}) @code{asorti} is a @command{gawk}-specific extension, returning the number of elements in the array @var{source}. It works similarly to @code{asort}, however, the @emph{indices} are sorted, instead of the values. As array indices are always strings, the comparison performed is always a string comparison. (Here too, @code{IGNORECASE} affects the sorting.) The @code{asorti} function is described in more detail in @ref{Array Sorting}. It was added in @command{gawk} 3.1.2. @code{asorti} is a @command{gawk} extension; it is not available in compatibility mode (@pxref{Options}). @item index(@var{in}, @var{find}) @cindex @code{index} function @cindex searching This searches the string @var{in} for the first occurrence of the string @var{find}, and returns the position in characters where that occurrence begins in the string @var{in}. Consider the following example: @example $ awk 'BEGIN @{ print index("peanut", "an") @}' @print{} 3 @end example @noindent If @var{find} is not found, @code{index} returns zero. (Remember that string indices in @command{awk} start at one.) @item length(@r{[}@var{string}@r{]}) @cindex @code{length} function This returns the number of characters in @var{string}. If @var{string} is a number, the length of the digit string representing that number is returned. For example, @code{length("abcde")} is 5. By contrast, @code{length(15 * 35)} works out to 3. In this example, 15 * 35 = 525, and 525 is then converted to the string @code{"525"}, which has three characters. If no argument is supplied, @code{length} returns the length of @code{$0}. @c @cindex historical features @cindex portability, @code{length} function @cindex POSIX @command{awk}, functions and, @code{length} @strong{Note:} In older versions of @command{awk}, the @code{length} function could be called without any parentheses. Doing so is marked as ``deprecated'' in the POSIX standard. This means that while a program can do this, it is a feature that can eventually be removed from a future version of the standard. Therefore, for programs to be maximally portable, always supply the parentheses. @item match(@var{string}, @var{regexp} @r{[}, @var{array}@r{]}) @cindex @code{match} function The @code{match} function searches @var{string} for the longest, leftmost substring matched by the regular expression, @var{regexp}. It returns the character position, or @dfn{index}, at which that substring begins (one, if it starts at the beginning of @var{string}). If no match is found, it returns zero. The @var{regexp} argument may be either a regexp constant (@samp{/@dots{}/}) or a string constant (@var{"@dots{}"}). In the latter case, the string is treated as a regexp to be matched. @ref{Computed Regexps}, for a discussion of the difference between the two forms, and the implications for writing your program correctly. The order of the first two arguments is backwards from most other string functions that work with regular expressions, such as @code{sub} and @code{gsub}. It might help to remember that for @code{match}, the order is the same as for the @samp{~} operator: @samp{@var{string} ~ @var{regexp}}. @cindex @code{RSTART} variable, @code{match} function and @cindex @code{RLENGTH} variable, @code{match} function and @cindex @code{match} function, @code{RSTART}/@code{RLENGTH} variables The @code{match} function sets the built-in variable @code{RSTART} to the index. It also sets the built-in variable @code{RLENGTH} to the length in characters of the matched substring. If no match is found, @code{RSTART} is set to zero, and @code{RLENGTH} to @minus{}1. For example: @example @c file eg/misc/findpat.awk @{ if ($1 == "FIND") regex = $2 else @{ where = match($0, regex) if (where != 0) print "Match of", regex, "found at", where, "in", $0 @} @} @c endfile @end example @noindent This program looks for lines that match the regular expression stored in the variable @code{regex}. This regular expression can be changed. If the first word on a line is @samp{FIND}, @code{regex} is changed to be the second word on that line. Therefore, if given: @example @c file eg/misc/findpat.data FIND ru+n My program runs but not very quickly FIND Melvin JF+KM This line is property of Reality Engineering Co. Melvin was here. @c endfile @end example @noindent @command{awk} prints: @example Match of ru+n found at 12 in My program runs Match of Melvin found at 1 in Melvin was here. @end example @cindex differences in @command{awk} and @command{gawk}, @code{match} function If @var{array} is present, it is cleared, and then the 0th element of @var{array} is set to the entire portion of @var{string} matched by @var{regexp}. If @var{regexp} contains parentheses, the integer-indexed elements of @var{array} are set to contain the portion of @var{string} matching the corresponding parenthesized subexpression. For example: @example $ echo foooobazbarrrrr | > gawk '@{ match($0, /(fo+).+(bar*)/, arr) > print arr[1], arr[2] @}' @print{} foooo barrrrr @end example In addition, beginning with @command{gawk} 3.1.2, multidimensional subscripts are available providing the start index and length of each matched subexpression: @example $ echo foooobazbarrrrr | > gawk '@{ match($0, /(fo+).+(bar*)/, arr) > print arr[1], arr[2] > print arr[1, "start"], arr[1, "length"] > print arr[2, "start"], arr[2, "length"] > @}' @print{} foooo barrrrr @print{} 1 5 @print{} 9 7 @end example There may not be subscripts for the start and index for every parenthesized subexpressions, since they may not all have matched text; thus they should be tested for with the @code{in} operator (@pxref{Reference to Elements}). @cindex troubleshooting, @code{match} function The @var{array} argument to @code{match} is a @command{gawk} extension. In compatibility mode (@pxref{Options}), using a third argument is a fatal error. @item split(@var{string}, @var{array} @r{[}, @var{fieldsep}@r{]}) @cindex @code{split} function This function divides @var{string} into pieces separated by @var{fieldsep} and stores the pieces in @var{array}. The first piece is stored in @code{@var{array}[1]}, the second piece in @code{@var{array}[2]}, and so forth. The string value of the third argument, @var{fieldsep}, is a regexp describing where to split @var{string} (much as @code{FS} can be a regexp describing where to split input records). If @var{fieldsep} is omitted, the value of @code{FS} is used. @code{split} returns the number of elements created. The @code{split} function splits strings into pieces in a manner similar to the way input lines are split into fields. For example: @example split("cul-de-sac", a, "-") @end example @noindent @cindex strings, splitting splits the string @samp{cul-de-sac} into three fields using @samp{-} as the separator. It sets the contents of the array @code{a} as follows: @example a[1] = "cul" a[2] = "de" a[3] = "sac" @end example @noindent The value returned by this call to @code{split} is three. @cindex differences in @command{awk} and @command{gawk}, @code{split} function As with input field-splitting, when the value of @var{fieldsep} is @w{@code{" "}}, leading and trailing whitespace is ignored, and the elements are separated by runs of whitespace. Also as with input field-splitting, if @var{fieldsep} is the null string, each individual character in the string is split into its own array element. (This is a @command{gawk}-specific extension.) Note, however, that @code{RS} has no effect on the way @code{split} works. Even though @samp{RS = ""} causes newline to also be an input field separator, this does not affect how @code{split} splits strings. @cindex dark corner, @code{split} function Modern implementations of @command{awk}, including @command{gawk}, allow the third argument to be a regexp constant (@code{/abc/}) as well as a string. @value{DARKCORNER} The POSIX standard allows this as well. @ref{Computed Regexps}, for a discussion of the difference between using a string constant or a regexp constant, and the implications for writing your program correctly. Before splitting the string, @code{split} deletes any previously existing elements in the array @var{array}. If @var{string} is null, the array has no elements. (So this is a portable way to delete an entire array with one statement. @xref{Delete}.) If @var{string} does not match @var{fieldsep} at all (but is not null), @var{array} has one element only. The value of that element is the original @var{string}. @item sprintf(@var{format}, @var{expression1}, @dots{}) @cindex @code{sprintf} function This returns (without printing) the string that @code{printf} would have printed out with the same arguments (@pxref{Printf}). For example: @example pival = sprintf("pi = %.2f (approx.)", 22/7) @end example @noindent assigns the string @w{@code{"pi = 3.14 (approx.)"}} to the variable @code{pival}. @cindex differences in @command{awk} and @command{gawk}, @code{strtonum} function (@command{gawk}) @cindex @code{strtonum} function (@command{gawk}) @item strtonum(@var{str}) # Examines @var{str} and returns its numeric value. If @var{str} begins with a leading @samp{0}, @code{strtonum} assumes that @var{str} is an octal number. If @var{str} begins with a leading @samp{0x} or @samp{0X}, @code{strtonum} assumes that @var{str} is a hexadecimal number. For example: @example $ echo 0x11 | > gawk '@{ printf "%d\n", strtonum($1) @}' @print{} 17 @end example Using the @code{strtonum} function is @emph{not} the same as adding zero to a string value; the automatic coercion of strings to numbers works only for decimal data, not for octal or hexadecimal.@footnote{Unless you use the @option{--non-decimal-data} option, which isn't recommended. @xref{Nondecimal Data}, for more information.} @cindex differences in @command{awk} and @command{gawk}, @code{strtonum} function (@command{gawk}) @code{strtonum} is a @command{gawk} extension; it is not available in compatibility mode (@pxref{Options}). @item sub(@var{regexp}, @var{replacement} @r{[}, @var{target}@r{]}) @cindex @code{sub} function The @code{sub} function alters the value of @var{target}. It searches this value, which is treated as a string, for the leftmost, longest substring matched by the regular expression @var{regexp}. Then the entire string is changed by replacing the matched text with @var{replacement}. The modified string becomes the new value of @var{target}. The @var{regexp} argument may be either a regexp constant (@samp{/@dots{}/}) or a string constant (@var{"@dots{}"}). In the latter case, the string is treated as a regexp to be matched. @ref{Computed Regexps}, for a discussion of the difference between the two forms, and the implications for writing your program correctly. This function is peculiar because @var{target} is not simply used to compute a value, and not just any expression will do---it must be a variable, field, or array element so that @code{sub} can store a modified value there. If this argument is omitted, then the default is to use and alter @code{$0}.@footnote{Note that this means that the record will first be regenerated using the value of @code{OFS} if any fields have been changed, and that the fields will be updated after the substituion, even if the operation is a ``no-op'' such as @samp{sub(/^/, "")}.} For example: @example str = "water, water, everywhere" sub(/at/, "ith", str) @end example @noindent sets @code{str} to @w{@code{"wither, water, everywhere"}}, by replacing the leftmost longest occurrence of @samp{at} with @samp{ith}. The @code{sub} function returns the number of substitutions made (either one or zero). If the special character @samp{&} appears in @var{replacement}, it stands for the precise substring that was matched by @var{regexp}. (If the regexp can match more than one string, then this precise substring may vary.) For example: @example @{ sub(/candidate/, "& and his wife"); print @} @end example @noindent changes the first occurrence of @samp{candidate} to @samp{candidate and his wife} on each input line. Here is another example: @example $ awk 'BEGIN @{ > str = "daabaaa" > sub(/a+/, "C&C", str) > print str > @}' @print{} dCaaCbaaa @end example @noindent This shows how @samp{&} can represent a nonconstant string and also illustrates the ``leftmost, longest'' rule in regexp matching (@pxref{Leftmost Longest}). The effect of this special character (@samp{&}) can be turned off by putting a backslash before it in the string. As usual, to insert one backslash in the string, you must write two backslashes. Therefore, write @samp{\\&} in a string constant to include a literal @samp{&} in the replacement. For example, the following shows how to replace the first @samp{|} on each line with an @samp{&}: @example @{ sub(/\|/, "\\&"); print @} @end example @cindex @code{sub} function, arguments of @cindex @code{gsub} function, arguments of As mentioned, the third argument to @code{sub} must be a variable, field or array reference. Some versions of @command{awk} allow the third argument to be an expression that is not an lvalue. In such a case, @code{sub} still searches for the pattern and returns zero or one, but the result of the substitution (if any) is thrown away because there is no place to put it. Such versions of @command{awk} accept expressions such as the following: @example sub(/USA/, "United States", "the USA and Canada") @end example @noindent @cindex troubleshooting, @code{gsub}/@code{sub} functions For historical compatibility, @command{gawk} accepts erroneous code, such as in the previous example. However, using any other nonchangeable object as the third parameter causes a fatal error and your program will not run. Finally, if the @var{regexp} is not a regexp constant, it is converted into a string, and then the value of that string is treated as the regexp to match. @item gsub(@var{regexp}, @var{replacement} @r{[}, @var{target}@r{]}) @cindex @code{gsub} function This is similar to the @code{sub} function, except @code{gsub} replaces @emph{all} of the longest, leftmost, @emph{nonoverlapping} matching substrings it can find. The @samp{g} in @code{gsub} stands for ``global,'' which means replace everywhere. For example: @example @{ gsub(/Britain/, "United Kingdom"); print @} @end example @noindent replaces all occurrences of the string @samp{Britain} with @samp{United Kingdom} for all input records. The @code{gsub} function returns the number of substitutions made. If the variable to search and alter (@var{target}) is omitted, then the entire input record (@code{$0}) is used. As in @code{sub}, the characters @samp{&} and @samp{\} are special, and the third argument must be assignable. @item gensub(@var{regexp}, @var{replacement}, @var{how} @r{[}, @var{target}@r{]}) # @cindex @code{gensub} function (@command{gawk}) @code{gensub} is a general substitution function. Like @code{sub} and @code{gsub}, it searches the target string @var{target} for matches of the regular expression @var{regexp}. Unlike @code{sub} and @code{gsub}, the modified string is returned as the result of the function and the original target string is @emph{not} changed. If @var{how} is a string beginning with @samp{g} or @samp{G}, then it replaces all matches of @var{regexp} with @var{replacement}. Otherwise, @var{how} is treated as a number that indicates which match of @var{regexp} to replace. If no @var{target} is supplied, @code{$0} is used. @code{gensub} provides an additional feature that is not available in @code{sub} or @code{gsub}: the ability to specify components of a regexp in the replacement text. This is done by using parentheses in the regexp to mark the components and then specifying @samp{\@var{N}} in the replacement text, where @var{N} is a digit from 1 to 9. For example: @example $ gawk ' > BEGIN @{ > a = "abc def" > b = gensub(/(.+) (.+)/, "\\2 \\1", "g", a) > print b > @}' @print{} def abc @end example @noindent As with @code{sub}, you must type two backslashes in order to get one into the string. In the replacement text, the sequence @samp{\0} represents the entire matched text, as does the character @samp{&}. The following example shows how you can use the third argument to control which match of the regexp should be changed: @example $ echo a b c a b c | > gawk '@{ print gensub(/a/, "AA", 2) @}' @print{} a b c AA b c @end example In this case, @code{$0} is used as the default target string. @code{gensub} returns the new string as its result, which is passed directly to @code{print} for printing. @c @cindex automatic warnings @c @cindex warnings, automatic If the @var{how} argument is a string that does not begin with @samp{g} or @samp{G}, or if it is a number that is less than or equal to zero, only one substitution is performed. If @var{how} is zero, @command{gawk} issues a warning message. If @var{regexp} does not match @var{target}, @code{gensub}'s return value is the original unchanged value of @var{target}. @code{gensub} is a @command{gawk} extension; it is not available in compatibility mode (@pxref{Options}). @item substr(@var{string}, @var{start} @r{[}, @var{length}@r{]}) @cindex @code{substr} function This returns a @var{length}-character-long substring of @var{string}, starting at character number @var{start}. The first character of a string is character number one.@footnote{This is different from C and C++, in which the first character is number zero.} For example, @code{substr("washington", 5, 3)} returns @code{"ing"}. If @var{length} is not present, this function returns the whole suffix of @var{string} that begins at character number @var{start}. For example, @code{substr("washington", 5)} returns @code{"ington"}. The whole suffix is also returned if @var{length} is greater than the number of characters remaining in the string, counting from character @var{start}. If @var{start} is less than one, @code{substr} treats it as if it was one. (POSIX doesn't specify what to do in this case: Unix @command{awk} acts this way, and therefore @command{gawk} does too.) If @var{start} is greater than the number of characters in the string, @code{substr} returns the null string. Similarly, if @var{length} is present but less than or equal to zero, the null string is returned. @cindex troubleshooting, @code{substr} function The string returned by @code{substr} @emph{cannot} be assigned. Thus, it is a mistake to attempt to change a portion of a string, as shown in the following example: @example string = "abcdef" # try to get "abCDEf", won't work substr(string, 3, 3) = "CDE" @end example @noindent It is also a mistake to use @code{substr} as the third argument of @code{sub} or @code{gsub}: @example gsub(/xyz/, "pdq", substr($0, 5, 20)) # WRONG @end example @cindex portability, @code{substr} function (Some commercial versions of @command{awk} do in fact let you use @code{substr} this way, but doing so is not portable.) If you need to replace bits and pieces of a string, combine @code{substr} with string concatenation, in the following manner: @example string = "abcdef" @dots{} string = substr(string, 1, 2) "CDE" substr(string, 6) @end example @cindex case sensitivity, converting case @cindex converting, case @item tolower(@var{string}) @cindex @code{tolower} function This returns a copy of @var{string}, with each uppercase character in the string replaced with its corresponding lowercase character. Nonalphabetic characters are left unchanged. For example, @code{tolower("MiXeD cAsE 123")} returns @code{"mixed case 123"}. @item toupper(@var{string}) @cindex @code{toupper} function This returns a copy of @var{string}, with each lowercase character in the string replaced with its corresponding uppercase character. Nonalphabetic characters are left unchanged. For example, @code{toupper("MiXeD cAsE 123")} returns @code{"MIXED CASE 123"}. @end table @node Gory Details @subsubsection More About @samp{\} and @samp{&} with @code{sub}, @code{gsub}, and @code{gensub} @cindex escape processing, @code{gsub}/@code{gensub}/@code{sub} functions @cindex @code{sub} function, escape processing @cindex @code{gsub} function, escape processing @cindex @code{gensub} function (@command{gawk}), escape processing @cindex @code{\} (backslash), @code{gsub}/@code{gensub}/@code{sub} functions and @cindex backslash (@code{\}), @code{gsub}/@code{gensub}/@code{sub} functions and @cindex @code{&} (ampersand), @code{gsub}/@code{gensub}/@code{sub} functions and @cindex ampersand (@code{&}), @code{gsub}/@code{gensub}/@code{sub} functions and When using @code{sub}, @code{gsub}, or @code{gensub}, and trying to get literal backslashes and ampersands into the replacement text, you need to remember that there are several levels of @dfn{escape processing} going on. First, there is the @dfn{lexical} level, which is when @command{awk} reads your program and builds an internal copy of it that can be executed. Then there is the runtime level, which is when @command{awk} actually scans the replacement string to determine what to generate. At both levels, @command{awk} looks for a defined set of characters that can come after a backslash. At the lexical level, it looks for the escape sequences listed in @ref{Escape Sequences}. Thus, for every @samp{\} that @command{awk} processes at the runtime level, type two backslashes at the lexical level. When a character that is not valid for an escape sequence follows the @samp{\}, Unix @command{awk} and @command{gawk} both simply remove the initial @samp{\} and put the next character into the string. Thus, for example, @code{"a\qb"} is treated as @code{"aqb"}. At the runtime level, the various functions handle sequences of @samp{\} and @samp{&} differently. The situation is (sadly) somewhat complex. Historically, the @code{sub} and @code{gsub} functions treated the two character sequence @samp{\&} specially; this sequence was replaced in the generated text with a single @samp{&}. Any other @samp{\} within the @var{replacement} string that did not precede an @samp{&} was passed through unchanged. To illustrate with a table: @c Thank to Karl Berry for help with the TeX stuff. @tex \vbox{\bigskip % This table has lots of &'s and \'s, so unspecialize them. \catcode`\& = \other \catcode`\\ = \other % But then we need character for escape and tab. @catcode`! = 4 @halign{@hfil#!@qquad@hfil#!@qquad#@hfil@cr You type!@code{sub} sees!@code{sub} generates@cr @hrulefill!@hrulefill!@hrulefill@cr @code{\&}! @code{&}!the matched text@cr @code{\\&}! @code{\&}!a literal @samp{&}@cr @code{\\\&}! @code{\&}!a literal @samp{&}@cr @code{\\\\&}! @code{\\&}!a literal @samp{\&}@cr @code{\\\\\&}! @code{\\&}!a literal @samp{\&}@cr @code{\\\\\\&}! @code{\\\&}!a literal @samp{\\&}@cr @code{\\q}! @code{\q}!a literal @samp{\q}@cr } @bigskip} @end tex @ifnottex @display You type @code{sub} sees @code{sub} generates -------- ---------- --------------- @code{\&} @code{&} the matched text @code{\\&} @code{\&} a literal @samp{&} @code{\\\&} @code{\&} a literal @samp{&} @code{\\\\&} @code{\\&} a literal @samp{\&} @code{\\\\\&} @code{\\&} a literal @samp{\&} @code{\\\\\\&} @code{\\\&} a literal @samp{\\&} @code{\\q} @code{\q} a literal @samp{\q} @end display @end ifnottex @noindent This table shows both the lexical-level processing, where an odd number of backslashes becomes an even number at the runtime level, as well as the runtime processing done by @code{sub}. (For the sake of simplicity, the rest of the following tables only show the case of even numbers of backslashes entered at the lexical level.) The problem with the historical approach is that there is no way to get a literal @samp{\} followed by the matched text. @c @cindex @command{awk} language, POSIX version @cindex POSIX @command{awk}, functions and, @code{gsub}/@code{sub} The 1992 POSIX standard attempted to fix this problem. The standard says that @code{sub} and @code{gsub} look for either a @samp{\} or an @samp{&} after the @samp{\}. If either one follows a @samp{\}, that character is output literally. The interpretation of @samp{\} and @samp{&} then becomes: @c thanks to Karl Berry for formatting this table @tex \vbox{\bigskip % This table has lots of &'s and \'s, so unspecialize them. \catcode`\& = \other \catcode`\\ = \other % But then we need character for escape and tab. @catcode`! = 4 @halign{@hfil#!@qquad@hfil#!@qquad#@hfil@cr You type!@code{sub} sees!@code{sub} generates@cr @hrulefill!@hrulefill!@hrulefill@cr @code{&}! @code{&}!the matched text@cr @code{\\&}! @code{\&}!a literal @samp{&}@cr @code{\\\\&}! @code{\\&}!a literal @samp{\}, then the matched text@cr @code{\\\\\\&}! @code{\\\&}!a literal @samp{\&}@cr } @bigskip} @end tex @ifnottex @display You type @code{sub} sees @code{sub} generates -------- ---------- --------------- @code{&} @code{&} the matched text @code{\\&} @code{\&} a literal @samp{&} @code{\\\\&} @code{\\&} a literal @samp{\}, then the matched text @code{\\\\\\&} @code{\\\&} a literal @samp{\&} @end display @end ifnottex @noindent This appears to solve the problem. Unfortunately, the phrasing of the standard is unusual. It says, in effect, that @samp{\} turns off the special meaning of any following character, but for anything other than @samp{\} and @samp{&}, such special meaning is undefined. This wording leads to two problems: @itemize @bullet @item Backslashes must now be doubled in the @var{replacement} string, breaking historical @command{awk} programs. @item To make sure that an @command{awk} program is portable, @emph{every} character in the @var{replacement} string must be preceded with a backslash.@footnote{This consequence was certainly unintended.} @c I can say that, 'cause I was involved in making this change @end itemize The POSIX standard is under revision. Because of the problems just listed, proposed text for the revised standard reverts to rules that correspond more closely to the original existing practice. The proposed rules have special cases that make it possible to produce a @samp{\} preceding the matched text: @tex \vbox{\bigskip % This table has lots of &'s and \'s, so unspecialize them. \catcode`\& = \other \catcode`\\ = \other % But then we need character for escape and tab. @catcode`! = 4 @halign{@hfil#!@qquad@hfil#!@qquad#@hfil@cr You type!@code{sub} sees!@code{sub} generates@cr @hrulefill!@hrulefill!@hrulefill@cr @code{\\\\\\&}! @code{\\\&}!a literal @samp{\&}@cr @code{\\\\&}! @code{\\&}!a literal @samp{\}, followed by the matched text@cr @code{\\&}! @code{\&}!a literal @samp{&}@cr @code{\\q}! @code{\q}!a literal @samp{\q}@cr } @bigskip} @end tex @ifinfo @display You type @code{sub} sees @code{sub} generates -------- ---------- --------------- @code{\\\\\\&} @code{\\\&} a literal @samp{\&} @code{\\\\&} @code{\\&} a literal @samp{\}, followed by the matched text @code{\\&} @code{\&} a literal @samp{&} @code{\\q} @code{\q} a literal @samp{\q} @end display @end ifinfo In a nutshell, at the runtime level, there are now three special sequences of characters (@samp{\\\&}, @samp{\\&} and @samp{\&}) whereas historically there was only one. However, as in the historical case, any @samp{\} that is not part of one of these three sequences is not special and appears in the output literally. @command{gawk} 3.0 and 3.1 follow these proposed POSIX rules for @code{sub} and @code{gsub}. @c As much as we think it's a lousy idea. You win some, you lose some. Sigh. Whether these proposed rules will actually become codified into the standard is unknown at this point. Subsequent @command{gawk} releases will track the standard and implement whatever the final version specifies; this @value{DOCUMENT} will be updated as well.@footnote{As this @value{DOCUMENT} was being finalized, we learned that the POSIX standard will not use these rules. However, it was too late to change @command{gawk} for the 3.1 release. @command{gawk} behaves as described here.} The rules for @code{gensub} are considerably simpler. At the runtime level, whenever @command{gawk} sees a @samp{\}, if the following character is a digit, then the text that matched the corresponding parenthesized subexpression is placed in the generated output. Otherwise, no matter what character follows the @samp{\}, it appears in the generated text and the @samp{\} does not: @tex \vbox{\bigskip % This table has lots of &'s and \'s, so unspecialize them. \catcode`\& = \other \catcode`\\ = \other % But then we need character for escape and tab. @catcode`! = 4 @halign{@hfil#!@qquad@hfil#!@qquad#@hfil@cr You type!@code{gensub} sees!@code{gensub} generates@cr @hrulefill!@hrulefill!@hrulefill@cr @code{&}! @code{&}!the matched text@cr @code{\\&}! @code{\&}!a literal @samp{&}@cr @code{\\\\}! @code{\\}!a literal @samp{\}@cr @code{\\\\&}! @code{\\&}!a literal @samp{\}, then the matched text@cr @code{\\\\\\&}! @code{\\\&}!a literal @samp{\&}@cr @code{\\q}! @code{\q}!a literal @samp{q}@cr } @bigskip} @end tex @ifnottex @display You type @code{gensub} sees @code{gensub} generates -------- ------------- ------------------ @code{&} @code{&} the matched text @code{\\&} @code{\&} a literal @samp{&} @code{\\\\} @code{\\} a literal @samp{\} @code{\\\\&} @code{\\&} a literal @samp{\}, then the matched text @code{\\\\\\&} @code{\\\&} a literal @samp{\&} @code{\\q} @code{\q} a literal @samp{q} @end display @end ifnottex Because of the complexity of the lexical and runtime level processing and the special cases for @code{sub} and @code{gsub}, we recommend the use of @command{gawk} and @code{gensub} when you have to do substitutions. @c fakenode --- for prepinfo @subheading Advanced Notes: Matching the Null String @c last comma does NOT start tertiary @cindex advanced features, null strings, matching @cindex matching, null strings @cindex null strings, matching @c last comma in next two is part of tertiary @cindex @code{*} (asterisk), @code{*} operator, null strings, matching @cindex asterisk (@code{*}), @code{*} operator, null strings, matching In @command{awk}, the @samp{*} operator can match the null string. This is particularly important for the @code{sub}, @code{gsub}, and @code{gensub} functions. For example: @example $ echo abc | awk '@{ gsub(/m*/, "X"); print @}' @print{} XaXbXcX @end example @noindent Although this makes a certain amount of sense, it can be surprising. @node I/O Functions @subsection Input/Output Functions The following functions relate to input/output (I/O). Optional parameters are enclosed in square brackets ([ ]): @table @code @item close(@var{filename} @r{[}, @var{how}@r{]}) @cindex @code{close} function @cindex files, closing Close the file @var{filename} for input or output. Alternatively, the argument may be a shell command that was used for creating a coprocess, or for redirecting to or from a pipe; then the coprocess or pipe is closed. @xref{Close Files And Pipes}, for more information. When closing a coprocess, it is occasionally useful to first close one end of the two-way pipe and then to close the other. This is done by providing a second argument to @code{close}. This second argument should be one of the two string values @code{"to"} or @code{"from"}, indicating which end of the pipe to close. Case in the string does not matter. @xref{Two-way I/O}, which discusses this feature in more detail and gives an example. @item fflush(@r{[}@var{filename}@r{]}) @cindex @code{fflush} function Flush any buffered output associated with @var{filename}, which is either a file opened for writing or a shell command for redirecting output to a pipe or coprocess. @cindex portability, @code{fflush} function and @cindex buffers, flushing @cindex output, buffering Many utility programs @dfn{buffer} their output; i.e., they save information to write to a disk file or terminal in memory until there is enough for it to be worthwhile to send the data to the output device. This is often more efficient than writing every little bit of information as soon as it is ready. However, sometimes it is necessary to force a program to @dfn{flush} its buffers; that is, write the information to its destination, even if a buffer is not full. This is the purpose of the @code{fflush} function---@command{gawk} also buffers its output and the @code{fflush} function forces @command{gawk} to flush its buffers. @code{fflush} was added to the Bell Laboratories research version of @command{awk} in 1994; it is not part of the POSIX standard and is not available if @option{--posix} has been specified on the command line (@pxref{Options}). @cindex @command{gawk}, @code{fflush} function in @command{gawk} extends the @code{fflush} function in two ways. The first is to allow no argument at all. In this case, the buffer for the standard output is flushed. The second is to allow the null string (@w{@code{""}}) as the argument. In this case, the buffers for @emph{all} open output files and pipes are flushed. @c @cindex automatic warnings @c @cindex warnings, automatic @cindex troubleshooting, @code{fflush} function @code{fflush} returns zero if the buffer is successfully flushed; otherwise, it returns @minus{}1. In the case where all buffers are flushed, the return value is zero only if all buffers were flushed successfully. Otherwise, it is @minus{}1, and @command{gawk} warns about the problem @var{filename}. @command{gawk} also issues a warning message if you attempt to flush a file or pipe that was opened for reading (such as with @code{getline}), or if @var{filename} is not an open file, pipe, or coprocess. In such a case, @code{fflush} returns @minus{}1, as well. @item system(@var{command}) @cindex @code{system} function @cindex interacting with other programs Executes operating-system commands and then returns to the @command{awk} program. The @code{system} function executes the command given by the string @var{command}. It returns the status returned by the command that was executed as its value. For example, if the following fragment of code is put in your @command{awk} program: @example END @{ system("date | mail -s 'awk run done' root") @} @end example @noindent the system administrator is sent mail when the @command{awk} program finishes processing input and begins its end-of-input processing. Note that redirecting @code{print} or @code{printf} into a pipe is often enough to accomplish your task. If you need to run many commands, it is more efficient to simply print them down a pipeline to the shell: @example while (@var{more stuff to do}) print @var{command} | "/bin/sh" close("/bin/sh") @end example @noindent @cindex troubleshooting, @code{system} function However, if your @command{awk} program is interactive, @code{system} is useful for cranking up large self-contained programs, such as a shell or an editor. Some operating systems cannot implement the @code{system} function. @code{system} causes a fatal error if it is not supported. @end table @c fakenode --- for prepinfo @subheading Advanced Notes: Interactive Versus Noninteractive Buffering @cindex advanced features, buffering @cindex buffering, interactive vs. noninteractive As a side point, buffering issues can be even more confusing, depending upon whether your program is @dfn{interactive}, i.e., communicating with a user sitting at a keyboard.@footnote{A program is interactive if the standard output is connected to a terminal device.} @c Thanks to Walter.Mecky@dresdnerbank.de for this example, and for @c motivating me to write this section. Interactive programs generally @dfn{line buffer} their output; i.e., they write out every line. Noninteractive programs wait until they have a full buffer, which may be many lines of output. Here is an example of the difference: @example $ awk '@{ print $1 + $2 @}' 1 1 @print{} 2 2 3 @print{} 5 @kbd{@value{CTL}-d} @end example @noindent Each line of output is printed immediately. Compare that behavior with this example: @example $ awk '@{ print $1 + $2 @}' | cat 1 1 2 3 @kbd{@value{CTL}-d} @print{} 2 @print{} 5 @end example @noindent Here, no output is printed until after the @kbd{@value{CTL}-d} is typed, because it is all buffered and sent down the pipe to @command{cat} in one shot. @c fakenode --- for prepinfo @subheading Advanced Notes: Controlling Output Buffering with @code{system} @cindex advanced features, buffering @cindex buffers, flushing @cindex buffering, input/output @cindex output, buffering The @code{fflush} function provides explicit control over output buffering for individual files and pipes. However, its use is not portable to many other @command{awk} implementations. An alternative method to flush output buffers is to call @code{system} with a null string as its argument: @example system("") # flush output @end example @noindent @command{gawk} treats this use of the @code{system} function as a special case and is smart enough not to run a shell (or other command interpreter) with the empty command. Therefore, with @command{gawk}, this idiom is not only useful, it is also efficient. While this method should work with other @command{awk} implementations, it does not necessarily avoid starting an unnecessary shell. (Other implementations may only flush the buffer associated with the standard output and not necessarily all buffered output.) If you think about what a programmer expects, it makes sense that @code{system} should flush any pending output. The following program: @example BEGIN @{ print "first print" system("echo system echo") print "second print" @} @end example @noindent must print: @example first print system echo second print @end example @noindent and not: @example system echo first print second print @end example If @command{awk} did not flush its buffers before calling @code{system}, you would see the latter (undesirable) output. @node Time Functions @subsection Using @command{gawk}'s Timestamp Functions @c STARTOFRANGE tst @cindex timestamps @c STARTOFRANGE logftst @cindex log files, timestamps in @c last comma does NOT start tertiary @c STARTOFRANGE filogtst @cindex files, log, timestamps in @c STARTOFRANGE gawtst @cindex @command{gawk}, timestamps @cindex POSIX @command{awk}, timestamps and @code{awk} programs are commonly used to process log files containing timestamp information, indicating when a particular log record was written. Many programs log their timestamp in the form returned by the @code{time} system call, which is the number of seconds since a particular epoch. On POSIX-compliant systems, it is the number of seconds since 1970-01-01 00:00:00 UTC, not counting leap seconds.@footnote{@xref{Glossary}, especially the entries ``Epoch'' and ``UTC.''} All known POSIX-compliant systems support timestamps from 0 through @math{2^31 - 1}, which is sufficient to represent times through 2038-01-19 03:14:07 UTC. Many systems support a wider range of timestamps, including negative timestamps that represent times before the epoch. @cindex @command{date} utility, GNU @cindex time, retrieving In order to make it easier to process such log files and to produce useful reports, @command{gawk} provides the following functions for working with timestamps. They are @command{gawk} extensions; they are not specified in the POSIX standard, nor are they in any other known version of @command{awk}.@footnote{The GNU @command{date} utility can also do many of the things described here. Its use may be preferable for simple time-related operations in shell scripts.} Optional parameters are enclosed in square brackets ([ ]): @table @code @item systime() @cindex @code{systime} function (@command{gawk}) @cindex timestamps This function returns the current time as the number of seconds since the system epoch. On POSIX systems, this is the number of seconds since 1970-01-01 00:00:00 UTC, not counting leap seconds. It may be a different number on other systems. @item mktime(@var{datespec}) @cindex @code{mktime} function (@command{gawk}) This function turns @var{datespec} into a timestamp in the same form as is returned by @code{systime}. It is similar to the function of the same name in ISO C. The argument, @var{datespec}, is a string of the form @w{@code{"@var{YYYY} @var{MM} @var{DD} @var{HH} @var{MM} @var{SS} [@var{DST}]"}}. The string consists of six or seven numbers representing, respectively, the full year including century, the month from 1 to 12, the day of the month from 1 to 31, the hour of the day from 0 to 23, the minute from 0 to 59, the second from 0 to 60,@footnote{Occasionally there are minutes in a year with a leap second, which is why the seconds can go up to 60.} and an optional daylight-savings flag. The values of these numbers need not be within the ranges specified; for example, an hour of @minus{}1 means 1 hour before midnight. The origin-zero Gregorian calendar is assumed, with year 0 preceding year 1 and year @minus{}1 preceding year 0. The time is assumed to be in the local timezone. If the daylight-savings flag is positive, the time is assumed to be daylight savings time; if zero, the time is assumed to be standard time; and if negative (the default), @code{mktime} attempts to determine whether daylight savings time is in effect for the specified time. If @var{datespec} does not contain enough elements or if the resulting time is out of range, @code{mktime} returns @minus{}1. @item strftime(@r{[}@var{format} @r{[}, @var{timestamp}@r{]]}) @c STARTOFRANGE strf @cindex @code{strftime} function (@command{gawk}) This function returns a string. It is similar to the function of the same name in ISO C. The time specified by @var{timestamp} is used to produce a string, based on the contents of the @var{format} string. The @var{timestamp} is in the same format as the value returned by the @code{systime} function. If no @var{timestamp} argument is supplied, @command{gawk} uses the current time of day as the timestamp. If no @var{format} argument is supplied, @code{strftime} uses @code{@w{"%a %b %d %H:%M:%S %Z %Y"}}. This format string produces output that is (almost) equivalent to that of the @command{date} utility. (Versions of @command{gawk} prior to 3.0 require the @var{format} argument.) @end table The @code{systime} function allows you to compare a timestamp from a log file with the current time of day. In particular, it is easy to determine how long ago a particular record was logged. It also allows you to produce log records using the ``seconds since the epoch'' format. @cindex converting, dates to timestamps @cindex dates, converting to timestamps @cindex timestamps, converting dates to The @code{mktime} function allows you to convert a textual representation of a date and time into a timestamp. This makes it easy to do before/after comparisons of dates and times, particularly when dealing with date and time data coming from an external source, such as a log file. The @code{strftime} function allows you to easily turn a timestamp into human-readable information. It is similar in nature to the @code{sprintf} function (@pxref{String Functions}), in that it copies nonformat specification characters verbatim to the returned string, while substituting date and time values for format specifications in the @var{format} string. @cindex format specifiers, @code{strftime} function (@command{gawk}) @code{strftime} is guaranteed by the 1999 ISO C standard@footnote{As this is a recent standard, not every system's @code{strftime} necessarily supports all of the conversions listed here.} to support the following date format specifications: @table @code @item %a The locale's abbreviated weekday name. @item %A The locale's full weekday name. @item %b The locale's abbreviated month name. @item %B The locale's full month name. @item %c The locale's ``appropriate'' date and time representation. (This is @samp{%A %B %d %T %Y} in the @code{"C"} locale.) @item %C The century. This is the year divided by 100 and truncated to the next lower integer. @item %d The day of the month as a decimal number (01--31). @item %D Equivalent to specifying @samp{%m/%d/%y}. @item %e The day of the month, padded with a space if it is only one digit. @item %F Equivalent to specifying @samp{%Y-%m-%d}. This is the ISO 8601 date format. @item %g The year modulo 100 of the ISO week number, as a decimal number (00--99). For example, January 1, 1993 is in week 53 of 1992. Thus, the year of its ISO week number is 1992, even though its year is 1993. Similarly, December 31, 1973 is in week 1 of 1974. Thus, the year of its ISO week number is 1974, even though its year is 1973. @item %G The full year of the ISO week number, as a decimal number. @item %h Equivalent to @samp{%b}. @item %H The hour (24-hour clock) as a decimal number (00--23). @item %I The hour (12-hour clock) as a decimal number (01--12). @item %j The day of the year as a decimal number (001--366). @item %m The month as a decimal number (01--12). @item %M The minute as a decimal number (00--59). @item %n A newline character (ASCII LF). @item %p The locale's equivalent of the AM/PM designations associated with a 12-hour clock. @item %r The locale's 12-hour clock time. (This is @samp{%I:%M:%S %p} in the @code{"C"} locale.) @item %R Equivalent to specifying @samp{%H:%M}. @item %S The second as a decimal number (00--60). @item %t A TAB character. @item %T Equivalent to specifying @samp{%H:%M:%S}. @item %u The weekday as a decimal number (1--7). Monday is day one. @item %U The week number of the year (the first Sunday as the first day of week one) as a decimal number (00--53). @c @cindex ISO 8601 @item %V The week number of the year (the first Monday as the first day of week one) as a decimal number (01--53). The method for determining the week number is as specified by ISO 8601. (To wit: if the week containing January 1 has four or more days in the new year, then it is week one; otherwise it is week 53 of the previous year and the next week is week one.) @item %w The weekday as a decimal number (0--6). Sunday is day zero. @item %W The week number of the year (the first Monday as the first day of week one) as a decimal number (00--53). @item %x The locale's ``appropriate'' date representation. (This is @samp{%A %B %d %Y} in the @code{"C"} locale.) @item %X The locale's ``appropriate'' time representation. (This is @samp{%T} in the @code{"C"} locale.) @item %y The year modulo 100 as a decimal number (00--99). @item %Y The full year as a decimal number (e.g., 1995). @c @cindex RFC 822 @c @cindex RFC 1036 @item %z The timezone offset in a +HHMM format (e.g., the format necessary to produce RFC 822/RFC 1036 date headers). @item %Z The time zone name or abbreviation; no characters if no time zone is determinable. @item %Ec %EC %Ex %EX %Ey %EY %Od %Oe %OH @itemx %OI %Om %OM %OS %Ou %OU %OV %Ow %OW %Oy ``Alternate representations'' for the specifications that use only the second letter (@samp{%c}, @samp{%C}, and so on).@footnote{If you don't understand any of this, don't worry about it; these facilities are meant to make it easier to ``internationalize'' programs. Other internationalization features are described in @ref{Internationalization}.} (These facilitate compliance with the POSIX @command{date} utility.) @item %% A literal @samp{%}. @end table If a conversion specifier is not one of the above, the behavior is undefined.@footnote{This is because ISO C leaves the behavior of the C version of @code{strftime} undefined and @command{gawk} uses the system's version of @code{strftime} if it's there. Typically, the conversion specifier either does not appear in the returned string or appears literally.} @c @cindex locale, definition of Informally, a @dfn{locale} is the geographic place in which a program is meant to run. For example, a common way to abbreviate the date September 4, 1991 in the United States is ``9/4/91.'' In many countries in Europe, however, it is abbreviated ``4.9.91.'' Thus, the @samp{%x} specification in a @code{"US"} locale might produce @samp{9/4/91}, while in a @code{"EUROPE"} locale, it might produce @samp{4.9.91}. The ISO C standard defines a default @code{"C"} locale, which is an environment that is typical of what most C programmers are used to. A public-domain C version of @code{strftime} is supplied with @command{gawk} for systems that are not yet fully standards-compliant. It supports all of the just listed format specifications. If that version is used to compile @command{gawk} (@pxref{Installation}), then the following additional format specifications are available: @table @code @item %k The hour (24-hour clock) as a decimal number (0--23). Single-digit numbers are padded with a space. @item %l The hour (12-hour clock) as a decimal number (1--12). Single-digit numbers are padded with a space. @item %N The ``Emperor/Era'' name. Equivalent to @code{%C}. @item %o The ``Emperor/Era'' year. Equivalent to @code{%y}. @item %s The time as a decimal timestamp in seconds since the epoch. @item %v The date in VMS format (e.g., @samp{20-JUN-1991}). @end table @c ENDOFRANGE strf Additionally, the alternate representations are recognized but their normal representations are used. @cindex @code{date} utility, POSIX @cindex POSIX @command{awk}, @code{date} utility and This example is an @command{awk} implementation of the POSIX @command{date} utility. Normally, the @command{date} utility prints the current date and time of day in a well-known format. However, if you provide an argument to it that begins with a @samp{+}, @command{date} copies nonformat specifier characters to the standard output and interprets the current time according to the format specifiers in the string. For example: @example $ date '+Today is %A, %B %d, %Y.' @print{} Today is Thursday, September 14, 2000. @end example Here is the @command{gawk} version of the @command{date} utility. It has a shell ``wrapper'' to handle the @option{-u} option, which requires that @command{date} run as if the time zone is set to UTC: @example #! /bin/sh # # date --- approximate the P1003.2 'date' command case $1 in -u) TZ=UTC0 # use UTC export TZ shift ;; esac @c FIXME: One day, change %d to %e, when C 99 is common. gawk 'BEGIN @{ format = "%a %b %d %H:%M:%S %Z %Y" exitval = 0 if (ARGC > 2) exitval = 1 else if (ARGC == 2) @{ format = ARGV[1] if (format ~ /^\+/) format = substr(format, 2) # remove leading + @} print strftime(format) exit exitval @}' "$@@" @end example @c ENDOFRANGE tst @c ENDOFRANGE logftst @c ENDOFRANGE filogtst @c ENDOFRANGE gawtst @node Bitwise Functions @subsection Bit-Manipulation Functions of @command{gawk} @c STARTOFRANGE bit @cindex bitwise, operations @c STARTOFRANGE and @cindex AND bitwise operation @c STARTOFRANGE oro @cindex OR bitwise operation @c STARTOFRANGE xor @cindex XOR bitwise operation @c STARTOFRANGE opbit @cindex operations, bitwise @quotation @i{I can explain it for you, but I can't understand it for you.}@* Anonymous @end quotation Many languages provide the ability to perform @dfn{bitwise} operations on two integer numbers. In other words, the operation is performed on each successive pair of bits in the operands. Three common operations are bitwise AND, OR, and XOR. The operations are described by the following table: @ifnottex @display Bit Operator | AND | OR | XOR |---+---+---+---+---+--- Operands | 0 | 1 | 0 | 1 | 0 | 1 ----------+---+---+---+---+---+--- 0 | 0 0 | 0 1 | 0 1 1 | 0 1 | 1 1 | 1 0 @end display @end ifnottex @tex \centerline{ \vbox{\bigskip % space above the table (about 1 linespace) % Because we have vertical rules, we can't let TeX insert interline space % in its usual way. \offinterlineskip \halign{\strut\hfil#\quad\hfil % operands &\vrule#&\quad#\quad % rule, 0 (of and) &\vrule#&\quad#\quad % rule, 1 (of and) &\vrule# % rule between and and or &\quad#\quad % 0 (of or) &\vrule#&\quad#\quad % rule, 1 (of of) &\vrule# % rule between or and xor &\quad#\quad % 0 of xor &\vrule#&\quad#\quad % rule, 1 of xor \cr &\omit&\multispan{11}\hfil\bf Bit operator\hfil\cr \noalign{\smallskip} & &\multispan3\hfil AND\hfil&&\multispan3\hfil OR\hfil &&\multispan3\hfil XOR\hfil\cr \bf Operands&&0&&1&&0&&1&&0&&1\cr \noalign{\hrule} \omit&height 2pt&&\omit&&&&\omit&&&&\omit\cr \noalign{\hrule height0pt}% without this the rule does not extend; why? 0&&0&\omit&0&&0&\omit&1&&0&\omit&1\cr 1&&0&\omit&1&&1&\omit&1&&1&\omit&0\cr }}} @end tex @cindex bitwise, complement @cindex complement, bitwise As you can see, the result of an AND operation is 1 only when @emph{both} bits are 1. The result of an OR operation is 1 if @emph{either} bit is 1. The result of an XOR operation is 1 if either bit is 1, but not both. The next operation is the @dfn{complement}; the complement of 1 is 0 and the complement of 0 is 1. Thus, this operation ``flips'' all the bits of a given value. @cindex bitwise, shift @cindex left shift, bitwise @cindex right shift, bitwise @cindex shift, bitwise Finally, two other common operations are to shift the bits left or right. For example, if you have a bit string @samp{10111001} and you shift it right by three bits, you end up with @samp{00010111}.@footnote{This example shows that 0's come in on the left side. For @command{gawk}, this is always true, but in some languages, it's possible to have the left side fill with 1's. Caveat emptor.} @c Purposely decided to use 0's and 1's here. 2/2001. If you start over again with @samp{10111001} and shift it left by three bits, you end up with @samp{11001000}. @command{gawk} provides built-in functions that implement the bitwise operations just described. They are: @ignore @table @code @cindex @code{and} function (@command{gawk}) @item and(@var{v1}, @var{v2}) Return the bitwise AND of the values provided by @var{v1} and @var{v2}. @cindex @code{or} function (@command{gawk}) @item or(@var{v1}, @var{v2}) Return the bitwise OR of the values provided by @var{v1} and @var{v2}. @cindex @code{xor} function (@command{gawk}) @item xor(@var{v1}, @var{v2}) Return the bitwise XOR of the values provided by @var{v1} and @var{v2}. @cindex @code{compl} function (@command{gawk}) @item compl(@var{val}) Return the bitwise complement of @var{val}. @cindex @code{lshift} function (@command{gawk}) @item lshift(@var{val}, @var{count}) Return the value of @var{val}, shifted left by @var{count} bits. @cindex @code{rshift} function (@command{gawk}) @item rshift(@var{val}, @var{count}) Return the value of @var{val}, shifted right by @var{count} bits. @end table @end ignore @cindex @command{gawk}, bitwise operations in @multitable {@code{rshift(@var{val}, @var{count})}} {Return the value of @var{val}, shifted right by @var{count} bits.} @cindex @code{and} function (@command{gawk}) @item @code{and(@var{v1}, @var{v2})} @tab Returns the bitwise AND of the values provided by @var{v1} and @var{v2}. @cindex @code{or} function (@command{gawk}) @item @code{or(@var{v1}, @var{v2})} @tab Returns the bitwise OR of the values provided by @var{v1} and @var{v2}. @cindex @code{xor} function (@command{gawk}) @item @code{xor(@var{v1}, @var{v2})} @tab Returns the bitwise XOR of the values provided by @var{v1} and @var{v2}. @cindex @code{compl} function (@command{gawk}) @item @code{compl(@var{val})} @tab Returns the bitwise complement of @var{val}. @cindex @code{lshift} function (@command{gawk}) @item @code{lshift(@var{val}, @var{count})} @tab Returns the value of @var{val}, shifted left by @var{count} bits. @cindex @code{rshift} function (@command{gawk}) @item @code{rshift(@var{val}, @var{count})} @tab Returns the value of @var{val}, shifted right by @var{count} bits. @end multitable For all of these functions, first the double-precision floating-point value is converted to the widest C unsigned integer type, then the bitwise operation is performed and then the result is converted back into a C @code{double}. (If you don't understand this paragraph, don't worry about it.) Here is a user-defined function (@pxref{User-defined}) that illustrates the use of these functions: @cindex @code{bits2str} user-defined function @cindex @code{testbits.awk} program @smallexample @group @c file eg/lib/bits2str.awk # bits2str --- turn a byte into readable 1's and 0's function bits2str(bits, data, mask) @{ if (bits == 0) return "0" mask = 1 for (; bits != 0; bits = rshift(bits, 1)) data = (and(bits, mask) ? "1" : "0") data while ((length(data) % 8) != 0) data = "0" data return data @} @c endfile @end group @c this is a hack to make testbits.awk self-contained @ignore @c file eg/prog/testbits.awk # bits2str --- turn a byte into readable 1's and 0's function bits2str(bits, data, mask) @{ if (bits == 0) return "0" mask = 1 for (; bits != 0; bits = rshift(bits, 1)) data = (and(bits, mask) ? "1" : "0") data while ((length(data) % 8) != 0) data = "0" data return data @} @c endfile @end ignore @c file eg/prog/testbits.awk BEGIN @{ printf "123 = %s\n", bits2str(123) printf "0123 = %s\n", bits2str(0123) printf "0x99 = %s\n", bits2str(0x99) comp = compl(0x99) printf "compl(0x99) = %#x = %s\n", comp, bits2str(comp) shift = lshift(0x99, 2) printf "lshift(0x99, 2) = %#x = %s\n", shift, bits2str(shift) shift = rshift(0x99, 2) printf "rshift(0x99, 2) = %#x = %s\n", shift, bits2str(shift) @} @c endfile @end smallexample @noindent This program produces the following output when run: @smallexample $ gawk -f testbits.awk @print{} 123 = 01111011 @print{} 0123 = 01010011 @print{} 0x99 = 10011001 @print{} compl(0x99) = 0xffffff66 = 11111111111111111111111101100110 @print{} lshift(0x99, 2) = 0x264 = 0000001001100100 @print{} rshift(0x99, 2) = 0x26 = 00100110 @end smallexample @cindex numbers, converting, to strings @cindex strings, converting, numbers to @cindex converting, numbers, to strings The @code{bits2str} function turns a binary number into a string. The number @code{1} represents a binary value where the rightmost bit is set to 1. Using this mask, the function repeatedly checks the rightmost bit. ANDing the mask with the value indicates whether the rightmost bit is 1 or not. If so, a @code{"1"} is concatenated onto the front of the string. Otherwise, a @code{"0"} is added. The value is then shifted right by one bit and the loop continues until there are no more 1 bits. If the initial value is zero it returns a simple @code{"0"}. Otherwise, at the end, it pads the value with zeros to represent multiples of 8-bit quantities. This is typical in modern computers. The main code in the @code{BEGIN} rule shows the difference between the decimal and octal values for the same numbers (@pxref{Nondecimal-numbers}), and then demonstrates the results of the @code{compl}, @code{lshift}, and @code{rshift} functions. @c ENDOFRANGE bit @c ENDOFRANGE and @c ENDOFRANGE oro @c ENDOFRANGE xor @c ENDOFRANGE opbit @node I18N Functions @subsection Using @command{gawk}'s String-Translation Functions @cindex @command{gawk}, string-translation functions @cindex functions, string-translation @cindex internationalization @cindex @command{awk} programs, internationalizing @command{gawk} provides facilities for internationalizing @command{awk} programs. These include the functions described in the following list. The descriptions here are purposely brief. @xref{Internationalization}, for the full story. Optional parameters are enclosed in square brackets ([ ]): @table @code @cindex @code{dcgettext} function (@command{gawk}) @item dcgettext(@var{string} @r{[}, @var{domain} @r{[}, @var{category}@r{]]}) This function returns the translation of @var{string} in text domain @var{domain} for locale category @var{category}. The default value for @var{domain} is the current value of @code{TEXTDOMAIN}. The default value for @var{category} is @code{"LC_MESSAGES"}. @cindex @code{dcngettext} function (@command{gawk}) @item dcngettext(@var{string1}, @var{string2}, @var{number} @r{[}, @var{domain} @r{[}, @var{category}@r{]]}) This function returns the plural form used for @var{number} of the translation of @var{string1} and @var{string2} in text domain @var{domain} for locale category @var{category}. @var{string1} is the English singular variant of a message, and @var{string2} the English plural variant of the same message. The default value for @var{domain} is the current value of @code{TEXTDOMAIN}. The default value for @var{category} is @code{"LC_MESSAGES"}. @cindex @code{bindtextdomain} function (@command{gawk}) @item bindtextdomain(@var{directory} @r{[}, @var{domain}@r{]}) This function allows you to specify the directory in which @command{gawk} will look for message translation files, in case they will not or cannot be placed in the ``standard'' locations (e.g., during testing). It returns the directory in which @var{domain} is ``bound.'' The default @var{domain} is the value of @code{TEXTDOMAIN}. If @var{directory} is the null string (@code{""}), then @code{bindtextdomain} returns the current binding for the given @var{domain}. @end table @c ENDOFRANGE funcbi @c ENDOFRANGE bifunc @node User-defined @section User-Defined Functions @c STARTOFRANGE udfunc @cindex user-defined, functions @c STARTOFRANGE funcud @cindex functions, user-defined Complicated @command{awk} programs can often be simplified by defining your own functions. User-defined functions can be called just like built-in ones (@pxref{Function Calls}), but it is up to you to define them, i.e., to tell @command{awk} what they should do. @menu * Definition Syntax:: How to write definitions and what they mean. * Function Example:: An example function definition and what it does. * Function Caveats:: Things to watch out for. * Return Statement:: Specifying the value a function returns. * Dynamic Typing:: How variable types can change at runtime. @end menu @node Definition Syntax @subsection Function Definition Syntax @c STARTOFRANGE fdef @cindex functions, defining Definitions of functions can appear anywhere between the rules of an @command{awk} program. Thus, the general form of an @command{awk} program is extended to include sequences of rules @emph{and} user-defined function definitions. There is no need to put the definition of a function before all uses of the function. This is because @command{awk} reads the entire program before starting to execute any of it. The definition of a function named @var{name} looks like this: @c NEXT ED: put [ ] around parameter list @example function @var{name}(@var{parameter-list}) @{ @var{body-of-function} @} @end example @cindex names, functions @cindex functions, names of @cindex namespace issues, functions @noindent @var{name} is the name of the function to define. A valid function name is like a valid variable name: a sequence of letters, digits, and underscores that doesn't start with a digit. Within a single @command{awk} program, any particular name can only be used as a variable, array, or function. @c NEXT ED: parameter-list is an OPTIONAL list of ... @var{parameter-list} is a list of the function's arguments and local variable names, separated by commas. When the function is called, the argument names are used to hold the argument values given in the call. The local variables are initialized to the empty string. A function cannot have two parameters with the same name, nor may it have a parameter with the same name as the function itself. The @var{body-of-function} consists of @command{awk} statements. It is the most important part of the definition, because it says what the function should actually @emph{do}. The argument names exist to give the body a way to talk about the arguments; local variables exist to give the body places to keep temporary values. Argument names are not distinguished syntactically from local variable names. Instead, the number of arguments supplied when the function is called determines how many argument variables there are. Thus, if three argument values are given, the first three names in @var{parameter-list} are arguments and the rest are local variables. It follows that if the number of arguments is not the same in all calls to the function, some of the names in @var{parameter-list} may be arguments on some occasions and local variables on others. Another way to think of this is that omitted arguments default to the null string. @cindex programming conventions, functions, writing Usually when you write a function, you know how many names you intend to use for arguments and how many you intend to use as local variables. It is conventional to place some extra space between the arguments and the local variables, in order to document how your function is supposed to be used. @cindex variables, shadowing During execution of the function body, the arguments and local variable values hide, or @dfn{shadow}, any variables of the same names used in the rest of the program. The shadowed variables are not accessible in the function definition, because there is no way to name them while their names have been taken away for the local variables. All other variables used in the @command{awk} program can be referenced or set normally in the function's body. The arguments and local variables last only as long as the function body is executing. Once the body finishes, you can once again access the variables that were shadowed while the function was running. @cindex recursive functions @cindex functions, recursive The function body can contain expressions that call functions. They can even call this function, either directly or by way of another function. When this happens, we say the function is @dfn{recursive}. The act of a function calling itself is called @dfn{recursion}. @c @cindex @command{awk} language, POSIX version @c @cindex POSIX @command{awk} @cindex POSIX @command{awk}, @code{function} keyword in In many @command{awk} implementations, including @command{gawk}, the keyword @code{function} may be abbreviated @code{func}. However, POSIX only specifies the use of the keyword @code{function}. This actually has some practical implications. If @command{gawk} is in POSIX-compatibility mode (@pxref{Options}), then the following statement does @emph{not} define a function: @example func foo() @{ a = sqrt($1) ; print a @} @end example @noindent Instead it defines a rule that, for each record, concatenates the value of the variable @samp{func} with the return value of the function @samp{foo}. If the resulting string is non-null, the action is executed. This is probably not what is desired. (@command{awk} accepts this input as syntactically valid, because functions may be used before they are defined in @command{awk} programs.) @c NEXT ED: This won't actually run, since foo() is undefined ... @c last comma does NOT start tertiary @cindex portability, functions, defining To ensure that your @command{awk} programs are portable, always use the keyword @code{function} when defining a function. @node Function Example @subsection Function Definition Examples Here is an example of a user-defined function, called @code{myprint}, that takes a number and prints it in a specific format: @example function myprint(num) @{ printf "%6.3g\n", num @} @end example @noindent To illustrate, here is an @command{awk} rule that uses our @code{myprint} function: @example $3 > 0 @{ myprint($3) @} @end example @noindent This program prints, in our special format, all the third fields that contain a positive number in our input. Therefore, when given the following: @example 1.2 3.4 5.6 7.8 9.10 11.12 -13.14 15.16 17.18 19.20 21.22 23.24 @end example @noindent this program, using our function to format the results, prints: @example 5.6 21.2 @end example This function deletes all the elements in an array: @example function delarray(a, i) @{ for (i in a) delete a[i] @} @end example When working with arrays, it is often necessary to delete all the elements in an array and start over with a new list of elements (@pxref{Delete}). Instead of having to repeat this loop everywhere that you need to clear out an array, your program can just call @code{delarray}. (This guarantees portability. The use of @samp{delete @var{array}} to delete the contents of an entire array is a nonstandard extension.) The following is an example of a recursive function. It takes a string as an input parameter and returns the string in backwards order. Recursive functions must always have a test that stops the recursion. In this case, the recursion terminates when the starting position is zero, i.e., when there are no more characters left in the string. @cindex @code{rev} user-defined function @example function rev(str, start) @{ if (start == 0) return "" return (substr(str, start, 1) rev(str, start - 1)) @} @end example If this function is in a file named @file{rev.awk}, it can be tested this way: @example $ echo "Don't Panic!" | > gawk --source '@{ print rev($0, length($0)) @}' -f rev.awk @print{} !cinaP t'noD @end example The C @code{ctime} function takes a timestamp and returns it in a string, formatted in a well-known fashion. The following example uses the built-in @code{strftime} function (@pxref{Time Functions}) to create an @command{awk} version of @code{ctime}: @cindex @code{ctime} user-defined function @c FIXME: One day, change %d to %e, when C 99 is common. @example @c file eg/lib/ctime.awk # ctime.awk # # awk version of C ctime(3) function function ctime(ts, format) @{ format = "%a %b %d %H:%M:%S %Z %Y" if (ts == 0) ts = systime() # use current time as default return strftime(format, ts) @} @c endfile @end example @c ENDOFRANGE fdef @node Function Caveats @subsection Calling User-Defined Functions @c STARTOFRANGE fudc @cindex functions, user-defined, calling @dfn{Calling a function} means causing the function to run and do its job. A function call is an expression and its value is the value returned by the function. A function call consists of the function name followed by the arguments in parentheses. @command{awk} expressions are what you write in the call for the arguments. Each time the call is executed, these expressions are evaluated, and the values are the actual arguments. For example, here is a call to @code{foo} with three arguments (the first being a string concatenation): @example foo(x y, "lose", 4 * z) @end example @strong{Caution:} Whitespace characters (spaces and tabs) are not allowed between the function name and the open-parenthesis of the argument list. If you write whitespace by mistake, @command{awk} might think that you mean to concatenate a variable with an expression in parentheses. However, it notices that you used a function name and not a variable name, and reports an error. @cindex call by value When a function is called, it is given a @emph{copy} of the values of its arguments. This is known as @dfn{call by value}. The caller may use a variable as the expression for the argument, but the called function does not know this---it only knows what value the argument had. For example, if you write the following code: @example foo = "bar" z = myfunc(foo) @end example @noindent then you should not think of the argument to @code{myfunc} as being ``the variable @code{foo}.'' Instead, think of the argument as the string value @code{"bar"}. If the function @code{myfunc} alters the values of its local variables, this has no effect on any other variables. Thus, if @code{myfunc} does this: @example function myfunc(str) @{ print str str = "zzz" print str @} @end example @noindent to change its first argument variable @code{str}, it does @emph{not} change the value of @code{foo} in the caller. The role of @code{foo} in calling @code{myfunc} ended when its value (@code{"bar"}) was computed. If @code{str} also exists outside of @code{myfunc}, the function body cannot alter this outer value, because it is shadowed during the execution of @code{myfunc} and cannot be seen or changed from there. @cindex call by reference @cindex arrays, as parameters to functions @cindex functions, arrays as parameters to However, when arrays are the parameters to functions, they are @emph{not} copied. Instead, the array itself is made available for direct manipulation by the function. This is usually called @dfn{call by reference}. Changes made to an array parameter inside the body of a function @emph{are} visible outside that function. @strong{Note:} Changing an array parameter inside a function can be very dangerous if you do not watch what you are doing. For example: @example function changeit(array, ind, nvalue) @{ array[ind] = nvalue @} BEGIN @{ a[1] = 1; a[2] = 2; a[3] = 3 changeit(a, 2, "two") printf "a[1] = %s, a[2] = %s, a[3] = %s\n", a[1], a[2], a[3] @} @end example @noindent prints @samp{a[1] = 1, a[2] = two, a[3] = 3}, because @code{changeit} stores @code{"two"} in the second element of @code{a}. @cindex undefined functions @cindex functions, undefined Some @command{awk} implementations allow you to call a function that has not been defined. They only report a problem at runtime when the program actually tries to call the function. For example: @example BEGIN @{ if (0) foo() else bar() @} function bar() @{ @dots{} @} # note that `foo' is not defined @end example @noindent Because the @samp{if} statement will never be true, it is not really a problem that @code{foo} has not been defined. Usually, though, it is a problem if a program calls an undefined function. @cindex lint checking, undefined functions If @option{--lint} is specified (@pxref{Options}), @command{gawk} reports calls to undefined functions. @cindex portability, @code{next} statement in user-defined functions Some @command{awk} implementations generate a runtime error if you use the @code{next} statement (@pxref{Next Statement}) inside a user-defined function. @command{gawk} does not have this limitation. @c ENDOFRANGE fudc @node Return Statement @subsection The @code{return} Statement @c comma does NOT start a secondary @cindex @code{return} statement, user-defined functions The body of a user-defined function can contain a @code{return} statement. This statement returns control to the calling part of the @command{awk} program. It can also be used to return a value for use in the rest of the @command{awk} program. It looks like this: @example return @r{[}@var{expression}@r{]} @end example The @var{expression} part is optional. If it is omitted, then the returned value is undefined, and therefore, unpredictable. A @code{return} statement with no value expression is assumed at the end of every function definition. So if control reaches the end of the function body, then the function returns an unpredictable value. @command{awk} does @emph{not} warn you if you use the return value of such a function. Sometimes, you want to write a function for what it does, not for what it returns. Such a function corresponds to a @code{void} function in C or to a @code{procedure} in Pascal. Thus, it may be appropriate to not return any value; simply bear in mind that if you use the return value of such a function, you do so at your own risk. The following is an example of a user-defined function that returns a value for the largest number among the elements of an array: @example function maxelt(vec, i, ret) @{ for (i in vec) @{ if (ret == "" || vec[i] > ret) ret = vec[i] @} return ret @} @end example @cindex programming conventions, function parameters @noindent You call @code{maxelt} with one argument, which is an array name. The local variables @code{i} and @code{ret} are not intended to be arguments; while there is nothing to stop you from passing more than one argument to @code{maxelt}, the results would be strange. The extra space before @code{i} in the function parameter list indicates that @code{i} and @code{ret} are not supposed to be arguments. You should follow this convention when defining functions. The following program uses the @code{maxelt} function. It loads an array, calls @code{maxelt}, and then reports the maximum number in that array: @example function maxelt(vec, i, ret) @{ for (i in vec) @{ if (ret == "" || vec[i] > ret) ret = vec[i] @} return ret @} # Load all fields of each record into nums. @{ for(i = 1; i <= NF; i++) nums[NR, i] = $i @} END @{ print maxelt(nums) @} @end example Given the following input: @example 1 5 23 8 16 44 3 5 2 8 26 256 291 1396 2962 100 -6 467 998 1101 99385 11 0 225 @end example @noindent the program reports (predictably) that @code{99385} is the largest number in the array. @node Dynamic Typing @subsection Functions and Their Effects on Variable Typing @command{awk} is a very fluid language. It is possible that @command{awk} can't tell if an identifier represents a regular variable or an array until runtime. Here is an annotated sample program: @example function foo(a) @{ a[1] = 1 # parameter is an array @} BEGIN @{ b = 1 foo(b) # invalid: fatal type mismatch foo(x) # x uninitialized, becomes an array dynamically x = 1 # now not allowed, runtime error @} @end example Usually, such things aren't a big issue, but it's worth being aware of them. @c ENDOFRANGE udfunc @c ENDOFRANGE funcud @node Internationalization @chapter Internationalization with @command{gawk} Once upon a time, computer makers wrote software that worked only in English. Eventually, hardware and software vendors noticed that if their systems worked in the native languages of non-English-speaking countries, they were able to sell more systems. As a result, internationalization and localization of programs and software systems became a common practice. @c STARTOFRANGE inloc @cindex internationalization, localization @cindex @command{gawk}, internationalization and, See internationalization @cindex internationalization, localization, @command{gawk} and Until recently, the ability to provide internationalization was largely restricted to programs written in C and C++. This @value{CHAPTER} describes the underlying library @command{gawk} uses for internationalization, as well as how @command{gawk} makes internationalization features available at the @command{awk} program level. Having internationalization available at the @command{awk} level gives software developers additional flexibility---they are no longer required to write in C when internationalization is a requirement. @menu * I18N and L10N:: Internationalization and Localization. * Explaining gettext:: How GNU @code{gettext} works. * Programmer i18n:: Features for the programmer. * Translator i18n:: Features for the translator. * I18N Example:: A simple i18n example. * Gawk I18N:: @command{gawk} is also internationalized. @end menu @node I18N and L10N @section Internationalization and Localization @cindex internationalization @c comma is part of see @cindex localization, See internationalization, localization @cindex localization @dfn{Internationalization} means writing (or modifying) a program once, in such a way that it can use multiple languages without requiring further source-code changes. @dfn{Localization} means providing the data necessary for an internationalized program to work in a particular language. Most typically, these terms refer to features such as the language used for printing error messages, the language used to read responses, and information related to how numerical and monetary values are printed and read. @node Explaining gettext @section GNU @code{gettext} @cindex internationalizing a program @c STARTOFRANGE gettex @cindex @code{gettext} library The facilities in GNU @code{gettext} focus on messages; strings printed by a program, either directly or via formatting with @code{printf} or @code{sprintf}.@footnote{For some operating systems, the @command{gawk} port doesn't support GNU @code{gettext}. This applies most notably to the PC operating systems. As such, these features are not available if you are using one of those operating systems. Sorry.} @cindex portability, @code{gettext} library and When using GNU @code{gettext}, each application has its own @dfn{text domain}. This is a unique name, such as @samp{kpilot} or @samp{gawk}, that identifies the application. A complete application may have multiple components---programs written in C or C++, as well as scripts written in @command{sh} or @command{awk}. All of the components use the same text domain. To make the discussion concrete, assume we're writing an application named @command{guide}. Internationalization consists of the following steps, in this order: @enumerate @item The programmer goes through the source for all of @command{guide}'s components and marks each string that is a candidate for translation. For example, @code{"`-F': option required"} is a good candidate for translation. A table with strings of option names is not (e.g., @command{gawk}'s @option{--profile} option should remain the same, no matter what the local language). @cindex @code{textdomain} function (C library) @item The programmer indicates the application's text domain (@code{"guide"}) to the @code{gettext} library, by calling the @code{textdomain} function. @item Messages from the application are extracted from the source code and collected into a portable object file (@file{guide.po}), which lists the strings and their translations. The translations are initially empty. The original (usually English) messages serve as the key for lookup of the translations. @cindex @code{.po} files @cindex files, @code{.po} @cindex portable object files @cindex files, portable object @item For each language with a translator, @file{guide.po} is copied and translations are created and shipped with the application. @cindex @code{.mo} files @cindex files, @code{.mo} @cindex message object files @cindex files, message object @item Each language's @file{.po} file is converted into a binary message object (@file{.mo}) file. A message object file contains the original messages and their translations in a binary format that allows fast lookup of translations at runtime. @item When @command{guide} is built and installed, the binary translation files are installed in a standard place. @cindex @code{bindtextdomain} function (C library) @item For testing and development, it is possible to tell @code{gettext} to use @file{.mo} files in a different directory than the standard one by using the @code{bindtextdomain} function. @cindex @code{.mo} files, specifying directory of @cindex files, @code{.mo}, specifying directory of @cindex message object files, specifying directory of @cindex files, message object, specifying directory of @item At runtime, @command{guide} looks up each string via a call to @code{gettext}. The returned string is the translated string if available, or the original string if not. @item If necessary, it is possible to access messages from a different text domain than the one belonging to the application, without having to switch the application's default text domain back and forth. @end enumerate @cindex @code{gettext} function (C library) In C (or C++), the string marking and dynamic translation lookup are accomplished by wrapping each string in a call to @code{gettext}: @example printf(gettext("Don't Panic!\n")); @end example The tools that extract messages from source code pull out all strings enclosed in calls to @code{gettext}. @cindex @code{_} (underscore), @code{_} C macro @cindex underscore (@code{_}), @code{_} C macro The GNU @code{gettext} developers, recognizing that typing @samp{gettext} over and over again is both painful and ugly to look at, use the macro @samp{_} (an underscore) to make things easier: @example /* In the standard header file: */ #define _(str) gettext(str) /* In the program text: */ printf(_("Don't Panic!\n")); @end example @cindex internationalization, localization, locale categories @cindex @code{gettext} library, locale categories @cindex locale categories @noindent This reduces the typing overhead to just three extra characters per string and is considerably easier to read as well. There are locale @dfn{categories} for different types of locale-related information. The defined locale categories that @code{gettext} knows about are: @table @code @cindex @code{LC_MESSAGES} locale category @item LC_MESSAGES Text messages. This is the default category for @code{gettext} operations, but it is possible to supply a different one explicitly, if necessary. (It is almost never necessary to supply a different category.) @cindex sorting characters in different languages @cindex @code{LC_COLLATE} locale category @item LC_COLLATE Text-collation information; i.e., how different characters and/or groups of characters sort in a given language. @cindex @code{LC_CTYPE} locale category @item LC_CTYPE Character-type information (alphabetic, digit, upper- or lowercase, and so on). This information is accessed via the POSIX character classes in regular expressions, such as @code{/[[:alnum:]]/} (@pxref{Regexp Operators}). @cindex monetary information, localization @cindex currency symbols, localization @cindex @code{LC_MONETARY} locale category @item LC_MONETARY Monetary information, such as the currency symbol, and whether the symbol goes before or after a number. @cindex @code{LC_NUMERIC} locale category @item LC_NUMERIC Numeric information, such as which characters to use for the decimal point and the thousands separator.@footnote{Americans use a comma every three decimal places and a period for the decimal point, while many Europeans do exactly the opposite: @code{1,234.56} versus @code{1.234,56}.} @cindex @code{LC_RESPONSE} locale category @item LC_RESPONSE Response information, such as how ``yes'' and ``no'' appear in the local language, and possibly other information as well. @cindex time, localization and @c last comma does NOT start a tertiary @cindex dates, information related to, localization @cindex @code{LC_TIME} locale category @item LC_TIME Time- and date-related information, such as 12- or 24-hour clock, month printed before or after day in a date, local month abbreviations, and so on. @cindex @code{LC_ALL} locale category @item LC_ALL All of the above. (Not too useful in the context of @code{gettext}.) @end table @c ENDOFRANGE gettex @node Programmer i18n @section Internationalizing @command{awk} Programs @c STARTOFRANGE inap @cindex @command{awk} programs, internationalizing @command{gawk} provides the following variables and functions for internationalization: @table @code @cindex @code{TEXTDOMAIN} variable @item TEXTDOMAIN This variable indicates the application's text domain. For compatibility with GNU @code{gettext}, the default value is @code{"messages"}. @cindex internationalization, localization, marked strings @cindex strings, for localization @item _"your message here" String constants marked with a leading underscore are candidates for translation at runtime. String constants without a leading underscore are not translated. @cindex @code{dcgettext} function (@command{gawk}) @item dcgettext(@var{string} @r{[}, @var{domain} @r{[}, @var{category}@r{]]}) This built-in function returns the translation of @var{string} in text domain @var{domain} for locale category @var{category}. The default value for @var{domain} is the current value of @code{TEXTDOMAIN}. The default value for @var{category} is @code{"LC_MESSAGES"}. If you supply a value for @var{category}, it must be a string equal to one of the known locale categories described in @ifnotinfo the previous @value{SECTION}. @end ifnotinfo @ifinfo @ref{Explaining gettext}. @end ifinfo You must also supply a text domain. Use @code{TEXTDOMAIN} if you want to use the current domain. @strong{Caution:} The order of arguments to the @command{awk} version of the @code{dcgettext} function is purposely different from the order for the C version. The @command{awk} version's order was chosen to be simple and to allow for reasonable @command{awk}-style default arguments. @cindex @code{dcngettext} function (@command{gawk}) @item dcngettext(@var{string1}, @var{string2}, @var{number} @r{[}, @var{domain} @r{[}, @var{category}@r{]]}) This built-in function returns the plural form used for @var{number} of the translation of @var{string1} and @var{string2} in text domain @var{domain} for locale category @var{category}. @var{string1} is the English singular variant of a message, and @var{string2} the English plural variant of the same message. The default value for @var{domain} is the current value of @code{TEXTDOMAIN}. The default value for @var{category} is @code{"LC_MESSAGES"}. The same remarks as for the @code{dcgettext} function apply. @cindex @code{.mo} files, specifying directory of @cindex files, @code{.mo}, specifying directory of @cindex message object files, specifying directory of @cindex files, message object, specifying directory of @cindex @code{bindtextdomain} function (@command{gawk}) @item bindtextdomain(@var{directory} @r{[}, @var{domain}@r{]}) This built-in function allows you to specify the directory in which @code{gettext} looks for @file{.mo} files, in case they will not or cannot be placed in the standard locations (e.g., during testing). It returns the directory in which @var{domain} is ``bound.'' The default @var{domain} is the value of @code{TEXTDOMAIN}. If @var{directory} is the null string (@code{""}), then @code{bindtextdomain} returns the current binding for the given @var{domain}. @end table To use these facilities in your @command{awk} program, follow the steps outlined in @ifnotinfo the previous @value{SECTION}, @end ifnotinfo @ifinfo @ref{Explaining gettext}, @end ifinfo like so: @enumerate @cindex @code{BEGIN} pattern, @code{TEXTDOMAIN} variable and @cindex @code{TEXTDOMAIN} variable, @code{BEGIN} pattern and @item Set the variable @code{TEXTDOMAIN} to the text domain of your program. This is best done in a @code{BEGIN} rule (@pxref{BEGIN/END}), or it can also be done via the @option{-v} command-line option (@pxref{Options}): @example BEGIN @{ TEXTDOMAIN = "guide" @dots{} @} @end example @cindex @code{_} (underscore), translatable string @cindex underscore (@code{_}), translatable string @item Mark all translatable strings with a leading underscore (@samp{_}) character. It @emph{must} be adjacent to the opening quote of the string. For example: @example print _"hello, world" x = _"you goofed" printf(_"Number of users is %d\n", nusers) @end example @item If you are creating strings dynamically, you can still translate them, using the @code{dcgettext} built-in function: @example message = nusers " users logged in" message = dcgettext(message, "adminprog") print message @end example Here, the call to @code{dcgettext} supplies a different text domain (@code{"adminprog"}) in which to find the message, but it uses the default @code{"LC_MESSAGES"} category. @cindex @code{LC_MESSAGES} locale category, @code{bindtextdomain} function (@command{gawk}) @item During development, you might want to put the @file{.mo} file in a private directory for testing. This is done with the @code{bindtextdomain} built-in function: @example BEGIN @{ TEXTDOMAIN = "guide" # our text domain if (Testing) @{ # where to find our files bindtextdomain("testdir") # joe is in charge of adminprog bindtextdomain("../joe/testdir", "adminprog") @} @dots{} @} @end example @end enumerate @xref{I18N Example}, for an example program showing the steps to create and use translations from @command{awk}. @node Translator i18n @section Translating @command{awk} Programs @cindex @code{.po} files @cindex files, @code{.po} @cindex portable object files @cindex files, portable object Once a program's translatable strings have been marked, they must be extracted to create the initial @file{.po} file. As part of translation, it is often helpful to rearrange the order in which arguments to @code{printf} are output. @command{gawk}'s @option{--gen-po} command-line option extracts the messages and is discussed next. After that, @code{printf}'s ability to rearrange the order for @code{printf} arguments at runtime is covered. @menu * String Extraction:: Extracting marked strings. * Printf Ordering:: Rearranging @code{printf} arguments. * I18N Portability:: @command{awk}-level portability issues. @end menu @node String Extraction @subsection Extracting Marked Strings @cindex strings, extracting @c comma does NOT start secondary @cindex marked strings, extracting @cindex @code{--gen-po} option @cindex command-line options, string extraction @cindex string extraction (internationalization) @cindex marked string extraction (internationalization) @cindex extraction, of marked strings (internationalization) @cindex @code{--gen-po} option Once your @command{awk} program is working, and all the strings have been marked and you've set (and perhaps bound) the text domain, it is time to produce translations. First, use the @option{--gen-po} command-line option to create the initial @file{.po} file: @example $ gawk --gen-po -f guide.awk > guide.po @end example @cindex @code{xgettext} utility When run with @option{--gen-po}, @command{gawk} does not execute your program. Instead, it parses it as usual and prints all marked strings to standard output in the format of a GNU @code{gettext} Portable Object file. Also included in the output are any constant strings that appear as the first argument to @code{dcgettext} or as the first and second argument to @code{dcngettext}.@footnote{Starting with @code{gettext} version 0.11.5, the @command{xgettext} utility that comes with GNU @code{gettext} can handle @file{.awk} files.} @xref{I18N Example}, for the full list of steps to go through to create and test translations for @command{guide}. @node Printf Ordering @subsection Rearranging @code{printf} Arguments @cindex @code{printf} statement, positional specifiers @c comma does NOT start secondary @cindex positional specifiers, @code{printf} statement Format strings for @code{printf} and @code{sprintf} (@pxref{Printf}) present a special problem for translation. Consider the following:@footnote{This example is borrowed from the GNU @code{gettext} manual.} @c line broken here only for smallbook format @example printf(_"String `%s' has %d characters\n", string, length(string))) @end example A possible German translation for this might be: @example "%d Zeichen lang ist die Zeichenkette `%s'\n" @end example The problem should be obvious: the order of the format specifications is different from the original! Even though @code{gettext} can return the translated string at runtime, it cannot change the argument order in the call to @code{printf}. To solve this problem, @code{printf} format specificiers may have an additional optional element, which we call a @dfn{positional specifier}. For example: @example "%2$d Zeichen lang ist die Zeichenkette `%1$s'\n" @end example Here, the positional specifier consists of an integer count, which indicates which argument to use, and a @samp{$}. Counts are one-based, and the format string itself is @emph{not} included. Thus, in the following example, @samp{string} is the first argument and @samp{length(string)} is the second: @example $ gawk 'BEGIN @{ > string = "Dont Panic" > printf _"%2$d characters live in \"%1$s\"\n", > string, length(string) > @}' @print{} 10 characters live in "Dont Panic" @end example If present, positional specifiers come first in the format specification, before the flags, the field width, and/or the precision. Positional specifiers can be used with the dynamic field width and precision capability: @example $ gawk 'BEGIN @{ > printf("%*.*s\n", 10, 20, "hello") > printf("%3$*2$.*1$s\n", 20, 10, "hello") > @}' @print{} hello @print{} hello @end example @noindent @strong{Note:} When using @samp{*} with a positional specifier, the @samp{*} comes first, then the integer position, and then the @samp{$}. This is somewhat counterintutive. @cindex @code{printf} statement, positional specifiers, mixing with regular formats @c first comma does is part of primary @cindex positional specifiers, @code{printf} statement, mixing with regular formats @cindex format specifiers, mixing regular with positional specifiers @command{gawk} does not allow you to mix regular format specifiers and those with positional specifiers in the same string: @smallexample $ gawk 'BEGIN @{ printf _"%d %3$s\n", 1, 2, "hi" @}' @error{} gawk: cmd. line:1: fatal: must use `count$' on all formats or none @end smallexample @strong{Note:} There are some pathological cases that @command{gawk} may fail to diagnose. In such cases, the output may not be what you expect. It's still a bad idea to try mixing them, even if @command{gawk} doesn't detect it. Although positional specifiers can be used directly in @command{awk} programs, their primary purpose is to help in producing correct translations of format strings into languages different from the one in which the program is first written. @node I18N Portability @subsection @command{awk} Portability Issues @cindex portability, internationalization and @cindex internationalization, localization, portability and @command{gawk}'s internationalization features were purposely chosen to have as little impact as possible on the portability of @command{awk} programs that use them to other versions of @command{awk}. Consider this program: @example BEGIN @{ TEXTDOMAIN = "guide" if (Test_Guide) # set with -v bindtextdomain("/test/guide/messages") print _"don't panic!" @} @end example @noindent As written, it won't work on other versions of @command{awk}. However, it is actually almost portable, requiring very little change: @itemize @bullet @cindex @code{TEXTDOMAIN} variable, portability and @item Assignments to @code{TEXTDOMAIN} won't have any effect, since @code{TEXTDOMAIN} is not special in other @command{awk} implementations. @item Non-GNU versions of @command{awk} treat marked strings as the concatenation of a variable named @code{_} with the string following it.@footnote{This is good fodder for an ``Obfuscated @command{awk}'' contest.} Typically, the variable @code{_} has the null string (@code{""}) as its value, leaving the original string constant as the result. @item By defining ``dummy'' functions to replace @code{dcgettext}, @code{dcngettext} and @code{bindtextdomain}, the @command{awk} program can be made to run, but all the messages are output in the original language. For example: @cindex @code{bindtextdomain} function (@command{gawk}), portability and @cindex @code{dcgettext} function (@command{gawk}), portability and @cindex @code{dcngettext} function (@command{gawk}), portability and @example @c file eg/lib/libintl.awk function bindtextdomain(dir, domain) @{ return dir @} function dcgettext(string, domain, category) @{ return string @} function dcngettext(string1, string2, number, domain, category) @{ return (number == 1 ? string1 : string2) @} @c endfile @end example @item The use of positional specifications in @code{printf} or @code{sprintf} is @emph{not} portable. To support @code{gettext} at the C level, many systems' C versions of @code{sprintf} do support positional specifiers. But it works only if enough arguments are supplied in the function call. Many versions of @command{awk} pass @code{printf} formats and arguments unchanged to the underlying C library version of @code{sprintf}, but only one format and argument at a time. What happens if a positional specification is used is anybody's guess. However, since the positional specifications are primarily for use in @emph{translated} format strings, and since non-GNU @command{awk}s never retrieve the translated string, this should not be a problem in practice. @end itemize @c ENDOFRANGE inap @node I18N Example @section A Simple Internationalization Example Now let's look at a step-by-step example of how to internationalize and localize a simple @command{awk} program, using @file{guide.awk} as our original source: @example @c file eg/prog/guide.awk BEGIN @{ TEXTDOMAIN = "guide" bindtextdomain(".") # for testing print _"Don't Panic" print _"The Answer Is", 42 print "Pardon me, Zaphod who?" @} @c endfile @end example @noindent Run @samp{gawk --gen-po} to create the @file{.po} file: @example $ gawk --gen-po -f guide.awk > guide.po @end example @noindent This produces: @example @c file eg/data/guide.po #: guide.awk:4 msgid "Don't Panic" msgstr "" #: guide.awk:5 msgid "The Answer Is" msgstr "" @c endfile @end example This original portable object file is saved and reused for each language into which the application is translated. The @code{msgid} is the original string and the @code{msgstr} is the translation. @strong{Note:} Strings not marked with a leading underscore do not appear in the @file{guide.po} file. Next, the messages must be translated. Here is a translation to a hypothetical dialect of English, called ``Mellow'':@footnote{Perhaps it would be better if it were called ``Hippy.'' Ah, well.} @example @group $ cp guide.po guide-mellow.po @var{Add translations to} guide-mellow.po @dots{} @end group @end example @noindent Following are the translations: @example @c file eg/data/guide-mellow.po #: guide.awk:4 msgid "Don't Panic" msgstr "Hey man, relax!" #: guide.awk:5 msgid "The Answer Is" msgstr "Like, the scoop is" @c endfile @end example @cindex Linux @cindex GNU/Linux The next step is to make the directory to hold the binary message object file and then to create the @file{guide.mo} file. The directory layout shown here is standard for GNU @code{gettext} on GNU/Linux systems. Other versions of @code{gettext} may use a different layout: @example $ mkdir en_US en_US/LC_MESSAGES @end example @cindex @code{.po} files, converting to @code{.mo} @cindex files, @code{.po}, converting to @code{.mo} @cindex @code{.mo} files, converting from @code{.po} @cindex files, @code{.mo}, converting from @code{.po} @cindex portable object files, converting to message object files @cindex files, portable object, converting to message object files @cindex message object files, converting from portable object files @cindex files, message object, converting from portable object files @cindex @command{msgfmt} utility The @command{msgfmt} utility does the conversion from human-readable @file{.po} file to machine-readable @file{.mo} file. By default, @command{msgfmt} creates a file named @file{messages}. This file must be renamed and placed in the proper directory so that @command{gawk} can find it: @example $ msgfmt guide-mellow.po $ mv messages en_US/LC_MESSAGES/guide.mo @end example Finally, we run the program to test it: @example $ gawk -f guide.awk @print{} Hey man, relax! @print{} Like, the scoop is 42 @print{} Pardon me, Zaphod who? @end example If the three replacement functions for @code{dcgettext}, @code{dcngettext} and @code{bindtextdomain} (@pxref{I18N Portability}) are in a file named @file{libintl.awk}, then we can run @file{guide.awk} unchanged as follows: @example $ gawk --posix -f guide.awk -f libintl.awk @print{} Don't Panic @print{} The Answer Is 42 @print{} Pardon me, Zaphod who? @end example @node Gawk I18N @section @command{gawk} Can Speak Your Language As of @value{PVERSION} 3.1, @command{gawk} itself has been internationalized using the GNU @code{gettext} package. @ifinfo (GNU @code{gettext} is described in complete detail in @ref{Top}.) @end ifinfo @ifnotinfo (GNU @code{gettext} is described in complete detail in @cite{GNU gettext tools}.) @end ifnotinfo As of this writing, the latest version of GNU @code{gettext} is @uref{ftp://ftp.gnu.org/gnu/gettext/gettext-0.11.5.tar.gz, @value{PVERSION} 0.11.5}. If a translation of @command{gawk}'s messages exists, then @command{gawk} produces usage messages, warnings, and fatal errors in the local language. @cindex @code{--with-included-gettext} configuration option @cindex configuration option, @code{--with-included-gettext} On systems that do not use @value{PVERSION} 2 (or later) of the GNU C library, you should configure @command{gawk} with the @option{--with-included-gettext} option before compiling and installing it. @xref{Additional Configuration Options}, for more information. @c ENDOFRANGE inloc @node Advanced Features @chapter Advanced Features of @command{gawk} @cindex advanced features, network connections, See Also networks, connections @c STARTOFRANGE gawadv @cindex @command{gawk}, features, advanced @c STARTOFRANGE advgaw @cindex advanced features, @command{gawk} @ignore Contributed by: Peter Langston Found in Steve English's "signature" line: "Write documentation as if whoever reads it is a violent psychopath who knows where you live." @end ignore @quotation @i{Write documentation as if whoever reads it is a violent psychopath who knows where you live.}@* Steve English, as quoted by Peter Langston @end quotation This @value{CHAPTER} discusses advanced features in @command{gawk}. It's a bit of a ``grab bag'' of items that are otherwise unrelated to each other. First, a command-line option allows @command{gawk} to recognize nondecimal numbers in input data, not just in @command{awk} programs. Next, two-way I/O, discussed briefly in earlier parts of this @value{DOCUMENT}, is described in full detail, along with the basics of TCP/IP networking and BSD portal files. Finally, @command{gawk} can @dfn{profile} an @command{awk} program, making it possible to tune it for performance. @ref{Dynamic Extensions}, discusses the ability to dynamically add new built-in functions to @command{gawk}. As this feature is still immature and likely to change, its description is relegated to an appendix. @menu * Nondecimal Data:: Allowing nondecimal input data. * Two-way I/O:: Two-way communications with another process. * TCP/IP Networking:: Using @command{gawk} for network programming. * Portal Files:: Using @command{gawk} with BSD portals. * Profiling:: Profiling your @command{awk} programs. @end menu @node Nondecimal Data @section Allowing Nondecimal Input Data @cindex @code{--non-decimal-data} option @cindex advanced features, @command{gawk}, nondecimal input data @c last comma does NOT start tertiary @cindex input, data, nondecimal @cindex constants, nondecimal If you run @command{gawk} with the @option{--non-decimal-data} option, you can have nondecimal constants in your input data: @c line break here for small book format @example $ echo 0123 123 0x123 | > gawk --non-decimal-data '@{ printf "%d, %d, %d\n", > $1, $2, $3 @}' @print{} 83, 123, 291 @end example For this feature to work, write your program so that @command{gawk} treats your data as numeric: @example $ echo 0123 123 0x123 | gawk '@{ print $1, $2, $3 @}' @print{} 0123 123 0x123 @end example @noindent The @code{print} statement treats its expressions as strings. Although the fields can act as numbers when necessary, they are still strings, so @code{print} does not try to treat them numerically. You may need to add zero to a field to force it to be treated as a number. For example: @example $ echo 0123 123 0x123 | gawk --non-decimal-data ' > @{ print $1, $2, $3 > print $1 + 0, $2 + 0, $3 + 0 @}' @print{} 0123 123 0x123 @print{} 83 123 291 @end example Because it is common to have decimal data with leading zeros, and because using it could lead to surprising results, the default is to leave this facility disabled. If you want it, you must explicitly request it. @cindex programming conventions, @code{--non-decimal-data} option @cindex @code{--non-decimal-data} option, @code{strtonum} function and @cindex @code{strtonum} function (@command{gawk}), @code{--non-decimal-data} option and @strong{Caution:} @emph{Use of this option is not recommended.} It can break old programs very badly. Instead, use the @code{strtonum} function to convert your data (@pxref{Nondecimal-numbers}). This makes your programs easier to write and easier to read, and leads to less surprising results. @node Two-way I/O @section Two-Way Communications with Another Process @cindex Brennan, Michael @cindex programmers, attractiveness of @smallexample @c Path: cssun.mathcs.emory.edu!gatech!newsxfer3.itd.umich.edu!news-peer.sprintlink.net!news-sea-19.sprintlink.net!news-in-west.sprintlink.net!news.sprintlink.net!Sprint!204.94.52.5!news.whidbey.com!brennan From: brennan@@whidbey.com (Mike Brennan) Newsgroups: comp.lang.awk Subject: Re: Learn the SECRET to Attract Women Easily Date: 4 Aug 1997 17:34:46 GMT @c Organization: WhidbeyNet @c Lines: 12 Message-ID: <5s53rm$eca@@news.whidbey.com> @c References: <5s20dn$2e1@chronicle.concentric.net> @c Reply-To: brennan@whidbey.com @c NNTP-Posting-Host: asn202.whidbey.com @c X-Newsreader: slrn (0.9.4.1 UNIX) @c Xref: cssun.mathcs.emory.edu comp.lang.awk:5403 On 3 Aug 1997 13:17:43 GMT, Want More Dates??? wrote: >Learn the SECRET to Attract Women Easily > >The SCENT(tm) Pheromone Sex Attractant For Men to Attract Women The scent of awk programmers is a lot more attractive to women than the scent of perl programmers. -- Mike Brennan @c brennan@@whidbey.com @end smallexample @c final comma is part of tertiary @cindex advanced features, @command{gawk}, processes, communicating with @cindex processes, two-way communications with It is often useful to be able to send data to a separate program for processing and then read the result. This can always be done with temporary files: @example # write the data for processing tempfile = ("mydata." PROCINFO["pid"]) while (@var{not done with data}) print @var{data} | ("subprogram > " tempfile) close("subprogram > " tempfile) # read the results, remove tempfile when done while ((getline newdata < tempfile) > 0) @var{process} newdata @var{appropriately} close(tempfile) system("rm " tempfile) @end example @noindent This works, but not elegantly. Among other things, it requires that the program be run in a directory that cannot be shared among users; for example, @file{/tmp} will not do, as another user might happen to be using a temporary file with the same name. @cindex coprocesses @cindex input/output, two-way @cindex @code{|} (vertical bar), @code{|&} operator (I/O) @cindex vertical bar (@code{|}), @code{|&} I/O operator (I/O) @cindex @command{csh} utility, @code{|&} operator, comparison with Starting with @value{PVERSION} 3.1 of @command{gawk}, it is possible to open a @emph{two-way} pipe to another process. The second process is termed a @dfn{coprocess}, since it runs in parallel with @command{gawk}. The two-way connection is created using the new @samp{|&} operator (borrowed from the Korn shell, @command{ksh}):@footnote{This is very different from the same operator in the C shell, @command{csh}.} @example do @{ print @var{data} |& "subprogram" "subprogram" |& getline results @} while (@var{data left to process}) close("subprogram") @end example The first time an I/O operation is executed using the @samp{|&} operator, @command{gawk} creates a two-way pipeline to a child process that runs the other program. Output created with @code{print} or @code{printf} is written to the program's standard input, and output from the program's standard output can be read by the @command{gawk} program using @code{getline}. As is the case with processes started by @samp{|}, the subprogram can be any program, or pipeline of programs, that can be started by the shell. There are some cautionary items to be aware of: @itemize @bullet @item As the code inside @command{gawk} currently stands, the coprocess's standard error goes to the same place that the parent @command{gawk}'s standard error goes. It is not possible to read the child's standard error separately. @cindex deadlocks @cindex buffering, input/output @cindex @code{getline} command, deadlock and @item I/O buffering may be a problem. @command{gawk} automatically flushes all output down the pipe to the child process. However, if the coprocess does not flush its output, @command{gawk} may hang when doing a @code{getline} in order to read the coprocess's results. This could lead to a situation known as @dfn{deadlock}, where each process is waiting for the other one to do something. @end itemize @cindex @code{close} function, two-way pipes and It is possible to close just one end of the two-way pipe to a coprocess, by supplying a second argument to the @code{close} function of either @code{"to"} or @code{"from"} (@pxref{Close Files And Pipes}). These strings tell @command{gawk} to close the end of the pipe that sends data to the process or the end that reads from it, respectively. @cindex @command{sort} utility, coprocesses and This is particularly necessary in order to use the system @command{sort} utility as part of a coprocess; @command{sort} must read @emph{all} of its input data before it can produce any output. The @command{sort} program does not receive an end-of-file indication until @command{gawk} closes the write end of the pipe. When you have finished writing data to the @command{sort} utility, you can close the @code{"to"} end of the pipe, and then start reading sorted data via @code{getline}. For example: @example BEGIN @{ command = "LC_ALL=C sort" n = split("abcdefghijklmnopqrstuvwxyz", a, "") for (i = n; i > 0; i--) print a[i] |& command close(command, "to") while ((command |& getline line) > 0) print "got", line close(command) @} @end example This program writes the letters of the alphabet in reverse order, one per line, down the two-way pipe to @command{sort}. It then closes the write end of the pipe, so that @command{sort} receives an end-of-file indication. This causes @command{sort} to sort the data and write the sorted data back to the @command{gawk} program. Once all of the data has been read, @command{gawk} terminates the coprocess and exits. As a side note, the assignment @samp{LC_ALL=C} in the @command{sort} command ensures traditional Unix (ASCII) sorting from @command{sort}. Beginning with @command{gawk} 3.1.2, you may use Pseudo-ttys (ptys) for two-way communication instead of pipes, if your system supports them. This is done on a per-command basis, by setting a special element in the @code{PROCINFO} array (@pxref{Auto-set}), like so: @example command = "sort -nr" # command, saved in variable for convenience PROCINFO[command, "pty"] = 1 # update PROCINFO print @dots{} |& command # start two-way pipe @dots{} @end example @noindent Using ptys avoids the buffer deadlock issues described earlier, at some loss in performance. If your system does not have ptys, or if all the system's ptys are in use, @command{gawk} automatically falls back to using regular pipes. @node TCP/IP Networking @section Using @command{gawk} for Network Programming @cindex advanced features, @command{gawk}, network programming @cindex networks, programming @c STARTOFRANGE tcpip @cindex TCP/IP @cindex @code{/inet/} files (@command{gawk}) @cindex files, @code{/inet/} (@command{gawk}) @cindex @code{EMISTERED} @quotation @code{EMISTERED}: @i{A host is a host from coast to coast,@* and no-one can talk to host that's close,@* unless the host that isn't close@* is busy hung or dead.} @end quotation In addition to being able to open a two-way pipeline to a coprocess on the same system (@pxref{Two-way I/O}), it is possible to make a two-way connection to another process on another system across an IP networking connection. You can think of this as just a @emph{very long} two-way pipeline to a coprocess. The way @command{gawk} decides that you want to use TCP/IP networking is by recognizing special @value{FN}s that begin with @samp{/inet/}. The full syntax of the special @value{FN} is @file{/inet/@var{protocol}/@var{local-port}/@var{remote-host}/@var{remote-port}}. The components are: @table @var @item protocol The protocol to use over IP. This must be either @samp{tcp}, @samp{udp}, or @samp{raw}, for a TCP, UDP, or raw IP connection, respectively. The use of TCP is recommended for most applications. @cindex raw sockets @cindex sockets @strong{Caution:} The use of raw sockets is not currently supported in @value{PVERSION} 3.1 of @command{gawk}. @item local-port @cindex @code{getservbyname} function (C library) The local TCP or UDP port number to use. Use a port number of @samp{0} when you want the system to pick a port. This is what you should do when writing a TCP or UDP client. You may also use a well-known service name, such as @samp{smtp} or @samp{http}, in which case @command{gawk} attempts to determine the predefined port number using the C @code{getservbyname} function. @item remote-host The IP address or fully-qualified domain name of the Internet host to which you want to connect. @item remote-port The TCP or UDP port number to use on the given @var{remote-host}. Again, use @samp{0} if you don't care, or else a well-known service name. @end table Consider the following very simple example: @example BEGIN @{ Service = "/inet/tcp/0/localhost/daytime" Service |& getline print $0 close(Service) @} @end example This program reads the current date and time from the local system's TCP @samp{daytime} server. It then prints the results and closes the connection. Because this topic is extensive, the use of @command{gawk} for TCP/IP programming is documented separately. @ifinfo @xref{Top}, @end ifinfo @ifnotinfo See @cite{TCP/IP Internetworking with @command{gawk}}, which comes as part of the @command{gawk} distribution, @end ifnotinfo for a much more complete introduction and discussion, as well as extensive examples. @node Portal Files @section Using @command{gawk} with BSD Portals @cindex advanced features, @command{gawk}, BSD portals @cindex portal files @cindex files, portal @cindex BSD portals @cindex @code{/p} files (@command{gawk}) @cindex files, @code{/p} (@command{gawk}) @cindex @code{--enable-portals} configuration option @cindex operating systems, BSD-based Similar to the @file{/inet} special files, if @command{gawk} is configured with the @option{--enable-portals} option (@pxref{Quick Installation}), then @command{gawk} treats files whose pathnames begin with @code{/p} as 4.4 BSD-style portals. @cindex @code{|} (vertical bar), @code{|&} operator (I/O), two-way communications @cindex vertical bar (@code{|}), @code{|&} operator (I/O), two-way communications When used with the @samp{|&} operator, @command{gawk} opens the file for two-way communications. The operating system's portal mechanism then manages creating the process associated with the portal and the corresponding communications with the portal's process. @c ENDOFRANGE tcpip @node Profiling @section Profiling Your @command{awk} Programs @c STARTOFRANGE awkp @cindex @command{awk} programs, profiling @c STARTOFRANGE proawk @cindex profiling @command{awk} programs @c STARTOFRANGE pgawk @cindex @command{pgawk} program @cindex profiling @command{gawk}, See @command{pgawk} program Beginning with @value{PVERSION} 3.1 of @command{gawk}, you may produce execution traces of your @command{awk} programs. This is done with a specially compiled version of @command{gawk}, called @command{pgawk} (``profiling @command{gawk}''). @cindex @code{awkprof.out} file @cindex files, @code{awkprof.out} @cindex @command{pgawk} program, @code{awkprof.out} file @command{pgawk} is identical in every way to @command{gawk}, except that when it has finished running, it creates a profile of your program in a file named @file{awkprof.out}. Because it is profiling, it also executes up to 45% slower than @command{gawk} normally does. @cindex @code{--profile} option As shown in the following example, the @option{--profile} option can be used to change the name of the file where @command{pgawk} will write the profile: @example $ pgawk --profile=myprog.prof -f myprog.awk data1 data2 @end example @noindent In the above example, @command{pgawk} places the profile in @file{myprog.prof} instead of in @file{awkprof.out}. Regular @command{gawk} also accepts this option. When called with just @option{--profile}, @command{gawk} ``pretty prints'' the program into @file{awkprof.out}, without any execution counts. You may supply an option to @option{--profile} to change the @value{FN}. Here is a sample session showing a simple @command{awk} program, its input data, and the results from running @command{pgawk}. First, the @command{awk} program: @example BEGIN @{ print "First BEGIN rule" @} END @{ print "First END rule" @} /foo/ @{ print "matched /foo/, gosh" for (i = 1; i <= 3; i++) sing() @} @{ if (/foo/) print "if is true" else print "else is true" @} BEGIN @{ print "Second BEGIN rule" @} END @{ print "Second END rule" @} function sing( dummy) @{ print "I gotta be me!" @} @end example Following is the input data: @example foo bar baz foo junk @end example Here is the @file{awkprof.out} that results from running @command{pgawk} on this program and data (this example also illustrates that @command{awk} programmers sometimes have to work late): @cindex @code{BEGIN} pattern, @command{pgawk} program @cindex @code{END} pattern, @command{pgawk} program @example # gawk profile, created Sun Aug 13 00:00:15 2000 # BEGIN block(s) BEGIN @{ 1 print "First BEGIN rule" 1 print "Second BEGIN rule" @} # Rule(s) 5 /foo/ @{ # 2 2 print "matched /foo/, gosh" 6 for (i = 1; i <= 3; i++) @{ 6 sing() @} @} 5 @{ 5 if (/foo/) @{ # 2 2 print "if is true" 3 @} else @{ 3 print "else is true" @} @} # END block(s) END @{ 1 print "First END rule" 1 print "Second END rule" @} # Functions, listed alphabetically 6 function sing(dummy) @{ 6 print "I gotta be me!" @} @end example This example illustrates many of the basic rules for profiling output. The rules are as follows: @itemize @bullet @item The program is printed in the order @code{BEGIN} rule, pattern/action rules, @code{END} rule and functions, listed alphabetically. Multiple @code{BEGIN} and @code{END} rules are merged together. @cindex patterns, counts @item Pattern-action rules have two counts. The first count, to the left of the rule, shows how many times the rule's pattern was @emph{tested}. The second count, to the right of the rule's opening left brace in a comment, shows how many times the rule's action was @emph{executed}. The difference between the two indicates how many times the rule's pattern evaluated to false. @item Similarly, the count for an @code{if}-@code{else} statement shows how many times the condition was tested. To the right of the opening left brace for the @code{if}'s body is a count showing how many times the condition was true. The count for the @code{else} indicates how many times the test failed. @cindex loops, count for header @item The count for a loop header (such as @code{for} or @code{while}) shows how many times the loop test was executed. (Because of this, you can't just look at the count on the first statement in a rule to determine how many times the rule was executed. If the first statement is a loop, the count is misleading.) @cindex functions, user-defined, counts @cindex user-defined, functions, counts @item For user-defined functions, the count next to the @code{function} keyword indicates how many times the function was called. The counts next to the statements in the body show how many times those statements were executed. @cindex @code{@{@}} (braces), @command{pgawk} program @cindex braces (@code{@{@}}), @command{pgawk} program @item The layout uses ``K&R'' style with tabs. Braces are used everywhere, even when the body of an @code{if}, @code{else}, or loop is only a single statement. @cindex @code{()} (parentheses), @command{pgawk} program @cindex parentheses @code{()}, @command{pgawk} program @item Parentheses are used only where needed, as indicated by the structure of the program and the precedence rules. @c extra verbiage here satisfies the copyeditor. ugh. For example, @samp{(3 + 5) * 4} means add three plus five, then multiply the total by four. However, @samp{3 + 5 * 4} has no parentheses, and means @samp{3 + (5 * 4)}. @item All string concatenations are parenthesized too. (This could be made a bit smarter.) @item Parentheses are used around the arguments to @code{print} and @code{printf} only when the @code{print} or @code{printf} statement is followed by a redirection. Similarly, if the target of a redirection isn't a scalar, it gets parenthesized. @item @command{pgawk} supplies leading comments in front of the @code{BEGIN} and @code{END} rules, the pattern/action rules, and the functions. @end itemize The profiled version of your program may not look exactly like what you typed when you wrote it. This is because @command{pgawk} creates the profiled version by ``pretty printing'' its internal representation of the program. The advantage to this is that @command{pgawk} can produce a standard representation. The disadvantage is that all source-code comments are lost, as are the distinctions among multiple @code{BEGIN} and @code{END} rules. Also, things such as: @example /foo/ @end example @noindent come out as: @example /foo/ @{ print $0 @} @end example @noindent which is correct, but possibly surprising. @cindex profiling @command{awk} programs, dynamically @cindex @command{pgawk} program, dynamic profiling Besides creating profiles when a program has completed, @command{pgawk} can produce a profile while it is running. This is useful if your @command{awk} program goes into an infinite loop and you want to see what has been executed. To use this feature, run @command{pgawk} in the background: @example $ pgawk -f myprog & [1] 13992 @end example @c comma does NOT start secondary @cindex @command{kill} command, dynamic profiling @cindex @code{USR1} signal @cindex signals, @code{USR1}/@code{SIGUSR1} @noindent The shell prints a job number and process ID number; in this case, 13992. Use the @command{kill} command to send the @code{USR1} signal to @command{pgawk}: @example $ kill -USR1 13992 @end example @noindent As usual, the profiled version of the program is written to @file{awkprof.out}, or to a different file if you use the @option{--profile} option. Along with the regular profile, as shown earlier, the profile includes a trace of any active functions: @example # Function Call Stack: # 3. baz # 2. bar # 1. foo # -- main -- @end example You may send @command{pgawk} the @code{USR1} signal as many times as you like. Each time, the profile and function call trace are appended to the output profile file. @cindex @code{HUP} signal @cindex signals, @code{HUP}/@code{SIGHUP} If you use the @code{HUP} signal instead of the @code{USR1} signal, @command{pgawk} produces the profile and the function call trace and then exits. @cindex @code{INT} signal (MS-DOS) @cindex signals, @code{INT}/@code{SIGINT} (MS-DOS) @cindex @code{QUIT} signal (MS-DOS) @cindex signals, @code{QUIT}/@code{SIGQUIT} (MS-DOS) When @command{pgawk} runs on MS-DOS or MS-Windows, it uses the @code{INT} and @code{QUIT} signals for producing the profile and, in the case of the @code{INT} signal, @command{pgawk} exits. This is because these systems don't support the @command{kill} command, so the only signals you can deliver to a program are those generated by the keyboard. The @code{INT} signal is generated by the @kbd{@value{CTL}-@key{C}} or @kbd{@value{CTL}-@key{BREAK}} key, while the @code{QUIT} signal is generated by the @kbd{@value{CTL}-@key{\}} key. @c ENDOFRANGE advgaw @c ENDOFRANGE gawadv @c ENDOFRANGE pgawk @c ENDOFRANGE awkp @c ENDOFRANGE proawk @node Invoking Gawk @chapter Running @command{awk} and @command{gawk} This @value{CHAPTER} covers how to run awk, both POSIX-standard and @command{gawk}-specific command-line options, and what @command{awk} and @command{gawk} do with non-option arguments. It then proceeds to cover how @command{gawk} searches for source files, obsolete options and/or features, and known bugs in @command{gawk}. This @value{CHAPTER} rounds out the discussion of @command{awk} as a program and as a language. While a number of the options and features described here were discussed in passing earlier in the book, this @value{CHAPTER} provides the full details. @menu * Command Line:: How to run @command{awk}. * Options:: Command-line options and their meanings. * Other Arguments:: Input file names and variable assignments. * AWKPATH Variable:: Searching directories for @command{awk} programs. * Obsolete:: Obsolete Options and/or features. * Undocumented:: Undocumented Options and Features. * Known Bugs:: Known Bugs in @command{gawk}. @end menu @node Command Line @section Invoking @command{awk} @cindex command line, invoking @command{awk} from @cindex @command{awk}, invoking @cindex arguments, command-line, invoking @command{awk} @cindex options, command-line, invoking @command{awk} There are two ways to run @command{awk}---with an explicit program or with one or more program files. Here are templates for both of them; items enclosed in [@dots{}] in these templates are optional: @example awk @r{[@var{options}]} -f progfile @r{[@code{--}]} @var{file} @dots{} awk @r{[@var{options}]} @r{[@code{--}]} '@var{program}' @var{file} @dots{} @end example @cindex GNU long options @cindex long options @cindex options, long Besides traditional one-letter POSIX-style options, @command{gawk} also supports GNU long options. @cindex dark corner, invoking @command{awk} @cindex lint checking, empty programs It is possible to invoke @command{awk} with an empty program: @example awk '' datafile1 datafile2 @end example @cindex @code{--lint} option @noindent Doing so makes little sense, though; @command{awk} exits silently when given an empty program. @value{DARKCORNER} If @option{--lint} has been specified on the command line, @command{gawk} issues a warning that the program is empty. @node Options @section Command-Line Options @c STARTOFRANGE ocl @cindex options, command-line @c STARTOFRANGE clo @cindex command line, options @c STARTOFRANGE gnulo @cindex GNU long options @c STARTOFRANGE longo @cindex options, long Options begin with a dash and consist of a single character. GNU-style long options consist of two dashes and a keyword. The keyword can be abbreviated, as long as the abbreviation allows the option to be uniquely identified. If the option takes an argument, then the keyword is either immediately followed by an equals sign (@samp{=}) and the argument's value, or the keyword and the argument's value are separated by whitespace. If a particular option with a value is given more than once, it is the last value that counts. @cindex POSIX @command{awk}, GNU long options and Each long option for @command{gawk} has a corresponding POSIX-style option. The long and short options are interchangeable in all contexts. The options and their meanings are as follows: @table @code @item -F @var{fs} @itemx --field-separator @var{fs} @cindex @code{-F} option @cindex @code{--field-separator} option @cindex @code{FS} variable, @code{--field-separator} option and Sets the @code{FS} variable to @var{fs} (@pxref{Field Separators}). @item -f @var{source-file} @itemx --file @var{source-file} @cindex @code{-f} option @cindex @code{--file} option @cindex @command{awk} programs, location of Indicates that the @command{awk} program is to be found in @var{source-file} instead of in the first non-option argument. @item -v @var{var}=@var{val} @itemx --assign @var{var}=@var{val} @cindex @code{-v} option @cindex @code{--assign} option @cindex variables, setting Sets the variable @var{var} to the value @var{val} @emph{before} execution of the program begins. Such variable values are available inside the @code{BEGIN} rule (@pxref{Other Arguments}). The @option{-v} option can only set one variable, but it can be used more than once, setting another variable each time, like this: @samp{awk @w{-v foo=1} @w{-v bar=2} @dots{}}. @c last comma is part of secondary @cindex built-in variables, @code{-v} option, setting with @c last comma is part of tertiary @cindex variables, built-in, @code{-v} option, setting with @strong{Caution:} Using @option{-v} to set the values of the built-in variables may lead to surprising results. @command{awk} will reset the values of those variables as it needs to, possibly ignoring any predefined value you may have given. @item -mf @var{N} @itemx -mr @var{N} @cindex @code{-mf}/@code{-mr} options @cindex memory, setting limits Sets various memory limits to the value @var{N}. The @samp{f} flag sets the maximum number of fields and the @samp{r} flag sets the maximum record size. These two flags and the @option{-m} option are from the Bell Laboratories research version of Unix @command{awk}. They are provided for compatibility but otherwise ignored by @command{gawk}, since @command{gawk} has no predefined limits. (The Bell Laboratories @command{awk} no longer needs these options; it continues to accept them to avoid breaking old programs.) @item -W @var{gawk-opt} @cindex @code{-W} option Following the POSIX standard, implementation-specific options are supplied as arguments to the @option{-W} option. These options also have corresponding GNU-style long options. Note that the long options may be abbreviated, as long as the abbreviations remain unique. The full list of @command{gawk}-specific options is provided next. @item -- @cindex command line, options, end of @cindex options, command-line, end of Signals the end of the command-line options. The following arguments are not treated as options even if they begin with @samp{-}. This interpretation of @option{--} follows the POSIX argument parsing conventions. @cindex @code{-} (hyphen), filenames beginning with @cindex hyphen (@code{-}), filenames beginning with This is useful if you have @value{FN}s that start with @samp{-}, or in shell scripts, if you have @value{FN}s that will be specified by the user that could start with @samp{-}. @end table @c ENDOFRANGE gnulo @c ENDOFRANGE longo The previous list described options mandated by the POSIX standard, as well as options available in the Bell Laboratories version of @command{awk}. The following list describes @command{gawk}-specific options: @table @code @item -W compat @itemx -W traditional @itemx --compat @itemx --traditional @cindex @code{--compat} option @cindex @code{--traditional} option @cindex compatibility mode (@command{gawk}), specifying Specifies @dfn{compatibility mode}, in which the GNU extensions to the @command{awk} language are disabled, so that @command{gawk} behaves just like the Bell Laboratories research version of Unix @command{awk}. @option{--traditional} is the preferred form of this option. @xref{POSIX/GNU}, which summarizes the extensions. Also see @ref{Compatibility Mode}. @item -W copyright @itemx --copyright @cindex @code{--copyright} option @cindex GPL (General Public License), printing Print the short version of the General Public License and then exit. @item -W copyleft @itemx --copyleft @cindex @code{--copyleft} option Just like @option{--copyright}. This option may disappear in a future version of @command{gawk}. @cindex @code{--dump-variables} option @cindex @code{awkvars.out} file @cindex files, @code{awkvars.out} @cindex variables, global, printing list of @item -W dump-variables@r{[}=@var{file}@r{]} @itemx --dump-variables@r{[}=@var{file}@r{]} Prints a sorted list of global variables, their types, and final values to @var{file}. If no @var{file} is provided, @command{gawk} prints this list to the file named @file{awkvars.out} in the current directory. @c last comma is part of secondary @cindex troubleshooting, typographical errors, global variables Having a list of all global variables is a good way to look for typographical errors in your programs. You would also use this option if you have a large program with a lot of functions, and you want to be sure that your functions don't inadvertently use global variables that you meant to be local. (This is a particularly easy mistake to make with simple variable names like @code{i}, @code{j}, etc.) @item -W gen-po @itemx --gen-po @cindex @code{--gen-po} option @cindex portable object files, generating @cindex files, portable object, generating Analyzes the source program and generates a GNU @code{gettext} Portable Object file on standard output for all string constants that have been marked for translation. @xref{Internationalization}, for information about this option. @item -W help @itemx -W usage @itemx --help @itemx --usage @cindex @code{--help} option @cindex @code{--usage} option @cindex GNU long options, printing list of @cindex options, printing list of @cindex printing, list of options Prints a ``usage'' message summarizing the short and long style options that @command{gawk} accepts and then exit. @item -W lint@r{[}=fatal@r{]} @itemx --lint@r{[}=fatal@r{]} @cindex @code{--lint} option @cindex lint checking, issuing warnings @cindex warnings, issuing Warns about constructs that are dubious or nonportable to other @command{awk} implementations. Some warnings are issued when @command{gawk} first reads your program. Others are issued at runtime, as your program executes. With an optional argument of @samp{fatal}, lint warnings become fatal errors. This may be drastic, but its use will certainly encourage the development of cleaner @command{awk} programs. With an optional argument of @samp{invalid}, only warnings about things that are actually invalid are issued. (This is not fully implemented yet.) @item -W lint-old @itemx --lint-old @cindex @code{--lint-old} option Warns about constructs that are not available in the original version of @command{awk} from Version 7 Unix (@pxref{V7/SVR3.1}). @item -W non-decimal-data @itemx --non-decimal-data @cindex @code{--non-decimal-data} option @cindex hexadecimal, values, enabling interpretation of @c comma is part of primary @cindex octal values, enabling interpretation of Enable automatic interpretation of octal and hexadecimal values in input data (@pxref{Nondecimal Data}). @cindex troubleshooting, @code{--non-decimal-data} option @strong{Caution:} This option can severely break old programs. Use with care. @item -W posix @itemx --posix @cindex @code{--posix} option @cindex POSIX mode @c last comma is part of tertiary @cindex @command{gawk}, extensions, disabling Operates in strict POSIX mode. This disables all @command{gawk} extensions (just like @option{--traditional}) and adds the following additional restrictions: @c IMPORTANT! Keep this list in sync with the one in node POSIX @itemize @bullet @cindex escape sequences, unrecognized @item @code{\x} escape sequences are not recognized (@pxref{Escape Sequences}). @cindex newlines @cindex whitespace, newlines as @item Newlines do not act as whitespace to separate fields when @code{FS} is equal to a single space (@pxref{Fields}). @item Newlines are not allowed after @samp{?} or @samp{:} (@pxref{Conditional Exp}). @item The synonym @code{func} for the keyword @code{function} is not recognized (@pxref{Definition Syntax}). @cindex @code{*} (asterisk), @code{**} operator @cindex asterisk (@code{*}), @code{**} operator @cindex @code{*} (asterisk), @code{**=} operator @cindex asterisk (@code{*}), @code{**=} operator @cindex @code{^} (caret), @code{^} operator @cindex caret (@code{^}), @code{^} operator @cindex @code{^} (caret), @code{^=} operator @cindex caret (@code{^}), @code{^=} operator @item The @samp{**} and @samp{**=} operators cannot be used in place of @samp{^} and @samp{^=} (@pxref{Arithmetic Ops}, and also @pxref{Assignment Ops}). @cindex @code{FS} variable, as TAB character @item Specifying @samp{-Ft} on the command-line does not set the value of @code{FS} to be a single TAB character (@pxref{Field Separators}). @c comma does not start secondary @cindex @code{fflush} function, unsupported @item The @code{fflush} built-in function is not supported (@pxref{I/O Functions}). @end itemize @c @cindex automatic warnings @c @cindex warnings, automatic @cindex @code{--traditional} option, @code{--posix} option and @cindex @code{--posix} option, @code{--traditional} option and If you supply both @option{--traditional} and @option{--posix} on the command line, @option{--posix} takes precedence. @command{gawk} also issues a warning if both options are supplied. @item -W profile@r{[}=@var{file}@r{]} @itemx --profile@r{[}=@var{file}@r{]} @cindex @code{--profile} option @cindex @command{awk} programs, profiling, enabling Enable profiling of @command{awk} programs (@pxref{Profiling}). By default, profiles are created in a file named @file{awkprof.out}. The optional @var{file} argument allows you to specify a different @value{FN} for the profile file. When run with @command{gawk}, the profile is just a ``pretty printed'' version of the program. When run with @command{pgawk}, the profile contains execution counts for each statement in the program in the left margin, and function call counts for each function. @item -W re-interval @itemx --re-interval @cindex @code{--re-interval} option @cindex regular expressions, interval expressions and Allows interval expressions (@pxref{Regexp Operators}) in regexps. Because interval expressions were traditionally not available in @command{awk}, @command{gawk} does not provide them by default. This prevents old @command{awk} programs from breaking. @item -W source @var{program-text} @itemx --source @var{program-text} @cindex @code{--source} option @cindex source code, mixing Allows you to mix source code in files with source code that you enter on the command line. Program source code is taken from the @var{program-text}. This is particularly useful when you have library functions that you want to use from your command-line programs (@pxref{AWKPATH Variable}). @item -W version @itemx --version @cindex @code{--version} option @c last comma is part of tertiary @cindex @command{gawk}, versions of, information about, printing Prints version information for this particular copy of @command{gawk}. This allows you to determine if your copy of @command{gawk} is up to date with respect to whatever the Free Software Foundation is currently distributing. It is also useful for bug reports (@pxref{Bugs}). @end table As long as program text has been supplied, any other options are flagged as invalid with a warning message but are otherwise ignored. @cindex @code{-F} option, @code{-Ft} sets @code{FS} to TAB In compatibility mode, as a special case, if the value of @var{fs} supplied to the @option{-F} option is @samp{t}, then @code{FS} is set to the TAB character (@code{"\t"}). This is true only for @option{--traditional} and not for @option{--posix} (@pxref{Field Separators}). @cindex @code{-f} option, on command line The @option{-f} option may be used more than once on the command line. If it is, @command{awk} reads its program source from all of the named files, as if they had been concatenated together into one big file. This is useful for creating libraries of @command{awk} functions. These functions can be written once and then retrieved from a standard place, instead of having to be included into each individual program. (As mentioned in @ref{Definition Syntax}, function names must be unique.) Library functions can still be used, even if the program is entered at the terminal, by specifying @samp{-f /dev/tty}. After typing your program, type @kbd{@value{CTL}-d} (the end-of-file character) to terminate it. (You may also use @samp{-f -} to read program source from the standard input but then you will not be able to also use the standard input as a source of data.) Because it is clumsy using the standard @command{awk} mechanisms to mix source file and command-line @command{awk} programs, @command{gawk} provides the @option{--source} option. This does not require you to pre-empt the standard input for your source code; it allows you to easily mix command-line and library source code (@pxref{AWKPATH Variable}). @cindex @code{--source} option If no @option{-f} or @option{--source} option is specified, then @command{gawk} uses the first non-option command-line argument as the text of the program source code. @cindex @code{POSIXLY_CORRECT} environment variable @cindex lint checking, @code{POSIXLY_CORRECT} environment variable @cindex POSIX mode If the environment variable @env{POSIXLY_CORRECT} exists, then @command{gawk} behaves in strict POSIX mode, exactly as if you had supplied the @option{--posix} command-line option. Many GNU programs look for this environment variable to turn on strict POSIX mode. If @option{--lint} is supplied on the command line and @command{gawk} turns on POSIX mode because of @env{POSIXLY_CORRECT}, then it issues a warning message indicating that POSIX mode is in effect. You would typically set this variable in your shell's startup file. For a Bourne-compatible shell (such as @command{bash}), you would add these lines to the @file{.profile} file in your home directory: @example POSIXLY_CORRECT=true export POSIXLY_CORRECT @end example @cindex @command{csh} utility, @code{POSIXLY_CORRECT} environment variable For a @command{csh}-compatible shell,@footnote{Not recommended.} you would add this line to the @file{.login} file in your home directory: @example setenv POSIXLY_CORRECT true @end example @cindex portability, @code{POSIXLY_CORRECT} environment variable Having @env{POSIXLY_CORRECT} set is not recommended for daily use, but it is good for testing the portability of your programs to other environments. @c ENDOFRANGE ocl @c ENDOFRANGE clo @node Other Arguments @section Other Command-Line Arguments @cindex command line, arguments @cindex arguments, command-line Any additional arguments on the command line are normally treated as input files to be processed in the order specified. However, an argument that has the form @code{@var{var}=@var{value}}, assigns the value @var{value} to the variable @var{var}---it does not specify a file at all. (This was discussed earlier in @ref{Assignment Options}.) @cindex @code{ARGIND} variable, command-line arguments @cindex @code{ARGC}/@code{ARGV} variables, command-line arguments All these arguments are made available to your @command{awk} program in the @code{ARGV} array (@pxref{Built-in Variables}). Command-line options and the program text (if present) are omitted from @code{ARGV}. All other arguments, including variable assignments, are included. As each element of @code{ARGV} is processed, @command{gawk} sets the variable @code{ARGIND} to the index in @code{ARGV} of the current element. @cindex input files, variable assignments and The distinction between @value{FN} arguments and variable-assignment arguments is made when @command{awk} is about to open the next input file. At that point in execution, it checks the @value{FN} to see whether it is really a variable assignment; if so, @command{awk} sets the variable instead of reading a file. Therefore, the variables actually receive the given values after all previously specified files have been read. In particular, the values of variables assigned in this fashion are @emph{not} available inside a @code{BEGIN} rule (@pxref{BEGIN/END}), because such rules are run before @command{awk} begins scanning the argument list. @cindex dark corner, escape sequences The variable values given on the command line are processed for escape sequences (@pxref{Escape Sequences}). @value{DARKCORNER} In some earlier implementations of @command{awk}, when a variable assignment occurred before any @value{FN}s, the assignment would happen @emph{before} the @code{BEGIN} rule was executed. @command{awk}'s behavior was thus inconsistent; some command-line assignments were available inside the @code{BEGIN} rule, while others were not. Unfortunately, some applications came to depend upon this ``feature.'' When @command{awk} was changed to be more consistent, the @option{-v} option was added to accommodate applications that depended upon the old behavior. The variable assignment feature is most useful for assigning to variables such as @code{RS}, @code{OFS}, and @code{ORS}, which control input and output formats before scanning the @value{DF}s. It is also useful for controlling state if multiple passes are needed over a @value{DF}. For example: @cindex files, multiple passes over @example awk 'pass == 1 @{ @var{pass 1 stuff} @} pass == 2 @{ @var{pass 2 stuff} @}' pass=1 mydata pass=2 mydata @end example Given the variable assignment feature, the @option{-F} option for setting the value of @code{FS} is not strictly necessary. It remains for historical compatibility. @node AWKPATH Variable @section The @env{AWKPATH} Environment Variable @cindex @env{AWKPATH} environment variable @cindex directories, searching @cindex search paths, for source files @cindex differences in @command{awk} and @command{gawk}, @code{AWKPATH} environment variable @ifinfo The previous @value{SECTION} described how @command{awk} program files can be named on the command-line with the @option{-f} option. @end ifinfo In most @command{awk} implementations, you must supply a precise path name for each program file, unless the file is in the current directory. But in @command{gawk}, if the @value{FN} supplied to the @option{-f} option does not contain a @samp{/}, then @command{gawk} searches a list of directories (called the @dfn{search path}), one by one, looking for a file with the specified name. The search path is a string consisting of directory names separated by colons. @command{gawk} gets its search path from the @env{AWKPATH} environment variable. If that variable does not exist, @command{gawk} uses a default path, @samp{.:/usr/local/share/awk}.@footnote{Your version of @command{gawk} may use a different directory; it will depend upon how @command{gawk} was built and installed. The actual directory is the value of @samp{$(datadir)} generated when @command{gawk} was configured. You probably don't need to worry about this, though.} (Programs written for use by system administrators should use an @env{AWKPATH} variable that does not include the current directory, @file{.}.) The search path feature is particularly useful for building libraries of useful @command{awk} functions. The library files can be placed in a standard directory in the default path and then specified on the command line with a short @value{FN}. Otherwise, the full @value{FN} would have to be typed for each file. By using both the @option{--source} and @option{-f} options, your command-line @command{awk} programs can use facilities in @command{awk} library files (@pxref{Library Functions}). Path searching is not done if @command{gawk} is in compatibility mode. This is true for both @option{--traditional} and @option{--posix}. @xref{Options}. @strong{Note:} If you want files in the current directory to be found, you must include the current directory in the path, either by including @file{.} explicitly in the path or by writing a null entry in the path. (A null entry is indicated by starting or ending the path with a colon or by placing two colons next to each other (@samp{::}).) If the current directory is not included in the path, then files cannot be found in the current directory. This path search mechanism is identical to the shell's. @c someday, @cite{The Bourne Again Shell}.... Starting with @value{PVERSION} 3.0, if @env{AWKPATH} is not defined in the environment, @command{gawk} places its default search path into @code{ENVIRON["AWKPATH"]}. This makes it easy to determine the actual search path that @command{gawk} will use from within an @command{awk} program. While you can change @code{ENVIRON["AWKPATH"]} within your @command{awk} program, this has no effect on the running program's behavior. This makes sense: the @env{AWKPATH} environment variable is used to find the program source files. Once your program is running, all the files have been found, and @command{gawk} no longer needs to use @env{AWKPATH}. @node Obsolete @section Obsolete Options and/or Features @cindex features, advanced, See advanced features @cindex options, deprecated @cindex features, deprecated @cindex obsolete features This @value{SECTION} describes features and/or command-line options from previous releases of @command{gawk} that are either not available in the current version or that are still supported but deprecated (meaning that they will @emph{not} be in the next release). @c update this section for each release! @cindex @code{next file} statement, deprecated @cindex @code{nextfile} statement, @code{next file} statement and For @value{PVERSION} @value{VERSION} of @command{gawk}, there are no deprecated command-line options @c or other deprecated features from the previous version of @command{gawk}. The use of @samp{next file} (two words) for @code{nextfile} was deprecated in @command{gawk} 3.0 but still worked. Starting with @value{PVERSION} 3.1, the two-word usage is no longer accepted. The process-related special files described in @ref{Special Process}, work as described, but are now considered deprecated. @command{gawk} prints a warning message every time they are used. (Use @code{PROCINFO} instead; see @ref{Auto-set}.) They will be removed from the next release of @command{gawk}. @ignore This @value{SECTION} is thus essentially a place holder, in case some option becomes obsolete in a future version of @command{gawk}. @end ignore @node Undocumented @section Undocumented Options and Features @cindex undocumented features @cindex features, undocumented @cindex Skywalker, Luke @cindex Kenobi, Obi-Wan @cindex Jedi knights @cindex Knights, jedi @quotation @i{Use the Source, Luke!}@* Obi-Wan @end quotation This @value{SECTION} intentionally left blank. @ignore @c If these came out in the Info file or TeX document, then they wouldn't @c be undocumented, would they? @command{gawk} has one undocumented option: @table @code @item -W nostalgia @itemx --nostalgia Print the message @code{"awk: bailing out near line 1"} and dump core. This option was inspired by the common behavior of very early versions of Unix @command{awk} and by a t--shirt. The message is @emph{not} subject to translation in non-English locales. @c so there! nyah, nyah. @end table Early versions of @command{awk} used to not require any separator (either a newline or @samp{;}) between the rules in @command{awk} programs. Thus, it was common to see one-line programs like: @example awk '@{ sum += $1 @} END @{ print sum @}' @end example @command{gawk} actually supports this but it is purposely undocumented because it is considered bad style. The correct way to write such a program is either @example awk '@{ sum += $1 @} ; END @{ print sum @}' @end example @noindent or @example awk '@{ sum += $1 @} END @{ print sum @}' data @end example @noindent @xref{Statements/Lines}, for a fuller explanation. You can insert newlines after the @samp{;} in @code{for} loops. This seems to have been a long-undocumented feature in Unix @command{awk}. Similarly, you may use @code{print} or @code{printf} statements in the @var{init} and @var{increment} parts of a @code{for} loop. This is another long-undocumented ``feature'' of Unix @code{awk}. If the environment variable @env{WHINY_USERS} exists when @command{gawk} is run, then the associative @code{for} loop will go through the array indices in sorted order. The comparison used for sorting is simple string comparison; any non-English or non-ASCII locales are not taken into account. @code{IGNORECASE} does not affect the comparison either. In addition, if @env{WHINY_USERS} is set, the profiled version of a program generated by @option{--profile} will print all 8-bit characters verbatim, instead of using the octal equivalent. @end ignore @node Known Bugs @section Known Bugs in @command{gawk} @cindex @command{gawk}, debugging @cindex debugging @command{gawk} @cindex troubleshooting, @command{gawk} @itemize @bullet @cindex troubleshooting, @code{-F} option @cindex @code{-F} option, troubleshooting @cindex @code{FS} variable, changing value of @item The @option{-F} option for changing the value of @code{FS} (@pxref{Options}) is not necessary given the command-line variable assignment feature; it remains only for backward compatibility. @item Syntactically invalid single-character programs tend to overflow the parse stack, generating a rather unhelpful message. Such programs are surprisingly difficult to diagnose in the completely general case, and the effort to do so really is not worth it. @end itemize @ignore @c Try this @iftex @page @headings off @majorheading II@ @ @ Using @command{awk} and @command{gawk} Part II shows how to use @command{awk} and @command{gawk} for problem solving. There is lots of code here for you to read and learn from. It contains the following chapters: @itemize @bullet @item @ref{Library Functions}. @item @ref{Sample Programs}. @end itemize @page @evenheading @thispage@ @ @ @strong{@value{TITLE}} @| @| @oddheading @| @| @strong{@thischapter}@ @ @ @thispage @end iftex @end ignore @node Library Functions @chapter A Library of @command{awk} Functions @c STARTOFRANGE libf @cindex libraries of @command{awk} functions @c STARTOFRANGE flib @cindex functions, library @c STARTOFRANGE fudlib @cindex functions, user-defined, library of @ref{User-defined}, describes how to write your own @command{awk} functions. Writing functions is important, because it allows you to encapsulate algorithms and program tasks in a single place. It simplifies programming, making program development more manageable, and making programs more readable. One valuable way to learn a new programming language is to @emph{read} programs in that language. To that end, this @value{CHAPTER} and @ref{Sample Programs}, provide a good-sized body of code for you to read, and hopefully, to learn from. @c 2e: USE TEXINFO-2 FUNCTION DEFINITION STUFF!!!!!!!!!!!!! This @value{CHAPTER} presents a library of useful @command{awk} functions. Many of the sample programs presented later in this @value{DOCUMENT} use these functions. The functions are presented here in a progression from simple to complex. @cindex Texinfo @ref{Extract Program}, presents a program that you can use to extract the source code for these example library functions and programs from the Texinfo source for this @value{DOCUMENT}. (This has already been done as part of the @command{gawk} distribution.) If you have written one or more useful, general-purpose @command{awk} functions and would like to contribute them to the author's collection of @command{awk} programs, see @ref{How To Contribute}, for more information. @cindex portability, example programs The programs in this @value{CHAPTER} and in @ref{Sample Programs}, freely use features that are @command{gawk}-specific. Rewriting these programs for different implementations of awk is pretty straightforward. Diagnostic error messages are sent to @file{/dev/stderr}. Use @samp{| "cat 1>&2"} instead of @samp{> "/dev/stderr"} if your system does not have a @file{/dev/stderr}, or if you cannot use @command{gawk}. A number of programs use @code{nextfile} (@pxref{Nextfile Statement}) to skip any remaining input in the input file. @ref{Nextfile Function}, shows you how to write a function that does the same thing. @c 12/2000: Thanks to Nelson Beebe for pointing out the output issue. @cindex case sensitivity, example programs @cindex @code{IGNORECASE} variable, in example programs Finally, some of the programs choose to ignore upper- and lowercase distinctions in their input. They do so by assigning one to @code{IGNORECASE}. You can achieve almost the same effect@footnote{The effects are not identical. Output of the transformed record will be in all lowercase, while @code{IGNORECASE} preserves the original contents of the input record.} by adding the following rule to the beginning of the program: @example # ignore case @{ $0 = tolower($0) @} @end example @noindent Also, verify that all regexp and string constants used in comparisons use only lowercase letters. @menu * Library Names:: How to best name private global variables in library functions. * General Functions:: Functions that are of general use. * Data File Management:: Functions for managing command-line data files. * Getopt Function:: A function for processing command-line arguments. * Passwd Functions:: Functions for getting user information. * Group Functions:: Functions for getting group information. @end menu @node Library Names @section Naming Library Function Global Variables @cindex names, arrays/variables @cindex names, functions @cindex namespace issues @cindex @command{awk} programs, documenting @cindex documentation, of @command{awk} programs Due to the way the @command{awk} language evolved, variables are either @dfn{global} (usable by the entire program) or @dfn{local} (usable just by a specific function). There is no intermediate state analogous to @code{static} variables in C. @cindex variables, global, for library functions @cindex private variables @cindex variables, private Library functions often need to have global variables that they can use to preserve state information between calls to the function---for example, @code{getopt}'s variable @code{_opti} (@pxref{Getopt Function}). Such variables are called @dfn{private}, since the only functions that need to use them are the ones in the library. When writing a library function, you should try to choose names for your private variables that will not conflict with any variables used by either another library function or a user's main program. For example, a name like @samp{i} or @samp{j} is not a good choice, because user programs often use variable names like these for their own purposes. @cindex programming conventions, private variable names The example programs shown in this @value{CHAPTER} all start the names of their private variables with an underscore (@samp{_}). Users generally don't use leading underscores in their variable names, so this convention immediately decreases the chances that the variable name will be accidentally shared with the user's program. @cindex @code{_} (underscore), in names of private variables @cindex underscore (@code{_}), in names of private variables In addition, several of the library functions use a prefix that helps indicate what function or set of functions use the variables---for example, @code{_pw_byname} in the user database routines (@pxref{Passwd Functions}). This convention is recommended, since it even further decreases the chance of inadvertent conflict among variable names. Note that this convention is used equally well for variable names and for private function names as well.@footnote{While all the library routines could have been rewritten to use this convention, this was not done, in order to show how my own @command{awk} programming style has evolved and to provide some basis for this discussion.} As a final note on variable naming, if a function makes global variables available for use by a main program, it is a good convention to start that variable's name with a capital letter---for example, @code{getopt}'s @code{Opterr} and @code{Optind} variables (@pxref{Getopt Function}). The leading capital letter indicates that it is global, while the fact that the variable name is not all capital letters indicates that the variable is not one of @command{awk}'s built-in variables, such as @code{FS}. @cindex @code{--dump-variables} option It is also important that @emph{all} variables in library functions that do not need to save state are, in fact, declared local.@footnote{@command{gawk}'s @option{--dump-variables} command-line option is useful for verifying this.} If this is not done, the variable could accidentally be used in the user's program, leading to bugs that are very difficult to track down: @example function lib_func(x, y, l1, l2) @{ @dots{} @var{use variable} some_var # some_var should be local @dots{} # but is not by oversight @} @end example @cindex arrays, associative, library functions and @cindex libraries of @command{awk} functions, associative arrays and @cindex functions, library, associative arrays and @cindex Tcl A different convention, common in the Tcl community, is to use a single associative array to hold the values needed by the library function(s), or ``package.'' This significantly decreases the number of actual global names in use. For example, the functions described in @ref{Passwd Functions}, might have used array elements @code{@w{PW_data["inited"]}}, @code{@w{PW_data["total"]}}, @code{@w{PW_data["count"]}}, and @code{@w{PW_data["awklib"]}}, instead of @code{@w{_pw_inited}}, @code{@w{_pw_awklib}}, @code{@w{_pw_total}}, and @code{@w{_pw_count}}. The conventions presented in this @value{SECTION} are exactly that: conventions. You are not required to write your programs this way---we merely recommend that you do so. @node General Functions @section General Programming This @value{SECTION} presents a number of functions that are of general programming use. @menu * Nextfile Function:: Two implementations of a @code{nextfile} function. * Assert Function:: A function for assertions in @command{awk} programs. * Round Function:: A function for rounding if @code{sprintf} does not do it correctly. * Cliff Random Function:: The Cliff Random Number Generator. * Ordinal Functions:: Functions for using characters as numbers and vice versa. * Join Function:: A function to join an array into a string. * Gettimeofday Function:: A function to get formatted times. @end menu @node Nextfile Function @subsection Implementing @code{nextfile} as a Function @cindex input files, skipping @c STARTOFRANGE libfnex @cindex libraries of @command{awk} functions, @code{nextfile} statement @c STARTOFRANGE flibnex @cindex functions, library, @code{nextfile} statement @c STARTOFRANGE nexim @cindex @code{nextfile} statement, implementing @cindex @command{gawk}, @code{nextfile} statement in The @code{nextfile} statement, presented in @ref{Nextfile Statement}, is a @command{gawk}-specific extension---it is not available in most other implementations of @command{awk}. This @value{SECTION} shows two versions of a @code{nextfile} function that you can use to simulate @command{gawk}'s @code{nextfile} statement if you cannot use @command{gawk}. A first attempt at writing a @code{nextfile} function is as follows: @example # nextfile --- skip remaining records in current file # this should be read in before the "main" awk program function nextfile() @{ _abandon_ = FILENAME; next @} _abandon_ == FILENAME @{ next @} @end example @cindex programming conventions, @code{nextfile} statement Because it supplies a rule that must be executed first, this file should be included before the main program. This rule compares the current @value{DF}'s name (which is always in the @code{FILENAME} variable) to a private variable named @code{_abandon_}. If the @value{FN} matches, then the action part of the rule executes a @code{next} statement to go on to the next record. (The use of @samp{_} in the variable name is a convention. It is discussed more fully in @ref{Library Names}.) The use of the @code{next} statement effectively creates a loop that reads all the records from the current @value{DF}. The end of the file is eventually reached and a new @value{DF} is opened, changing the value of @code{FILENAME}. Once this happens, the comparison of @code{_abandon_} to @code{FILENAME} fails, and execution continues with the first rule of the ``real'' program. The @code{nextfile} function itself simply sets the value of @code{_abandon_} and then executes a @code{next} statement to start the loop. @ignore @c If the function can't be used on other versions of awk, this whole @c section is pointless, no? Sigh. @footnote{@command{gawk} is the only known @command{awk} implementation that allows you to execute @code{next} from within a function body. Some other workaround is necessary if you are not using @command{gawk}.} @end ignore @cindex @code{nextfile} user-defined function This initial version has a subtle problem. If the same @value{DF} is listed @emph{twice} on the commandline, one right after the other or even with just a variable assignment between them, this code skips right through the file a second time, even though it should stop when it gets to the end of the first occurrence. A second version of @code{nextfile} that remedies this problem is shown here: @example @c file eg/lib/nextfile.awk # nextfile --- skip remaining records in current file # correctly handle successive occurrences of the same file @c endfile @ignore @c file eg/lib/nextfile.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # May, 1993 @c endfile @end ignore @c file eg/lib/nextfile.awk # this should be read in before the "main" awk program function nextfile() @{ _abandon_ = FILENAME; next @} _abandon_ == FILENAME @{ if (FNR == 1) _abandon_ = "" else next @} @c endfile @end example The @code{nextfile} function has not changed. It makes @code{_abandon_} equal to the current @value{FN} and then executes a @code{next} statement. The @code{next} statement reads the next record and increments @code{FNR} so that @code{FNR} is guaranteed to have a value of at least two. However, if @code{nextfile} is called for the last record in the file, then @command{awk} closes the current @value{DF} and moves on to the next one. Upon doing so, @code{FILENAME} is set to the name of the new file and @code{FNR} is reset to one. If this next file is the same as the previous one, @code{_abandon_} is still equal to @code{FILENAME}. However, @code{FNR} is equal to one, telling us that this is a new occurrence of the file and not the one we were reading when the @code{nextfile} function was executed. In that case, @code{_abandon_} is reset to the empty string, so that further executions of this rule fail (until the next time that @code{nextfile} is called). If @code{FNR} is not one, then we are still in the original @value{DF} and the program executes a @code{next} statement to skip through it. An important question to ask at this point is: given that the functionality of @code{nextfile} can be provided with a library file, why is it built into @command{gawk}? Adding features for little reason leads to larger, slower programs that are harder to maintain. The answer is that building @code{nextfile} into @command{gawk} provides significant gains in efficiency. If the @code{nextfile} function is executed at the beginning of a large @value{DF}, @command{awk} still has to scan the entire file, splitting it up into records, @c at least conceptually just to skip over it. The built-in @code{nextfile} can simply close the file immediately and proceed to the next one, which saves a lot of time. This is particularly important in @command{awk}, because @command{awk} programs are generally I/O-bound (i.e., they spend most of their time doing input and output, instead of performing computations). @c ENDOFRANGE libfnex @c ENDOFRANGE flibnex @c ENDOFRANGE nexim @node Assert Function @subsection Assertions @c STARTOFRANGE asse @cindex assertions @c STARTOFRANGE assef @cindex @code{assert} function (C library) @c STARTOFRANGE libfass @cindex libraries of @command{awk} functions, assertions @c STARTOFRANGE flibass @cindex functions, library, assertions @cindex @command{awk} programs, lengthy, assertions When writing large programs, it is often useful to know that a condition or set of conditions is true. Before proceeding with a particular computation, you make a statement about what you believe to be the case. Such a statement is known as an @dfn{assertion}. The C language provides an @code{} header file and corresponding @code{assert} macro that the programmer can use to make assertions. If an assertion fails, the @code{assert} macro arranges to print a diagnostic message describing the condition that should have been true but was not, and then it kills the program. In C, using @code{assert} looks this: @example #include int myfunc(int a, double b) @{ assert(a <= 5 && b >= 17.1); @dots{} @} @end example If the assertion fails, the program prints a message similar to this: @example prog.c:5: assertion failed: a <= 5 && b >= 17.1 @end example @cindex @code{assert} user-defined function The C language makes it possible to turn the condition into a string for use in printing the diagnostic message. This is not possible in @command{awk}, so this @code{assert} function also requires a string version of the condition that is being tested. Following is the function: @example @c file eg/lib/assert.awk # assert --- assert that a condition is true. Otherwise exit. @c endfile @ignore @c file eg/lib/assert.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # May, 1993 @c endfile @end ignore @c file eg/lib/assert.awk function assert(condition, string) @{ if (! condition) @{ printf("%s:%d: assertion failed: %s\n", FILENAME, FNR, string) > "/dev/stderr" _assert_exit = 1 exit 1 @} @} @group END @{ if (_assert_exit) exit 1 @} @end group @c endfile @end example The @code{assert} function tests the @code{condition} parameter. If it is false, it prints a message to standard error, using the @code{string} parameter to describe the failed condition. It then sets the variable @code{_assert_exit} to one and executes the @code{exit} statement. The @code{exit} statement jumps to the @code{END} rule. If the @code{END} rules finds @code{_assert_exit} to be true, it exits immediately. The purpose of the test in the @code{END} rule is to keep any other @code{END} rules from running. When an assertion fails, the program should exit immediately. If no assertions fail, then @code{_assert_exit} is still false when the @code{END} rule is run normally, and the rest of the program's @code{END} rules execute. For all of this to work correctly, @file{assert.awk} must be the first source file read by @command{awk}. The function can be used in a program in the following way: @example function myfunc(a, b) @{ assert(a <= 5 && b >= 17.1, "a <= 5 && b >= 17.1") @dots{} @} @end example @noindent If the assertion fails, you see a message similar to the following: @example mydata:1357: assertion failed: a <= 5 && b >= 17.1 @end example @cindex @code{END} pattern, @code{assert} user-defined function and There is a small problem with this version of @code{assert}. An @code{END} rule is automatically added to the program calling @code{assert}. Normally, if a program consists of just a @code{BEGIN} rule, the input files and/or standard input are not read. However, now that the program has an @code{END} rule, @command{awk} attempts to read the input @value{DF}s or standard input (@pxref{Using BEGIN/END}), most likely causing the program to hang as it waits for input. @cindex @code{BEGIN} pattern, @code{assert} user-defined function and There is a simple workaround to this: make sure the @code{BEGIN} rule always ends with an @code{exit} statement. @c ENDOFRANGE asse @c ENDOFRANGE assef @c ENDOFRANGE flibass @c ENDOFRANGE libfass @node Round Function @subsection Rounding Numbers @cindex rounding @cindex rounding numbers @cindex numbers, rounding @cindex libraries of @command{awk} functions, rounding numbers @cindex functions, library, rounding numbers @cindex @code{print} statement, @code{sprintf} function and @cindex @code{printf} statement, @code{sprintf} function and @cindex @code{sprintf} function, @code{print}/@code{printf} statements and The way @code{printf} and @code{sprintf} (@pxref{Printf}) perform rounding often depends upon the system's C @code{sprintf} subroutine. On many machines, @code{sprintf} rounding is ``unbiased,'' which means it doesn't always round a trailing @samp{.5} up, contrary to naive expectations. In unbiased rounding, @samp{.5} rounds to even, rather than always up, so 1.5 rounds to 2 but 4.5 rounds to 4. This means that if you are using a format that does rounding (e.g., @code{"%.0f"}), you should check what your system does. The following function does traditional rounding; it might be useful if your awk's @code{printf} does unbiased rounding: @cindex @code{round} user-defined function @example @c file eg/lib/round.awk # round.awk --- do normal rounding @c endfile @ignore @c file eg/lib/round.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # August, 1996 @c endfile @end ignore @c file eg/lib/round.awk function round(x, ival, aval, fraction) @{ ival = int(x) # integer part, int() truncates # see if fractional part if (ival == x) # no fraction return x if (x < 0) @{ aval = -x # absolute value ival = int(aval) fraction = aval - ival if (fraction >= .5) return int(x) - 1 # -2.5 --> -3 else return int(x) # -2.3 --> -2 @} else @{ fraction = x - ival if (fraction >= .5) return ival + 1 else return ival @} @} # test harness @{ print $0, round($0) @} @c endfile @end example @node Cliff Random Function @subsection The Cliff Random Number Generator @cindex random numbers, Cliff @cindex Cliff random numbers @cindex numbers, Cliff random @cindex functions, library, Cliff random numbers The Cliff random number generator@footnote{@uref{http://mathworld.wolfram.com/CliffRandomNumberGenerator.hmtl}} is a very simple random number generator that ``passes the noise sphere test for randomness by showing no structure.'' It is easily programmed, in less than 10 lines of @command{awk} code: @cindex @code{cliff_rand} user-defined function @example @c file eg/lib/cliff_rand.awk # cliff_rand.awk --- generate Cliff random numbers @c endfile @ignore @c file eg/lib/cliff_rand.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # December 2000 @c endfile @end ignore @c file eg/lib/cliff_rand.awk BEGIN @{ _cliff_seed = 0.1 @} function cliff_rand() @{ _cliff_seed = (100 * log(_cliff_seed)) % 1 if (_cliff_seed < 0) _cliff_seed = - _cliff_seed return _cliff_seed @} @c endfile @end example This algorithm requires an initial ``seed'' of 0.1. Each new value uses the current seed as input for the calculation. If the built-in @code{rand} function (@pxref{Numeric Functions}) isn't random enough, you might try using this function instead. @node Ordinal Functions @subsection Translating Between Characters and Numbers @cindex libraries of @command{awk} functions, character values as numbers @cindex functions, library, character values as numbers @cindex characters, values of as numbers @cindex numbers, as values of characters One commercial implementation of @command{awk} supplies a built-in function, @code{ord}, which takes a character and returns the numeric value for that character in the machine's character set. If the string passed to @code{ord} has more than one character, only the first one is used. The inverse of this function is @code{chr} (from the function of the same name in Pascal), which takes a number and returns the corresponding character. Both functions are written very nicely in @command{awk}; there is no real reason to build them into the @command{awk} interpreter: @cindex @code{ord} user-defined function @cindex @code{chr} user-defined function @example @c file eg/lib/ord.awk # ord.awk --- do ord and chr # Global identifiers: # _ord_: numerical values indexed by characters # _ord_init: function to initialize _ord_ @c endfile @ignore @c file eg/lib/ord.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # 16 January, 1992 # 20 July, 1992, revised @c endfile @end ignore @c file eg/lib/ord.awk BEGIN @{ _ord_init() @} function _ord_init( low, high, i, t) @{ low = sprintf("%c", 7) # BEL is ascii 7 if (low == "\a") @{ # regular ascii low = 0 high = 127 @} else if (sprintf("%c", 128 + 7) == "\a") @{ # ascii, mark parity low = 128 high = 255 @} else @{ # ebcdic(!) low = 0 high = 255 @} for (i = low; i <= high; i++) @{ t = sprintf("%c", i) _ord_[t] = i @} @} @c endfile @end example @cindex character sets @cindex character encodings @cindex ASCII @cindex EBCDIC @cindex mark parity Some explanation of the numbers used by @code{chr} is worthwhile. The most prominent character set in use today is ASCII. Although an 8-bit byte can hold 256 distinct values (from 0 to 255), ASCII only defines characters that use the values from 0 to 127.@footnote{ASCII has been extended in many countries to use the values from 128 to 255 for country-specific characters. If your system uses these extensions, you can simplify @code{_ord_init} to simply loop from 0 to 255.} In the now distant past, at least one minicomputer manufacturer @c Pr1me, blech used ASCII, but with mark parity, meaning that the leftmost bit in the byte is always 1. This means that on those systems, characters have numeric values from 128 to 255. Finally, large mainframe systems use the EBCDIC character set, which uses all 256 values. While there are other character sets in use on some older systems, they are not really worth worrying about: @example @c file eg/lib/ord.awk function ord(str, c) @{ # only first character is of interest c = substr(str, 1, 1) return _ord_[c] @} function chr(c) @{ # force c to be numeric by adding 0 return sprintf("%c", c + 0) @} @c endfile #### test code #### # BEGIN \ # @{ # for (;;) @{ # printf("enter a character: ") # if (getline var <= 0) # break # printf("ord(%s) = %d\n", var, ord(var)) # @} # @} @c endfile @end example An obvious improvement to these functions is to move the code for the @code{@w{_ord_init}} function into the body of the @code{BEGIN} rule. It was written this way initially for ease of development. There is a ``test program'' in a @code{BEGIN} rule, to test the function. It is commented out for production use. @node Join Function @subsection Merging an Array into a String @cindex libraries of @command{awk} functions, merging arrays into strings @cindex functions, library, merging arrays into strings @cindex strings, merging arrays into @cindex arrays, merging into strings When doing string processing, it is often useful to be able to join all the strings in an array into one long string. The following function, @code{join}, accomplishes this task. It is used later in several of the application programs (@pxref{Sample Programs}). Good function design is important; this function needs to be general but it should also have a reasonable default behavior. It is called with an array as well as the beginning and ending indices of the elements in the array to be merged. This assumes that the array indices are numeric---a reasonable assumption since the array was likely created with @code{split} (@pxref{String Functions}): @cindex @code{join} user-defined function @example @c file eg/lib/join.awk # join.awk --- join an array into a string @c endfile @ignore @c file eg/lib/join.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # May 1993 @c endfile @end ignore @c file eg/lib/join.awk function join(array, start, end, sep, result, i) @{ if (sep == "") sep = " " else if (sep == SUBSEP) # magic value sep = "" result = array[start] for (i = start + 1; i <= end; i++) result = result sep array[i] return result @} @c endfile @end example An optional additional argument is the separator to use when joining the strings back together. If the caller supplies a nonempty value, @code{join} uses it; if it is not supplied, it has a null value. In this case, @code{join} uses a single blank as a default separator for the strings. If the value is equal to @code{SUBSEP}, then @code{join} joins the strings with no separator between them. @code{SUBSEP} serves as a ``magic'' value to indicate that there should be no separation between the component strings.@footnote{It would be nice if @command{awk} had an assignment operator for concatenation. The lack of an explicit operator for concatenation makes string operations more difficult than they really need to be.} @node Gettimeofday Function @subsection Managing the Time of Day @cindex libraries of @command{awk} functions, managing, time @cindex functions, library, managing time @cindex timestamps, formatted @cindex time, managing The @code{systime} and @code{strftime} functions described in @ref{Time Functions}, provide the minimum functionality necessary for dealing with the time of day in human readable form. While @code{strftime} is extensive, the control formats are not necessarily easy to remember or intuitively obvious when reading a program. The following function, @code{gettimeofday}, populates a user-supplied array with preformatted time information. It returns a string with the current time formatted in the same way as the @command{date} utility: @cindex @code{gettimeofday} user-defined function @example @c file eg/lib/gettime.awk # gettimeofday.awk --- get the time of day in a usable format @c endfile @ignore @c file eg/lib/gettime.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain, May 1993 # @c endfile @end ignore @c file eg/lib/gettime.awk # Returns a string in the format of output of date(1) # Populates the array argument time with individual values: # time["second"] -- seconds (0 - 59) # time["minute"] -- minutes (0 - 59) # time["hour"] -- hours (0 - 23) # time["althour"] -- hours (0 - 12) # time["monthday"] -- day of month (1 - 31) # time["month"] -- month of year (1 - 12) # time["monthname"] -- name of the month # time["shortmonth"] -- short name of the month # time["year"] -- year modulo 100 (0 - 99) # time["fullyear"] -- full year # time["weekday"] -- day of week (Sunday = 0) # time["altweekday"] -- day of week (Monday = 0) # time["dayname"] -- name of weekday # time["shortdayname"] -- short name of weekday # time["yearday"] -- day of year (0 - 365) # time["timezone"] -- abbreviation of timezone name # time["ampm"] -- AM or PM designation # time["weeknum"] -- week number, Sunday first day # time["altweeknum"] -- week number, Monday first day function gettimeofday(time, ret, now, i) @{ # get time once, avoids unnecessary system calls now = systime() # return date(1)-style output ret = strftime("%a %b %d %H:%M:%S %Z %Y", now) # clear out target array delete time # fill in values, force numeric values to be # numeric by adding 0 time["second"] = strftime("%S", now) + 0 time["minute"] = strftime("%M", now) + 0 time["hour"] = strftime("%H", now) + 0 time["althour"] = strftime("%I", now) + 0 time["monthday"] = strftime("%d", now) + 0 time["month"] = strftime("%m", now) + 0 time["monthname"] = strftime("%B", now) time["shortmonth"] = strftime("%b", now) time["year"] = strftime("%y", now) + 0 time["fullyear"] = strftime("%Y", now) + 0 time["weekday"] = strftime("%w", now) + 0 time["altweekday"] = strftime("%u", now) + 0 time["dayname"] = strftime("%A", now) time["shortdayname"] = strftime("%a", now) time["yearday"] = strftime("%j", now) + 0 time["timezone"] = strftime("%Z", now) time["ampm"] = strftime("%p", now) time["weeknum"] = strftime("%U", now) + 0 time["altweeknum"] = strftime("%W", now) + 0 return ret @} @c endfile @end example The string indices are easier to use and read than the various formats required by @code{strftime}. The @code{alarm} program presented in @ref{Alarm Program}, uses this function. A more general design for the @code{gettimeofday} function would have allowed the user to supply an optional timestamp value to use instead of the current time. @node Data File Management @section @value{DDF} Management @c STARTOFRANGE dataf @cindex files, managing @c STARTOFRANGE libfdataf @cindex libraries of @command{awk} functions, managing, @value{DF}s @c STARTOFRANGE flibdataf @cindex functions, library, managing @value{DF}s This @value{SECTION} presents functions that are useful for managing command-line @value{DF}s. @menu * Filetrans Function:: A function for handling data file transitions. * Rewind Function:: A function for rereading the current file. * File Checking:: Checking that data files are readable. * Empty Files:: Checking for zero-length files. * Ignoring Assigns:: Treating assignments as file names. @end menu @node Filetrans Function @subsection Noting @value{DDF} Boundaries @cindex files, managing, @value{DF} boundaries @cindex files, initialization and cleanup The @code{BEGIN} and @code{END} rules are each executed exactly once at the beginning and end of your @command{awk} program, respectively (@pxref{BEGIN/END}). We (the @command{gawk} authors) once had a user who mistakenly thought that the @code{BEGIN} rule is executed at the beginning of each @value{DF} and the @code{END} rule is executed at the end of each @value{DF}. When informed that this was not the case, the user requested that we add new special patterns to @command{gawk}, named @code{BEGIN_FILE} and @code{END_FILE}, that would have the desired behavior. He even supplied us the code to do so. Adding these special patterns to @command{gawk} wasn't necessary; the job can be done cleanly in @command{awk} itself, as illustrated by the following library program. It arranges to call two user-supplied functions, @code{beginfile} and @code{endfile}, at the beginning and end of each @value{DF}. Besides solving the problem in only nine(!) lines of code, it does so @emph{portably}; this works with any implementation of @command{awk}: @example # transfile.awk # # Give the user a hook for filename transitions # # The user must supply functions beginfile() and endfile() # that each take the name of the file being started or # finished, respectively. @c # @c # Arnold Robbins, arnold@@gnu.org, Public Domain @c # January 1992 FILENAME != _oldfilename \ @{ if (_oldfilename != "") endfile(_oldfilename) _oldfilename = FILENAME beginfile(FILENAME) @} END @{ endfile(FILENAME) @} @end example This file must be loaded before the user's ``main'' program, so that the rule it supplies is executed first. This rule relies on @command{awk}'s @code{FILENAME} variable that automatically changes for each new @value{DF}. The current @value{FN} is saved in a private variable, @code{_oldfilename}. If @code{FILENAME} does not equal @code{_oldfilename}, then a new @value{DF} is being processed and it is necessary to call @code{endfile} for the old file. Because @code{endfile} should only be called if a file has been processed, the program first checks to make sure that @code{_oldfilename} is not the null string. The program then assigns the current @value{FN} to @code{_oldfilename} and calls @code{beginfile} for the file. Because, like all @command{awk} variables, @code{_oldfilename} is initialized to the null string, this rule executes correctly even for the first @value{DF}. The program also supplies an @code{END} rule to do the final processing for the last file. Because this @code{END} rule comes before any @code{END} rules supplied in the ``main'' program, @code{endfile} is called first. Once again the value of multiple @code{BEGIN} and @code{END} rules should be clear. @cindex @code{beginfile} user-defined function @cindex @code{endfile} user-defined function This version has same problem as the first version of @code{nextfile} (@pxref{Nextfile Function}). If the same @value{DF} occurs twice in a row on the command line, then @code{endfile} and @code{beginfile} are not executed at the end of the first pass and at the beginning of the second pass. The following version solves the problem: @example @c file eg/lib/ftrans.awk # ftrans.awk --- handle data file transitions # # user supplies beginfile() and endfile() functions @c endfile @ignore @c file eg/lib/ftrans.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # November 1992 @c endfile @end ignore @c file eg/lib/ftrans.awk FNR == 1 @{ if (_filename_ != "") endfile(_filename_) _filename_ = FILENAME beginfile(FILENAME) @} END @{ endfile(_filename_) @} @c endfile @end example @ref{Wc Program}, shows how this library function can be used and how it simplifies writing the main program. @node Rewind Function @subsection Rereading the Current File @cindex files, reading Another request for a new built-in function was for a @code{rewind} function that would make it possible to reread the current file. The requesting user didn't want to have to use @code{getline} (@pxref{Getline}) inside a loop. However, as long as you are not in the @code{END} rule, it is quite easy to arrange to immediately close the current input file and then start over with it from the top. For lack of a better name, we'll call it @code{rewind}: @cindex @code{rewind} user-defined function @example @c file eg/lib/rewind.awk # rewind.awk --- rewind the current file and start over @c endfile @ignore @c file eg/lib/rewind.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # September 2000 @c endfile @end ignore @c file eg/lib/rewind.awk function rewind( i) @{ # shift remaining arguments up for (i = ARGC; i > ARGIND; i--) ARGV[i] = ARGV[i-1] # make sure gawk knows to keep going ARGC++ # make current file next to get done ARGV[ARGIND+1] = FILENAME # do it nextfile @} @c endfile @end example This code relies on the @code{ARGIND} variable (@pxref{Auto-set}), which is specific to @command{gawk}. If you are not using @command{gawk}, you can use ideas presented in @ifnotinfo the previous @value{SECTION} @end ifnotinfo @ifinfo @ref{Filetrans Function}, @end ifinfo to either update @code{ARGIND} on your own or modify this code as appropriate. The @code{rewind} function also relies on the @code{nextfile} keyword (@pxref{Nextfile Statement}). @xref{Nextfile Function}, for a function version of @code{nextfile}. @node File Checking @subsection Checking for Readable @value{DDF}s @cindex troubleshooting, readable @value{DF}s @c comma is part of primary @cindex readable @value{DF}s, checking @cindex files, skipping Normally, if you give @command{awk} a @value{DF} that isn't readable, it stops with a fatal error. There are times when you might want to just ignore such files and keep going. You can do this by prepending the following program to your @command{awk} program: @cindex @code{readable.awk} program @example @c file eg/lib/readable.awk # readable.awk --- library file to skip over unreadable files @c endfile @ignore @c file eg/lib/readable.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # October 2000 @c endfile @end ignore @c file eg/lib/readable.awk BEGIN @{ for (i = 1; i < ARGC; i++) @{ if (ARGV[i] ~ /^[A-Za-z_][A-Za-z0-9_]*=.*/ \ || ARGV[i] == "-") continue # assignment or standard input else if ((getline junk < ARGV[i]) < 0) # unreadable delete ARGV[i] else close(ARGV[i]) @} @} @c endfile @end example @cindex troubleshooting, @code{getline} function In @command{gawk}, the @code{getline} won't be fatal (unless @option{--posix} is in force). Removing the element from @code{ARGV} with @code{delete} skips the file (since it's no longer in the list). @c This doesn't handle /dev/stdin etc. Not worth the hassle to mention or fix. @node Empty Files @subsection Checking For Zero-length Files All known @command{awk} implementations silently skip over zero-length files. This is a by-product of @command{awk}'s implicit read-a-record-and-match-against-the-rules loop: when @command{awk} tries to read a record from an empty file, it immediately receives an end of file indication, closes the file, and proceeds on to the next command-line @value{DF}, @emph{without} executing any user-level @command{awk} program code. Using @command{gawk}'s @code{ARGIND} variable (@pxref{Built-in Variables}), it is possible to detect when an empty @value{DF} has been skipped. Similar to the library file presented in @ref{Filetrans Function}, the following library file calls a function named @code{zerofile} that the user must provide. The arguments passed are the @value{FN} and the position in @code{ARGV} where it was found: @cindex @code{zerofile.awk} program @example @c file eg/lib/zerofile.awk # zerofile.awk --- library file to process empty input files @c endfile @ignore @c file eg/lib/zerofile.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # June 2003 @c endfile @end ignore @c file eg/lib/zerofile.awk BEGIN @{ Argind = 0 @} ARGIND > Argind + 1 @{ for (Argind++; Argind < ARGIND; Argind++) zerofile(ARGV[Argind], Argind) @} ARGIND != Argind @{ Argind = ARGIND @} END @{ if (ARGIND > Argind) for (Argind++; Argind <= ARGIND; Argind++) zerofile(ARGV[Argind], Argind) @} @c endfile @end example The user-level variable @code{Argind} allows the @command{awk} program to track its progress through @code{ARGV}. Whenever the program detects that @code{ARGIND} is greater than @samp{Argind + 1}, it means that one or more empty files were skipped. The action then calls @code{zerofile} for each such file, incrementing @code{Argind} along the way. The @samp{Argind != ARGIND} rule simply keeps @code{Argind} up to date in the normal case. Finally, the @code{END} rule catches the case of any empty files at the end of the command-line arguments. Note that the test in the condition of the @code{for} loop uses the @samp{<=} operator, not @code{<}. As an exercise, you might consider whether this same problem can be solved without relying on @command{gawk}'s @code{ARGIND} variable. As a second exercise, revise this code to handle the case where an intervening value in @code{ARGV} is a variable assignment. @ignore # zerofile2.awk --- same thing, portably BEGIN @{ ARGIND = Argind = 0 for (i = 1; i < ARGC; i++) Fnames[ARGV[i]]++ @} FNR == 1 @{ while (ARGV[ARGIND] != FILENAME) ARGIND++ Seen[FILENAME]++ if (Seen[FILENAME] == Fnames[FILENAME]) do ARGIND++ while (ARGV[ARGIND] != FILENAME) @} ARGIND > Argind + 1 @{ for (Argind++; Argind < ARGIND; Argind++) zerofile(ARGV[Argind], Argind) @} ARGIND != Argind @{ Argind = ARGIND @} END @{ if (ARGIND < ARGC - 1) ARGIND = ARGC - 1 if (ARGIND > Argind) for (Argind++; Argind <= ARGIND; Argind++) zerofile(ARGV[Argind], Argind) @} @end ignore @node Ignoring Assigns @subsection Treating Assignments as @value{FFN}s @cindex assignments as filenames @cindex filenames, assignments as Occasionally, you might not want @command{awk} to process command-line variable assignments (@pxref{Assignment Options}). In particular, if you have @value{FN}s that contain an @samp{=} character, @command{awk} treats the @value{FN} as an assignment, and does not process it. Some users have suggested an additional command-line option for @command{gawk} to disable command-line assignments. However, some simple programming with a library file does the trick: @cindex @code{noassign.awk} program @example @c file eg/lib/noassign.awk # noassign.awk --- library file to avoid the need for a # special option that disables command-line assignments @c endfile @ignore @c file eg/lib/noassign.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # October 1999 @c endfile @end ignore @c file eg/lib/noassign.awk function disable_assigns(argc, argv, i) @{ for (i = 1; i < argc; i++) if (argv[i] ~ /^[A-Za-z_][A-Za-z_0-9]*=.*/) argv[i] = ("./" argv[i]) @} BEGIN @{ if (No_command_assign) disable_assigns(ARGC, ARGV) @} @c endfile @end example You then run your program this way: @example awk -v No_command_assign=1 -f noassign.awk -f yourprog.awk * @end example The function works by looping through the arguments. It prepends @samp{./} to any argument that matches the form of a variable assignment, turning that argument into a @value{FN}. The use of @code{No_command_assign} allows you to disable command-line assignments at invocation time, by giving the variable a true value. When not set, it is initially zero (i.e., false), so the command-line arguments are left alone. @c ENDOFRANGE dataf @c ENDOFRANGE flibdataf @c ENDOFRANGE libfdataf @node Getopt Function @section Processing Command-Line Options @c STARTOFRANGE libfclo @cindex libraries of @command{awk} functions, command-line options @c STARTOFRANGE flibclo @cindex functions, library, command-line options @c STARTOFRANGE clop @cindex command-line options, processing @c STARTOFRANGE oclp @cindex options, command-line, processing @c STARTOFRANGE clibf @cindex functions, library, C library @cindex arguments, processing Most utilities on POSIX compatible systems take options, or ``switches,'' on the command line that can be used to change the way a program behaves. @command{awk} is an example of such a program (@pxref{Options}). Often, options take @dfn{arguments}; i.e., data that the program needs to correctly obey the command-line option. For example, @command{awk}'s @option{-F} option requires a string to use as the field separator. The first occurrence on the command line of either @option{--} or a string that does not begin with @samp{-} ends the options. @cindex @code{getopt} function (C library) Modern Unix systems provide a C function named @code{getopt} for processing command-line arguments. The programmer provides a string describing the one-letter options. If an option requires an argument, it is followed in the string with a colon. @code{getopt} is also passed the count and values of the command-line arguments and is called in a loop. @code{getopt} processes the command-line arguments for option letters. Each time around the loop, it returns a single character representing the next option letter that it finds, or @samp{?} if it finds an invalid option. When it returns @minus{}1, there are no options left on the command line. When using @code{getopt}, options that do not take arguments can be grouped together. Furthermore, options that take arguments require that the argument is present. The argument can immediately follow the option letter, or it can be a separate command-line argument. Given a hypothetical program that takes three command-line options, @option{-a}, @option{-b}, and @option{-c}, where @option{-b} requires an argument, all of the following are valid ways of invoking the program: @example prog -a -b foo -c data1 data2 data3 prog -ac -bfoo -- data1 data2 data3 prog -acbfoo data1 data2 data3 @end example Notice that when the argument is grouped with its option, the rest of the argument is considered to be the option's argument. In this example, @option{-acbfoo} indicates that all of the @option{-a}, @option{-b}, and @option{-c} options were supplied, and that @samp{foo} is the argument to the @option{-b} option. @code{getopt} provides four external variables that the programmer can use: @table @code @item optind The index in the argument value array (@code{argv}) where the first nonoption command-line argument can be found. @item optarg The string value of the argument to an option. @item opterr Usually @code{getopt} prints an error message when it finds an invalid option. Setting @code{opterr} to zero disables this feature. (An application might want to print its own error message.) @item optopt The letter representing the command-line option. @c While not usually documented, most versions supply this variable. @end table The following C fragment shows how @code{getopt} might process command-line arguments for @command{awk}: @example int main(int argc, char *argv[]) @{ @dots{} /* print our own message */ opterr = 0; while ((c = getopt(argc, argv, "v:f:F:W:")) != -1) @{ switch (c) @{ case 'f': /* file */ @dots{} break; case 'F': /* field separator */ @dots{} break; case 'v': /* variable assignment */ @dots{} break; case 'W': /* extension */ @dots{} break; case '?': default: usage(); break; @} @} @dots{} @} @end example As a side point, @command{gawk} actually uses the GNU @code{getopt_long} function to process both normal and GNU-style long options (@pxref{Options}). The abstraction provided by @code{getopt} is very useful and is quite handy in @command{awk} programs as well. Following is an @command{awk} version of @code{getopt}. This function highlights one of the greatest weaknesses in @command{awk}, which is that it is very poor at manipulating single characters. Repeated calls to @code{substr} are necessary for accessing individual characters (@pxref{String Functions}).@footnote{This function was written before @command{gawk} acquired the ability to split strings into single characters using @code{""} as the separator. We have left it alone, since using @code{substr} is more portable.} The discussion that follows walks through the code a bit at a time: @cindex @code{getopt} user-defined function @example @c file eg/lib/getopt.awk # getopt.awk --- do C library getopt(3) function in awk @c endfile @ignore @c file eg/lib/getopt.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # # Initial version: March, 1991 # Revised: May, 1993 @c endfile @end ignore @c file eg/lib/getopt.awk # External variables: # Optind -- index in ARGV of first nonoption argument # Optarg -- string value of argument to current option # Opterr -- if nonzero, print our own diagnostic # Optopt -- current option letter # Returns: # -1 at end of options # ? for unrecognized option # a character representing the current option # Private Data: # _opti -- index in multi-flag option, e.g., -abc @c endfile @end example The function starts out with a list of the global variables it uses, what the return values are, what they mean, and any global variables that are ``private'' to this library function. Such documentation is essential for any program, and particularly for library functions. The @code{getopt} function first checks that it was indeed called with a string of options (the @code{options} parameter). If @code{options} has a zero length, @code{getopt} immediately returns @minus{}1: @cindex @code{getopt} user-defined function @example @c file eg/lib/getopt.awk function getopt(argc, argv, options, thisopt, i) @{ if (length(options) == 0) # no options given return -1 @group if (argv[Optind] == "--") @{ # all done Optind++ _opti = 0 return -1 @end group @} else if (argv[Optind] !~ /^-[^: \t\n\f\r\v\b]/) @{ _opti = 0 return -1 @} @c endfile @end example The next thing to check for is the end of the options. A @option{--} ends the command-line options, as does any command-line argument that does not begin with a @samp{-}. @code{Optind} is used to step through the array of command-line arguments; it retains its value across calls to @code{getopt}, because it is a global variable. The regular expression that is used, @code{@w{/^-[^: \t\n\f\r\v\b]/}}, is perhaps a bit of overkill; it checks for a @samp{-} followed by anything that is not whitespace and not a colon. If the current command-line argument does not match this pattern, it is not an option, and it ends option processing: @example @c file eg/lib/getopt.awk if (_opti == 0) _opti = 2 thisopt = substr(argv[Optind], _opti, 1) Optopt = thisopt i = index(options, thisopt) if (i == 0) @{ if (Opterr) printf("%c -- invalid option\n", thisopt) > "/dev/stderr" if (_opti >= length(argv[Optind])) @{ Optind++ _opti = 0 @} else _opti++ return "?" @} @c endfile @end example The @code{_opti} variable tracks the position in the current command-line argument (@code{argv[Optind]}). If multiple options are grouped together with one @samp{-} (e.g., @option{-abx}), it is necessary to return them to the user one at a time. If @code{_opti} is equal to zero, it is set to two, which is the index in the string of the next character to look at (we skip the @samp{-}, which is at position one). The variable @code{thisopt} holds the character, obtained with @code{substr}. It is saved in @code{Optopt} for the main program to use. If @code{thisopt} is not in the @code{options} string, then it is an invalid option. If @code{Opterr} is nonzero, @code{getopt} prints an error message on the standard error that is similar to the message from the C version of @code{getopt}. Because the option is invalid, it is necessary to skip it and move on to the next option character. If @code{_opti} is greater than or equal to the length of the current command-line argument, it is necessary to move on to the next argument, so @code{Optind} is incremented and @code{_opti} is reset to zero. Otherwise, @code{Optind} is left alone and @code{_opti} is merely incremented. In any case, because the option is invalid, @code{getopt} returns @samp{?}. The main program can examine @code{Optopt} if it needs to know what the invalid option letter actually is. Continuing on: @example @c file eg/lib/getopt.awk if (substr(options, i + 1, 1) == ":") @{ # get option argument if (length(substr(argv[Optind], _opti + 1)) > 0) Optarg = substr(argv[Optind], _opti + 1) else Optarg = argv[++Optind] _opti = 0 @} else Optarg = "" @c endfile @end example If the option requires an argument, the option letter is followed by a colon in the @code{options} string. If there are remaining characters in the current command-line argument (@code{argv[Optind]}), then the rest of that string is assigned to @code{Optarg}. Otherwise, the next command-line argument is used (@samp{-xFOO} versus @samp{@w{-x FOO}}). In either case, @code{_opti} is reset to zero, because there are no more characters left to examine in the current command-line argument. Continuing: @example @c file eg/lib/getopt.awk if (_opti == 0 || _opti >= length(argv[Optind])) @{ Optind++ _opti = 0 @} else _opti++ return thisopt @} @c endfile @end example Finally, if @code{_opti} is either zero or greater than the length of the current command-line argument, it means this element in @code{argv} is through being processed, so @code{Optind} is incremented to point to the next element in @code{argv}. If neither condition is true, then only @code{_opti} is incremented, so that the next option letter can be processed on the next call to @code{getopt}. The @code{BEGIN} rule initializes both @code{Opterr} and @code{Optind} to one. @code{Opterr} is set to one, since the default behavior is for @code{getopt} to print a diagnostic message upon seeing an invalid option. @code{Optind} is set to one, since there's no reason to look at the program name, which is in @code{ARGV[0]}: @example @c file eg/lib/getopt.awk BEGIN @{ Opterr = 1 # default is to diagnose Optind = 1 # skip ARGV[0] # test program if (_getopt_test) @{ while ((_go_c = getopt(ARGC, ARGV, "ab:cd")) != -1) printf("c = <%c>, optarg = <%s>\n", _go_c, Optarg) printf("non-option arguments:\n") for (; Optind < ARGC; Optind++) printf("\tARGV[%d] = <%s>\n", Optind, ARGV[Optind]) @} @} @c endfile @end example The rest of the @code{BEGIN} rule is a simple test program. Here is the result of two sample runs of the test program: @example $ awk -f getopt.awk -v _getopt_test=1 -- -a -cbARG bax -x @print{} c = , optarg = <> @print{} c = , optarg = <> @print{} c = , optarg = @print{} non-option arguments: @print{} ARGV[3] = @print{} ARGV[4] = <-x> $ awk -f getopt.awk -v _getopt_test=1 -- -a -x -- xyz abc @print{} c = , optarg = <> @error{} x -- invalid option @print{} c = , optarg = <> @print{} non-option arguments: @print{} ARGV[4] = @print{} ARGV[5] = @end example In both runs, the first @option{--} terminates the arguments to @command{awk}, so that it does not try to interpret the @option{-a}, etc., as its own options. Several of the sample programs presented in @ref{Sample Programs}, use @code{getopt} to process their arguments. @c ENDOFRANGE libfclo @c ENDOFRANGE flibclo @c ENDOFRANGE clop @c ENDOFRANGE oclp @node Passwd Functions @section Reading the User Database @c STARTOFRANGE libfudata @cindex libraries of @command{awk} functions, user database, reading @c STARTOFRANGE flibudata @cindex functions, library, user database, reading @c last comma is part of primary @c STARTOFRANGE udatar @cindex user database, reading @c last comma is part of secondary @c STARTOFRANGE dataur @cindex database, users, reading @cindex @code{PROCINFO} array The @code{PROCINFO} array (@pxref{Built-in Variables}) provides access to the current user's real and effective user and group ID numbers, and if available, the user's supplementary group set. However, because these are numbers, they do not provide very useful information to the average user. There needs to be some way to find the user information associated with the user and group ID numbers. This @value{SECTION} presents a suite of functions for retrieving information from the user database. @xref{Group Functions}, for a similar suite that retrieves information from the group database. @cindex @code{getpwent} function (C library) @cindex @code{getpwent} user-defined function @cindex users, information about, retrieving @cindex login information @cindex account information @cindex password file @cindex files, password The POSIX standard does not define the file where user information is kept. Instead, it provides the @code{} header file and several C language subroutines for obtaining user information. The primary function is @code{getpwent}, for ``get password entry.'' The ``password'' comes from the original user database file, @file{/etc/passwd}, which stores user information, along with the encrypted passwords (hence the name). @cindex @command{pwcat} program While an @command{awk} program could simply read @file{/etc/passwd} directly, this file may not contain complete information about the system's set of users.@footnote{It is often the case that password information is stored in a network database.} To be sure you are able to produce a readable and complete version of the user database, it is necessary to write a small C program that calls @code{getpwent}. @code{getpwent} is defined as returning a pointer to a @code{struct passwd}. Each time it is called, it returns the next entry in the database. When there are no more entries, it returns @code{NULL}, the null pointer. When this happens, the C program should call @code{endpwent} to close the database. Following is @command{pwcat}, a C program that ``cats'' the password database: @c Use old style function header for portability to old systems (SunOS, HP/UX). @example @c file eg/lib/pwcat.c /* * pwcat.c * * Generate a printable version of the password database */ @c endfile @ignore @c file eg/lib/pwcat.c /* * Arnold Robbins, arnold@@gnu.org, May 1993 * Public Domain */ #if HAVE_CONFIG_H #include #endif @c endfile @end ignore @c file eg/lib/pwcat.c #include #include @c endfile @ignore @c file eg/lib/pwcat.c #if defined (STDC_HEADERS) #include #endif @c endfile @end ignore @c file eg/lib/pwcat.c int main(argc, argv) int argc; char **argv; @{ struct passwd *p; while ((p = getpwent()) != NULL) printf("%s:%s:%ld:%ld:%s:%s:%s\n", p->pw_name, p->pw_passwd, (long) p->pw_uid, (long) p->pw_gid, p->pw_gecos, p->pw_dir, p->pw_shell); endpwent(); return 0; @} @c endfile @end example If you don't understand C, don't worry about it. The output from @command{pwcat} is the user database, in the traditional @file{/etc/passwd} format of colon-separated fields. The fields are: @ignore @table @asis @item Login name The user's login name. @item Encrypted password The user's encrypted password. This may not be available on some systems. @item User-ID The user's numeric user ID number. (On some systems it's a C @code{long}, and not an @code{int}. Thus we cast it to @code{long} for all cases.) @item Group-ID The user's numeric group ID number. (Similar comments about @code{long} vs.@: @code{int} apply here.) @item Full name The user's full name, and perhaps other information associated with the user. @item Home directory The user's login (or ``home'') directory (familiar to shell programmers as @code{$HOME}). @item Login shell The program that is run when the user logs in. This is usually a shell, such as @command{bash}. @end table @end ignore @multitable {Encrypted password} {1234567890123456789012345678901234567890123456} @item Login name @tab The user's login name. @item Encrypted password @tab The user's encrypted password. This may not be available on some systems. @item User-ID @tab The user's numeric user ID number. @item Group-ID @tab The user's numeric group ID number. @item Full name @tab The user's full name, and perhaps other information associated with the user. @item Home directory @tab The user's login (or ``home'') directory (familiar to shell programmers as @code{$HOME}). @item Login shell @tab The program that is run when the user logs in. This is usually a shell, such as @command{bash}. @end multitable A few lines representative of @command{pwcat}'s output are as follows: @cindex Jacobs, Andrew @cindex Robbins, Arnold @cindex Robbins, Miriam @example $ pwcat @print{} root:3Ov02d5VaUPB6:0:1:Operator:/:/bin/sh @print{} nobody:*:65534:65534::/: @print{} daemon:*:1:1::/: @print{} sys:*:2:2::/:/bin/csh @print{} bin:*:3:3::/bin: @print{} arnold:xyzzy:2076:10:Arnold Robbins:/home/arnold:/bin/sh @print{} miriam:yxaay:112:10:Miriam Robbins:/home/miriam:/bin/sh @print{} andy:abcca2:113:10:Andy Jacobs:/home/andy:/bin/sh @dots{} @end example With that introduction, following is a group of functions for getting user information. There are several functions here, corresponding to the C functions of the same names: @c Exercise: simplify all these functions that return values. @c Answer: return foo[key] returns "" if key not there, no need to check with `in'. @cindex @code{_pw_init} user-defined function @example @c file eg/lib/passwdawk.in # passwd.awk --- access password file information @c endfile @ignore @c file eg/lib/passwdawk.in # # Arnold Robbins, arnold@@gnu.org, Public Domain # May 1993 # Revised October 2000 @c endfile @end ignore @c file eg/lib/passwdawk.in BEGIN @{ # tailor this to suit your system _pw_awklib = "/usr/local/libexec/awk/" @} function _pw_init( oldfs, oldrs, olddol0, pwcat, using_fw) @{ if (_pw_inited) return oldfs = FS oldrs = RS olddol0 = $0 using_fw = (PROCINFO["FS"] == "FIELDWIDTHS") FS = ":" RS = "\n" pwcat = _pw_awklib "pwcat" while ((pwcat | getline) > 0) @{ _pw_byname[$1] = $0 _pw_byuid[$3] = $0 _pw_bycount[++_pw_total] = $0 @} close(pwcat) _pw_count = 0 _pw_inited = 1 FS = oldfs if (using_fw) FIELDWIDTHS = FIELDWIDTHS RS = oldrs $0 = olddol0 @} @c endfile @end example @cindex @code{BEGIN} pattern, @code{pwcat} program The @code{BEGIN} rule sets a private variable to the directory where @command{pwcat} is stored. Because it is used to help out an @command{awk} library routine, we have chosen to put it in @file{/usr/local/libexec/awk}; however, you might want it to be in a different directory on your system. The function @code{_pw_init} keeps three copies of the user information in three associative arrays. The arrays are indexed by username (@code{_pw_byname}), by user ID number (@code{_pw_byuid}), and by order of occurrence (@code{_pw_bycount}). The variable @code{_pw_inited} is used for efficiency; @code{_pw_init} needs only to be called once. @cindex @code{getline} command, @code{_pw_init} function Because this function uses @code{getline} to read information from @command{pwcat}, it first saves the values of @code{FS}, @code{RS}, and @code{$0}. It notes in the variable @code{using_fw} whether field splitting with @code{FIELDWIDTHS} is in effect or not. Doing so is necessary, since these functions could be called from anywhere within a user's program, and the user may have his or her own way of splitting records and fields. The @code{using_fw} variable checks @code{PROCINFO["FS"]}, which is @code{"FIELDWIDTHS"} if field splitting is being done with @code{FIELDWIDTHS}. This makes it possible to restore the correct field-splitting mechanism later. The test can only be true for @command{gawk}. It is false if using @code{FS} or on some other @command{awk} implementation. The main part of the function uses a loop to read database lines, split the line into fields, and then store the line into each array as necessary. When the loop is done, @code{@w{_pw_init}} cleans up by closing the pipeline, setting @code{@w{_pw_inited}} to one, and restoring @code{FS} (and @code{FIELDWIDTHS} if necessary), @code{RS}, and @code{$0}. The use of @code{@w{_pw_count}} is explained shortly. @c NEXT ED: All of these functions don't need the ... in ... test. Just @c return the array element, which will be "" if not already there. Duh. @cindex @code{getpwnam} function (C library) The @code{getpwnam} function takes a username as a string argument. If that user is in the database, it returns the appropriate line. Otherwise, it returns the null string: @cindex @code{getpwnam} user-defined function @example @group @c file eg/lib/passwdawk.in function getpwnam(name) @{ _pw_init() if (name in _pw_byname) return _pw_byname[name] return "" @} @c endfile @end group @end example @cindex @code{getpwuid} function (C library) Similarly, the @code{getpwuid} function takes a user ID number argument. If that user number is in the database, it returns the appropriate line. Otherwise, it returns the null string: @cindex @code{getpwuid} user-defined function @example @c file eg/lib/passwdawk.in function getpwuid(uid) @{ _pw_init() if (uid in _pw_byuid) return _pw_byuid[uid] return "" @} @c endfile @end example @cindex @code{getpwent} function (C library) The @code{getpwent} function simply steps through the database, one entry at a time. It uses @code{_pw_count} to track its current position in the @code{_pw_bycount} array: @cindex @code{getpwent} user-defined function @example @c file eg/lib/passwdawk.in function getpwent() @{ _pw_init() if (_pw_count < _pw_total) return _pw_bycount[++_pw_count] return "" @} @c endfile @end example @cindex @code{endpwent} function (C library) The @code{@w{endpwent}} function resets @code{@w{_pw_count}} to zero, so that subsequent calls to @code{getpwent} start over again: @cindex @code{endpwent} user-defined function @example @c file eg/lib/passwdawk.in function endpwent() @{ _pw_count = 0 @} @c endfile @end example A conscious design decision in this suite was made that each subroutine calls @code{@w{_pw_init}} to initialize the database arrays. The overhead of running a separate process to generate the user database, and the I/O to scan it, are only incurred if the user's main program actually calls one of these functions. If this library file is loaded along with a user's program, but none of the routines are ever called, then there is no extra runtime overhead. (The alternative is move the body of @code{@w{_pw_init}} into a @code{BEGIN} rule, which always runs @command{pwcat}. This simplifies the code but runs an extra process that may never be needed.) In turn, calling @code{_pw_init} is not too expensive, because the @code{_pw_inited} variable keeps the program from reading the data more than once. If you are worried about squeezing every last cycle out of your @command{awk} program, the check of @code{_pw_inited} could be moved out of @code{_pw_init} and duplicated in all the other functions. In practice, this is not necessary, since most @command{awk} programs are I/O-bound, and it clutters up the code. The @command{id} program in @ref{Id Program}, uses these functions. @c ENDOFRANGE libfudata @c ENDOFRANGE flibudata @c ENDOFRANGE udatar @c ENDOFRANGE dataur @node Group Functions @section Reading the Group Database @c STARTOFRANGE libfgdata @cindex libraries of @command{awk} functions, group database, reading @c STARTOFRANGE flibgdata @cindex functions, library, group database, reading @c STARTOFRANGE gdatar @cindex group database, reading @c STARTOFRANGE datagr @cindex database, group, reading @cindex @code{PROCINFO} array @cindex @code{getgrent} function (C library) @cindex @code{getgrent} user-defined function @c comma is part of primary @cindex groups, information about @cindex account information @cindex group file @cindex files, group Much of the discussion presented in @ref{Passwd Functions}, applies to the group database as well. Although there has traditionally been a well-known file (@file{/etc/group}) in a well-known format, the POSIX standard only provides a set of C library routines (@code{} and @code{getgrent}) for accessing the information. Even though this file may exist, it likely does not have complete information. Therefore, as with the user database, it is necessary to have a small C program that generates the group database as its output. @cindex @command{grcat} program @command{grcat}, a C program that ``cats'' the group database, is as follows: @example @c file eg/lib/grcat.c /* * grcat.c * * Generate a printable version of the group database */ @c endfile @ignore @c file eg/lib/grcat.c /* * Arnold Robbins, arnold@@gnu.org, May 1993 * Public Domain */ /* For OS/2, do nothing. */ #if HAVE_CONFIG_H #include #endif #if defined (STDC_HEADERS) #include #endif #ifndef HAVE_GETGRENT int main() { return 0; } #else @c endfile @end ignore @c file eg/lib/grcat.c #include #include int main(argc, argv) int argc; char **argv; @{ struct group *g; int i; while ((g = getgrent()) != NULL) @{ printf("%s:%s:%ld:", g->gr_name, g->gr_passwd, (long) g->gr_gid); for (i = 0; g->gr_mem[i] != NULL; i++) @{ printf("%s", g->gr_mem[i]); @group if (g->gr_mem[i+1] != NULL) putchar(','); @} @end group putchar('\n'); @} endgrent(); return 0; @} @c endfile @end example @ignore @c file eg/lib/grcat.c #endif /* HAVE_GETGRENT */ @c endfile @end ignore Each line in the group database represents one group. The fields are separated with colons and represent the following information: @ignore @table @asis @item Group Name The name of the group. @item Group Password The encrypted group password. In practice, this field is never used. It is usually empty or set to @samp{*}. @item Group ID Number The numeric group ID number. This number is unique within the file. (On some systems it's a C @code{long}, and not an @code{int}. Thus we cast it to @code{long} for all cases.) @item Group Member List A comma-separated list of usernames. These users are members of the group. Modern Unix systems allow users to be members of several groups simultaneously. If your system does, then there are elements @code{"group1"} through @code{"group@var{N}"} in @code{PROCINFO} for those group ID numbers. (Note that @code{PROCINFO} is a @command{gawk} extension; @pxref{Built-in Variables}.) @end table @end ignore @multitable {Encrypted password} {1234567890123456789012345678901234567890123456} @item Group name @tab The group's name. @item Group password @tab The group's encrypted password. In practice, this field is never used; it is usually empty or set to @samp{*}. @item Group-ID @tab The group's numeric group ID number; this number should be unique within the file. @item Group member list @tab A comma-separated list of usernames. These users are members of the group. Modern Unix systems allow users to be members of several groups simultaneously. If your system does, then there are elements @code{"group1"} through @code{"group@var{N}"} in @code{PROCINFO} for those group ID numbers. (Note that @code{PROCINFO} is a @command{gawk} extension; @pxref{Built-in Variables}.) @end multitable Here is what running @command{grcat} might produce: @example $ grcat @print{} wheel:*:0:arnold @print{} nogroup:*:65534: @print{} daemon:*:1: @print{} kmem:*:2: @print{} staff:*:10:arnold,miriam,andy @print{} other:*:20: @dots{} @end example Here are the functions for obtaining information from the group database. There are several, modeled after the C library functions of the same names: @cindex @code{getline} command, @code{_gr_init} user-defined function @cindex @code{_gr_init} user-defined function @example @c file eg/lib/groupawk.in # group.awk --- functions for dealing with the group file @c endfile @ignore @c file eg/lib/groupawk.in # # Arnold Robbins, arnold@@gnu.org, Public Domain # May 1993 # Revised October 2000 @c endfile @end ignore @c line break on _gr_init for smallbook @c file eg/lib/groupawk.in BEGIN \ @{ # Change to suit your system _gr_awklib = "/usr/local/libexec/awk/" @} function _gr_init( oldfs, oldrs, olddol0, grcat, using_fw, n, a, i) @{ if (_gr_inited) return oldfs = FS oldrs = RS olddol0 = $0 using_fw = (PROCINFO["FS"] == "FIELDWIDTHS") FS = ":" RS = "\n" grcat = _gr_awklib "grcat" while ((grcat | getline) > 0) @{ if ($1 in _gr_byname) _gr_byname[$1] = _gr_byname[$1] "," $4 else _gr_byname[$1] = $0 if ($3 in _gr_bygid) _gr_bygid[$3] = _gr_bygid[$3] "," $4 else _gr_bygid[$3] = $0 n = split($4, a, "[ \t]*,[ \t]*") for (i = 1; i <= n; i++) if (a[i] in _gr_groupsbyuser) _gr_groupsbyuser[a[i]] = \ _gr_groupsbyuser[a[i]] " " $1 else _gr_groupsbyuser[a[i]] = $1 _gr_bycount[++_gr_count] = $0 @} close(grcat) _gr_count = 0 _gr_inited++ FS = oldfs if (using_fw) FIELDWIDTHS = FIELDWIDTHS RS = oldrs $0 = olddol0 @} @c endfile @end example The @code{BEGIN} rule sets a private variable to the directory where @command{grcat} is stored. Because it is used to help out an @command{awk} library routine, we have chosen to put it in @file{/usr/local/libexec/awk}. You might want it to be in a different directory on your system. These routines follow the same general outline as the user database routines (@pxref{Passwd Functions}). The @code{@w{_gr_inited}} variable is used to ensure that the database is scanned no more than once. The @code{@w{_gr_init}} function first saves @code{FS}, @code{FIELDWIDTHS}, @code{RS}, and @code{$0}, and then sets @code{FS} and @code{RS} to the correct values for scanning the group information. The group information is stored is several associative arrays. The arrays are indexed by group name (@code{@w{_gr_byname}}), by group ID number (@code{@w{_gr_bygid}}), and by position in the database (@code{@w{_gr_bycount}}). There is an additional array indexed by username (@code{@w{_gr_groupsbyuser}}), which is a space-separated list of groups to which each user belongs. Unlike the user database, it is possible to have multiple records in the database for the same group. This is common when a group has a large number of members. A pair of such entries might look like the following: @example tvpeople:*:101:johnny,jay,arsenio tvpeople:*:101:david,conan,tom,joan @end example For this reason, @code{_gr_init} looks to see if a group name or group ID number is already seen. If it is, then the usernames are simply concatenated onto the previous list of users. (There is actually a subtle problem with the code just presented. Suppose that the first time there were no names. This code adds the names with a leading comma. It also doesn't check that there is a @code{$4}.) Finally, @code{_gr_init} closes the pipeline to @command{grcat}, restores @code{FS} (and @code{FIELDWIDTHS} if necessary), @code{RS}, and @code{$0}, initializes @code{_gr_count} to zero (it is used later), and makes @code{_gr_inited} nonzero. @cindex @code{getgrnam} function (C library) The @code{getgrnam} function takes a group name as its argument, and if that group exists, it is returned. Otherwise, @code{getgrnam} returns the null string: @cindex @code{getgrnam} user-defined function @example @c file eg/lib/groupawk.in function getgrnam(group) @{ _gr_init() if (group in _gr_byname) return _gr_byname[group] return "" @} @c endfile @end example @cindex @code{getgrgid} function (C library) The @code{getgrgid} function is similar, it takes a numeric group ID and looks up the information associated with that group ID: @cindex @code{getgrgid} user-defined function @example @c file eg/lib/groupawk.in function getgrgid(gid) @{ _gr_init() if (gid in _gr_bygid) return _gr_bygid[gid] return "" @} @c endfile @end example @cindex @code{getgruser} function (C library) The @code{getgruser} function does not have a C counterpart. It takes a username and returns the list of groups that have the user as a member: @cindex @code{getgruser} function, user-defined @example @c file eg/lib/groupawk.in function getgruser(user) @{ _gr_init() if (user in _gr_groupsbyuser) return _gr_groupsbyuser[user] return "" @} @c endfile @end example @cindex @code{getgrent} function (C library) The @code{getgrent} function steps through the database one entry at a time. It uses @code{_gr_count} to track its position in the list: @cindex @code{getgrent} user-defined function @example @c file eg/lib/groupawk.in function getgrent() @{ _gr_init() if (++_gr_count in _gr_bycount) return _gr_bycount[_gr_count] return "" @} @c endfile @end example @c ENDOFRANGE clibf @cindex @code{endgrent} function (C library) The @code{endgrent} function resets @code{_gr_count} to zero so that @code{getgrent} can start over again: @cindex @code{endgrent} user-defined function @example @c file eg/lib/groupawk.in function endgrent() @{ _gr_count = 0 @} @c endfile @end example As with the user database routines, each function calls @code{_gr_init} to initialize the arrays. Doing so only incurs the extra overhead of running @command{grcat} if these functions are used (as opposed to moving the body of @code{_gr_init} into a @code{BEGIN} rule). Most of the work is in scanning the database and building the various associative arrays. The functions that the user calls are themselves very simple, relying on @command{awk}'s associative arrays to do work. The @command{id} program in @ref{Id Program}, uses these functions. @c ENDOFRANGE libfgdata @c ENDOFRANGE flibgdata @c ENDOFRANGE gdatar @c ENDOFRANGE libf @c ENDOFRANGE flib @c ENDOFRANGE fudlib @c ENDOFRANGE datagr @node Sample Programs @chapter Practical @command{awk} Programs @c STARTOFRANGE awkpex @cindex @command{awk} programs, examples of @ref{Library Functions}, presents the idea that reading programs in a language contributes to learning that language. This @value{CHAPTER} continues that theme, presenting a potpourri of @command{awk} programs for your reading enjoyment. @ifnotinfo There are three sections. The first describes how to run the programs presented in this @value{CHAPTER}. The second presents @command{awk} versions of several common POSIX utilities. These are programs that you are hopefully already familiar with, and therefore, whose problems are understood. By reimplementing these programs in @command{awk}, you can focus on the @command{awk}-related aspects of solving the programming problem. The third is a grab bag of interesting programs. These solve a number of different data-manipulation and management problems. Many of the programs are short, which emphasizes @command{awk}'s ability to do a lot in just a few lines of code. @end ifnotinfo Many of these programs use the library functions presented in @ref{Library Functions}. @menu * Running Examples:: How to run these examples. * Clones:: Clones of common utilities. * Miscellaneous Programs:: Some interesting @command{awk} programs. @end menu @node Running Examples @section Running the Example Programs To run a given program, you would typically do something like this: @example awk -f @var{program} -- @var{options} @var{files} @end example @noindent Here, @var{program} is the name of the @command{awk} program (such as @file{cut.awk}), @var{options} are any command-line options for the program that start with a @samp{-}, and @var{files} are the actual @value{DF}s. If your system supports the @samp{#!} executable interpreter mechanism (@pxref{Executable Scripts}), you can instead run your program directly: @example cut.awk -c1-8 myfiles > results @end example If your @command{awk} is not @command{gawk}, you may instead need to use this: @example cut.awk -- -c1-8 myfiles > results @end example @node Clones @section Reinventing Wheels for Fun and Profit @c last comma is part of secondary @c STARTOFRANGE posimawk @cindex POSIX, programs, implementing in @command{awk} This @value{SECTION} presents a number of POSIX utilities that are implemented in @command{awk}. Reinventing these programs in @command{awk} is often enjoyable, because the algorithms can be very clearly expressed, and the code is usually very concise and simple. This is true because @command{awk} does so much for you. It should be noted that these programs are not necessarily intended to replace the installed versions on your system. Instead, their purpose is to illustrate @command{awk} language programming for ``real world'' tasks. The programs are presented in alphabetical order. @menu * Cut Program:: The @command{cut} utility. * Egrep Program:: The @command{egrep} utility. * Id Program:: The @command{id} utility. * Split Program:: The @command{split} utility. * Tee Program:: The @command{tee} utility. * Uniq Program:: The @command{uniq} utility. * Wc Program:: The @command{wc} utility. @end menu @node Cut Program @subsection Cutting out Fields and Columns @cindex @command{cut} utility @c STARTOFRANGE cut @cindex @command{cut} utility @c STARTOFRANGE ficut @cindex fields, cutting @c STARTOFRANGE colcut @cindex columns, cutting The @command{cut} utility selects, or ``cuts,'' characters or fields from its standard input and sends them to its standard output. Fields are separated by tabs by default, but you may supply a command-line option to change the field @dfn{delimiter} (i.e., the field-separator character). @command{cut}'s definition of fields is less general than @command{awk}'s. A common use of @command{cut} might be to pull out just the login name of logged-on users from the output of @command{who}. For example, the following pipeline generates a sorted, unique list of the logged-on users: @example who | cut -c1-8 | sort | uniq @end example The options for @command{cut} are: @table @code @item -c @var{list} Use @var{list} as the list of characters to cut out. Items within the list may be separated by commas, and ranges of characters can be separated with dashes. The list @samp{1-8,15,22-35} specifies characters 1 through 8, 15, and 22 through 35. @item -f @var{list} Use @var{list} as the list of fields to cut out. @item -d @var{delim} Use @var{delim} as the field-separator character instead of the tab character. @item -s Suppress printing of lines that do not contain the field delimiter. @end table The @command{awk} implementation of @command{cut} uses the @code{getopt} library function (@pxref{Getopt Function}) and the @code{join} library function (@pxref{Join Function}). The program begins with a comment describing the options, the library functions needed, and a @code{usage} function that prints out a usage message and exits. @code{usage} is called if invalid arguments are supplied: @cindex @code{cut.awk} program @example @c file eg/prog/cut.awk # cut.awk --- implement cut in awk @c endfile @ignore @c file eg/prog/cut.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # May 1993 @c endfile @end ignore @c file eg/prog/cut.awk # Options: # -f list Cut fields # -d c Field delimiter character # -c list Cut characters # # -s Suppress lines without the delimiter # # Requires getopt and join library functions @group function usage( e1, e2) @{ e1 = "usage: cut [-f list] [-d c] [-s] [files...]" e2 = "usage: cut [-c list] [files...]" print e1 > "/dev/stderr" print e2 > "/dev/stderr" exit 1 @} @end group @c endfile @end example @noindent The variables @code{e1} and @code{e2} are used so that the function fits nicely on the @ifnotinfo page. @end ifnotinfo @ifnottex screen. @end ifnottex @cindex @code{BEGIN} pattern, running @command{awk} programs and @cindex @code{FS} variable, running @command{awk} programs and Next comes a @code{BEGIN} rule that parses the command-line options. It sets @code{FS} to a single TAB character, because that is @command{cut}'s default field separator. The output field separator is also set to be the same as the input field separator. Then @code{getopt} is used to step through the command-line options. Exactly one of the variables @code{by_fields} or @code{by_chars} is set to true, to indicate that processing should be done by fields or by characters, respectively. When cutting by characters, the output field separator is set to the null string: @example @c file eg/prog/cut.awk BEGIN \ @{ FS = "\t" # default OFS = FS while ((c = getopt(ARGC, ARGV, "sf:c:d:")) != -1) @{ if (c == "f") @{ by_fields = 1 fieldlist = Optarg @} else if (c == "c") @{ by_chars = 1 fieldlist = Optarg OFS = "" @} else if (c == "d") @{ if (length(Optarg) > 1) @{ printf("Using first character of %s" \ " for delimiter\n", Optarg) > "/dev/stderr" Optarg = substr(Optarg, 1, 1) @} FS = Optarg OFS = FS if (FS == " ") # defeat awk semantics FS = "[ ]" @} else if (c == "s") suppress++ else usage() @} for (i = 1; i < Optind; i++) ARGV[i] = "" @c endfile @end example @cindex field separators, spaces as Special care is taken when the field delimiter is a space. Using a single space (@code{@w{" "}}) for the value of @code{FS} is incorrect---@command{awk} would separate fields with runs of spaces, tabs, and/or newlines, and we want them to be separated with individual spaces. Also, note that after @code{getopt} is through, we have to clear out all the elements of @code{ARGV} from 1 to @code{Optind}, so that @command{awk} does not try to process the command-line options as @value{FN}s. After dealing with the command-line options, the program verifies that the options make sense. Only one or the other of @option{-c} and @option{-f} should be used, and both require a field list. Then the program calls either @code{set_fieldlist} or @code{set_charlist} to pull apart the list of fields or characters: @example @c file eg/prog/cut.awk if (by_fields && by_chars) usage() if (by_fields == 0 && by_chars == 0) by_fields = 1 # default if (fieldlist == "") @{ print "cut: needs list for -c or -f" > "/dev/stderr" exit 1 @} if (by_fields) set_fieldlist() else set_charlist() @} @c endfile @end example @code{set_fieldlist} is used to split the field list apart at the commas and into an array. Then, for each element of the array, it looks to see if it is actually a range, and if so, splits it apart. The range is verified to make sure the first number is smaller than the second. Each number in the list is added to the @code{flist} array, which simply lists the fields that will be printed. Normal field splitting is used. The program lets @command{awk} handle the job of doing the field splitting: @example @c file eg/prog/cut.awk function set_fieldlist( n, m, i, j, k, f, g) @{ n = split(fieldlist, f, ",") j = 1 # index in flist for (i = 1; i <= n; i++) @{ if (index(f[i], "-") != 0) @{ # a range m = split(f[i], g, "-") @group if (m != 2 || g[1] >= g[2]) @{ printf("bad field list: %s\n", f[i]) > "/dev/stderr" exit 1 @} @end group for (k = g[1]; k <= g[2]; k++) flist[j++] = k @} else flist[j++] = f[i] @} nfields = j - 1 @} @c endfile @end example The @code{set_charlist} function is more complicated than @code{set_fieldlist}. The idea here is to use @command{gawk}'s @code{FIELDWIDTHS} variable (@pxref{Constant Size}), which describes constant-width input. When using a character list, that is exactly what we have. Setting up @code{FIELDWIDTHS} is more complicated than simply listing the fields that need to be printed. We have to keep track of the fields to print and also the intervening characters that have to be skipped. For example, suppose you wanted characters 1 through 8, 15, and 22 through 35. You would use @samp{-c 1-8,15,22-35}. The necessary value for @code{FIELDWIDTHS} is @code{@w{"8 6 1 6 14"}}. This yields five fields, and the fields to print are @code{$1}, @code{$3}, and @code{$5}. The intermediate fields are @dfn{filler}, which is stuff in between the desired data. @code{flist} lists the fields to print, and @code{t} tracks the complete field list, including filler fields: @example @c file eg/prog/cut.awk function set_charlist( field, i, j, f, g, t, filler, last, len) @{ field = 1 # count total fields n = split(fieldlist, f, ",") j = 1 # index in flist for (i = 1; i <= n; i++) @{ if (index(f[i], "-") != 0) @{ # range m = split(f[i], g, "-") if (m != 2 || g[1] >= g[2]) @{ printf("bad character list: %s\n", f[i]) > "/dev/stderr" exit 1 @} len = g[2] - g[1] + 1 if (g[1] > 1) # compute length of filler filler = g[1] - last - 1 else filler = 0 @group if (filler) t[field++] = filler @end group t[field++] = len # length of field last = g[2] flist[j++] = field - 1 @} else @{ if (f[i] > 1) filler = f[i] - last - 1 else filler = 0 if (filler) t[field++] = filler t[field++] = 1 last = f[i] flist[j++] = field - 1 @} @} FIELDWIDTHS = join(t, 1, field - 1) nfields = j - 1 @} @c endfile @end example Next is the rule that actually processes the data. If the @option{-s} option is given, then @code{suppress} is true. The first @code{if} statement makes sure that the input record does have the field separator. If @command{cut} is processing fields, @code{suppress} is true, and the field separator character is not in the record, then the record is skipped. If the record is valid, then @command{gawk} has split the data into fields, either using the character in @code{FS} or using fixed-length fields and @code{FIELDWIDTHS}. The loop goes through the list of fields that should be printed. The corresponding field is printed if it contains data. If the next field also has data, then the separator character is written out between the fields: @example @c file eg/prog/cut.awk @{ if (by_fields && suppress && index($0, FS) != 0) next for (i = 1; i <= nfields; i++) @{ if ($flist[i] != "") @{ printf "%s", $flist[i] if (i < nfields && $flist[i+1] != "") printf "%s", OFS @} @} print "" @} @c endfile @end example This version of @command{cut} relies on @command{gawk}'s @code{FIELDWIDTHS} variable to do the character-based cutting. While it is possible in other @command{awk} implementations to use @code{substr} (@pxref{String Functions}), it is also extremely painful. The @code{FIELDWIDTHS} variable supplies an elegant solution to the problem of picking the input line apart by characters. @c ENDOFRANGE cut @c ENDOFRANGE ficut @c ENDOFRANGE colcut @c Exercise: Rewrite using split with "". @node Egrep Program @subsection Searching for Regular Expressions in Files @c STARTOFRANGE regexps @cindex regular expressions, searching for @c STARTOFRANGE sfregexp @cindex searching, files for regular expressions @c STARTOFRANGE fsregexp @cindex files, searching for regular expressions @cindex @command{egrep} utility The @command{egrep} utility searches files for patterns. It uses regular expressions that are almost identical to those available in @command{awk} (@pxref{Regexp}). It is used in the following manner: @example egrep @r{[} @var{options} @r{]} '@var{pattern}' @var{files} @dots{} @end example The @var{pattern} is a regular expression. In typical usage, the regular expression is quoted to prevent the shell from expanding any of the special characters as @value{FN} wildcards. Normally, @command{egrep} prints the lines that matched. If multiple @value{FN}s are provided on the command line, each output line is preceded by the name of the file and a colon. The options to @command{egrep} are as follows: @table @code @item -c Print out a count of the lines that matched the pattern, instead of the lines themselves. @item -s Be silent. No output is produced and the exit value indicates whether the pattern was matched. @item -v Invert the sense of the test. @command{egrep} prints the lines that do @emph{not} match the pattern and exits successfully if the pattern is not matched. @item -i Ignore case distinctions in both the pattern and the input data. @item -l Only print (list) the names of the files that matched, not the lines that matched. @item -e @var{pattern} Use @var{pattern} as the regexp to match. The purpose of the @option{-e} option is to allow patterns that start with a @samp{-}. @end table This version uses the @code{getopt} library function (@pxref{Getopt Function}) and the file transition library program (@pxref{Filetrans Function}). The program begins with a descriptive comment and then a @code{BEGIN} rule that processes the command-line arguments with @code{getopt}. The @option{-i} (ignore case) option is particularly easy with @command{gawk}; we just use the @code{IGNORECASE} built-in variable (@pxref{Built-in Variables}): @cindex @code{egrep.awk} program @example @c file eg/prog/egrep.awk # egrep.awk --- simulate egrep in awk @c endfile @ignore @c file eg/prog/egrep.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # May 1993 @c endfile @end ignore @c file eg/prog/egrep.awk # Options: # -c count of lines # -s silent - use exit value # -v invert test, success if no match # -i ignore case # -l print filenames only # -e argument is pattern # # Requires getopt and file transition library functions BEGIN @{ while ((c = getopt(ARGC, ARGV, "ce:svil")) != -1) @{ if (c == "c") count_only++ else if (c == "s") no_print++ else if (c == "v") invert++ else if (c == "i") IGNORECASE = 1 else if (c == "l") filenames_only++ else if (c == "e") pattern = Optarg else usage() @} @c endfile @end example Next comes the code that handles the @command{egrep}-specific behavior. If no pattern is supplied with @option{-e}, the first nonoption on the command line is used. The @command{awk} command-line arguments up to @code{ARGV[Optind]} are cleared, so that @command{awk} won't try to process them as files. If no files are specified, the standard input is used, and if multiple files are specified, we make sure to note this so that the @value{FN}s can precede the matched lines in the output: @example @c file eg/prog/egrep.awk if (pattern == "") pattern = ARGV[Optind++] for (i = 1; i < Optind; i++) ARGV[i] = "" if (Optind >= ARGC) @{ ARGV[1] = "-" ARGC = 2 @} else if (ARGC - Optind > 1) do_filenames++ # if (IGNORECASE) # pattern = tolower(pattern) @} @c endfile @end example The last two lines are commented out, since they are not needed in @command{gawk}. They should be uncommented if you have to use another version of @command{awk}. The next set of lines should be uncommented if you are not using @command{gawk}. This rule translates all the characters in the input line into lowercase if the @option{-i} option is specified.@footnote{It also introduces a subtle bug; if a match happens, we output the translated line, not the original.} The rule is commented out since it is not necessary with @command{gawk}: @c Exercise: Fix this, w/array and new line as key to original line @example @c file eg/prog/egrep.awk #@{ # if (IGNORECASE) # $0 = tolower($0) #@} @c endfile @end example The @code{beginfile} function is called by the rule in @file{ftrans.awk} when each new file is processed. In this case, it is very simple; all it does is initialize a variable @code{fcount} to zero. @code{fcount} tracks how many lines in the current file matched the pattern (naming the parameter @code{junk} shows we know that @code{beginfile} is called with a parameter, but that we're not interested in its value): @example @c file eg/prog/egrep.awk function beginfile(junk) @{ fcount = 0 @} @c endfile @end example The @code{endfile} function is called after each file has been processed. It affects the output only when the user wants a count of the number of lines that matched. @code{no_print} is true only if the exit status is desired. @code{count_only} is true if line counts are desired. @command{egrep} therefore only prints line counts if printing and counting are enabled. The output format must be adjusted depending upon the number of files to process. Finally, @code{fcount} is added to @code{total}, so that we know the total number of lines that matched the pattern: @example @c file eg/prog/egrep.awk function endfile(file) @{ if (! no_print && count_only) if (do_filenames) print file ":" fcount else print fcount total += fcount @} @c endfile @end example The following rule does most of the work of matching lines. The variable @code{matches} is true if the line matched the pattern. If the user wants lines that did not match, the sense of @code{matches} is inverted using the @samp{!} operator. @code{fcount} is incremented with the value of @code{matches}, which is either one or zero, depending upon a successful or unsuccessful match. If the line does not match, the @code{next} statement just moves on to the next record. @cindex @code{!} (exclamation point), @code{!} operator @cindex exclamation point (@code{!}), @code{!} operator A number of additional tests are made, but they are only done if we are not counting lines. First, if the user only wants exit status (@code{no_print} is true), then it is enough to know that @emph{one} line in this file matched, and we can skip on to the next file with @code{nextfile}. Similarly, if we are only printing @value{FN}s, we can print the @value{FN}, and then skip to the next file with @code{nextfile}. Finally, each line is printed, with a leading @value{FN} and colon if necessary: @cindex @code{!} operator @example @c file eg/prog/egrep.awk @{ matches = ($0 ~ pattern) if (invert) matches = ! matches fcount += matches # 1 or 0 if (! matches) next if (! count_only) @{ if (no_print) nextfile if (filenames_only) @{ print FILENAME nextfile @} if (do_filenames) print FILENAME ":" $0 else print @} @} @c endfile @end example The @code{END} rule takes care of producing the correct exit status. If there are no matches, the exit status is one; otherwise it is zero: @example @c file eg/prog/egrep.awk END \ @{ if (total == 0) exit 1 exit 0 @} @c endfile @end example The @code{usage} function prints a usage message in case of invalid options, and then exits: @example @c file eg/prog/egrep.awk function usage( e) @{ e = "Usage: egrep [-csvil] [-e pat] [files ...]" e = e "\n\tegrep [-csvil] pat [files ...]" print e > "/dev/stderr" exit 1 @} @c endfile @end example The variable @code{e} is used so that the function fits nicely on the printed page. @cindex @code{END} pattern, backslash continuation and @cindex @code{\} (backslash), continuing lines and @cindex backslash (@code{\}), continuing lines and Just a note on programming style: you may have noticed that the @code{END} rule uses backslash continuation, with the open brace on a line by itself. This is so that it more closely resembles the way functions are written. Many of the examples in this @value{CHAPTER} use this style. You can decide for yourself if you like writing your @code{BEGIN} and @code{END} rules this way or not. @c ENDOFRANGE regexps @c ENDOFRANGE sfregexp @c ENDOFRANGE fsregexp @node Id Program @subsection Printing out User Information @cindex printing, user information @cindex users, information about, printing @cindex @command{id} utility The @command{id} utility lists a user's real and effective user ID numbers, real and effective group ID numbers, and the user's group set, if any. @command{id} only prints the effective user ID and group ID if they are different from the real ones. If possible, @command{id} also supplies the corresponding user and group names. The output might look like this: @example $ id @print{} uid=2076(arnold) gid=10(staff) groups=10(staff),4(tty) @end example This information is part of what is provided by @command{gawk}'s @code{PROCINFO} array (@pxref{Built-in Variables}). However, the @command{id} utility provides a more palatable output than just individual numbers. Here is a simple version of @command{id} written in @command{awk}. It uses the user database library functions (@pxref{Passwd Functions}) and the group database library functions (@pxref{Group Functions}): The program is fairly straightforward. All the work is done in the @code{BEGIN} rule. The user and group ID numbers are obtained from @code{PROCINFO}. The code is repetitive. The entry in the user database for the real user ID number is split into parts at the @samp{:}. The name is the first field. Similar code is used for the effective user ID number and the group numbers: @cindex @code{id.awk} program @example @c file eg/prog/id.awk # id.awk --- implement id in awk # # Requires user and group library functions @c endfile @ignore @c file eg/prog/id.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # May 1993 # Revised February 1996 @c endfile @end ignore @c file eg/prog/id.awk # output is: # uid=12(foo) euid=34(bar) gid=3(baz) \ # egid=5(blat) groups=9(nine),2(two),1(one) @group BEGIN \ @{ uid = PROCINFO["uid"] euid = PROCINFO["euid"] gid = PROCINFO["gid"] egid = PROCINFO["egid"] @end group printf("uid=%d", uid) pw = getpwuid(uid) if (pw != "") @{ split(pw, a, ":") printf("(%s)", a[1]) @} if (euid != uid) @{ printf(" euid=%d", euid) pw = getpwuid(euid) if (pw != "") @{ split(pw, a, ":") printf("(%s)", a[1]) @} @} printf(" gid=%d", gid) pw = getgrgid(gid) if (pw != "") @{ split(pw, a, ":") printf("(%s)", a[1]) @} if (egid != gid) @{ printf(" egid=%d", egid) pw = getgrgid(egid) if (pw != "") @{ split(pw, a, ":") printf("(%s)", a[1]) @} @} for (i = 1; ("group" i) in PROCINFO; i++) @{ if (i == 1) printf(" groups=") group = PROCINFO["group" i] printf("%d", group) pw = getgrgid(group) if (pw != "") @{ split(pw, a, ":") printf("(%s)", a[1]) @} if (("group" (i+1)) in PROCINFO) printf(",") @} print "" @} @c endfile @end example @cindex @code{in} operator The test in the @code{for} loop is worth noting. Any supplementary groups in the @code{PROCINFO} array have the indices @code{"group1"} through @code{"group@var{N}"} for some @var{N}, i.e., the total number of supplementary groups. However, we don't know in advance how many of these groups there are. This loop works by starting at one, concatenating the value with @code{"group"}, and then using @code{in} to see if that value is in the array. Eventually, @code{i} is incremented past the last group in the array and the loop exits. The loop is also correct if there are @emph{no} supplementary groups; then the condition is false the first time it's tested, and the loop body never executes. @c exercise!!! @ignore The POSIX version of @command{id} takes arguments that control which information is printed. Modify this version to accept the same arguments and perform in the same way. @end ignore @node Split Program @subsection Splitting a Large File into Pieces @c STARTOFRANGE filspl @cindex files, splitting @cindex @code{split} utility The @code{split} program splits large text files into smaller pieces. Usage is as follows: @example split @r{[}-@var{count}@r{]} file @r{[} @var{prefix} @r{]} @end example By default, the output files are named @file{xaa}, @file{xab}, and so on. Each file has 1000 lines in it, with the likely exception of the last file. To change the number of lines in each file, supply a number on the command line preceded with a minus; e.g., @samp{-500} for files with 500 lines in them instead of 1000. To change the name of the output files to something like @file{myfileaa}, @file{myfileab}, and so on, supply an additional argument that specifies the @value{FN} prefix. Here is a version of @code{split} in @command{awk}. It uses the @code{ord} and @code{chr} functions presented in @ref{Ordinal Functions}. The program first sets its defaults, and then tests to make sure there are not too many arguments. It then looks at each argument in turn. The first argument could be a minus sign followed by a number. If it is, this happens to look like a negative number, so it is made positive, and that is the count of lines. The data @value{FN} is skipped over and the final argument is used as the prefix for the output @value{FN}s: @cindex @code{split.awk} program @example @c file eg/prog/split.awk # split.awk --- do split in awk # # Requires ord and chr library functions @c endfile @ignore @c file eg/prog/split.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # May 1993 @c endfile @end ignore @c file eg/prog/split.awk # usage: split [-num] [file] [outname] BEGIN @{ outfile = "x" # default count = 1000 if (ARGC > 4) usage() i = 1 if (ARGV[i] ~ /^-[0-9]+$/) @{ count = -ARGV[i] ARGV[i] = "" i++ @} # test argv in case reading from stdin instead of file if (i in ARGV) i++ # skip data file name if (i in ARGV) @{ outfile = ARGV[i] ARGV[i] = "" @} s1 = s2 = "a" out = (outfile s1 s2) @} @c endfile @end example The next rule does most of the work. @code{tcount} (temporary count) tracks how many lines have been printed to the output file so far. If it is greater than @code{count}, it is time to close the current file and start a new one. @code{s1} and @code{s2} track the current suffixes for the @value{FN}. If they are both @samp{z}, the file is just too big. Otherwise, @code{s1} moves to the next letter in the alphabet and @code{s2} starts over again at @samp{a}: @c else on separate line here for page breaking @example @c file eg/prog/split.awk @{ if (++tcount > count) @{ close(out) if (s2 == "z") @{ if (s1 == "z") @{ printf("split: %s is too large to split\n", FILENAME) > "/dev/stderr" exit 1 @} s1 = chr(ord(s1) + 1) s2 = "a" @} @group else s2 = chr(ord(s2) + 1) @end group out = (outfile s1 s2) tcount = 1 @} print > out @} @c endfile @end example @c Exercise: do this with just awk builtin functions, index("abc..."), substr, etc. @noindent The @code{usage} function simply prints an error message and exits: @example @c file eg/prog/split.awk function usage( e) @{ e = "usage: split [-num] [file] [outname]" print e > "/dev/stderr" exit 1 @} @c endfile @end example @noindent The variable @code{e} is used so that the function fits nicely on the @ifinfo screen. @end ifinfo @ifnotinfo page. @end ifnotinfo This program is a bit sloppy; it relies on @command{awk} to automatically close the last file instead of doing it in an @code{END} rule. It also assumes that letters are contiguous in the character set, which isn't true for EBCDIC systems. @c BFD... @c ENDOFRANGE filspl @node Tee Program @subsection Duplicating Output into Multiple Files @c last comma is part of secondary @cindex files, multiple, duplicating output into @cindex output, duplicating into files @cindex @code{tee} utility The @code{tee} program is known as a ``pipe fitting.'' @code{tee} copies its standard input to its standard output and also duplicates it to the files named on the command line. Its usage is as follows: @example tee @r{[}-a@r{]} file @dots{} @end example The @option{-a} option tells @code{tee} to append to the named files, instead of truncating them and starting over. The @code{BEGIN} rule first makes a copy of all the command-line arguments into an array named @code{copy}. @code{ARGV[0]} is not copied, since it is not needed. @code{tee} cannot use @code{ARGV} directly, since @command{awk} attempts to process each @value{FN} in @code{ARGV} as input data. @cindex flag variables If the first argument is @option{-a}, then the flag variable @code{append} is set to true, and both @code{ARGV[1]} and @code{copy[1]} are deleted. If @code{ARGC} is less than two, then no @value{FN}s were supplied and @code{tee} prints a usage message and exits. Finally, @command{awk} is forced to read the standard input by setting @code{ARGV[1]} to @code{"-"} and @code{ARGC} to two: @c NEXT ED: Add more leading commentary in this program @cindex @code{tee.awk} program @example @c file eg/prog/tee.awk # tee.awk --- tee in awk @c endfile @ignore @c file eg/prog/tee.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # May 1993 # Revised December 1995 @c endfile @end ignore @c file eg/prog/tee.awk BEGIN \ @{ for (i = 1; i < ARGC; i++) copy[i] = ARGV[i] if (ARGV[1] == "-a") @{ append = 1 delete ARGV[1] delete copy[1] ARGC-- @} if (ARGC < 2) @{ print "usage: tee [-a] file ..." > "/dev/stderr" exit 1 @} ARGV[1] = "-" ARGC = 2 @} @c endfile @end example The single rule does all the work. Since there is no pattern, it is executed for each line of input. The body of the rule simply prints the line into each file on the command line, and then to the standard output: @example @c file eg/prog/tee.awk @{ # moving the if outside the loop makes it run faster if (append) for (i in copy) print >> copy[i] else for (i in copy) print > copy[i] print @} @c endfile @end example @noindent It is also possible to write the loop this way: @example for (i in copy) if (append) print >> copy[i] else print > copy[i] @end example @noindent This is more concise but it is also less efficient. The @samp{if} is tested for each record and for each output file. By duplicating the loop body, the @samp{if} is only tested once for each input record. If there are @var{N} input records and @var{M} output files, the first method only executes @var{N} @samp{if} statements, while the second executes @var{N}@code{*}@var{M} @samp{if} statements. Finally, the @code{END} rule cleans up by closing all the output files: @example @c file eg/prog/tee.awk END \ @{ for (i in copy) close(copy[i]) @} @c endfile @end example @node Uniq Program @subsection Printing Nonduplicated Lines of Text @c STARTOFRANGE prunt @cindex printing, unduplicated lines of text @c first comma is part of primary @c STARTOFRANGE tpul @cindex text, printing, unduplicated lines of @cindex @command{uniq} utility The @command{uniq} utility reads sorted lines of data on its standard input, and by default removes duplicate lines. In other words, it only prints unique lines---hence the name. @command{uniq} has a number of options. The usage is as follows: @example uniq @r{[}-udc @r{[}-@var{n}@r{]]} @r{[}+@var{n}@r{]} @r{[} @var{input file} @r{[} @var{output file} @r{]]} @end example The options for @command{uniq} are: @table @code @item -d Pnly print only repeated lines. @item -u Print only nonrepeated lines. @item -c Count lines. This option overrides @option{-d} and @option{-u}. Both repeated and nonrepeated lines are counted. @item -@var{n} Skip @var{n} fields before comparing lines. The definition of fields is similar to @command{awk}'s default: nonwhitespace characters separated by runs of spaces and/or tabs. @item +@var{n} Skip @var{n} characters before comparing lines. Any fields specified with @samp{-@var{n}} are skipped first. @item @var{input file} Data is read from the input file named on the command line, instead of from the standard input. @item @var{output file} The generated output is sent to the named output file, instead of to the standard output. @end table Normally @command{uniq} behaves as if both the @option{-d} and @option{-u} options are provided. @command{uniq} uses the @code{getopt} library function (@pxref{Getopt Function}) and the @code{join} library function (@pxref{Join Function}). The program begins with a @code{usage} function and then a brief outline of the options and their meanings in a comment. The @code{BEGIN} rule deals with the command-line arguments and options. It uses a trick to get @code{getopt} to handle options of the form @samp{-25}, treating such an option as the option letter @samp{2} with an argument of @samp{5}. If indeed two or more digits are supplied (@code{Optarg} looks like a number), @code{Optarg} is concatenated with the option digit and then the result is added to zero to make it into a number. If there is only one digit in the option, then @code{Optarg} is not needed. In this case, @code{Optind} must be decremented so that @code{getopt} processes it next time. This code is admittedly a bit tricky. If no options are supplied, then the default is taken, to print both repeated and nonrepeated lines. The output file, if provided, is assigned to @code{outputfile}. Early on, @code{outputfile} is initialized to the standard output, @file{/dev/stdout}: @cindex @code{uniq.awk} program @example @c file eg/prog/uniq.awk @group # uniq.awk --- do uniq in awk # # Requires getopt and join library functions @end group @c endfile @ignore @c file eg/prog/uniq.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # May 1993 @c endfile @end ignore @c file eg/prog/uniq.awk function usage( e) @{ e = "Usage: uniq [-udc [-n]] [+n] [ in [ out ]]" print e > "/dev/stderr" exit 1 @} # -c count lines. overrides -d and -u # -d only repeated lines # -u only non-repeated lines # -n skip n fields # +n skip n characters, skip fields first BEGIN \ @{ count = 1 outputfile = "/dev/stdout" opts = "udc0:1:2:3:4:5:6:7:8:9:" while ((c = getopt(ARGC, ARGV, opts)) != -1) @{ if (c == "u") non_repeated_only++ else if (c == "d") repeated_only++ else if (c == "c") do_count++ else if (index("0123456789", c) != 0) @{ # getopt requires args to options # this messes us up for things like -5 if (Optarg ~ /^[0-9]+$/) fcount = (c Optarg) + 0 else @{ fcount = c + 0 Optind-- @} @} else usage() @} if (ARGV[Optind] ~ /^\+[0-9]+$/) @{ charcount = substr(ARGV[Optind], 2) + 0 Optind++ @} for (i = 1; i < Optind; i++) ARGV[i] = "" if (repeated_only == 0 && non_repeated_only == 0) repeated_only = non_repeated_only = 1 if (ARGC - Optind == 2) @{ outputfile = ARGV[ARGC - 1] ARGV[ARGC - 1] = "" @} @} @c endfile @end example The following function, @code{are_equal}, compares the current line, @code{$0}, to the previous line, @code{last}. It handles skipping fields and characters. If no field count and no character count are specified, @code{are_equal} simply returns one or zero depending upon the result of a simple string comparison of @code{last} and @code{$0}. Otherwise, things get more complicated. If fields have to be skipped, each line is broken into an array using @code{split} (@pxref{String Functions}); the desired fields are then joined back into a line using @code{join}. The joined lines are stored in @code{clast} and @code{cline}. If no fields are skipped, @code{clast} and @code{cline} are set to @code{last} and @code{$0}, respectively. Finally, if characters are skipped, @code{substr} is used to strip off the leading @code{charcount} characters in @code{clast} and @code{cline}. The two strings are then compared and @code{are_equal} returns the result: @example @c file eg/prog/uniq.awk function are_equal( n, m, clast, cline, alast, aline) @{ if (fcount == 0 && charcount == 0) return (last == $0) if (fcount > 0) @{ n = split(last, alast) m = split($0, aline) clast = join(alast, fcount+1, n) cline = join(aline, fcount+1, m) @} else @{ clast = last cline = $0 @} if (charcount) @{ clast = substr(clast, charcount + 1) cline = substr(cline, charcount + 1) @} return (clast == cline) @} @c endfile @end example The following two rules are the body of the program. The first one is executed only for the very first line of data. It sets @code{last} equal to @code{$0}, so that subsequent lines of text have something to be compared to. The second rule does the work. The variable @code{equal} is one or zero, depending upon the results of @code{are_equal}'s comparison. If @command{uniq} is counting repeated lines, and the lines are equal, then it increments the @code{count} variable. Otherwise, it prints the line and resets @code{count}, since the two lines are not equal. If @command{uniq} is not counting, and if the lines are equal, @code{count} is incremented. Nothing is printed, since the point is to remove duplicates. Otherwise, if @command{uniq} is counting repeated lines and more than one line is seen, or if @command{uniq} is counting nonrepeated lines and only one line is seen, then the line is printed, and @code{count} is reset. Finally, similar logic is used in the @code{END} rule to print the final line of input data: @example @c file eg/prog/uniq.awk NR == 1 @{ last = $0 next @} @{ equal = are_equal() if (do_count) @{ # overrides -d and -u if (equal) count++ else @{ printf("%4d %s\n", count, last) > outputfile last = $0 count = 1 # reset @} next @} if (equal) count++ else @{ if ((repeated_only && count > 1) || (non_repeated_only && count == 1)) print last > outputfile last = $0 count = 1 @} @} END @{ if (do_count) printf("%4d %s\n", count, last) > outputfile else if ((repeated_only && count > 1) || (non_repeated_only && count == 1)) print last > outputfile @} @c endfile @end example @c ENDOFRANGE prunt @c ENDOFRANGE tpul @node Wc Program @subsection Counting Things @c STARTOFRANGE count @cindex counting @c STARTOFRANGE infco @cindex input files, counting elements in @c STARTOFRANGE woco @cindex words, counting @c STARTOFRANGE chco @cindex characters, counting @c STARTOFRANGE lico @cindex lines, counting @cindex @command{wc} utility The @command{wc} (word count) utility counts lines, words, and characters in one or more input files. Its usage is as follows: @example wc @r{[}-lwc@r{]} @r{[} @var{files} @dots{} @r{]} @end example If no files are specified on the command line, @command{wc} reads its standard input. If there are multiple files, it also prints total counts for all the files. The options and their meanings are shown in the following list: @table @code @item -l Count only lines. @item -w Count only words. A ``word'' is a contiguous sequence of nonwhitespace characters, separated by spaces and/or tabs. Luckily, this is the normal way @command{awk} separates fields in its input data. @item -c Count only characters. @end table Implementing @command{wc} in @command{awk} is particularly elegant, since @command{awk} does a lot of the work for us; it splits lines into words (i.e., fields) and counts them, it counts lines (i.e., records), and it can easily tell us how long a line is. This uses the @code{getopt} library function (@pxref{Getopt Function}) and the file-transition functions (@pxref{Filetrans Function}). This version has one notable difference from traditional versions of @command{wc}: it always prints the counts in the order lines, words, and characters. Traditional versions note the order of the @option{-l}, @option{-w}, and @option{-c} options on the command line, and print the counts in that order. The @code{BEGIN} rule does the argument processing. The variable @code{print_total} is true if more than one file is named on the command line: @cindex @code{wc.awk} program @example @c file eg/prog/wc.awk # wc.awk --- count lines, words, characters @c endfile @ignore @c file eg/prog/wc.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # May 1993 @c endfile @end ignore @c file eg/prog/wc.awk # Options: # -l only count lines # -w only count words # -c only count characters # # Default is to count lines, words, characters # # Requires getopt and file transition library functions BEGIN @{ # let getopt print a message about # invalid options. we ignore them while ((c = getopt(ARGC, ARGV, "lwc")) != -1) @{ if (c == "l") do_lines = 1 else if (c == "w") do_words = 1 else if (c == "c") do_chars = 1 @} for (i = 1; i < Optind; i++) ARGV[i] = "" # if no options, do all if (! do_lines && ! do_words && ! do_chars) do_lines = do_words = do_chars = 1 print_total = (ARGC - i > 2) @} @c endfile @end example The @code{beginfile} function is simple; it just resets the counts of lines, words, and characters to zero, and saves the current @value{FN} in @code{fname}: @c NEXT ED: make it lines = words = chars = 0 @example @c file eg/prog/wc.awk function beginfile(file) @{ chars = lines = words = 0 fname = FILENAME @} @c endfile @end example The @code{endfile} function adds the current file's numbers to the running totals of lines, words, and characters.@footnote{@command{wc} can't just use the value of @code{FNR} in @code{endfile}. If you examine the code in @ref{Filetrans Function} you will see that @code{FNR} has already been reset by the time @code{endfile} is called.} It then prints out those numbers for the file that was just read. It relies on @code{beginfile} to reset the numbers for the following @value{DF}: @c ONE DAY: make the above footnote an exercise, instead of giving away the answer. @c NEXT ED: make order for += be lines, words, chars @example @c file eg/prog/wc.awk function endfile(file) @{ tchars += chars tlines += lines twords += words if (do_lines) printf "\t%d", lines @group if (do_words) printf "\t%d", words @end group if (do_chars) printf "\t%d", chars printf "\t%s\n", fname @} @c endfile @end example There is one rule that is executed for each line. It adds the length of the record, plus one, to @code{chars}. Adding one plus the record length is needed because the newline character separating records (the value of @code{RS}) is not part of the record itself, and thus not included in its length. Next, @code{lines} is incremented for each line read, and @code{words} is incremented by the value of @code{NF}, which is the number of ``words'' on this line: @example @c file eg/prog/wc.awk # do per line @{ chars += length($0) + 1 # get newline lines++ words += NF @} @c endfile @end example Finally, the @code{END} rule simply prints the totals for all the files: @example @c file eg/prog/wc.awk END @{ if (print_total) @{ if (do_lines) printf "\t%d", tlines if (do_words) printf "\t%d", twords if (do_chars) printf "\t%d", tchars print "\ttotal" @} @} @c endfile @end example @c ENDOFRANGE count @c ENDOFRANGE infco @c ENDOFRANGE lico @c ENDOFRANGE woco @c ENDOFRANGE chco @c ENDOFRANGE posimawk @node Miscellaneous Programs @section A Grab Bag of @command{awk} Programs This @value{SECTION} is a large ``grab bag'' of miscellaneous programs. We hope you find them both interesting and enjoyable. @menu * Dupword Program:: Finding duplicated words in a document. * Alarm Program:: An alarm clock. * Translate Program:: A program similar to the @command{tr} utility. * Labels Program:: Printing mailing labels. * Word Sorting:: A program to produce a word usage count. * History Sorting:: Eliminating duplicate entries from a history file. * Extract Program:: Pulling out programs from Texinfo source files. * Simple Sed:: A Simple Stream Editor. * Igawk Program:: A wrapper for @command{awk} that includes files. @end menu @node Dupword Program @subsection Finding Duplicated Words in a Document @c last comma is part of secondary @cindex words, duplicate, searching for @cindex searching, for words @c first comma is part of primary @cindex documents, searching A common error when writing large amounts of prose is to accidentally duplicate words. Typically you will see this in text as something like ``the the program does the following@dots{}'' When the text is online, often the duplicated words occur at the end of one line and the beginning of another, making them very difficult to spot. @c as here! This program, @file{dupword.awk}, scans through a file one line at a time and looks for adjacent occurrences of the same word. It also saves the last word on a line (in the variable @code{prev}) for comparison with the first word on the next line. @cindex Texinfo The first two statements make sure that the line is all lowercase, so that, for example, ``The'' and ``the'' compare equal to each other. The next statement replaces nonalphanumeric and nonwhitespace characters with spaces, so that punctuation does not affect the comparison either. The characters are replaced with spaces so that formatting controls don't create nonsense words (e.g., the Texinfo @samp{@@code@{NF@}} becomes @samp{codeNF} if punctuation is simply deleted). The record is then resplit into fields, yielding just the actual words on the line, and ensuring that there are no empty fields. If there are no fields left after removing all the punctuation, the current record is skipped. Otherwise, the program loops through each word, comparing it to the previous one: @cindex @code{dupword.awk} program @example @c file eg/prog/dupword.awk # dupword.awk --- find duplicate words in text @c endfile @ignore @c file eg/prog/dupword.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # December 1991 # Revised October 2000 @c endfile @end ignore @c file eg/prog/dupword.awk @{ $0 = tolower($0) gsub(/[^[:alnum:][:blank:]]/, " "); $0 = $0 # re-split if (NF == 0) next if ($1 == prev) printf("%s:%d: duplicate %s\n", FILENAME, FNR, $1) for (i = 2; i <= NF; i++) if ($i == $(i-1)) printf("%s:%d: duplicate %s\n", FILENAME, FNR, $i) prev = $NF @} @c endfile @end example @node Alarm Program @subsection An Alarm Clock Program @cindex insomnia, cure for @cindex Robbins, Arnold @quotation @i{Nothing cures insomnia like a ringing alarm clock.}@* Arnold Robbins @end quotation @c STARTOFRANGE tialarm @cindex time, alarm clock example program @c STARTOFRANGE alaex @cindex alarm clock example program The following program is a simple ``alarm clock'' program. You give it a time of day and an optional message. At the specified time, it prints the message on the standard output. In addition, you can give it the number of times to repeat the message as well as a delay between repetitions. This program uses the @code{gettimeofday} function from @ref{Gettimeofday Function}. All the work is done in the @code{BEGIN} rule. The first part is argument checking and setting of defaults: the delay, the count, and the message to print. If the user supplied a message without the ASCII BEL character (known as the ``alert'' character, @code{"\a"}), then it is added to the message. (On many systems, printing the ASCII BEL generates an audible alert. Thus when the alarm goes off, the system calls attention to itself in case the user is not looking at the computer or terminal.) Here is the program: @cindex @code{alarm.awk} program @example @c file eg/prog/alarm.awk # alarm.awk --- set an alarm # # Requires gettimeofday library function @c endfile @ignore @c file eg/prog/alarm.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # May 1993 @c endfile @end ignore @c file eg/prog/alarm.awk # usage: alarm time [ "message" [ count [ delay ] ] ] BEGIN \ @{ # Initial argument sanity checking usage1 = "usage: alarm time ['message' [count [delay]]]" usage2 = sprintf("\t(%s) time ::= hh:mm", ARGV[1]) if (ARGC < 2) @{ print usage1 > "/dev/stderr" print usage2 > "/dev/stderr" exit 1 @} else if (ARGC == 5) @{ delay = ARGV[4] + 0 count = ARGV[3] + 0 message = ARGV[2] @} else if (ARGC == 4) @{ count = ARGV[3] + 0 message = ARGV[2] @} else if (ARGC == 3) @{ message = ARGV[2] @} else if (ARGV[1] !~ /[0-9]?[0-9]:[0-9][0-9]/) @{ print usage1 > "/dev/stderr" print usage2 > "/dev/stderr" exit 1 @} # set defaults for once we reach the desired time if (delay == 0) delay = 180 # 3 minutes @group if (count == 0) count = 5 @end group if (message == "") message = sprintf("\aIt is now %s!\a", ARGV[1]) else if (index(message, "\a") == 0) message = "\a" message "\a" @c endfile @end example The next @value{SECTION} of code turns the alarm time into hours and minutes, converts it (if necessary) to a 24-hour clock, and then turns that time into a count of the seconds since midnight. Next it turns the current time into a count of seconds since midnight. The difference between the two is how long to wait before setting off the alarm: @example @c file eg/prog/alarm.awk # split up alarm time split(ARGV[1], atime, ":") hour = atime[1] + 0 # force numeric minute = atime[2] + 0 # force numeric # get current broken down time gettimeofday(now) # if time given is 12-hour hours and it's after that # hour, e.g., `alarm 5:30' at 9 a.m. means 5:30 p.m., # then add 12 to real hour if (hour < 12 && now["hour"] > hour) hour += 12 # set target time in seconds since midnight target = (hour * 60 * 60) + (minute * 60) # get current time in seconds since midnight current = (now["hour"] * 60 * 60) + \ (now["minute"] * 60) + now["second"] # how long to sleep for naptime = target - current if (naptime <= 0) @{ print "time is in the past!" > "/dev/stderr" exit 1 @} @c endfile @end example @cindex @command{sleep} utility Finally, the program uses the @code{system} function (@pxref{I/O Functions}) to call the @command{sleep} utility. The @command{sleep} utility simply pauses for the given number of seconds. If the exit status is not zero, the program assumes that @command{sleep} was interrupted and exits. If @command{sleep} exited with an OK status (zero), then the program prints the message in a loop, again using @command{sleep} to delay for however many seconds are necessary: @example @c file eg/prog/alarm.awk # zzzzzz..... go away if interrupted if (system(sprintf("sleep %d", naptime)) != 0) exit 1 # time to notify! command = sprintf("sleep %d", delay) for (i = 1; i <= count; i++) @{ print message # if sleep command interrupted, go away if (system(command) != 0) break @} exit 0 @} @c endfile @end example @c ENDOFRANGE tialarm @c ENDOFRANGE alaex @node Translate Program @subsection Transliterating Characters @c STARTOFRANGE chtra @cindex characters, transliterating @cindex @command{tr} utility The system @command{tr} utility transliterates characters. For example, it is often used to map uppercase letters into lowercase for further processing: @example @var{generate data} | tr 'A-Z' 'a-z' | @var{process data} @dots{} @end example @command{tr} requires two lists of characters.@footnote{On some older System V systems, @ifset ORA including Solaris, @end ifset @command{tr} may require that the lists be written as range expressions enclosed in square brackets (@samp{[a-z]}) and quoted, to prevent the shell from attempting a @value{FN} expansion. This is not a feature.} When processing the input, the first character in the first list is replaced with the first character in the second list, the second character in the first list is replaced with the second character in the second list, and so on. If there are more characters in the ``from'' list than in the ``to'' list, the last character of the ``to'' list is used for the remaining characters in the ``from'' list. Some time ago, @c early or mid-1989! a user proposed that a transliteration function should be added to @command{gawk}. @c Wishing to avoid gratuitous new features, @c at least theoretically The following program was written to prove that character transliteration could be done with a user-level function. This program is not as complete as the system @command{tr} utility but it does most of the job. The @command{translate} program demonstrates one of the few weaknesses of standard @command{awk}: dealing with individual characters is very painful, requiring repeated use of the @code{substr}, @code{index}, and @code{gsub} built-in functions (@pxref{String Functions}).@footnote{This program was written before @command{gawk} acquired the ability to split each character in a string into separate array elements.} @c Exercise: How might you use this new feature to simplify the program? There are two functions. The first, @code{stranslate}, takes three arguments: @table @code @item from A list of characters from which to translate. @item to A list of characters to which to translate. @item target The string on which to do the translation. @end table Associative arrays make the translation part fairly easy. @code{t_ar} holds the ``to'' characters, indexed by the ``from'' characters. Then a simple loop goes through @code{from}, one character at a time. For each character in @code{from}, if the character appears in @code{target}, @code{gsub} is used to change it to the corresponding @code{to} character. The @code{translate} function simply calls @code{stranslate} using @code{$0} as the target. The main program sets two global variables, @code{FROM} and @code{TO}, from the command line, and then changes @code{ARGV} so that @command{awk} reads from the standard input. Finally, the processing rule simply calls @code{translate} for each record: @cindex @code{translate.awk} program @example @c file eg/prog/translate.awk # translate.awk --- do tr-like stuff @c endfile @ignore @c file eg/prog/translate.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # August 1989 @c endfile @end ignore @c file eg/prog/translate.awk # Bugs: does not handle things like: tr A-Z a-z, it has # to be spelled out. However, if `to' is shorter than `from', # the last character in `to' is used for the rest of `from'. function stranslate(from, to, target, lf, lt, t_ar, i, c) @{ lf = length(from) lt = length(to) for (i = 1; i <= lt; i++) t_ar[substr(from, i, 1)] = substr(to, i, 1) if (lt < lf) for (; i <= lf; i++) t_ar[substr(from, i, 1)] = substr(to, lt, 1) for (i = 1; i <= lf; i++) @{ c = substr(from, i, 1) if (index(target, c) > 0) gsub(c, t_ar[c], target) @} return target @} function translate(from, to) @{ return $0 = stranslate(from, to, $0) @} # main program BEGIN @{ @group if (ARGC < 3) @{ print "usage: translate from to" > "/dev/stderr" exit @} @end group FROM = ARGV[1] TO = ARGV[2] ARGC = 2 ARGV[1] = "-" @} @{ translate(FROM, TO) print @} @c endfile @end example While it is possible to do character transliteration in a user-level function, it is not necessarily efficient, and we (the @command{gawk} authors) started to consider adding a built-in function. However, shortly after writing this program, we learned that the System V Release 4 @command{awk} had added the @code{toupper} and @code{tolower} functions (@pxref{String Functions}). These functions handle the vast majority of the cases where character transliteration is necessary, and so we chose to simply add those functions to @command{gawk} as well and then leave well enough alone. An obvious improvement to this program would be to set up the @code{t_ar} array only once, in a @code{BEGIN} rule. However, this assumes that the ``from'' and ``to'' lists will never change throughout the lifetime of the program. @c ENDOFRANGE chtra @node Labels Program @subsection Printing Mailing Labels @c STARTOFRANGE prml @cindex printing, mailing labels @c comma is part of primary @c STARTOFRANGE mlprint @cindex mailing labels, printing Here is a ``real world''@footnote{``Real world'' is defined as ``a program actually used to get something done.''} program. This script reads lists of names and addresses and generates mailing labels. Each page of labels has 20 labels on it, 2 across and 10 down. The addresses are guaranteed to be no more than 5 lines of data. Each address is separated from the next by a blank line. The basic idea is to read 20 labels worth of data. Each line of each label is stored in the @code{line} array. The single rule takes care of filling the @code{line} array and printing the page when 20 labels have been read. The @code{BEGIN} rule simply sets @code{RS} to the empty string, so that @command{awk} splits records at blank lines (@pxref{Records}). It sets @code{MAXLINES} to 100, since 100 is the maximum number of lines on the page (20 * 5 = 100). Most of the work is done in the @code{printpage} function. The label lines are stored sequentially in the @code{line} array. But they have to print horizontally; @code{line[1]} next to @code{line[6]}, @code{line[2]} next to @code{line[7]}, and so on. Two loops are used to accomplish this. The outer loop, controlled by @code{i}, steps through every 10 lines of data; this is each row of labels. The inner loop, controlled by @code{j}, goes through the lines within the row. As @code{j} goes from 0 to 4, @samp{i+j} is the @code{j}-th line in the row, and @samp{i+j+5} is the entry next to it. The output ends up looking something like this: @example line 1 line 6 line 2 line 7 line 3 line 8 line 4 line 9 line 5 line 10 @dots{} @end example As a final note, an extra blank line is printed at lines 21 and 61, to keep the output lined up on the labels. This is dependent on the particular brand of labels in use when the program was written. You will also note that there are 2 blank lines at the top and 2 blank lines at the bottom. The @code{END} rule arranges to flush the final page of labels; there may not have been an even multiple of 20 labels in the data: @cindex @code{labels.awk} program @example @c file eg/prog/labels.awk # labels.awk --- print mailing labels @c endfile @ignore @c file eg/prog/labels.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # June 1992 @c endfile @end ignore @c file eg/prog/labels.awk # Each label is 5 lines of data that may have blank lines. # The label sheets have 2 blank lines at the top and 2 at # the bottom. BEGIN @{ RS = "" ; MAXLINES = 100 @} function printpage( i, j) @{ if (Nlines <= 0) return printf "\n\n" # header for (i = 1; i <= Nlines; i += 10) @{ if (i == 21 || i == 61) print "" for (j = 0; j < 5; j++) @{ if (i + j > MAXLINES) break printf " %-41s %s\n", line[i+j], line[i+j+5] @} print "" @} printf "\n\n" # footer for (i in line) line[i] = "" @} # main rule @{ if (Count >= 20) @{ printpage() Count = 0 Nlines = 0 @} n = split($0, a, "\n") for (i = 1; i <= n; i++) line[++Nlines] = a[i] for (; i <= 5; i++) line[++Nlines] = "" Count++ @} END \ @{ printpage() @} @c endfile @end example @c ENDOFRANGE prml @c ENDOFRANGE mlprint @node Word Sorting @subsection Generating Word-Usage Counts @c last comma is part of secondary @c STARTOFRANGE worus @cindex words, usage counts, generating @c NEXT ED: Rewrite this whole section and example The following @command{awk} program prints the number of occurrences of each word in its input. It illustrates the associative nature of @command{awk} arrays by using strings as subscripts. It also demonstrates the @samp{for @var{index} in @var{array}} mechanism. Finally, it shows how @command{awk} is used in conjunction with other utility programs to do a useful task of some complexity with a minimum of effort. Some explanations follow the program listing: @example # Print list of word frequencies @{ for (i = 1; i <= NF; i++) freq[$i]++ @} END @{ for (word in freq) printf "%s\t%d\n", word, freq[word] @} @end example @c Exercise: Use asort() here This program has two rules. The first rule, because it has an empty pattern, is executed for every input line. It uses @command{awk}'s field-accessing mechanism (@pxref{Fields}) to pick out the individual words from the line, and the built-in variable @code{NF} (@pxref{Built-in Variables}) to know how many fields are available. For each input word, it increments an element of the array @code{freq} to reflect that the word has been seen an additional time. The second rule, because it has the pattern @code{END}, is not executed until the input has been exhausted. It prints out the contents of the @code{freq} table that has been built up inside the first action. This program has several problems that would prevent it from being useful by itself on real text files: @itemize @bullet @item Words are detected using the @command{awk} convention that fields are separated just by whitespace. Other characters in the input (except newlines) don't have any special meaning to @command{awk}. This means that punctuation characters count as part of words. @item The @command{awk} language considers upper- and lowercase characters to be distinct. Therefore, ``bartender'' and ``Bartender'' are not treated as the same word. This is undesirable, since in normal text, words are capitalized if they begin sentences, and a frequency analyzer should not be sensitive to capitalization. @item The output does not come out in any useful order. You're more likely to be interested in which words occur most frequently or in having an alphabetized table of how frequently each word occurs. @end itemize @cindex @command{sort} utility The way to solve these problems is to use some of @command{awk}'s more advanced features. First, we use @code{tolower} to remove case distinctions. Next, we use @code{gsub} to remove punctuation characters. Finally, we use the system @command{sort} utility to process the output of the @command{awk} script. Here is the new version of the program: @cindex @code{wordfreq.awk} program @example @c file eg/prog/wordfreq.awk # wordfreq.awk --- print list of word frequencies @{ $0 = tolower($0) # remove case distinctions # remove punctuation gsub(/[^[:alnum:]_[:blank:]]/, "", $0) for (i = 1; i <= NF; i++) freq[$i]++ @} END @{ for (word in freq) printf "%s\t%d\n", word, freq[word] @} @c endfile @end example Assuming we have saved this program in a file named @file{wordfreq.awk}, and that the data is in @file{file1}, the following pipeline: @example awk -f wordfreq.awk file1 | sort -k 2nr @end example @noindent produces a table of the words appearing in @file{file1} in order of decreasing frequency. The @command{awk} program suitably massages the data and produces a word frequency table, which is not ordered. The @command{awk} script's output is then sorted by the @command{sort} utility and printed on the terminal. The options given to @command{sort} specify a sort that uses the second field of each input line (skipping one field), that the sort keys should be treated as numeric quantities (otherwise @samp{15} would come before @samp{5}), and that the sorting should be done in descending (reverse) order. The @command{sort} could even be done from within the program, by changing the @code{END} action to: @example @c file eg/prog/wordfreq.awk END @{ sort = "sort -k 2nr" for (word in freq) printf "%s\t%d\n", word, freq[word] | sort close(sort) @} @c endfile @end example This way of sorting must be used on systems that do not have true pipes at the command-line (or batch-file) level. See the general operating system documentation for more information on how to use the @command{sort} program. @c ENDOFRANGE worus @node History Sorting @subsection Removing Duplicates from Unsorted Text @c last comma is part of secondary @c STARTOFRANGE lidu @cindex lines, duplicate, removing The @command{uniq} program (@pxref{Uniq Program}), removes duplicate lines from @emph{sorted} data. Suppose, however, you need to remove duplicate lines from a @value{DF} but that you want to preserve the order the lines are in. A good example of this might be a shell history file. The history file keeps a copy of all the commands you have entered, and it is not unusual to repeat a command several times in a row. Occasionally you might want to compact the history by removing duplicate entries. Yet it is desirable to maintain the order of the original commands. This simple program does the job. It uses two arrays. The @code{data} array is indexed by the text of each line. For each line, @code{data[$0]} is incremented. If a particular line has not been seen before, then @code{data[$0]} is zero. In this case, the text of the line is stored in @code{lines[count]}. Each element of @code{lines} is a unique command, and the indices of @code{lines} indicate the order in which those lines are encountered. The @code{END} rule simply prints out the lines, in order: @cindex Rakitzis, Byron @cindex @code{histsort.awk} program @example @c file eg/prog/histsort.awk # histsort.awk --- compact a shell history file # Thanks to Byron Rakitzis for the general idea @c endfile @ignore @c file eg/prog/histsort.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # May 1993 @c endfile @end ignore @c file eg/prog/histsort.awk @group @{ if (data[$0]++ == 0) lines[++count] = $0 @} @end group END @{ for (i = 1; i <= count; i++) print lines[i] @} @c endfile @end example This program also provides a foundation for generating other useful information. For example, using the following @code{print} statement in the @code{END} rule indicates how often a particular command is used: @example print data[lines[i]], lines[i] @end example This works because @code{data[$0]} is incremented each time a line is seen. @c ENDOFRANGE lidu @node Extract Program @subsection Extracting Programs from Texinfo Source Files @c STARTOFRANGE texse @cindex Texinfo, extracting programs from source files @c last comma is part of secondary @c STARTOFRANGE fitex @cindex files, Texinfo, extracting programs from @ifnotinfo Both this chapter and the previous chapter (@ref{Library Functions}) present a large number of @command{awk} programs. @end ifnotinfo @ifinfo The nodes @ref{Library Functions}, and @ref{Sample Programs}, are the top level nodes for a large number of @command{awk} programs. @end ifinfo If you want to experiment with these programs, it is tedious to have to type them in by hand. Here we present a program that can extract parts of a Texinfo input file into separate files. @cindex Texinfo This @value{DOCUMENT} is written in Texinfo, the GNU project's document formatting language. A single Texinfo source file can be used to produce both printed and online documentation. @ifnotinfo Texinfo is fully documented in the book @cite{Texinfo---The GNU Documentation Format}, available from the Free Software Foundation. @end ifnotinfo @ifinfo The Texinfo language is described fully, starting with @ref{Top}. @end ifinfo For our purposes, it is enough to know three things about Texinfo input files: @itemize @bullet @item The ``at'' symbol (@samp{@@}) is special in Texinfo, much as the backslash (@samp{\}) is in C or @command{awk}. Literal @samp{@@} symbols are represented in Texinfo source files as @samp{@@@@}. @item Comments start with either @samp{@@c} or @samp{@@comment}. The file-extraction program works by using special comments that start at the beginning of a line. @item Lines containing @samp{@@group} and @samp{@@end group} commands bracket example text that should not be split across a page boundary. (Unfortunately, @TeX{} isn't always smart enough to do things exactly right, and we have to give it some help.) @end itemize The following program, @file{extract.awk}, reads through a Texinfo source file and does two things, based on the special comments. Upon seeing @samp{@w{@@c system @dots{}}}, it runs a command, by extracting the command text from the control line and passing it on to the @code{system} function (@pxref{I/O Functions}). Upon seeing @samp{@@c file @var{filename}}, each subsequent line is sent to the file @var{filename}, until @samp{@@c endfile} is encountered. The rules in @file{extract.awk} match either @samp{@@c} or @samp{@@comment} by letting the @samp{omment} part be optional. Lines containing @samp{@@group} and @samp{@@end group} are simply removed. @file{extract.awk} uses the @code{join} library function (@pxref{Join Function}). The example programs in the online Texinfo source for @cite{@value{TITLE}} (@file{gawk.texi}) have all been bracketed inside @samp{file} and @samp{endfile} lines. The @command{gawk} distribution uses a copy of @file{extract.awk} to extract the sample programs and install many of them in a standard directory where @command{gawk} can find them. The Texinfo file looks something like this: @example @dots{} This program has a @@code@{BEGIN@} rule, that prints a nice message: @@example @@c file examples/messages.awk BEGIN @@@{ print "Don't panic!" @@@} @@c end file @@end example It also prints some final advice: @@example @@c file examples/messages.awk END @@@{ print "Always avoid bored archeologists!" @@@} @@c end file @@end example @dots{} @end example @file{extract.awk} begins by setting @code{IGNORECASE} to one, so that mixed upper- and lowercase letters in the directives won't matter. The first rule handles calling @code{system}, checking that a command is given (@code{NF} is at least three) and also checking that the command exits with a zero exit status, signifying OK: @cindex @code{extract.awk} program @example @c file eg/prog/extract.awk # extract.awk --- extract files and run programs # from texinfo files @c endfile @ignore @c file eg/prog/extract.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # May 1993 # Revised September 2000 @c endfile @end ignore @c file eg/prog/extract.awk BEGIN @{ IGNORECASE = 1 @} /^@@c(omment)?[ \t]+system/ \ @{ if (NF < 3) @{ e = (FILENAME ":" FNR) e = (e ": badly formed `system' line") print e > "/dev/stderr" next @} $1 = "" $2 = "" stat = system($0) if (stat != 0) @{ e = (FILENAME ":" FNR) e = (e ": warning: system returned " stat) print e > "/dev/stderr" @} @} @c endfile @end example @noindent The variable @code{e} is used so that the function fits nicely on the @ifnotinfo page. @end ifnotinfo @ifnottex screen. @end ifnottex The second rule handles moving data into files. It verifies that a @value{FN} is given in the directive. If the file named is not the current file, then the current file is closed. Keeping the current file open until a new file is encountered allows the use of the @samp{>} redirection for printing the contents, keeping open file management simple. The @samp{for} loop does the work. It reads lines using @code{getline} (@pxref{Getline}). For an unexpected end of file, it calls the @code{@w{unexpected_eof}} function. If the line is an ``endfile'' line, then it breaks out of the loop. If the line is an @samp{@@group} or @samp{@@end group} line, then it ignores it and goes on to the next line. Similarly, comments within examples are also ignored. Most of the work is in the following few lines. If the line has no @samp{@@} symbols, the program can print it directly. Otherwise, each leading @samp{@@} must be stripped off. To remove the @samp{@@} symbols, the line is split into separate elements of the array @code{a}, using the @code{split} function (@pxref{String Functions}). The @samp{@@} symbol is used as the separator character. Each element of @code{a} that is empty indicates two successive @samp{@@} symbols in the original line. For each two empty elements (@samp{@@@@} in the original file), we have to add a single @samp{@@} symbol back in. When the processing of the array is finished, @code{join} is called with the value of @code{SUBSEP}, to rejoin the pieces back into a single line. That line is then printed to the output file: @example @c file eg/prog/extract.awk /^@@c(omment)?[ \t]+file/ \ @{ if (NF != 3) @{ e = (FILENAME ":" FNR ": badly formed `file' line") print e > "/dev/stderr" next @} if ($3 != curfile) @{ if (curfile != "") close(curfile) curfile = $3 @} for (;;) @{ if ((getline line) <= 0) unexpected_eof() if (line ~ /^@@c(omment)?[ \t]+endfile/) break else if (line ~ /^@@(end[ \t]+)?group/) continue else if (line ~ /^@@c(omment+)?[ \t]+/) continue if (index(line, "@@") == 0) @{ print line > curfile continue @} n = split(line, a, "@@") # if a[1] == "", means leading @@, # don't add one back in. for (i = 2; i <= n; i++) @{ if (a[i] == "") @{ # was an @@@@ a[i] = "@@" if (a[i+1] == "") i++ @} @} print join(a, 1, n, SUBSEP) > curfile @} @} @c endfile @end example An important thing to note is the use of the @samp{>} redirection. Output done with @samp{>} only opens the file once; it stays open and subsequent output is appended to the file (@pxref{Redirection}). This makes it easy to mix program text and explanatory prose for the same sample source file (as has been done here!) without any hassle. The file is only closed when a new data @value{FN} is encountered or at the end of the input file. Finally, the function @code{@w{unexpected_eof}} prints an appropriate error message and then exits. The @code{END} rule handles the final cleanup, closing the open file: @c function lb put on same line for page breaking. sigh @example @c file eg/prog/extract.awk @group function unexpected_eof() @{ printf("%s:%d: unexpected EOF or error\n", FILENAME, FNR) > "/dev/stderr" exit 1 @} @end group END @{ if (curfile) close(curfile) @} @c endfile @end example @c ENDOFRANGE texse @c ENDOFRANGE fitex @node Simple Sed @subsection A Simple Stream Editor @cindex @command{sed} utility @cindex stream editors The @command{sed} utility is a stream editor, a program that reads a stream of data, makes changes to it, and passes it on. It is often used to make global changes to a large file or to a stream of data generated by a pipeline of commands. While @command{sed} is a complicated program in its own right, its most common use is to perform global substitutions in the middle of a pipeline: @example command1 < orig.data | sed 's/old/new/g' | command2 > result @end example Here, @samp{s/old/new/g} tells @command{sed} to look for the regexp @samp{old} on each input line and globally replace it with the text @samp{new}, i.e., all the occurrences on a line. This is similar to @command{awk}'s @code{gsub} function (@pxref{String Functions}). The following program, @file{awksed.awk}, accepts at least two command-line arguments: the pattern to look for and the text to replace it with. Any additional arguments are treated as data @value{FN}s to process. If none are provided, the standard input is used: @cindex Brennan, Michael @cindex @command{awksed.awk} program @c @cindex simple stream editor @c @cindex stream editor, simple @example @c file eg/prog/awksed.awk # awksed.awk --- do s/foo/bar/g using just print # Thanks to Michael Brennan for the idea @c endfile @ignore @c file eg/prog/awksed.awk # # Arnold Robbins, arnold@@gnu.org, Public Domain # August 1995 @c endfile @end ignore @c file eg/prog/awksed.awk function usage() @{ print "usage: awksed pat repl [files...]" > "/dev/stderr" exit 1 @} BEGIN @{ # validate arguments if (ARGC < 3) usage() RS = ARGV[1] ORS = ARGV[2] # don't use arguments as files ARGV[1] = ARGV[2] = "" @} @group # look ma, no hands! @{ if (RT == "") printf "%s", $0 else print @} @end group @c endfile @end example The program relies on @command{gawk}'s ability to have @code{RS} be a regexp, as well as on the setting of @code{RT} to the actual text that terminates the record (@pxref{Records}). The idea is to have @code{RS} be the pattern to look for. @command{gawk} automatically sets @code{$0} to the text between matches of the pattern. This is text that we want to keep, unmodified. Then, by setting @code{ORS} to the replacement text, a simple @code{print} statement outputs the text we want to keep, followed by the replacement text. There is one wrinkle to this scheme, which is what to do if the last record doesn't end with text that matches @code{RS}. Using a @code{print} statement unconditionally prints the replacement text, which is not correct. However, if the file did not end in text that matches @code{RS}, @code{RT} is set to the null string. In this case, we can print @code{$0} using @code{printf} (@pxref{Printf}). The @code{BEGIN} rule handles the setup, checking for the right number of arguments and calling @code{usage} if there is a problem. Then it sets @code{RS} and @code{ORS} from the command-line arguments and sets @code{ARGV[1]} and @code{ARGV[2]} to the null string, so that they are not treated as @value{FN}s (@pxref{ARGC and ARGV}). The @code{usage} function prints an error message and exits. Finally, the single rule handles the printing scheme outlined above, using @code{print} or @code{printf} as appropriate, depending upon the value of @code{RT}. @ignore Exercise, compare the performance of this version with the more straightforward: BEGIN { pat = ARGV[1] repl = ARGV[2] ARGV[1] = ARGV[2] = "" } { gsub(pat, repl); print } Exercise: what are the advantages and disadvantages of this version versus sed? Advantage: egrep regexps speed (?) Disadvantage: no & in replacement text Others? @end ignore @node Igawk Program @subsection An Easy Way to Use Library Functions @c STARTOFRANGE libfex @cindex libraries of @command{awk} functions, example program for using @c STARTOFRANGE flibex @cindex functions, library, example program for using Using library functions in @command{awk} can be very beneficial. It encourages code reuse and the writing of general functions. Programs are smaller and therefore clearer. However, using library functions is only easy when writing @command{awk} programs; it is painful when running them, requiring multiple @option{-f} options. If @command{gawk} is unavailable, then so too is the @env{AWKPATH} environment variable and the ability to put @command{awk} functions into a library directory (@pxref{Options}). It would be nice to be able to write programs in the following manner: @example # library functions @@include getopt.awk @@include join.awk @dots{} # main program BEGIN @{ while ((c = getopt(ARGC, ARGV, "a:b:cde")) != -1) @dots{} @dots{} @} @end example The following program, @file{igawk.sh}, provides this service. It simulates @command{gawk}'s searching of the @env{AWKPATH} variable and also allows @dfn{nested} includes; i.e., a file that is included with @samp{@@include} can contain further @samp{@@include} statements. @command{igawk} makes an effort to only include files once, so that nested includes don't accidentally include a library function twice. @command{igawk} should behave just like @command{gawk} externally. This means it should accept all of @command{gawk}'s command-line arguments, including the ability to have multiple source files specified via @option{-f}, and the ability to mix command-line and library source files. The program is written using the POSIX Shell (@command{sh}) command language.@footnote{Fully explaining the @command{sh} language is beyond the scope of this book. We provide some minimal explanations, but see a good shell programming book if you wish to understand things in more depth.} It works as follows: @enumerate @item Loop through the arguments, saving anything that doesn't represent @command{awk} source code for later, when the expanded program is run. @item For any arguments that do represent @command{awk} text, put the arguments into a shell variable that will be expanded. There are two cases: @enumerate a @item Literal text, provided with @option{--source} or @option{--source=}. This text is just appended directly. @item Source @value{FN}s, provided with @option{-f}. We use a neat trick and append @samp{@@include @var{filename}} to the shell variable's contents. Since the file-inclusion program works the way @command{gawk} does, this gets the text of the file included into the program at the correct point. @end enumerate @item Run an @command{awk} program (naturally) over the shell variable's contents to expand @samp{@@include} statements. The expanded program is placed in a second shell variable. @item Run the expanded program with @command{gawk} and any other original command-line arguments that the user supplied (such as the data @value{FN}s). @end enumerate This program uses shell variables extensively; for storing command line arguments, the text of the @command{awk} program that will expand the user's program, for the user's original program, and for the expanded program. Doing so removes some potential problems that might arise were we to use temporary files instead, at the cost of making the script somewhat more complicated. The initial part of the program turns on shell tracing if the first argument is @samp{debug}. The next part loops through all the command-line arguments. There are several cases of interest: @table @code @item -- This ends the arguments to @command{igawk}. Anything else should be passed on to the user's @command{awk} program without being evaluated. @item -W This indicates that the next option is specific to @command{gawk}. To make argument processing easier, the @option{-W} is appended to the front of the remaining arguments and the loop continues. (This is an @command{sh} programming trick. Don't worry about it if you are not familiar with @command{sh}.) @item -v@r{,} -F These are saved and passed on to @command{gawk}. @item -f@r{,} --file@r{,} --file=@r{,} -Wfile= The @value{FN} is appended to the shell variable @code{program} with an @samp{@@include} statement. The @command{expr} utility is used to remove the leading option part of the argument (e.g., @samp{--file=}). (Typical @command{sh} usage would be to use the @command{echo} and @command{sed} utilities to do this work. Unfortunately, some versions of @command{echo} evaluate escape sequences in their arguments, possibly mangling the program text. Using @command{expr} avoids this problem.) @item --source@r{,} --source=@r{,} -Wsource= The source text is appended to @code{program}. @item --version@r{,} -Wversion @command{igawk} prints its version number, runs @samp{gawk --version} to get the @command{gawk} version information, and then exits. @end table If none of the @option{-f}, @option{--file}, @option{-Wfile}, @option{--source}, or @option{-Wsource} arguments are supplied, then the first nonoption argument should be the @command{awk} program. If there are no command-line arguments left, @command{igawk} prints an error message and exits. Otherwise, the first argument is appended to @code{program}. In any case, after the arguments have been processed, @code{program} contains the complete text of the original @command{awk} program. The program is as follows: @cindex @code{igawk.sh} program @example @c file eg/prog/igawk.sh #! /bin/sh # igawk --- like gawk but do @@include processing @c endfile @ignore @c file eg/prog/igawk.sh # # Arnold Robbins, arnold@@gnu.org, Public Domain # July 1993 @c endfile @end ignore @c file eg/prog/igawk.sh if [ "$1" = debug ] then set -x shift fi # A literal newline, so that program text is formmatted correctly n=' ' # Initialize variables to empty program= opts= while [ $# -ne 0 ] # loop over arguments do case $1 in --) shift; break;; -W) shift # The $@{x?'message here'@} construct prints a # diagnostic if $x is the null string set -- -W"$@{@@?'missing operand'@}" continue;; -[vF]) opts="$opts $1 '$@{2?'missing operand'@}'" shift;; -[vF]*) opts="$opts '$1'" ;; -f) program="$program$n@@include $@{2?'missing operand'@}" shift;; -f*) f=`expr "$1" : '-f\(.*\)'` program="$program$n@@include $f";; -[W-]file=*) f=`expr "$1" : '-.file=\(.*\)'` program="$program$n@@include $f";; -[W-]file) program="$program$n@@include $@{2?'missing operand'@}" shift;; -[W-]source=*) t=`expr "$1" : '-.source=\(.*\)'` program="$program$n$t";; -[W-]source) program="$program$n$@{2?'missing operand'@}" shift;; -[W-]version) echo igawk: version 2.0 1>&2 gawk --version exit 0 ;; -[W-]*) opts="$opts '$1'" ;; *) break;; esac shift done if [ -z "$program" ] then program=$@{1?'missing program'@} shift fi # At this point, `program' has the program. @c endfile @end example The @command{awk} program to process @samp{@@include} directives is stored in the shell variable @code{expand_prog}. Doing this keeps the shell script readable. The @command{awk} program reads through the user's program, one line at a time, using @code{getline} (@pxref{Getline}). The input @value{FN}s and @samp{@@include} statements are managed using a stack. As each @samp{@@include} is encountered, the current @value{FN} is ``pushed'' onto the stack and the file named in the @samp{@@include} directive becomes the current @value{FN}. As each file is finished, the stack is ``popped,'' and the previous input file becomes the current input file again. The process is started by making the original file the first one on the stack. The @code{pathto} function does the work of finding the full path to a file. It simulates @command{gawk}'s behavior when searching the @env{AWKPATH} environment variable (@pxref{AWKPATH Variable}). If a @value{FN} has a @samp{/} in it, no path search is done. Otherwise, the @value{FN} is concatenated with the name of each directory in the path, and an attempt is made to open the generated @value{FN}. The only way to test if a file can be read in @command{awk} is to go ahead and try to read it with @code{getline}; this is what @code{pathto} does.@footnote{On some very old versions of @command{awk}, the test @samp{getline junk < t} can loop forever if the file exists but is empty. Caveat emptor.} If the file can be read, it is closed and the @value{FN} is returned: @ignore An alternative way to test for the file's existence would be to call @samp{system("test -r " t)}, which uses the @command{test} utility to see if the file exists and is readable. The disadvantage to this method is that it requires creating an extra process and can thus be slightly slower. @end ignore @example @c file eg/prog/igawk.sh expand_prog=' function pathto(file, i, t, junk) @{ if (index(file, "/") != 0) return file for (i = 1; i <= ndirs; i++) @{ t = (pathlist[i] "/" file) @group if ((getline junk < t) > 0) @{ # found it close(t) return t @} @end group @} return "" @} @c endfile @end example The main program is contained inside one @code{BEGIN} rule. The first thing it does is set up the @code{pathlist} array that @code{pathto} uses. After splitting the path on @samp{:}, null elements are replaced with @code{"."}, which represents the current directory: @example @c file eg/prog/igawk.sh BEGIN @{ path = ENVIRON["AWKPATH"] ndirs = split(path, pathlist, ":") for (i = 1; i <= ndirs; i++) @{ if (pathlist[i] == "") pathlist[i] = "." @} @c endfile @end example The stack is initialized with @code{ARGV[1]}, which will be @file{/dev/stdin}. The main loop comes next. Input lines are read in succession. Lines that do not start with @samp{@@include} are printed verbatim. If the line does start with @samp{@@include}, the @value{FN} is in @code{$2}. @code{pathto} is called to generate the full path. If it cannot, then we print an error message and continue. The next thing to check is if the file is included already. The @code{processed} array is indexed by the full @value{FN} of each included file and it tracks this information for us. If the file is seen again, a warning message is printed. Otherwise, the new @value{FN} is pushed onto the stack and processing continues. Finally, when @code{getline} encounters the end of the input file, the file is closed and the stack is popped. When @code{stackptr} is less than zero, the program is done: @example @c file eg/prog/igawk.sh stackptr = 0 input[stackptr] = ARGV[1] # ARGV[1] is first file for (; stackptr >= 0; stackptr--) @{ while ((getline < input[stackptr]) > 0) @{ if (tolower($1) != "@@include") @{ print continue @} fpath = pathto($2) @group if (fpath == "") @{ printf("igawk:%s:%d: cannot find %s\n", input[stackptr], FNR, $2) > "/dev/stderr" continue @} @end group if (! (fpath in processed)) @{ processed[fpath] = input[stackptr] input[++stackptr] = fpath # push onto stack @} else print $2, "included in", input[stackptr], "already included in", processed[fpath] > "/dev/stderr" @} close(input[stackptr]) @} @}' # close quote ends `expand_prog' variable processed_program=`gawk -- "$expand_prog" /dev/stdin < @c and Darrel Hankerson @node PC Installation @appendixsubsec Installation on PC Operating Systems @c first comma is part of primary @cindex PC operating systems, @command{gawk} on, installing @c {PC, gawk on} is the secondary term @cindex operating systems, PC, @command{gawk} on, installing This @value{SECTION} covers installation and usage of @command{gawk} on x86 machines running DOS, any version of Windows, or OS/2. In this @value{SECTION}, the term ``Windows32'' refers to any of Windows-95/98/ME/NT/2000. The limitations of DOS (and DOS shells under Windows or OS/2) has meant that various ``DOS extenders'' are often used with programs such as @command{gawk}. The varying capabilities of Microsoft Windows 3.1 and Windows32 can add to the confusion. For an overview of the considerations, please refer to @file{README_d/README.pc} in the distribution. @menu * PC Binary Installation:: Installing a prepared distribution. * PC Compiling:: Compiling @command{gawk} for MS-DOS, Windows32, and OS/2. * PC Dynamic:: Compiling @command{gawk} for dynamic libraries. * PC Using:: Running @command{gawk} on MS-DOS, Windows32 and OS/2. * Cygwin:: Building and running @command{gawk} for Cygwin. @end menu @node PC Binary Installation @appendixsubsubsec Installing a Prepared Distribution for PC Systems If you have received a binary distribution prepared by the DOS maintainers, then @command{gawk} and the necessary support files appear under the @file{gnu} directory, with executables in @file{gnu/bin}, libraries in @file{gnu/lib/awk}, and manual pages under @file{gnu/man}. This is designed for easy installation to a @file{/gnu} directory on your drive---however, the files can be installed anywhere provided @env{AWKPATH} is set properly. Regardless of the installation directory, the first line of @file{igawk.cmd} and @file{igawk.bat} (in @file{gnu/bin}) may need to be edited. The binary distribution contains a separate file describing the contents. In particular, it may include more than one version of the @command{gawk} executable. OS/2 (32 bit, EMX) binary distributions are prepared for the @file{/usr} directory of your preferred drive. Set @env{UNIXROOT} to your installation drive (e.g., @samp{e:}) if you want to install @command{gawk} onto another drive than the hardcoded default @samp{c:}. Executables appear in @file{/usr/bin}, libraries under @file{/usr/share/awk}, manual pages under @file{/usr/man}, Texinfo documentation under @file{/usr/info} and NLS files under @file{/usr/share/locale}. If you already have a file @file{/usr/info/dir} from another package @emph{do not overwrite it!} Instead enter the following commands at your prompt (replace @samp{x:} by your installation drive): @example install-info --info-dir=x:/usr/info x:/usr/info/awk.info install-info --info-dir=x:/usr/info x:/usr/info/gawkinet.info @end example However, the files can be installed anywhere provided @env{AWKPATH} is set properly. The binary distribution may contain a separate file containing additional or more detailed installation instructions. @node PC Compiling @appendixsubsubsec Compiling @command{gawk} for PC Operating Systems @command{gawk} can be compiled for MS-DOS, Windows32, and OS/2 using the GNU development tools from DJ Delorie (DJGPP; MS-DOS only) or Eberhard Mattes (EMX; MS-DOS, Windows32 and OS/2). Microsoft Visual C/C++ can be used to build a Windows32 version, and Microsoft C/C++ can be used to build 16-bit versions for MS-DOS and OS/2. @c FIXME: (As of @command{gawk} 3.1.2, the MSC version doesn't work. However, the maintainer is working on fixing it.) The file @file{README_d/README.pc} in the @command{gawk} distribution contains additional notes, and @file{pc/Makefile} contains important information on compilation options. To build @command{gawk} for MS-DOS, Windows32, and OS/2 (16 bit only; for 32 bit (EMX) you can use the @command{configure} script and skip the following paragraphs; for details see below), copy the files in the @file{pc} directory (@emph{except} for @file{ChangeLog}) to the directory with the rest of the @command{gawk} sources. The @file{Makefile} contains a configuration section with comments and may need to be edited in order to work with your @command{make} utility. The @file{Makefile} contains a number of targets for building various MS-DOS, Windows32, and OS/2 versions. A list of targets is printed if the @command{make} command is given without a target. As an example, to build @command{gawk} using the DJGPP tools, enter @samp{make djgpp}. Using @command{make} to run the standard tests and to install @command{gawk} requires additional Unix-like tools, including @command{sh}, @command{sed}, and @command{cp}. In order to run the tests, the @file{test/*.ok} files may need to be converted so that they have the usual DOS-style end-of-line markers. Most of the tests work properly with Stewartson's shell along with the companion utilities or appropriate GNU utilities. However, some editing of @file{test/Makefile} is required. It is recommended that you copy the file @file{pc/Makefile.tst} over the file @file{test/Makefile} as a replacement. Details can be found in @file{README_d/README.pc} and in the file @file{pc/Makefile.tst}. The 32 bit EMX version of @command{gawk} works ``out of the box'' under OS/2. In principle, it is possible to compile @command{gawk} the following way: @example $ ./configure $ make @end example This is not recommended, though. To get an OMF executable you should use the following commands at your @command{sh} prompt: @example $ CPPFLAGS="-D__ST_MT_ERRNO__" $ export CPPFLAGS $ CFLAGS="-O2 -Zomf -Zmt" $ export CFLAGS $ LDFLAGS="-s -Zcrtdll -Zlinker /exepack:2 -Zlinker /pm:vio -Zstack 0x8000" $ export LDFLAGS $ RANLIB="echo" $ export RANLIB $ ./configure --prefix=c:/usr --without-included-gettext $ make AR=emxomfar @end example These are just suggestions. You may use any other set of (self-consistent) environment variables and compiler flags. To get an FHS-compliant file hierarchy it is recommended to use the additional @command{configure} options @option{--infodir=c:/usr/share/info}, @option{--mandir=c:/usr/share/man} and @option{--libexecdir=c:/usr/lib}. The internal @code{gettext} library tends to be problematic. It is therefore recommended to use either an external one (@option{--without-included-gettext}) or to disable NLS entirely (@option{--disable-nls}). If you use GCC 2.95 or newer it is recommended to use also: @example $ LIBS="-lgcc" $ export LIBS @end example You can also get an @code{a.out} executable if you prefer: @example $ CPPFLAGS="-D__ST_MT_ERRNO__" $ export CPPFLAGS $ CFLAGS="-O2 -Zmt" $ export CFLAGS $ LDFLAGS="-s -Zstack 0x8000" $ LIBS="-lgcc" $ unset RANLIB $ ./configure --prefix=c:/usr --without-included-gettext $ make @end example @strong{Note:} Even if the compiled @command{gawk.exe} (@code{a.out}) executable contains a DOS header, it does @emph{not} work under DOS. To compile an executable that runs under DOS, @code{"-DPIPES_SIMULATED"} must be added to @env{CPPFLAGS}. But then some nonstandard extensions of @command{gawk} (e.g., @samp{|&}) do not work! After compilation the internal tests can be performed. Enter @samp{make check CMP="diff -a"} at your command prompt. All tests but the @code{pid} test are expected to work properly. The @code{pid} test fails because child processes are not started by @code{fork()}. @samp{make install} works as expected. @strong{Note:} Most OS/2 ports of GNU @command{make} are not able to handle the Makefiles of this package. If you encounter any problems with @command{make} try GNU Make 3.79.1 or later versions. You should find the latest version on @uref{http://www.unixos2.org/sw/pub/binary/make/} or on @uref{ftp://hobbes.nmsu.edu/pub/os2/}. @node PC Dynamic @appendixsubsubsec Compiling @command{gawk} For Dynamic Libraries @c From README_d/README.pcdynamic @c 11 June 2003 To compile @command{gawk} with dynamic extension support, uncomment the definitions of @code{DYN_FLAGS}, @code{DYN_EXP}, @code{DYN_OBJ}, and @code{DYN_MAKEXP} in the configuration section of the @file{Makefile}. There are two definitions for @code{DYN_MAKEXP}: pick the one that matches your target. To build some of the example extension libraries, @command{cd} to the extension directory and copy @file{Makefile.pc} to @file{Makefile}. You can then build using the same two targets. To run the example @command{awk} scripts, you'll need to either change the call to the @code{extension} function to match the name of the library (for instance, change @code{"./ordchr.so"} to @code{"ordchr.dll"} or simply @code{"ordchr"}), or rename the library to match the call (for instance, rename @file{ordchr.dll} to @file{ordchr.so}). If you build @command{gawk.exe} with one compiler but want to build an extension library with the other, you need to copy the import library. Visual C uses a library called @file{gawk.lib}, while MinGW uses a library called @file{libgawk.a}. These files are equivalent and will interoperate if you give them the correct name. The resulting shared libraries are also interoperable. To create your own extension library, you can use the examples as models, but you're essentially on your own. Post to @code{comp.lang.awk} or send electronic mail to @email{ptjm@@interlog.com} if you have problems getting started. If you need to access functions or variables which are not exported by @command{gawk.exe}, add them to @file{gawkw32.def} and rebuild. You should also add @code{ATTRIBUTE_EXPORTED} to the declaration in @file{awk.h} of any variables you add to @file{gawkw32.def}. Note that extension libraries have the name of the @command{awk} executable embedded in them at link time, so they will work only with @command{gawk.exe}. In particular, they won't work if you rename @command{gawk.exe} to @command{awk.exe} or if you try to use @command{pgawk.exe}. You can perform profiling by temporarily renaming @command{pgawk.exe} to @command{gawk.exe}. You can resolve this problem by changing the program name in the definition of @code{DYN_MAKEXP} for your compiler. On Windows32, libraries are sought first in the current directory, then in the directory containing @command{gawk.exe}, and finally through the @env{PATH} environment variable. @node PC Using @appendixsubsubsec Using @command{gawk} on PC Operating Systems @c STARTOFRANGE opgawx @cindex operating systems, PC, @command{gawk} on @c STARTOFRANGE pcgawon @cindex PC operating systems, @command{gawk} on With the exception of the Cygwin environment, the @samp{|&} operator and TCP/IP networking (@pxref{TCP/IP Networking}) are not supported for MS-DOS or MS-Windows. EMX (OS/2 only) does support at least the @samp{|&} operator. @cindex search paths @cindex @command{gawk}, OS/2 version of @cindex @command{gawk}, MS-DOS version of @cindex @code{;} (semicolon), @code{AWKPATH} variable and @cindex semicolon (@code{;}), @code{AWKPATH} variable and @cindex @code{AWKPATH} environment variable The OS/2 and MS-DOS versions of @command{gawk} search for program files as described in @ref{AWKPATH Variable}. However, semicolons (rather than colons) separate elements in the @env{AWKPATH} variable. If @env{AWKPATH} is not set or is empty, then the default search path for OS/2 (16 bit) and MS-DOS versions is @code{@w{".;c:/lib/awk;c:/gnu/lib/awk"}}. The search path for OS/2 (32 bit, EMX) is determined by the prefix directory (most likely @file{/usr} or @file{c:/usr}) that has been specified as an option of the @command{configure} script like it is the case for the Unix versions. If @file{c:/usr} is the prefix directory then the default search path contains @file{.} and @file{c:/usr/share/awk}. Additionally, to support binary distributions of @command{gawk} for OS/2 systems whose drive @samp{c:} might not support long file names or might not exist at all, there is a special environment variable. If @env{UNIXROOT} specifies a drive then this specific drive is also searched for program files. E.g., if @env{UNIXROOT} is set to @file{e:} the complete default search path is @code{@w{".;c:/usr/share/awk;e:/usr/share/awk"}}. An @command{sh}-like shell (as opposed to @command{command.com} under MS-DOS or @command{cmd.exe} under OS/2) may be useful for @command{awk} programming. Ian Stewartson has written an excellent shell for MS-DOS and OS/2, Daisuke Aoyama has ported GNU @command{bash} to MS-DOS using the DJGPP tools, and several shells are available for OS/2, including @command{ksh}. The file @file{README_d/README.pc} in the @command{gawk} distribution contains information on these shells. Users of Stewartson's shell on DOS should examine its documentation for handling command lines; in particular, the setting for @command{gawk} in the shell configuration may need to be changed and the @code{ignoretype} option may also be of interest. @cindex differences in @command{awk} and @command{gawk}, @code{BINMODE} variable @cindex @code{BINMODE} variable Under OS/2 and DOS, @command{gawk} (and many other text programs) silently translate end-of-line @code{"\r\n"} to @code{"\n"} on input and @code{"\n"} to @code{"\r\n"} on output. A special @code{BINMODE} variable allows control over these translations and is interpreted as follows: @itemize @bullet @item If @code{BINMODE} is @samp{"r"}, or @code{(BINMODE & 1)} is nonzero, then binary mode is set on read (i.e., no translations on reads). @item If @code{BINMODE} is @code{"w"}, or @code{(BINMODE & 2)} is nonzero, then binary mode is set on write (i.e., no translations on writes). @item If @code{BINMODE} is @code{"rw"} or @code{"wr"}, binary mode is set for both read and write (same as @code{(BINMODE & 3)}). @item @code{BINMODE=@var{non-null-string}} is the same as @samp{BINMODE=3} (i.e., no translations on reads or writes). However, @command{gawk} issues a warning message if the string is not one of @code{"rw"} or @code{"wr"}. @end itemize @noindent The modes for standard input and standard output are set one time only (after the command line is read, but before processing any of the @command{awk} program). Setting @code{BINMODE} for standard input or standard output is accomplished by using an appropriate @samp{-v BINMODE=@var{N}} option on the command line. @code{BINMODE} is set at the time a file or pipe is opened and cannot be changed mid-stream. The name @code{BINMODE} was chosen to match @command{mawk} (@pxref{Other Versions}). Both @command{mawk} and @command{gawk} handle @code{BINMODE} similarly; however, @command{mawk} adds a @samp{-W BINMODE=@var{N}} option and an environment variable that can set @code{BINMODE}, @code{RS}, and @code{ORS}. The files @file{binmode[1-3].awk} (under @file{gnu/lib/awk} in some of the prepared distributions) have been chosen to match @command{mawk}'s @samp{-W BINMODE=@var{N}} option. These can be changed or discarded; in particular, the setting of @code{RS} giving the fewest ``surprises'' is open to debate. @command{mawk} uses @samp{RS = "\r\n"} if binary mode is set on read, which is appropriate for files with the DOS-style end-of-line. To illustrate, the following examples set binary mode on writes for standard output and other files, and set @code{ORS} as the ``usual'' DOS-style end-of-line: @example gawk -v BINMODE=2 -v ORS="\r\n" @dots{} @end example @noindent or: @example gawk -v BINMODE=w -f binmode2.awk @dots{} @end example @noindent These give the same result as the @samp{-W BINMODE=2} option in @command{mawk}. The following changes the record separator to @code{"\r\n"} and sets binary mode on reads, but does not affect the mode on standard input: @example gawk -v RS="\r\n" --source "BEGIN @{ BINMODE = 1 @}" @dots{} @end example @noindent or: @example gawk -f binmode1.awk @dots{} @end example @noindent With proper quoting, in the first example the setting of @code{RS} can be moved into the @code{BEGIN} rule. @node Cygwin @appendixsubsubsec Using @command{gawk} In The Cygwin Environment @command{gawk} can be used ``out of the box'' under Windows if you are using the Cygwin environment.@footnote{@uref{http://www.cygwin.com}} This environment provides an excellent simulation of Unix, using the GNU tools, such as @command{bash}, the GNU Compiler Collection (GCC), GNU Make, and other GNU tools. Compilation and installation for Cygwin is the same as for a Unix system: @example tar -xvpzf gawk-@value{VERSION}.@value{PATCHLEVEL}.tar.gz cd gawk-@value{VERSION}.@value{PATCHLEVEL} ./configure make @end example When compared to GNU/Linux on the same system, the @samp{configure} step on Cygwin takes considerably longer. However, it does finish, and then the @samp{make} proceeds as usual. @strong{Note:} The @samp{|&} operator and TCP/IP networking (@pxref{TCP/IP Networking}) are fully supported in the Cygwin environment. This is not true for any other environment for MS-DOS or MS-Windows. @node VMS Installation @appendixsubsec How to Compile and Install @command{gawk} on VMS @c based on material from Pat Rankin @c now rankin@pactechdata.com @cindex installation, vms This @value{SUBSECTION} describes how to compile and install @command{gawk} under VMS. @menu * VMS Compilation:: How to compile @command{gawk} under VMS. * VMS Installation Details:: How to install @command{gawk} under VMS. * VMS Running:: How to run @command{gawk} under VMS. * VMS POSIX:: Alternate instructions for VMS POSIX. @end menu @node VMS Compilation @appendixsubsubsec Compiling @command{gawk} on VMS To compile @command{gawk} under VMS, there is a @code{DCL} command procedure that issues all the necessary @code{CC} and @code{LINK} commands. There is also a @file{Makefile} for use with the @code{MMS} utility. From the source directory, use either: @example $ @@[.VMS]VMSBUILD.COM @end example @noindent or: @example $ MMS/DESCRIPTION=[.VMS]DESCRIP.MMS GAWK @end example Depending upon which C compiler you are using, follow one of the sets of instructions in this table: @table @asis @item VAX C V3.x Use either @file{vmsbuild.com} or @file{descrip.mms} as is. These use @code{CC/OPTIMIZE=NOLINE}, which is essential for Version 3.0. @item VAX C V2.x You must have Version 2.3 or 2.4; older ones won't work. Edit either @file{vmsbuild.com} or @file{descrip.mms} according to the comments in them. For @file{vmsbuild.com}, this just entails removing two @samp{!} delimiters. Also edit @file{config.h} (which is a copy of file @file{[.config]vms-conf.h}) and comment out or delete the two lines @samp{#define __STDC__ 0} and @samp{#define VAXC_BUILTINS} near the end. @item GNU C Edit @file{vmsbuild.com} or @file{descrip.mms}; the changes are different from those for VAX C V2.x but equally straightforward. No changes to @file{config.h} are needed. @item DEC C Edit @file{vmsbuild.com} or @file{descrip.mms} according to their comments. No changes to @file{config.h} are needed. @end table @command{gawk} has been tested under VAX/VMS 5.5-1 using VAX C V3.2, and GNU C 1.40 and 2.3. It should work without modifications for VMS V4.6 and up. @node VMS Installation Details @appendixsubsubsec Installing @command{gawk} on VMS To install @command{gawk}, all you need is a ``foreign'' command, which is a @code{DCL} symbol whose value begins with a dollar sign. For example: @example $ GAWK :== $disk1:[gnubin]GAWK @end example @noindent Substitute the actual location of @command{gawk.exe} for @samp{$disk1:[gnubin]}. The symbol should be placed in the @file{login.com} of any user who wants to run @command{gawk}, so that it is defined every time the user logs on. Alternatively, the symbol may be placed in the system-wide @file{sylogin.com} procedure, which allows all users to run @command{gawk}. Optionally, the help entry can be loaded into a VMS help library: @example $ LIBRARY/HELP SYS$HELP:HELPLIB [.VMS]GAWK.HLP @end example @noindent (You may want to substitute a site-specific help library rather than the standard VMS library @samp{HELPLIB}.) After loading the help text, the command: @example $ HELP GAWK @end example @noindent provides information about both the @command{gawk} implementation and the @command{awk} programming language. The logical name @samp{AWK_LIBRARY} can designate a default location for @command{awk} program files. For the @option{-f} option, if the specified @value{FN} has no device or directory path information in it, @command{gawk} looks in the current directory first, then in the directory specified by the translation of @samp{AWK_LIBRARY} if the file is not found. If, after searching in both directories, the file still is not found, @command{gawk} appends the suffix @samp{.awk} to the filename and retries the file search. If @samp{AWK_LIBRARY} is not defined, that portion of the file search fails benignly. @node VMS Running @appendixsubsubsec Running @command{gawk} on VMS Command-line parsing and quoting conventions are significantly different on VMS, so examples in this @value{DOCUMENT} or from other sources often need minor changes. They @emph{are} minor though, and all @command{awk} programs should run correctly. Here are a couple of trivial tests: @example $ gawk -- "BEGIN @{print ""Hello, World!""@}" $ gawk -"W" version ! could also be -"W version" or "-W version" @end example @noindent Note that uppercase and mixed-case text must be quoted. The VMS port of @command{gawk} includes a @code{DCL}-style interface in addition to the original shell-style interface (see the help entry for details). One side effect of dual command-line parsing is that if there is only a single parameter (as in the quoted string program above), the command becomes ambiguous. To work around this, the normally optional @option{--} flag is required to force Unix style rather than @code{DCL} parsing. If any other dash-type options (or multiple parameters such as @value{DF}s to process) are present, there is no ambiguity and @option{--} can be omitted. @c @cindex directory search @c @cindex path, search @cindex search paths @cindex search paths, for source files The default search path, when looking for @command{awk} program files specified by the @option{-f} option, is @code{"SYS$DISK:[],AWK_LIBRARY:"}. The logical name @samp{AWKPATH} can be used to override this default. The format of @samp{AWKPATH} is a comma-separated list of directory specifications. When defining it, the value should be quoted so that it retains a single translation and not a multitranslation @code{RMS} searchlist. @node VMS POSIX @appendixsubsubsec Building and Using @command{gawk} on VMS POSIX Ignore the instructions above, although @file{vms/gawk.hlp} should still be made available in a help library. The source tree should be unpacked into a container file subsystem rather than into the ordinary VMS filesystem. Make sure that the two scripts, @file{configure} and @file{vms/posix-cc.sh}, are executable; use @samp{chmod +x} on them if necessary. Then execute the following two commands: @example psx> CC=vms/posix-cc.sh configure psx> make CC=c89 gawk @end example @noindent The first command constructs files @file{config.h} and @file{Makefile} out of templates, using a script to make the C compiler fit @command{configure}'s expectations. The second command compiles and links @command{gawk} using the C compiler directly; ignore any warnings from @command{make} about being unable to redefine @code{CC}. @command{configure} takes a very long time to execute, but at least it provides incremental feedback as it runs. This has been tested with VAX/VMS V6.2, VMS POSIX V2.0, and DEC C V5.2. Once built, @command{gawk} works like any other shell utility. Unlike the normal VMS port of @command{gawk}, no special command-line manipulation is needed in the VMS POSIX environment. @node Unsupported @appendixsec Unsupported Operating System Ports This sections describes systems for which the @command{gawk} port is no longer supported. @menu * Atari Installation:: Installing @command{gawk} on the Atari ST. * Tandem Installation:: Installing @command{gawk} on a Tandem. @end menu @node Atari Installation @appendixsubsec Installing @command{gawk} on the Atari ST The Atari port is no longer supported. It is included for those who might want to use it but it is no longer being actively maintained. @c based on material from Michal Jaegermann @cindex atari @cindex installation, atari There are no substantial differences when installing @command{gawk} on various Atari models. Compiled @command{gawk} executables do not require a large amount of memory with most @command{awk} programs, and should run on all Motorola processor-based models (called further ST, even if that is not exactly right). In order to use @command{gawk}, you need to have a shell, either text or graphics, that does not map all the characters of a command line to uppercase. Maintaining case distinction in option flags is very important (@pxref{Options}). These days this is the default and it may only be a problem for some very old machines. If your system does not preserve the case of option flags, you need to upgrade your tools. Support for I/O redirection is necessary to make it easy to import @command{awk} programs from other environments. Pipes are nice to have but not vital. @menu * Atari Compiling:: Compiling @command{gawk} on Atari. * Atari Using:: Running @command{gawk} on Atari. @end menu @node Atari Compiling @appendixsubsubsec Compiling @command{gawk} on the Atari ST A proper compilation of @command{gawk} sources when @code{sizeof(int)} differs from @code{sizeof(void *)} requires an ISO C compiler. An initial port was done with @command{gcc}. You may actually prefer executables where @code{int}s are four bytes wide but the other variant works as well. You may need quite a bit of memory when trying to recompile the @command{gawk} sources, as some source files (@file{regex.c} in particular) are quite big. If you run out of memory compiling such a file, try reducing the optimization level for this particular file, which may help. @cindex Linux @cindex GNU/Linux With a reasonable shell (@command{bash} will do), you have a pretty good chance that the @command{configure} utility will succeed, and in particular if you run GNU/Linux, MiNT or a similar operating system. Otherwise sample versions of @file{config.h} and @file{Makefile.st} are given in the @file{atari} subdirectory and can be edited and copied to the corresponding files in the main source directory. Even if @command{configure} produces something, it might be advisable to compare its results with the sample versions and possibly make adjustments. Some @command{gawk} source code fragments depend on a preprocessor define @samp{atarist}. This basically assumes the TOS environment with @command{gcc}. Modify these sections as appropriate if they are not right for your environment. Also see the remarks about @env{AWKPATH} and @code{envsep} in @ref{Atari Using}. As shipped, the sample @file{config.h} claims that the @code{system} function is missing from the libraries, which is not true, and an alternative implementation of this function is provided in @file{unsupported/atari/system.c}. Depending upon your particular combination of shell and operating system, you might want to change the file to indicate that @code{system} is available. @node Atari Using @appendixsubsubsec Running @command{gawk} on the Atari ST An executable version of @command{gawk} should be placed, as usual, anywhere in your @env{PATH} where your shell can find it. While executing, the Atari version of @command{gawk} creates a number of temporary files. When using @command{gcc} libraries for TOS, @command{gawk} looks for either of the environment variables, @env{TEMP} or @env{TMPDIR}, in that order. If either one is found, its value is assumed to be a directory for temporary files. This directory must exist, and if you can spare the memory, it is a good idea to put it on a RAM drive. If neither @env{TEMP} nor @env{TMPDIR} are found, then @command{gawk} uses the current directory for its temporary files. The ST version of @command{gawk} searches for its program files, as described in @ref{AWKPATH Variable}. The default value for the @env{AWKPATH} variable is taken from @code{DEFPATH} defined in @file{Makefile}. The sample @command{gcc}/TOS @file{Makefile} for the ST in the distribution sets @code{DEFPATH} to @code{@w{".,c:\lib\awk,c:\gnu\lib\awk"}}. The search path can be modified by explicitly setting @env{AWKPATH} to whatever you want. Note that colons cannot be used on the ST to separate elements in the @env{AWKPATH} variable, since they have another reserved meaning. Instead, you must use a comma to separate elements in the path. When recompiling, the separating character can be modified by initializing the @code{envsep} variable in @file{unsupported/atari/gawkmisc.atr} to another value. Although @command{awk} allows great flexibility in doing I/O redirections from within a program, this facility should be used with care on the ST running under TOS. In some circumstances, the OS routines for file-handle pool processing lose track of certain events, causing the computer to crash and requiring a reboot. Often a warm reboot is sufficient. Fortunately, this happens infrequently and in rather esoteric situations. In particular, avoid having one part of an @command{awk} program using @code{print} statements explicitly redirected to @file{/dev/stdout}, while other @code{print} statements use the default standard output, and a calling shell has redirected standard output to a file. @c 10/2000: Is this still true, now that gawk does /dev/stdout internally? When @command{gawk} is compiled with the ST version of @command{gcc} and its usual libraries, it accepts both @samp{/} and @samp{\} as path separators. While this is convenient, it should be remembered that this removes one technically valid character (@samp{/}) from your @value{FN}. It may also create problems for external programs called via the @code{system} function, which may not support this convention. Whenever it is possible that a file created by @command{gawk} will be used by some other program, use only backslashes. Also remember that in @command{awk}, backslashes in strings have to be doubled in order to get literal backslashes (@pxref{Escape Sequences}). @node Tandem Installation @appendixsubsec Installing @command{gawk} on a Tandem @cindex tandem @cindex installation, tandem The Tandem port is only minimally supported. The port's contributor no longer has access to a Tandem system. @c This section based on README.Tandem by Stephen Davies (scldad@sdc.com.au) The Tandem port was done on a Cyclone machine running D20. The port is pretty clean and all facilities seem to work except for the I/O piping facilities (@pxref{Getline/Pipe}, @ref{Getline/Variable/Pipe}, and @ref{Redirection}), which is just too foreign a concept for Tandem. To build a Tandem executable from source, download all of the files so that the @value{FN}s on the Tandem box conform to the restrictions of D20. For example, @file{array.c} becomes @file{ARRAYC}, and @file{awk.h} becomes @file{AWKH}. The totally Tandem-specific files are in the @file{tandem} ``subvolume'' (@file{unsupported/tandem} in the @command{gawk} distribution) and should be copied to the main source directory before building @command{gawk}. The file @file{compit} can then be used to compile and bind an executable. Alas, there is no @command{configure} or @command{make}. Usage is the same as for Unix, except that D20 requires all @samp{@{} and @samp{@}} characters to be escaped with @samp{~} on the command line (but @emph{not} in script files). Also, the standard Tandem syntax for @samp{/in filename,out filename/} must be used instead of the usual Unix @samp{<} and @samp{>} for file redirection. (Redirection options on @code{getline}, @code{print} etc., are supported.) The @samp{-mr @var{val}} option (@pxref{Options}) has been ``stolen'' to enable Tandem users to process fixed-length records with no ``end-of-line'' character. That is, @samp{-mr 74} tells @command{gawk} to read the input file as fixed 74-byte records. @c ENDOFRANGE opgawx @c ENDOFRANGE pcgawon @node Bugs @appendixsec Reporting Problems and Bugs @cindex archeologists @quotation @i{There is nothing more dangerous than a bored archeologist.}@* The Hitchhiker's Guide to the Galaxy @end quotation @c the radio show, not the book. :-) @c STARTOFRANGE dbugg @cindex debugging @command{gawk}, bug reports @c STARTOFRANGE tblgawb @cindex troubleshooting, @command{gawk}, bug reports If you have problems with @command{gawk} or think that you have found a bug, please report it to the developers; we cannot promise to do anything but we might well want to fix it. Before reporting a bug, make sure you have actually found a real bug. Carefully reread the documentation and see if it really says you can do what you're trying to do. If it's not clear whether you should be able to do something or not, report that too; it's a bug in the documentation! Before reporting a bug or trying to fix it yourself, try to isolate it to the smallest possible @command{awk} program and input @value{DF} that reproduces the problem. Then send us the program and @value{DF}, some idea of what kind of Unix system you're using, the compiler you used to compile @command{gawk}, and the exact results @command{gawk} gave you. Also say what you expected to occur; this helps us decide whether the problem is really in the documentation. @cindex @code{bug-gawk@@gnu.org} bug reporting address @cindex email address for bug reports, @code{bug-gawk@@gnu.org} @cindex bug reports, email address, @code{bug-gawk@@gnu.org} Once you have a precise problem, send email to @email{bug-gawk@@gnu.org}. @cindex Robbins, Arnold Please include the version number of @command{gawk} you are using. You can get this information with the command @samp{gawk --version}. Using this address automatically sends a carbon copy of your mail to me. If necessary, I can be reached directly at @email{arnold@@gnu.org}. The bug reporting address is preferred since the email list is archived at the GNU Project. @emph{All email should be in English, since that is my native language.} @cindex @code{comp.lang.awk} newsgroup @strong{Caution:} Do @emph{not} try to report bugs in @command{gawk} by posting to the Usenet/Internet newsgroup @code{comp.lang.awk}. While the @command{gawk} developers do occasionally read this newsgroup, there is no guarantee that we will see your posting. The steps described above are the official recognized ways for reporting bugs. Non-bug suggestions are always welcome as well. If you have questions about things that are unclear in the documentation or are just obscure features, ask me; I will try to help you out, although I may not have the time to fix the problem. You can send me electronic mail at the Internet address noted previously. If you find bugs in one of the non-Unix ports of @command{gawk}, please send an electronic mail message to the person who maintains that port. They are named in the following list, as well as in the @file{README} file in the @command{gawk} distribution. Information in the @file{README} file should be considered authoritative if it conflicts with this @value{DOCUMENT}. The people maintaining the non-Unix ports of @command{gawk} are as follows: @ignore @table @asis @cindex Fish, Fred @item Amiga Fred Fish, @email{fnf@@ninemoons.com}. @cindex Brown, Martin @item BeOS Martin Brown, @email{mc@@whoever.com}. @cindex Deifik, Scott @cindex Hankerson, Darrel @item MS-DOS Scott Deifik, @email{scottd@@amgen.com} and Darrel Hankerson, @email{hankedr@@mail.auburn.edu}. @cindex Grigera, Juan @item MS-Windows Juan Grigera, @email{juan@@biophnet.unlp.edu.ar}. @item OS/2 The Unix for OS/2 team, @email{gawk-maintainer@@unixos2.org}. @cindex Davies, Stephen @item Tandem Stephen Davies, @email{scldad@@sdc.com.au}. @cindex Rankin, Pat @item VMS Pat Rankin, @email{rankin@@pactechdata.com}. @end table @end ignore @multitable {MS-Windows} {123456789012345678901234567890123456789001234567890} @cindex Fish, Fred @item Amiga @tab Fred Fish, @email{fnf@@ninemoons.com}. @cindex Brown, Martin @item BeOS @tab Martin Brown, @email{mc@@whoever.com}. @cindex Deifik, Scott @cindex Hankerson, Darrel @item MS-DOS @tab Scott Deifik, @email{scottd@@amgen.com} and Darrel Hankerson, @email{hankedr@@mail.auburn.edu}. @cindex Grigera, Juan @item MS-Windows @tab Juan Grigera, @email{juan@@biophnet.unlp.edu.ar}. @item OS/2 @tab The Unix for OS/2 team, @email{gawk-maintainer@@unixos2.org}. @cindex Davies, Stephen @item Tandem @tab Stephen Davies, @email{scldad@@sdc.com.au}. @cindex Rankin, Pat @item VMS @tab Pat Rankin, @email{rankin@@pactechdata.com}. @end multitable If your bug is also reproducible under Unix, please send a copy of your report to the @email{bug-gawk@@gnu.org} email list as well. @c ENDOFRANGE dbugg @c ENDOFRANGE tblgawb @node Other Versions @appendixsec Other Freely Available @command{awk} Implementations @c STARTOFRANGE awkim @cindex @command{awk}, implementations @ignore From: emory!amc.com!brennan (Michael Brennan) Subject: C++ comments in awk programs To: arnold@gnu.ai.mit.edu (Arnold Robbins) Date: Wed, 4 Sep 1996 08:11:48 -0700 (PDT) @end ignore @cindex Brennan, Michael @quotation @i{It's kind of fun to put comments like this in your awk code.}@* @ @ @ @ @ @ @code{// Do C++ comments work? answer: yes! of course}@* Michael Brennan @end quotation There are three other freely available @command{awk} implementations. This @value{SECTION} briefly describes where to get them: @table @asis @cindex Kernighan, Brian @cindex source code, Bell Laboratories @command{awk} @item Unix @command{awk} Brian Kernighan has made his implementation of @command{awk} freely available. You can retrieve this version via the World Wide Web from his home page.@footnote{@uref{http://cm.bell-labs.com/who/bwk}} It is available in several archive formats: @table @asis @item Shell archive @uref{http://cm.bell-labs.com/who/bwk/awk.shar} @item Compressed @command{tar} file @uref{http://cm.bell-labs.com/who/bwk/awk.tar.gz} @item Zip file @uref{http://cm.bell-labs.com/who/bwk/awk.zip} @end table This version requires an ISO C (1990 standard) compiler; the C compiler from GCC (the GNU Compiler Collection) works quite nicely. @xref{BTL}, for a list of extensions in this @command{awk} that are not in POSIX @command{awk}. @cindex Brennan, Michael @cindex @command{mawk} program @cindex source code, @command{mawk} @item @command{mawk} Michael Brennan has written an independent implementation of @command{awk}, called @command{mawk}. It is available under the GPL (@pxref{Copying}), just as @command{gawk} is. You can get it via anonymous @command{ftp} to the host @code{@w{ftp.whidbey.net}}. Change directory to @file{/pub/brennan}. Use ``binary'' or ``image'' mode, and retrieve @file{mawk1.3.3.tar.gz} (or the latest version that is there). @command{gunzip} may be used to decompress this file. Installation is similar to @command{gawk}'s (@pxref{Unix Installation}). @cindex extensions, @command{mawk} @command{mawk} has the following extensions that are not in POSIX @command{awk}: @itemize @bullet @item The @code{fflush} built-in function for flushing buffered output (@pxref{I/O Functions}). @item The @samp{**} and @samp{**=} operators (@pxref{Arithmetic Ops} and also see @ref{Assignment Ops}). @item The use of @code{func} as an abbreviation for @code{function} (@pxref{Definition Syntax}). @item The @samp{\x} escape sequence (@pxref{Escape Sequences}). @item The @file{/dev/stdout}, and @file{/dev/stderr} special files (@pxref{Special Files}). Use @code{"-"} instead of @code{"/dev/stdin"} with @command{mawk}. @item The ability for @code{FS} and for the third argument to @code{split} to be null strings (@pxref{Single Character Fields}). @item The ability to delete all of an array at once with @samp{delete @var{array}} (@pxref{Delete}). @item The ability for @code{RS} to be a regexp (@pxref{Records}). @item The @code{BINMODE} special variable for non-Unix operating systems (@pxref{PC Using}). @end itemize The next version of @command{mawk} will support @code{nextfile}. @cindex Sumner, Andrew @cindex @command{awka} compiler for @command{awk} @cindex source code, @command{awka} @item @command{awka} Written by Andrew Sumner, @command{awka} translates @command{awk} programs into C, compiles them, and links them with a library of functions that provides the core @command{awk} functionality. It also has a number of extensions. The @command{awk} translator is released under the GPL, and the library is under the LGPL. To get @command{awka}, go to @uref{http://awka.sourceforge.net}. You can reach Andrew Sumner at @email{andrew@@zbcom.net}. @cindex Beebe, Nelson H.F. @cindex @command{pawk} profiling Bell Labs @command{awk} @item @command{pawk} Nelson H.F.@: Beebe at the University of Utah has modified the Bell Labs @command{awk} to provide timing and profiling information. It is different from @command{pgawk} (@pxref{Profiling}), in that it uses CPU-based profiling, not line-count profiling. You may find it at either @uref{ftp://ftp.math.utah.edu/pub/pawk/pawk-20020210.tar.gz} or @uref{http://www.math.utah.edu/pub/pawk/pawk-20020210.tar.gz}. @end table @c ENDOFRANGE gligawk @c ENDOFRANGE ingawk @c ENDOFRANGE awkim @node Notes @appendix Implementation Notes @c STARTOFRANGE gawii @cindex @command{gawk}, implementation issues @c STARTOFRANGE impis @cindex implementation issues, @command{gawk} This appendix contains information mainly of interest to implementors and maintainers of @command{gawk}. Everything in it applies specifically to @command{gawk} and not to other implementations. @menu * Compatibility Mode:: How to disable certain @command{gawk} extensions. * Additions:: Making Additions To @command{gawk}. * Dynamic Extensions:: Adding new built-in functions to @command{gawk}. * Future Extensions:: New features that may be implemented one day. @end menu @node Compatibility Mode @appendixsec Downward Compatibility and Debugging @cindex @command{gawk}, implementation issues, downward compatibility @cindex @command{gawk}, implementation issues, debugging @cindex troubleshooting, @command{gawk} @c first comma is part of primary @cindex implementation issues, @command{gawk}, debugging @xref{POSIX/GNU}, for a summary of the GNU extensions to the @command{awk} language and program. All of these features can be turned off by invoking @command{gawk} with the @option{--traditional} option or with the @option{--posix} option. If @command{gawk} is compiled for debugging with @samp{-DDEBUG}, then there is one more option available on the command line: @table @code @item -W parsedebug @itemx --parsedebug Prints out the parse stack information as the program is being parsed. @end table This option is intended only for serious @command{gawk} developers and not for the casual user. It probably has not even been compiled into your version of @command{gawk}, since it slows down execution. @node Additions @appendixsec Making Additions to @command{gawk} If you find that you want to enhance @command{gawk} in a significant fashion, you are perfectly free to do so. That is the point of having free software; the source code is available and you are free to change it as you want (@pxref{Copying}). This @value{SECTION} discusses the ways you might want to change @command{gawk} as well as any considerations you should bear in mind. @menu * Adding Code:: Adding code to the main body of @command{gawk}. * New Ports:: Porting @command{gawk} to a new operating system. @end menu @node Adding Code @appendixsubsec Adding New Features @c STARTOFRANGE adfgaw @cindex adding, features to @command{gawk} @c STARTOFRANGE fadgaw @cindex features, adding to @command{gawk} @c STARTOFRANGE gawadf @cindex @command{gawk}, features, adding You are free to add any new features you like to @command{gawk}. However, if you want your changes to be incorporated into the @command{gawk} distribution, there are several steps that you need to take in order to make it possible for me to include your changes: @enumerate 1 @item Before building the new feature into @command{gawk} itself, consider writing it as an extension module (@pxref{Dynamic Extensions}). If that's not possible, continue with the rest of the steps in this list. @item Get the latest version. It is much easier for me to integrate changes if they are relative to the most recent distributed version of @command{gawk}. If your version of @command{gawk} is very old, I may not be able to integrate them at all. (@xref{Getting}, for information on getting the latest version of @command{gawk}.) @item @ifnotinfo Follow the @cite{GNU Coding Standards}. @end ifnotinfo @ifinfo See @inforef{Top, , Version, standards, GNU Coding Standards}. @end ifinfo This document describes how GNU software should be written. If you haven't read it, please do so, preferably @emph{before} starting to modify @command{gawk}. (The @cite{GNU Coding Standards} are available from the GNU Project's @command{ftp} site, at @uref{ftp://ftp.gnu.org/gnu/GNUinfo/standards.text}. An HTML version, suitable for reading with a WWW browser, is available at @uref{http://www.gnu.org/prep/standards_toc.html}. Texinfo, Info, and DVI versions are also available.) @cindex @command{gawk}, coding style in @item Use the @command{gawk} coding style. The C code for @command{gawk} follows the instructions in the @cite{GNU Coding Standards}, with minor exceptions. The code is formatted using the traditional ``K&R'' style, particularly as regards to the placement of braces and the use of tabs. In brief, the coding rules for @command{gawk} are as follows: @itemize @bullet @item Use ANSI/ISO style (prototype) function headers when defining functions. @item Put the name of the function at the beginning of its own line. @item Put the return type of the function, even if it is @code{int}, on the line above the line with the name and arguments of the function. @item Put spaces around parentheses used in control structures (@code{if}, @code{while}, @code{for}, @code{do}, @code{switch}, and @code{return}). @item Do not put spaces in front of parentheses used in function calls. @item Put spaces around all C operators and after commas in function calls. @item Do not use the comma operator to produce multiple side effects, except in @code{for} loop initialization and increment parts, and in macro bodies. @item Use real tabs for indenting, not spaces. @item Use the ``K&R'' brace layout style. @item Use comparisons against @code{NULL} and @code{'\0'} in the conditions of @code{if}, @code{while}, and @code{for} statements, as well as in the @code{case}s of @code{switch} statements, instead of just the plain pointer or character value. @item Use the @code{TRUE}, @code{FALSE} and @code{NULL} symbolic constants and the character constant @code{'\0'} where appropriate, instead of @code{1} and @code{0}. @item Use the @code{ISALPHA}, @code{ISDIGIT}, etc.@: macros, instead of the traditional lowercase versions; these macros are better behaved for non-ASCII character sets. @item Provide one-line descriptive comments for each function. @item Do not use @samp{#elif}. Many older Unix C compilers cannot handle it. @item Do not use the @code{alloca} function for allocating memory off the stack. Its use causes more portability trouble than is worth the minor benefit of not having to free the storage. Instead, use @code{malloc} and @code{free}. @end itemize @strong{Note:} If I have to reformat your code to follow the coding style used in @command{gawk}, I may not bother to integrate your changes at all. @item Be prepared to sign the appropriate paperwork. In order for the FSF to distribute your changes, you must either place those changes in the public domain and submit a signed statement to that effect, or assign the copyright in your changes to the FSF. Both of these actions are easy to do and @emph{many} people have done so already. If you have questions, please contact me (@pxref{Bugs}), or @email{gnu@@gnu.org}. @cindex Texinfo @item Update the documentation. Along with your new code, please supply new sections and/or chapters for this @value{DOCUMENT}. If at all possible, please use real Texinfo, instead of just supplying unformatted ASCII text (although even that is better than no documentation at all). Conventions to be followed in @cite{@value{TITLE}} are provided after the @samp{@@bye} at the end of the Texinfo source file. If possible, please update the @command{man} page as well. You will also have to sign paperwork for your documentation changes. @item Submit changes as context diffs or unified diffs. Use @samp{diff -c -r -N} or @samp{diff -u -r -N} to compare the original @command{gawk} source tree with your version. (I find context diffs to be more readable but unified diffs are more compact.) I recommend using the GNU version of @command{diff}. Send the output produced by either run of @command{diff} to me when you submit your changes. (@xref{Bugs}, for the electronic mail information.) Using this format makes it easy for me to apply your changes to the master version of the @command{gawk} source code (using @code{patch}). If I have to apply the changes manually, using a text editor, I may not do so, particularly if there are lots of changes. @item Include an entry for the @file{ChangeLog} file with your submission. This helps further minimize the amount of work I have to do, making it easier for me to accept patches. @end enumerate Although this sounds like a lot of work, please remember that while you may write the new code, I have to maintain it and support it. If it isn't possible for me to do that with a minimum of extra work, then I probably will not. @c ENDOFRANGE adfgaw @c ENDOFRANGE gawadf @c ENDOFRANGE fadgaw @node New Ports @appendixsubsec Porting @command{gawk} to a New Operating System @cindex portability, @command{gawk} @cindex operating systems, porting @command{gawk} to @cindex porting @command{gawk} If you want to port @command{gawk} to a new operating system, there are several steps: @enumerate 1 @item Follow the guidelines in @ifinfo @ref{Adding Code}, @end ifinfo @ifnotinfo the previous @value{SECTION} @end ifnotinfo concerning coding style, submission of diffs, and so on. @item When doing a port, bear in mind that your code must coexist peacefully with the rest of @command{gawk} and the other ports. Avoid gratuitous changes to the system-independent parts of the code. If at all possible, avoid sprinkling @samp{#ifdef}s just for your port throughout the code. If the changes needed for a particular system affect too much of the code, I probably will not accept them. In such a case, you can, of course, distribute your changes on your own, as long as you comply with the GPL (@pxref{Copying}). @item A number of the files that come with @command{gawk} are maintained by other people at the Free Software Foundation. Thus, you should not change them unless it is for a very good reason; i.e., changes are not out of the question, but changes to these files are scrutinized extra carefully. The files are @file{getopt.h}, @file{getopt.c}, @file{getopt1.c}, @file{regex.h}, @file{regex.c}, @file{dfa.h}, @file{dfa.c}, @file{install-sh}, and @file{mkinstalldirs}. @item Be willing to continue to maintain the port. Non-Unix operating systems are supported by volunteers who maintain the code needed to compile and run @command{gawk} on their systems. If noone volunteers to maintain a port, it becomes unsupported and it may be necessary to remove it from the distribution. @item Supply an appropriate @file{gawkmisc.???} file. Each port has its own @file{gawkmisc.???} that implements certain operating system specific functions. This is cleaner than a plethora of @samp{#ifdef}s scattered throughout the code. The @file{gawkmisc.c} in the main source directory includes the appropriate @file{gawkmisc.???} file from each subdirectory. Be sure to update it as well. Each port's @file{gawkmisc.???} file has a suffix reminiscent of the machine or operating system for the port---for example, @file{pc/gawkmisc.pc} and @file{vms/gawkmisc.vms}. The use of separate suffixes, instead of plain @file{gawkmisc.c}, makes it possible to move files from a port's subdirectory into the main subdirectory, without accidentally destroying the real @file{gawkmisc.c} file. (Currently, this is only an issue for the PC operating system ports.) @item Supply a @file{Makefile} as well as any other C source and header files that are necessary for your operating system. All your code should be in a separate subdirectory, with a name that is the same as, or reminiscent of, either your operating system or the computer system. If possible, try to structure things so that it is not necessary to move files out of the subdirectory into the main source directory. If that is not possible, then be sure to avoid using names for your files that duplicate the names of files in the main source directory. @item Update the documentation. Please write a section (or sections) for this @value{DOCUMENT} describing the installation and compilation steps needed to compile and/or install @command{gawk} for your system. @item Be prepared to sign the appropriate paperwork. In order for the FSF to distribute your code, you must either place your code in the public domain and submit a signed statement to that effect, or assign the copyright in your code to the FSF. @ifinfo Both of these actions are easy to do and @emph{many} people have done so already. If you have questions, please contact me, or @email{gnu@@gnu.org}. @end ifinfo @end enumerate Following these steps makes it much easier to integrate your changes into @command{gawk} and have them coexist happily with other operating systems' code that is already there. In the code that you supply and maintain, feel free to use a coding style and brace layout that suits your taste. @node Dynamic Extensions @appendixsec Adding New Built-in Functions to @command{gawk} @cindex Robinson, Will @cindex robot, the @cindex Lost In Space @quotation @i{Danger Will Robinson! Danger!!@* Warning! Warning!}@* The Robot @end quotation @c STARTOFRANGE gladfgaw @cindex @command{gawk}, functions, adding @c STARTOFRANGE adfugaw @cindex adding, functions to @command{gawk} @c STARTOFRANGE fubadgaw @cindex functions, built-in, adding to @command{gawk} Beginning with @command{gawk} 3.1, it is possible to add new built-in functions to @command{gawk} using dynamically loaded libraries. This facility is available on systems (such as GNU/Linux) that support the @code{dlopen} and @code{dlsym} functions. This @value{SECTION} describes how to write and use dynamically loaded extentions for @command{gawk}. Experience with programming in C or C++ is necessary when reading this @value{SECTION}. @strong{Caution:} The facilities described in this @value{SECTION} are very much subject to change in the next @command{gawk} release. Be aware that you may have to re-do everything, perhaps from scratch, upon the next release. @menu * Internals:: A brief look at some @command{gawk} internals. * Sample Library:: A example of new functions. @end menu @node Internals @appendixsubsec A Minimal Introduction to @command{gawk} Internals @c STARTOFRANGE gawint @cindex @command{gawk}, internals The truth is that @command{gawk} was not designed for simple extensibility. The facilities for adding functions using shared libraries work, but are something of a ``bag on the side.'' Thus, this tour is brief and simplistic; would-be @command{gawk} hackers are encouraged to spend some time reading the source code before trying to write extensions based on the material presented here. Of particular note are the files @file{awk.h}, @file{builtin.c}, and @file{eval.c}. Reading @file{awk.y} in order to see how the parse tree is built would also be of use. @cindex @code{awk.h} file (internal) With the disclaimers out of the way, the following types, structure members, functions, and macros are declared in @file{awk.h} and are of use when writing extensions. The next @value{SECTION} shows how they are used: @table @code @cindex floating-point, numbers, @code{AWKNUM} internal type @cindex numbers, floating-point, @code{AWKNUM} internal type @cindex @code{AWKNUM} internal type @item AWKNUM An @code{AWKNUM} is the internal type of @command{awk} floating-point numbers. Typically, it is a C @code{double}. @cindex @code{NODE} internal type @cindex strings, @code{NODE} internal type @cindex numbers, @code{NODE} internal type @item NODE Just about everything is done using objects of type @code{NODE}. These contain both strings and numbers, as well as variables and arrays. @cindex @code{force_number} internal function @cindex numeric, values @item AWKNUM force_number(NODE *n) This macro forces a value to be numeric. It returns the actual numeric value contained in the node. It may end up calling an internal @command{gawk} function. @cindex @code{force_string} internal function @item void force_string(NODE *n) This macro guarantees that a @code{NODE}'s string value is current. It may end up calling an internal @command{gawk} function. It also guarantees that the string is zero-terminated. @c comma is part of primary @cindex parameters, number of @cindex @code{param_cnt} internal variable @item n->param_cnt The number of parameters actually passed in a function call at runtime. @cindex @code{stptr} internal variable @cindex @code{stlen} internal variable @item n->stptr @itemx n->stlen The data and length of a @code{NODE}'s string value, respectively. The string is @emph{not} guaranteed to be zero-terminated. If you need to pass the string value to a C library function, save the value in @code{n->stptr[n->stlen]}, assign @code{'\0'} to it, call the routine, and then restore the value. @cindex @code{type} internal variable @item n->type The type of the @code{NODE}. This is a C @code{enum}. Values should be either @code{Node_var} or @code{Node_var_array} for function parameters. @cindex @code{vname} internal variable @item n->vname The ``variable name'' of a node. This is not of much use inside externally written extensions. @cindex arrays, associative, clearing @cindex @code{assoc_clear} internal function @item void assoc_clear(NODE *n) Clears the associative array pointed to by @code{n}. Make sure that @samp{n->type == Node_var_array} first. @cindex arrays, elements, installing @cindex @code{assoc_lookup} internal function @item NODE **assoc_lookup(NODE *symbol, NODE *subs, int reference) Finds, and installs if necessary, array elements. @code{symbol} is the array, @code{subs} is the subscript. This is usually a value created with @code{tmp_string} (see below). @code{reference} should be @code{TRUE} if it is an error to use the value before it is created. Typically, @code{FALSE} is the correct value to use from extension functions. @cindex strings @cindex @code{make_string} internal function @item NODE *make_string(char *s, size_t len) Take a C string and turn it into a pointer to a @code{NODE} that can be stored appropriately. This is permanent storage; understanding of @command{gawk} memory management is helpful. @cindex numbers @cindex @code{make_number} internal function @item NODE *make_number(AWKNUM val) Take an @code{AWKNUM} and turn it into a pointer to a @code{NODE} that can be stored appropriately. This is permanent storage; understanding of @command{gawk} memory management is helpful. @cindex @code{tmp_string} internal function @item NODE *tmp_string(char *s, size_t len); Take a C string and turn it into a pointer to a @code{NODE} that can be stored appropriately. This is temporary storage; understanding of @command{gawk} memory management is helpful. @cindex @code{tmp_number} internal function @item NODE *tmp_number(AWKNUM val) Take an @code{AWKNUM} and turn it into a pointer to a @code{NODE} that can be stored appropriately. This is temporary storage; understanding of @command{gawk} memory management is helpful. @c comma is part of primary @cindex nodes, duplicating @cindex @code{dupnode} internal function @item NODE *dupnode(NODE *n) Duplicate a node. In most cases, this increments an internal reference count instead of actually duplicating the entire @code{NODE}; understanding of @command{gawk} memory management is helpful. @cindex memory, releasing @cindex @code{free_temp} internal macro @item void free_temp(NODE *n) This macro releases the memory associated with a @code{NODE} allocated with @code{tmp_string} or @code{tmp_number}. Understanding of @command{gawk} memory management is helpful. @cindex @code{make_builtin} internal function @item void make_builtin(char *name, NODE *(*func)(NODE *), int count) Register a C function pointed to by @code{func} as new built-in function @code{name}. @code{name} is a regular C string. @code{count} is the maximum number of arguments that the function takes. The function should be written in the following manner: @example /* do_xxx --- do xxx function for gawk */ NODE * do_xxx(NODE *tree) @{ @dots{} @} @end example @cindex arguments, retrieving @cindex @code{get_argument} internal function @item NODE *get_argument(NODE *tree, int i) This function is called from within a C extension function to get the @code{i}-th argument from the function call. The first argument is argument zero. @c last comma is part of secondary @cindex functions, return values, setting @cindex @code{set_value} internal function @item void set_value(NODE *tree) This function is called from within a C extension function to set the return value from the extension function. This value is what the @command{awk} program sees as the return value from the new @command{awk} function. @cindex @code{ERRNO} variable @cindex @code{update_ERRNO} internal function @item void update_ERRNO(void) This function is called from within a C extension function to set the value of @command{gawk}'s @code{ERRNO} variable, based on the current value of the C @code{errno} variable. It is provided as a convenience. @end table An argument that is supposed to be an array needs to be handled with some extra code, in case the array being passed in is actually from a function parameter. In versions of @command{gawk} up to and including 3.1.2, the following boilerplate code shows how to do this: @smallexample NODE *the_arg; the_arg = get_argument(tree, 2); /* assume need 3rd arg, 0-based */ /* if a parameter, get it off the stack */ if (the_arg->type == Node_param_list) the_arg = stack_ptr[the_arg->param_cnt]; /* parameter referenced an array, get it */ if (the_arg->type == Node_array_ref) the_arg = the_arg->orig_array; /* check type */ if (the_arg->type != Node_var && the_arg->type != Node_var_array) fatal("newfunc: third argument is not an array"); /* force it to be an array, if necessary, clear it */ the_arg->type = Node_var_array; assoc_clear(the_arg); @end smallexample For versions 3.1.3 and later, the internals changed. In particular, the interface was actually @emph{simplified} drastically. The following boilerplate code now suffices: @smallexample NODE *the_arg; the_arg = get_argument(tree, 2); /* assume need 3rd arg, 0-based */ /* force it to be an array: */ the_arg = get_array(the_arg); /* if necessary, clear it: */ assoc_clear(the_arg); @end smallexample Again, you should spend time studying the @command{gawk} internals; don't just blindly copy this code. @c ENDOFRANGE gawint @node Sample Library @appendixsubsec Directory and File Operation Built-ins @c comma is part of primary @c STARTOFRANGE chdirg @cindex @code{chdir} function, implementing in @command{gawk} @c comma is part of primary @c STARTOFRANGE statg @cindex @code{stat} function, implementing in @command{gawk} @c last comma is part of secondary @c STARTOFRANGE filre @cindex files, information about, retrieving @c STARTOFRANGE dirch @cindex directories, changing Two useful functions that are not in @command{awk} are @code{chdir} (so that an @command{awk} program can change its directory) and @code{stat} (so that an @command{awk} program can gather information about a file). This @value{SECTION} implements these functions for @command{gawk} in an external extension library. @menu * Internal File Description:: What the new functions will do. * Internal File Ops:: The code for internal file operations. * Using Internal File Ops:: How to use an external extension. @end menu @node Internal File Description @appendixsubsubsec Using @code{chdir} and @code{stat} This @value{SECTION} shows how to use the new functions at the @command{awk} level once they've been integrated into the running @command{gawk} interpreter. Using @code{chdir} is very straightforward. It takes one argument, the new directory to change to: @example @dots{} newdir = "/home/arnold/funstuff" ret = chdir(newdir) if (ret < 0) @{ printf("could not change to %s: %s\n", newdir, ERRNO) > "/dev/stderr" exit 1 @} @dots{} @end example The return value is negative if the @code{chdir} failed, and @code{ERRNO} (@pxref{Built-in Variables}) is set to a string indicating the error. Using @code{stat} is a bit more complicated. The C @code{stat} function fills in a structure that has a fair amount of information. The right way to model this in @command{awk} is to fill in an associative array with the appropriate information: @c broke printf for page breaking @example file = "/home/arnold/.profile" fdata[1] = "x" # force `fdata' to be an array ret = stat(file, fdata) if (ret < 0) @{ printf("could not stat %s: %s\n", file, ERRNO) > "/dev/stderr" exit 1 @} printf("size of %s is %d bytes\n", file, fdata["size"]) @end example The @code{stat} function always clears the data array, even if the @code{stat} fails. It fills in the following elements: @table @code @item "name" The name of the file that was @code{stat}'ed. @item "dev" @itemx "ino" The file's device and inode numbers, respectively. @item "mode" The file's mode, as a numeric value. This includes both the file's type and its permissions. @item "nlink" The number of hard links (directory entries) the file has. @item "uid" @itemx "gid" The numeric user and group ID numbers of the file's owner. @item "size" The size in bytes of the file. @item "blocks" The number of disk blocks the file actually occupies. This may not be a function of the file's size if the file has holes. @item "atime" @itemx "mtime" @itemx "ctime" The file's last access, modification, and inode update times, respectively. These are numeric timestamps, suitable for formatting with @code{strftime} (@pxref{Built-in}). @item "pmode" The file's ``printable mode.'' This is a string representation of the file's type and permissions, such as what is produced by @samp{ls -l}---for example, @code{"drwxr-xr-x"}. @item "type" A printable string representation of the file's type. The value is one of the following: @table @code @item "blockdev" @itemx "chardev" The file is a block or character device (``special file''). @ignore @item "door" The file is a Solaris ``door'' (special file used for interprocess communications). @end ignore @item "directory" The file is a directory. @item "fifo" The file is a named-pipe (also known as a FIFO). @item "file" The file is just a regular file. @item "socket" The file is an @code{AF_UNIX} (``Unix domain'') socket in the filesystem. @item "symlink" The file is a symbolic link. @end table @end table Several additional elements may be present depending upon the operating system and the type of the file. You can test for them in your @command{awk} program by using the @code{in} operator (@pxref{Reference to Elements}): @table @code @item "blksize" The preferred block size for I/O to the file. This field is not present on all POSIX-like systems in the C @code{stat} structure. @item "linkval" If the file is a symbolic link, this element is the name of the file the link points to (i.e., the value of the link). @item "rdev" @itemx "major" @itemx "minor" If the file is a block or character device file, then these values represent the numeric device number and the major and minor components of that number, respectively. @end table @node Internal File Ops @appendixsubsubsec C Code for @code{chdir} and @code{stat} Here is the C code for these extensions. They were written for GNU/Linux. The code needs some more work for complete portability to other POSIX-compliant systems:@footnote{This version is edited slightly for presentation. The complete version can be found in @file{extension/filefuncs.c} in the @command{gawk} distribution.} @c break line for page breaking @example #include "awk.h" #include /* do_chdir --- provide dynamically loaded chdir() builtin for gawk */ static NODE * do_chdir(tree) NODE *tree; @{ NODE *newdir; int ret = -1; newdir = get_argument(tree, 0); @end example The file includes the @code{"awk.h"} header file for definitions for the @command{gawk} internals. It includes @code{} for access to the @code{major} and @code{minor} macros. @cindex programming conventions, @command{gawk} internals By convention, for an @command{awk} function @code{foo}, the function that implements it is called @samp{do_foo}. The function should take a @samp{NODE *} argument, usually called @code{tree}, that represents the argument list to the function. The @code{newdir} variable represents the new directory to change to, retrieved with @code{get_argument}. Note that the first argument is numbered zero. This code actually accomplishes the @code{chdir}. It first forces the argument to be a string and passes the string value to the @code{chdir} system call. If the @code{chdir} fails, @code{ERRNO} is updated. The result of @code{force_string} has to be freed with @code{free_temp}: @example if (newdir != NULL) @{ (void) force_string(newdir); ret = chdir(newdir->stptr); if (ret < 0) update_ERRNO(); free_temp(newdir); @} @end example Finally, the function returns the return value to the @command{awk} level, using @code{set_value}. Then it must return a value from the call to the new built-in (this value ignored by the interpreter): @example /* Set the return value */ set_value(tmp_number((AWKNUM) ret)); /* Just to make the interpreter happy */ return tmp_number((AWKNUM) 0); @} @end example The @code{stat} built-in is more involved. First comes a function that turns a numeric mode into a printable representation (e.g., 644 becomes @samp{-rw-r--r--}). This is omitted here for brevity: @c break line for page breaking @example /* format_mode --- turn a stat mode field into something readable */ static char * format_mode(fmode) unsigned long fmode; @{ @dots{} @} @end example Next comes the actual @code{do_stat} function itself. First come the variable declarations and argument checking: @ignore Changed message for page breaking. Used to be: "stat: called with incorrect number of arguments (%d), should be 2", @end ignore @example /* do_stat --- provide a stat() function for gawk */ static NODE * do_stat(tree) NODE *tree; @{ NODE *file, *array; struct stat sbuf; int ret; char *msg; NODE **aptr; char *pmode; /* printable mode */ char *type = "unknown"; /* check arg count */ if (tree->param_cnt != 2) fatal( "stat: called with %d arguments, should be 2", tree->param_cnt); @end example Then comes the actual work. First, we get the arguments. Then, we always clear the array. To get the file information, we use @code{lstat}, in case the file is a symbolic link. If there's an error, we set @code{ERRNO} and return: @c comment made multiline for page breaking @example /* * directory is first arg, * array to hold results is second */ file = get_argument(tree, 0); array = get_argument(tree, 1); /* empty out the array */ assoc_clear(array); /* lstat the file, if error, set ERRNO and return */ (void) force_string(file); ret = lstat(file->stptr, & sbuf); if (ret < 0) @{ update_ERRNO(); set_value(tmp_number((AWKNUM) ret)); free_temp(file); return tmp_number((AWKNUM) 0); @} @end example Now comes the tedious part: filling in the array. Only a few of the calls are shown here, since they all follow the same pattern: @example /* fill in the array */ aptr = assoc_lookup(array, tmp_string("name", 4), FALSE); *aptr = dupnode(file); aptr = assoc_lookup(array, tmp_string("mode", 4), FALSE); *aptr = make_number((AWKNUM) sbuf.st_mode); aptr = assoc_lookup(array, tmp_string("pmode", 5), FALSE); pmode = format_mode(sbuf.st_mode); *aptr = make_string(pmode, strlen(pmode)); @end example When done, we free the temporary value containing the @value{FN}, set the return value, and return: @example free_temp(file); /* Set the return value */ set_value(tmp_number((AWKNUM) ret)); /* Just to make the interpreter happy */ return tmp_number((AWKNUM) 0); @} @end example @cindex programming conventions, @command{gawk} internals Finally, it's necessary to provide the ``glue'' that loads the new function(s) into @command{gawk}. By convention, each library has a routine named @code{dlload} that does the job: @example /* dlload --- load new builtins in this library */ NODE * dlload(tree, dl) NODE *tree; void *dl; @{ make_builtin("chdir", do_chdir, 1); make_builtin("stat", do_stat, 2); return tmp_number((AWKNUM) 0); @} @end example And that's it! As an exercise, consider adding functions to implement system calls such as @code{chown}, @code{chmod}, and @code{umask}. @node Using Internal File Ops @appendixsubsubsec Integrating the Extensions @c last comma is part of secondary @cindex @command{gawk}, interpreter, adding code to Now that the code is written, it must be possible to add it at runtime to the running @command{gawk} interpreter. First, the code must be compiled. Assuming that the functions are in a file named @file{filefuncs.c}, and @var{idir} is the location of the @command{gawk} include files, the following steps create a GNU/Linux shared library: @example $ gcc -shared -DHAVE_CONFIG_H -c -O -g -I@var{idir} filefuncs.c $ ld -o filefuncs.so -shared filefuncs.o @end example @cindex @code{extension} function (@command{gawk}) Once the library exists, it is loaded by calling the @code{extension} built-in function. This function takes two arguments: the name of the library to load and the name of a function to call when the library is first loaded. This function adds the new functions to @command{gawk}. It returns the value returned by the initialization function within the shared library: @example # file testff.awk BEGIN @{ extension("./filefuncs.so", "dlload") chdir(".") # no-op data[1] = 1 # force `data' to be an array print "Info for testff.awk" ret = stat("testff.awk", data) print "ret =", ret for (i in data) printf "data[\"%s\"] = %s\n", i, data[i] print "testff.awk modified:", strftime("%m %d %y %H:%M:%S", data["mtime"]) @} @end example Here are the results of running the program: @example $ gawk -f testff.awk @print{} Info for testff.awk @print{} ret = 0 @print{} data["blksize"] = 4096 @print{} data["mtime"] = 932361936 @print{} data["mode"] = 33188 @print{} data["type"] = file @print{} data["dev"] = 2065 @print{} data["gid"] = 10 @print{} data["ino"] = 878597 @print{} data["ctime"] = 971431797 @print{} data["blocks"] = 2 @print{} data["nlink"] = 1 @print{} data["name"] = testff.awk @print{} data["atime"] = 971608519 @print{} data["pmode"] = -rw-r--r-- @print{} data["size"] = 607 @print{} data["uid"] = 2076 @print{} testff.awk modified: 07 19 99 08:25:36 @end example @c ENDOFRANGE filre @c ENDOFRANGE dirch @c ENDOFRANGE statg @c ENDOFRANGE chdirg @c ENDOFRANGE gladfgaw @c ENDOFRANGE adfugaw @c ENDOFRANGE fubadgaw @node Future Extensions @appendixsec Probable Future Extensions @ignore From emory!scalpel.netlabs.com!lwall Tue Oct 31 12:43:17 1995 Return-Path: Message-Id: <9510311732.AA28472@scalpel.netlabs.com> To: arnold@skeeve.atl.ga.us (Arnold D. Robbins) Subject: Re: May I quote you? In-Reply-To: Your message of "Tue, 31 Oct 95 09:11:00 EST." Date: Tue, 31 Oct 95 09:32:46 -0800 From: Larry Wall : Greetings. I am working on the release of gawk 3.0. Part of it will be a : thoroughly updated manual. One of the sections deals with planned future : extensions and enhancements. I have the following at the beginning : of it: : : @cindex PERL : @cindex Wall, Larry : @display : @i{AWK is a language similar to PERL, only considerably more elegant.} @* : Arnold Robbins : @sp 1 : @i{Hey!} @* : Larry Wall : @end display : : Before I actually release this for publication, I wanted to get your : permission to quote you. (Hopefully, in the spirit of much of GNU, the : implied humor is visible... :-) I think that would be fine. Larry @end ignore @cindex PERL @cindex Wall, Larry @cindex Robbins, Arnold @quotation @i{AWK is a language similar to PERL, only considerably more elegant.}@* Arnold Robbins @i{Hey!}@* Larry Wall @end quotation This @value{SECTION} briefly lists extensions and possible improvements that indicate the directions we are currently considering for @command{gawk}. The file @file{FUTURES} in the @command{gawk} distribution lists these extensions as well. Following is a list of probable future changes visible at the @command{awk} language level: @c these are ordered by likelihood @table @asis @item Loadable module interface It is not clear that the @command{awk}-level interface to the modules facility is as good as it should be. The interface needs to be redesigned, particularly taking namespace issues into account, as well as possibly including issues such as library search path order and versioning. @item @code{RECLEN} variable for fixed-length records Along with @code{FIELDWIDTHS}, this would speed up the processing of fixed-length records. @code{PROCINFO["RS"]} would be @code{"RS"} or @code{"RECLEN"}, depending upon which kind of record processing is in effect. @item Additional @code{printf} specifiers The 1999 ISO C standard added a number of additional @code{printf} format specifiers. These should be evaluated for possible inclusion in @command{gawk}. @ignore @item A @samp{%'d} flag Add @samp{%'d} for putting in commas in formatting numeric values. @end ignore @item Databases It may be possible to map a GDBM/NDBM/SDBM file into an @command{awk} array. @item Large character sets It would be nice if @command{gawk} could handle UTF-8 and other character sets that are larger than eight bits. @item More @code{lint} warnings There are more things that could be checked for portability. @end table Following is a list of probable improvements that will make @command{gawk}'s source code easier to work with: @table @asis @item Loadable module mechanics The current extension mechanism works (@pxref{Dynamic Extensions}), but is rather primitive. It requires a fair amount of manual work to create and integrate a loadable module. Nor is the current mechanism as portable as might be desired. The GNU @command{libtool} package provides a number of features that would make using loadable modules much easier. @command{gawk} should be changed to use @command{libtool}. @item Loadable module internals The API to its internals that @command{gawk} ``exports'' should be revised. Too many things are needlessly exposed. A new API should be designed and implemented to make module writing easier. @item Better array subscript management @command{gawk}'s management of array subscript storage could use revamping, so that using the same value to index multiple arrays only stores one copy of the index value. @item Integrating the DBUG library Integrating Fred Fish's DBUG library would be helpful during development, but it's a lot of work to do. @end table Following is a list of probable improvements that will make @command{gawk} perform better: @table @asis @c NEXT ED: remove this item. awka and mawk do these respectively @item Compilation of @command{awk} programs @command{gawk} uses a Bison (YACC-like) parser to convert the script given it into a syntax tree; the syntax tree is then executed by a simple recursive evaluator. This method incurs a lot of overhead, since the recursive evaluator performs many procedure calls to do even the simplest things. It should be possible for @command{gawk} to convert the script's parse tree into a C program which the user would then compile, using the normal C compiler and a special @command{gawk} library to provide all the needed functions (regexps, fields, associative arrays, type coercion, and so on). @c last comma is part of secondary @cindex @command{gawk}, interpreter, adding code to An easier possibility might be for an intermediate phase of @command{gawk} to convert the parse tree into a linear byte code form like the one used in GNU Emacs Lisp. The recursive evaluator would then be replaced by a straight line byte code interpreter that would be intermediate in speed between running a compiled program and doing what @command{gawk} does now. @end table Finally, the programs in the test suite could use documenting in this @value{DOCUMENT}. @xref{Additions}, if you are interested in tackling any of these projects. @c ENDOFRANGE impis @c ENDOFRANGE gawii @node Basic Concepts @appendix Basic Programming Concepts @cindex programming, concepts @c STARTOFRANGE procon @cindex programming, concepts This @value{APPENDIX} attempts to define some of the basic concepts and terms that are used throughout the rest of this @value{DOCUMENT}. As this @value{DOCUMENT} is specifically about @command{awk}, and not about computer programming in general, the coverage here is by necessity fairly cursory and simplistic. (If you need more background, there are many other introductory texts that you should refer to instead.) @menu * Basic High Level:: The high level view. * Basic Data Typing:: A very quick intro to data types. * Floating Point Issues:: Stuff to know about floating-point numbers. @end menu @node Basic High Level @appendixsec What a Program Does @cindex processing data At the most basic level, the job of a program is to process some input data and produce results. @c NEXT ED: Use real images here @iftex @tex \expandafter\ifx\csname graph\endcsname\relax \csname newbox\endcsname\graph\fi \expandafter\ifx\csname graphtemp\endcsname\relax \csname newdimen\endcsname\graphtemp\fi \setbox\graph=\vtop{\vskip 0pt\hbox{% \special{pn 20}% \special{pa 2425 200}% \special{pa 2850 200}% \special{fp}% \special{sh 1.000}% \special{pn 20}% \special{pa 2750 175}% \special{pa 2850 200}% \special{pa 2750 225}% \special{pa 2750 175}% \special{fp}% \special{pn 20}% \special{pa 850 200}% \special{pa 1250 200}% \special{fp}% \special{sh 1.000}% \special{pn 20}% \special{pa 1150 175}% \special{pa 1250 200}% \special{pa 1150 225}% \special{pa 1150 175}% \special{fp}% \special{pn 20}% \special{pa 2950 400}% \special{pa 3650 400}% \special{pa 3650 0}% \special{pa 2950 0}% \special{pa 2950 400}% \special{fp}% \special{pn 10}% \special{ar 1800 200 450 200 0 6.28319}% \graphtemp=.5ex\advance\graphtemp by 0.200in \rlap{\kern 3.300in\lower\graphtemp\hbox to 0pt{\hss Results\hss}}% \graphtemp=.5ex\advance\graphtemp by 0.200in \rlap{\kern 1.800in\lower\graphtemp\hbox to 0pt{\hss Program\hss}}% \special{pn 10}% \special{pa 0 400}% \special{pa 700 400}% \special{pa 700 0}% \special{pa 0 0}% \special{pa 0 400}% \special{fp}% \graphtemp=.5ex\advance\graphtemp by 0.200in \rlap{\kern 0.350in\lower\graphtemp\hbox to 0pt{\hss Data\hss}}% \hbox{\vrule depth0.400in width0pt height 0pt}% \kern 3.650in }% }% \centerline{\box\graph} @end tex @end iftex @ifnottex @example _______ +------+ / \ +---------+ | Data | -----> < Program > -----> | Results | +------+ \_______/ +---------+ @end example @end ifnottex @cindex compiled programs @cindex interpreted programs The ``program'' in the figure can be either a compiled program@footnote{Compiled programs are typically written in lower-level languages such as C, C++, Fortran, or Ada, and then translated, or @dfn{compiled}, into a form that the computer can execute directly.} (such as @command{ls}), or it may be @dfn{interpreted}. In the latter case, a machine-executable program such as @command{awk} reads your program, and then uses the instructions in your program to process the data. @cindex programming, basic steps When you write a program, it usually consists of the following, very basic set of steps: @c NEXT ED: Use real images here @iftex @tex \expandafter\ifx\csname graph\endcsname\relax \csname newbox\endcsname\graph\fi \expandafter\ifx\csname graphtemp\endcsname\relax \csname newdimen\endcsname\graphtemp\fi \setbox\graph=\vtop{\vskip 0pt\hbox{% \graphtemp=.5ex\advance\graphtemp by 0.600in \rlap{\kern 2.800in\lower\graphtemp\hbox to 0pt{\hss Yes\hss}}% \graphtemp=.5ex\advance\graphtemp by 0.100in \rlap{\kern 3.300in\lower\graphtemp\hbox to 0pt{\hss No\hss}}% \special{pn 8}% \special{pa 2100 1000}% \special{pa 1600 1000}% \special{pa 1600 1000}% \special{pa 1600 300}% \special{fp}% \special{sh 1.000}% \special{pn 8}% \special{pa 1575 400}% \special{pa 1600 300}% \special{pa 1625 400}% \special{pa 1575 400}% \special{fp}% \special{pn 8}% \special{pa 2600 500}% \special{pa 2600 900}% \special{fp}% \special{sh 1.000}% \special{pn 8}% \special{pa 2625 800}% \special{pa 2600 900}% \special{pa 2575 800}% \special{pa 2625 800}% \special{fp}% \special{pn 8}% \special{pa 3200 200}% \special{pa 4000 200}% \special{fp}% \special{sh 1.000}% \special{pn 8}% \special{pa 3900 175}% \special{pa 4000 200}% \special{pa 3900 225}% \special{pa 3900 175}% \special{fp}% \special{pn 8}% \special{pa 1400 200}% \special{pa 2100 200}% \special{fp}% \special{sh 1.000}% \special{pn 8}% \special{pa 2000 175}% \special{pa 2100 200}% \special{pa 2000 225}% \special{pa 2000 175}% \special{fp}% \special{pn 8}% \special{ar 2600 1000 400 100 0 6.28319}% \graphtemp=.5ex\advance\graphtemp by 1.000in \rlap{\kern 2.600in\lower\graphtemp\hbox to 0pt{\hss Process\hss}}% \special{pn 8}% \special{pa 2200 400}% \special{pa 3100 400}% \special{pa 3100 0}% \special{pa 2200 0}% \special{pa 2200 400}% \special{fp}% \graphtemp=.5ex\advance\graphtemp by 0.200in \rlap{\kern 2.688in\lower\graphtemp\hbox to 0pt{\hss More Data?\hss}}% \special{pn 8}% \special{ar 650 200 650 200 0 6.28319}% \graphtemp=.5ex\advance\graphtemp by 0.200in \rlap{\kern 0.613in\lower\graphtemp\hbox to 0pt{\hss Initialization\hss}}% \special{pn 8}% \special{ar 0 200 0 0 0 6.28319}% \special{pn 8}% \special{ar 4550 200 450 100 0 6.28319}% \graphtemp=.5ex\advance\graphtemp by 0.200in \rlap{\kern 4.600in\lower\graphtemp\hbox to 0pt{\hss Clean Up\hss}}% \hbox{\vrule depth1.100in width0pt height 0pt}% \kern 5.000in }% }% \centerline{\box\graph} @end tex @end iftex @ifnottex @example ______ +----------------+ / More \ No +----------+ | Initialization | -------> < Data > -------> | Clean Up | +----------------+ ^ \ ? / +----------+ | +--+-+ | | Yes | | | V | +---------+ +-----+ Process | +---------+ @end example @end ifnottex @table @asis @item Initialization These are the things you do before actually starting to process data, such as checking arguments, initializing any data you need to work with, and so on. This step corresponds to @command{awk}'s @code{BEGIN} rule (@pxref{BEGIN/END}). If you were baking a cake, this might consist of laying out all the mixing bowls and the baking pan, and making sure you have all the ingredients that you need. @item Processing This is where the actual work is done. Your program reads data, one logical chunk at a time, and processes it as appropriate. In most programming languages, you have to manually manage the reading of data, checking to see if there is more each time you read a chunk. @command{awk}'s pattern-action paradigm (@pxref{Getting Started}) handles the mechanics of this for you. In baking a cake, the processing corresponds to the actual labor: breaking eggs, mixing the flour, water, and other ingredients, and then putting the cake into the oven. @item Clean Up Once you've processed all the data, you may have things you need to do before exiting. This step corresponds to @command{awk}'s @code{END} rule (@pxref{BEGIN/END}). After the cake comes out of the oven, you still have to wrap it in plastic wrap to keep anyone from tasting it, as well as wash the mixing bowls and utensils. @end table @cindex algorithms An @dfn{algorithm} is a detailed set of instructions necessary to accomplish a task, or process data. It is much the same as a recipe for baking a cake. Programs implement algorithms. Often, it is up to you to design the algorithm and implement it, simultaneously. @cindex records @cindex fields The ``logical chunks'' we talked about previously are called @dfn{records}, similar to the records a company keeps on employees, a school keeps for students, or a doctor keeps for patients. Each record has many component parts, such as first and last names, date of birth, address, and so on. The component parts are referred to as the @dfn{fields} of the record. The act of reading data is termed @dfn{input}, and that of generating results, not too surprisingly, is termed @dfn{output}. They are often referred to together as ``input/output,'' and even more often, as ``I/O'' for short. (You will also see ``input'' and ``output'' used as verbs.) @cindex data-driven languages @c comma is part of primary @cindex languages, data-driven @command{awk} manages the reading of data for you, as well as the breaking it up into records and fields. Your program's job is to tell @command{awk} what to with the data. You do this by describing @dfn{patterns} in the data to look for, and @dfn{actions} to execute when those patterns are seen. This @dfn{data-driven} nature of @command{awk} programs usually makes them both easier to write and easier to read. @node Basic Data Typing @appendixsec Data Values in a Computer @cindex variables In a program, you keep track of information and values in things called @dfn{variables}. A variable is just a name for a given value, such as @code{first_name}, @code{last_name}, @code{address}, and so on. @command{awk} has several predefined variables, and it has special names to refer to the current input record and the fields of the record. You may also group multiple associated values under one name, as an array. @cindex values, numeric @cindex values, string @cindex scalar values Data, particularly in @command{awk}, consists of either numeric values, such as 42 or 3.1415927, or string values. String values are essentially anything that's not a number, such as a name. Strings are sometimes referred to as @dfn{character data}, since they store the individual characters that comprise them. Individual variables, as well as numeric and string variables, are referred to as @dfn{scalar} values. Groups of values, such as arrays, are not scalars. @cindex integers @cindex floating-point, numbers @cindex numbers, floating-point Within computers, there are two kinds of numeric values: @dfn{integers} and @dfn{floating-point}. In school, integer values were referred to as ``whole'' numbers---that is, numbers without any fractional part, such as 1, 42, or @minus{}17. The advantage to integer numbers is that they represent values exactly. The disadvantage is that their range is limited. On most modern systems, this range is @minus{}2,147,483,648 to 2,147,483,647. @cindex unsigned integers @cindex integers, unsigned Integer values come in two flavors: @dfn{signed} and @dfn{unsigned}. Signed values may be negative or positive, with the range of values just described. Unsigned values are always positive. On most modern systems, the range is from 0 to 4,294,967,295. @cindex double-precision floating-point @cindex single-precision floating-point Floating-point numbers represent what are called ``real'' numbers; i.e., those that do have a fractional part, such as 3.1415927. The advantage to floating-point numbers is that they can represent a much larger range of values. The disadvantage is that there are numbers that they cannot represent exactly. @command{awk} uses @dfn{double-precision} floating-point numbers, which can hold more digits than @dfn{single-precision} floating-point numbers. Floating-point issues are discussed more fully in @ref{Floating Point Issues}. At the very lowest level, computers store values as groups of binary digits, or @dfn{bits}. Modern computers group bits into groups of eight, called @dfn{bytes}. Advanced applications sometimes have to manipulate bits directly, and @command{gawk} provides functions for doing so. @cindex null strings While you are probably used to the idea of a number without a value (i.e., zero), it takes a bit more getting used to the idea of zero-length character data. Nevertheless, such a thing exists. It is called the @dfn{null string}. The null string is character data that has no value. In other words, it is empty. It is written in @command{awk} programs like this: @code{""}. Humans are used to working in decimal; i.e., base 10. In base 10, numbers go from 0 to 9, and then ``roll over'' into the next column. (Remember grade school? 42 is 4 times 10 plus 2.) There are other number bases though. Computers commonly use base 2 or @dfn{binary}, base 8 or @dfn{octal}, and base 16 or @dfn{hexadecimal}. In binary, each column represents two times the value in the column to its right. Each column may contain either a 0 or a 1. Thus, binary 1010 represents 1 times 8, plus 0 times 4, plus 1 times 2, plus 0 times 1, or decimal 10. Octal and hexadecimal are discussed more in @ref{Nondecimal-numbers}. Programs are written in programming languages. Hundreds, if not thousands, of programming languages exist. One of the most popular is the C programming language. The C language had a very strong influence on the design of the @command{awk} language. @cindex Kernighan, Brian @cindex Ritchie, Dennis There have been several versions of C. The first is often referred to as ``K&R'' C, after the initials of Brian Kernighan and Dennis Ritchie, the authors of the first book on C. (Dennis Ritchie created the language, and Brian Kernighan was one of the creators of @command{awk}.) In the mid-1980s, an effort began to produce an international standard for C. This work culminated in 1989, with the production of the ANSI standard for C. This standard became an ISO standard in 1990. Where it makes sense, POSIX @command{awk} is compatible with 1990 ISO C. In 1999, a revised ISO C standard was approved and released. Future versions of @command{gawk} will be as compatible as possible with this standard. @node Floating Point Issues @appendixsec Floating-Point Number Caveats As mentioned earlier, floating-point numbers represent what are called ``real'' numbers, i.e., those that have a fractional part. @command{awk} uses double-precision floating-point numbers to represent all numeric values. This @value{SECTION} describes some of the issues involved in using floating-point numbers. There is a very nice paper on floating-point arithmetic by David Goldberg, ``What Every Computer Scientist Should Know About Floating-point Arithmetic,'' @cite{ACM Computing Surveys} @strong{23}, 1 (1991-03), 5-48.@footnote{@uref{http://www.validlab.com/goldberg/paper.ps}.} This is worth reading if you are interested in the details, but it does require a background in computer science. Internally, @command{awk} keeps both the numeric value (double-precision floating-point) and the string value for a variable. Separately, @command{awk} keeps track of what type the variable has (@pxref{Typing and Comparison}), which plays a role in how variables are used in comparisons. It is important to note that the string value for a number may not reflect the full value (all the digits) that the numeric value actually contains. The following program (@file{values.awk}) illustrates this: @example @{ $1 = $2 + $3 # see it for what it is printf("$1 = %.12g\n", $1) # use CONVFMT a = "<" $1 ">" print "a =", a @group # use OFMT print "$1 =", $1 @end group @} @end example @noindent This program shows the full value of the sum of @code{$2} and @code{$3} using @code{printf}, and then prints the string values obtained from both automatic conversion (via @code{CONVFMT}) and from printing (via @code{OFMT}). Here is what happens when the program is run: @example $ echo 2 3.654321 1.2345678 | awk -f values.awk @print{} $1 = 4.8888888 @print{} a = <4.88889> @print{} $1 = 4.88889 @end example This makes it clear that the full numeric value is different from what the default string representations show. @code{CONVFMT}'s default value is @code{"%.6g"}, which yields a value with at least six significant digits. For some applications, you might want to change it to specify more precision. On most modern machines, most of the time, 17 digits is enough to capture a floating-point number's value exactly.@footnote{Pathological cases can require up to 752 digits (!), but we doubt that you need to worry about this.} @cindex floating-point Unlike numbers in the abstract sense (such as what you studied in high school or college math), numbers stored in computers are limited in certain ways. They cannot represent an infinite number of digits, nor can they always represent things exactly. In particular, floating-point numbers cannot always represent values exactly. Here is an example: @example $ awk '@{ printf("%010d\n", $1 * 100) @}' 515.79 @print{} 0000051579 515.80 @print{} 0000051579 515.81 @print{} 0000051580 515.82 @print{} 0000051582 @kbd{@value{CTL}-d} @end example @noindent This shows that some values can be represented exactly, whereas others are only approximated. This is not a ``bug'' in @command{awk}, but simply an artifact of how computers represent numbers. @cindex negative zero @cindex positive zero @c comma is part of primary @cindex zero, negative vs.@: positive Another peculiarity of floating-point numbers on modern systems is that they often have more than one representation for the number zero! In particular, it is possible to represent ``minus zero'' as well as regular, or ``positive'' zero. This example shows that negative and positive zero are distinct values when stored internally, but that they are in fact equal to each other, as well as to ``regular'' zero: @smallexample $ gawk 'BEGIN @{ mz = -0 ; pz = 0 > printf "-0 = %g, +0 = %g, (-0 == +0) -> %d\n", mz, pz, mz == pz > printf "mz == 0 -> %d, pz == 0 -> %d\n", mz == 0, pz == 0 > @}' @print{} -0 = -0, +0 = 0, (-0 == +0) -> 1 @print{} mz == 0 -> 1, pz == 0 -> 1 @end smallexample It helps to keep this in mind should you process numeric data that contains negative zero values; the fact that the zero is negative is noted and can affect comparisons. @c ENDOFRANGE procon @node Glossary @unnumbered Glossary @table @asis @item Action A series of @command{awk} statements attached to a rule. If the rule's pattern matches an input record, @command{awk} executes the rule's action. Actions are always enclosed in curly braces. (@xref{Action Overview}.) @cindex Spencer, Henry @cindex @command{sed} utility @cindex amazing @command{awk} assembler (@command{aaa}) @item Amazing @command{awk} Assembler Henry Spencer at the University of Toronto wrote a retargetable assembler completely as @command{sed} and @command{awk} scripts. It is thousands of lines long, including machine descriptions for several eight-bit microcomputers. It is a good example of a program that would have been better written in another language. You can get it from @uref{ftp://ftp.freefriends.org/arnold/Awkstuff/aaa.tgz}. @cindex amazingly workable formatter (@command{awf}) @cindex @command{awf} (amazingly workable formatter) program @item Amazingly Workable Formatter (@command{awf}) Henry Spencer at the University of Toronto wrote a formatter that accepts a large subset of the @samp{nroff -ms} and @samp{nroff -man} formatting commands, using @command{awk} and @command{sh}. It is available over the Internet from @uref{ftp://ftp.freefriends.org/arnold/Awkstuff/awf.tgz}. @item Anchor The regexp metacharacters @samp{^} and @samp{$}, which force the match to the beginning or end of the string, respectively. @cindex ANSI @item ANSI The American National Standards Institute. This organization produces many standards, among them the standards for the C and C++ programming languages. These standards often become international standards as well. See also ``ISO.'' @item Array A grouping of multiple values under the same name. Most languages just provide sequential arrays. @command{awk} provides associative arrays. @item Assertion A statement in a program that a condition is true at this point in the program. Useful for reasoning about how a program is supposed to behave. @item Assignment An @command{awk} expression that changes the value of some @command{awk} variable or data object. An object that you can assign to is called an @dfn{lvalue}. The assigned values are called @dfn{rvalues}. @xref{Assignment Ops}. @item Associative Array Arrays in which the indices may be numbers or strings, not just sequential integers in a fixed range. @item @command{awk} Language The language in which @command{awk} programs are written. @item @command{awk} Program An @command{awk} program consists of a series of @dfn{patterns} and @dfn{actions}, collectively known as @dfn{rules}. For each input record given to the program, the program's rules are all processed in turn. @command{awk} programs may also contain function definitions. @item @command{awk} Script Another name for an @command{awk} program. @item Bash The GNU version of the standard shell @ifnotinfo (the @b{B}ourne-@b{A}gain @b{SH}ell). @end ifnotinfo @ifinfo (the Bourne-Again SHell). @end ifinfo See also ``Bourne Shell.'' @item BBS See ``Bulletin Board System.'' @item Bit Short for ``Binary Digit.'' All values in computer memory ultimately reduce to binary digits: values that are either zero or one. Groups of bits may be interpreted differently---as integers, floating-point numbers, character data, addresses of other memory objects, or other data. @command{awk} lets you work with floating-point numbers and strings. @command{gawk} lets you manipulate bit values with the built-in functions described in @ref{Bitwise Functions}. Computers are often defined by how many bits they use to represent integer values. Typical systems are 32-bit systems, but 64-bit systems are becoming increasingly popular, and 16-bit systems are waning in popularity. @item Boolean Expression Named after the English mathematician Boole. See also ``Logical Expression.'' @item Bourne Shell The standard shell (@file{/bin/sh}) on Unix and Unix-like systems, originally written by Steven R.@: Bourne. Many shells (@command{bash}, @command{ksh}, @command{pdksh}, @command{zsh}) are generally upwardly compatible with the Bourne shell. @item Built-in Function The @command{awk} language provides built-in functions that perform various numerical, I/O-related, and string computations. Examples are @code{sqrt} (for the square root of a number) and @code{substr} (for a substring of a string). @command{gawk} provides functions for timestamp management, bit manipulation, and runtime string translation. (@xref{Built-in}.) @item Built-in Variable @code{ARGC}, @code{ARGV}, @code{CONVFMT}, @code{ENVIRON}, @code{FILENAME}, @code{FNR}, @code{FS}, @code{NF}, @code{NR}, @code{OFMT}, @code{OFS}, @code{ORS}, @code{RLENGTH}, @code{RSTART}, @code{RS}, and @code{SUBSEP} are the variables that have special meaning to @command{awk}. In addition, @code{ARGIND}, @code{BINMODE}, @code{ERRNO}, @code{FIELDWIDTHS}, @code{IGNORECASE}, @code{LINT}, @code{PROCINFO}, @code{RT}, and @code{TEXTDOMAIN} are the variables that have special meaning to @command{gawk}. Changing some of them affects @command{awk}'s running environment. (@xref{Built-in Variables}.) @item Braces See ``Curly Braces.'' @item Bulletin Board System A computer system allowing users to log in and read and/or leave messages for other users of the system, much like leaving paper notes on a bulletin board. @item C The system programming language that most GNU software is written in. The @command{awk} programming language has C-like syntax, and this @value{DOCUMENT} points out similarities between @command{awk} and C when appropriate. In general, @command{gawk} attempts to be as similar to the 1990 version of ISO C as makes sense. Future versions of @command{gawk} may adopt features from the newer 1999 standard, as appropriate. @item C++ A popular object-oriented programming language derived from C. @cindex ISO 8859-1 @cindex ISO Latin-1 @cindex character sets (machine character encodings) @item Character Set The set of numeric codes used by a computer system to represent the characters (letters, numbers, punctuation, etc.) of a particular country or place. The most common character set in use today is ASCII (American Standard Code for Information Interchange). Many European countries use an extension of ASCII known as ISO-8859-1 (ISO Latin-1). @cindex @command{chem} utility @item CHEM A preprocessor for @command{pic} that reads descriptions of molecules and produces @command{pic} input for drawing them. It was written in @command{awk} by Brian Kernighan and Jon Bentley, and is available from @uref{http://cm.bell-labs.com/netlib/typesetting/chem.gz}. @item Coprocess A subordinate program with which two-way communications is possible. @cindex compiled programs @item Compiler A program that translates human-readable source code into machine-executable object code. The object code is then executed directly by the computer. See also ``Interpreter.'' @item Compound Statement A series of @command{awk} statements, enclosed in curly braces. Compound statements may be nested. (@xref{Statements}.) @item Concatenation Concatenating two strings means sticking them together, one after another, producing a new string. For example, the string @samp{foo} concatenated with the string @samp{bar} gives the string @samp{foobar}. (@xref{Concatenation}.) @item Conditional Expression An expression using the @samp{?:} ternary operator, such as @samp{@var{expr1} ? @var{expr2} : @var{expr3}}. The expression @var{expr1} is evaluated; if the result is true, the value of the whole expression is the value of @var{expr2}; otherwise the value is @var{expr3}. In either case, only one of @var{expr2} and @var{expr3} is evaluated. (@xref{Conditional Exp}.) @item Comparison Expression A relation that is either true or false, such as @samp{(a < b)}. Comparison expressions are used in @code{if}, @code{while}, @code{do}, and @code{for} statements, and in patterns to select which input records to process. (@xref{Typing and Comparison}.) @item Curly Braces The characters @samp{@{} and @samp{@}}. Curly braces are used in @command{awk} for delimiting actions, compound statements, and function bodies. @cindex dark corner @item Dark Corner An area in the language where specifications often were (or still are) not clear, leading to unexpected or undesirable behavior. Such areas are marked in this @value{DOCUMENT} with @iftex the picture of a flashlight in the margin @end iftex @ifnottex ``(d.c.)'' in the text @end ifnottex and are indexed under the heading ``dark corner.'' @item Data Driven A description of @command{awk} programs, where you specify the data you are interested in processing, and what to do when that data is seen. @item Data Objects These are numbers and strings of characters. Numbers are converted into strings and vice versa, as needed. (@xref{Conversion}.) @item Deadlock The situation in which two communicating processes are each waiting for the other to perform an action. @item Double-Precision An internal representation of numbers that can have fractional parts. Double-precision numbers keep track of more digits than do single-precision numbers, but operations on them are sometimes more expensive. This is the way @command{awk} stores numeric values. It is the C type @code{double}. @item Dynamic Regular Expression A dynamic regular expression is a regular expression written as an ordinary expression. It could be a string constant, such as @code{"foo"}, but it may also be an expression whose value can vary. (@xref{Computed Regexps}.) @item Environment A collection of strings, of the form @var{name@code{=}val}, that each program has available to it. Users generally place values into the environment in order to provide information to various programs. Typical examples are the environment variables @env{HOME} and @env{PATH}. @item Empty String See ``Null String.'' @cindex epoch, definition of @item Epoch The date used as the ``beginning of time'' for timestamps. Time values in Unix systems are represented as seconds since the epoch, with library functions available for converting these values into standard date and time formats. The epoch on Unix and POSIX systems is 1970-01-01 00:00:00 UTC. See also ``GMT'' and ``UTC.'' @item Escape Sequences A special sequence of characters used for describing nonprinting characters, such as @samp{\n} for newline or @samp{\033} for the ASCII ESC (Escape) character. (@xref{Escape Sequences}.) @item FDL See ``Free Documentation License.'' @item Field When @command{awk} reads an input record, it splits the record into pieces separated by whitespace (or by a separator regexp that you can change by setting the built-in variable @code{FS}). Such pieces are called fields. If the pieces are of fixed length, you can use the built-in variable @code{FIELDWIDTHS} to describe their lengths. (@xref{Field Separators}, and @ref{Constant Size}.) @item Flag A variable whose truth value indicates the existence or nonexistence of some condition. @item Floating-Point Number Often referred to in mathematical terms as a ``rational'' or real number, this is just a number that can have a fractional part. See also ``Double-Precision'' and ``Single-Precision.'' @item Format Format strings are used to control the appearance of output in the @code{strftime} and @code{sprintf} functions, and are used in the @code{printf} statement as well. Also, data conversions from numbers to strings are controlled by the format string contained in the built-in variable @code{CONVFMT}. (@xref{Control Letters}.) @item Free Documentation License This document describes the terms under which this @value{DOCUMENT} is published and may be copied. (@xref{GNU Free Documentation License}.) @item Function A specialized group of statements used to encapsulate general or program-specific tasks. @command{awk} has a number of built-in functions, and also allows you to define your own. (@xref{Functions}.) @item FSF See ``Free Software Foundation.'' @cindex FSF (Free Software Foundation) @cindex Free Software Foundation (FSF) @cindex Stallman, Richard @item Free Software Foundation A nonprofit organization dedicated to the production and distribution of freely distributable software. It was founded by Richard M.@: Stallman, the author of the original Emacs editor. GNU Emacs is the most widely used version of Emacs today. @item @command{gawk} The GNU implementation of @command{awk}. @cindex GPL (General Public License) @cindex General Public License (GPL) @cindex GNU General Public License @item General Public License This document describes the terms under which @command{gawk} and its source code may be distributed. (@xref{Copying}.) @item GMT ``Greenwich Mean Time.'' This is the old term for UTC. It is the time of day used as the epoch for Unix and POSIX systems. See also ``Epoch'' and ``UTC.'' @cindex FSF (Free Software Foundation) @cindex Free Software Foundation (FSF) @cindex GNU Project @item GNU ``GNU's not Unix''. An on-going project of the Free Software Foundation to create a complete, freely distributable, POSIX-compliant computing environment. @item GNU/Linux A variant of the GNU system using the Linux kernel, instead of the Free Software Foundation's Hurd kernel. Linux is a stable, efficient, full-featured clone of Unix that has been ported to a variety of architectures. It is most popular on PC-class systems, but runs well on a variety of other systems too. The Linux kernel source code is available under the terms of the GNU General Public License, which is perhaps its most important aspect. @item GPL See ``General Public License.'' @item Hexadecimal Base 16 notation, where the digits are @code{0}--@code{9} and @code{A}--@code{F}, with @samp{A} representing 10, @samp{B} representing 11, and so on, up to @samp{F} for 15. Hexadecimal numbers are written in C using a leading @samp{0x}, to indicate their base. Thus, @code{0x12} is 18 (1 times 16 plus 2). @item I/O Abbreviation for ``Input/Output,'' the act of moving data into and/or out of a running program. @item Input Record A single chunk of data that is read in by @command{awk}. Usually, an @command{awk} input record consists of one line of text. (@xref{Records}.) @item Integer A whole number, i.e., a number that does not have a fractional part. @item Internationalization The process of writing or modifying a program so that it can use multiple languages without requiring further source code changes. @cindex interpreted programs @item Interpreter A program that reads human-readable source code directly, and uses the instructions in it to process data and produce results. @command{awk} is typically (but not always) implemented as an interpreter. See also ``Compiler.'' @item Interval Expression A component of a regular expression that lets you specify repeated matches of some part of the regexp. Interval expressions were not traditionally available in @command{awk} programs. @cindex ISO @item ISO The International Standards Organization. This organization produces international standards for many things, including programming languages, such as C and C++. In the computer arena, important standards like those for C, C++, and POSIX become both American national and ISO international standards simultaneously. This @value{DOCUMENT} refers to Standard C as ``ISO C'' throughout. @item Keyword In the @command{awk} language, a keyword is a word that has special meaning. Keywords are reserved and may not be used as variable names. @command{gawk}'s keywords are: @code{BEGIN}, @code{END}, @code{if}, @code{else}, @code{while}, @code{do@dots{}while}, @code{for}, @code{for@dots{}in}, @code{break}, @code{continue}, @code{delete}, @code{next}, @code{nextfile}, @code{function}, @code{func}, and @code{exit}. @cindex LGPL (Lesser General Public License) @cindex Lesser General Public License (LGPL) @cindex GNU Lesser General Public License @item Lesser General Public License This document describes the terms under which binary library archives or shared objects, and their source code may be distributed. @item Linux See ``GNU/Linux.'' @item LGPL See ``Lesser General Public License.'' @item Localization The process of providing the data necessary for an internationalized program to work in a particular language. @item Logical Expression An expression using the operators for logic, AND, OR, and NOT, written @samp{&&}, @samp{||}, and @samp{!} in @command{awk}. Often called Boolean expressions, after the mathematician who pioneered this kind of mathematical logic. @item Lvalue An expression that can appear on the left side of an assignment operator. In most languages, lvalues can be variables or array elements. In @command{awk}, a field designator can also be used as an lvalue. @item Matching The act of testing a string against a regular expression. If the regexp describes the contents of the string, it is said to @dfn{match} it. @item Metacharacters Characters used within a regexp that do not stand for themselves. Instead, they denote regular expression operations, such as repetition, grouping, or alternation. @item Null String A string with no characters in it. It is represented explicitly in @command{awk} programs by placing two double quote characters next to each other (@code{""}). It can appear in input data by having two successive occurrences of the field separator appear next to each other. @item Number A numeric-valued data object. Modern @command{awk} implementations use double-precision floating-point to represent numbers. Very old @command{awk} implementations use single-precision floating-point. @item Octal Base-eight notation, where the digits are @code{0}--@code{7}. Octal numbers are written in C using a leading @samp{0}, to indicate their base. Thus, @code{013} is 11 (one times 8 plus 3). @cindex P1003.2 POSIX standard @item P1003.2 See ``POSIX.'' @item Pattern Patterns tell @command{awk} which input records are interesting to which rules. A pattern is an arbitrary conditional expression against which input is tested. If the condition is satisfied, the pattern is said to @dfn{match} the input record. A typical pattern might compare the input record against a regular expression. (@xref{Pattern Overview}.) @item POSIX The name for a series of standards @c being developed by the IEEE that specify a Portable Operating System interface. The ``IX'' denotes the Unix heritage of these standards. The main standard of interest for @command{awk} users is @cite{IEEE Standard for Information Technology, Standard 1003.2-1992, Portable Operating System Interface (POSIX) Part 2: Shell and Utilities}. Informally, this standard is often referred to as simply ``P1003.2.'' @item Precedence The order in which operations are performed when operators are used without explicit parentheses. @item Private Variables and/or functions that are meant for use exclusively by library functions and not for the main @command{awk} program. Special care must be taken when naming such variables and functions. (@xref{Library Names}.) @item Range (of input lines) A sequence of consecutive lines from the input file(s). A pattern can specify ranges of input lines for @command{awk} to process or it can specify single lines. (@xref{Pattern Overview}.) @item Recursion When a function calls itself, either directly or indirectly. If this isn't clear, refer to the entry for ``recursion.'' @item Redirection Redirection means performing input from something other than the standard input stream, or performing output to something other than the standard output stream. You can redirect the output of the @code{print} and @code{printf} statements to a file or a system command, using the @samp{>}, @samp{>>}, @samp{|}, and @samp{|&} operators. You can redirect input to the @code{getline} statement using the @samp{<}, @samp{|}, and @samp{|&} operators. (@xref{Redirection}, and @ref{Getline}.) @item Regexp Short for @dfn{regular expression}. A regexp is a pattern that denotes a set of strings, possibly an infinite set. For example, the regexp @samp{R.*xp} matches any string starting with the letter @samp{R} and ending with the letters @samp{xp}. In @command{awk}, regexps are used in patterns and in conditional expressions. Regexps may contain escape sequences. (@xref{Regexp}.) @item Regular Expression See ``regexp.'' @item Regular Expression Constant A regular expression constant is a regular expression written within slashes, such as @code{/foo/}. This regular expression is chosen when you write the @command{awk} program and cannot be changed during its execution. (@xref{Regexp Usage}.) @item Rule A segment of an @command{awk} program that specifies how to process single input records. A rule consists of a @dfn{pattern} and an @dfn{action}. @command{awk} reads an input record; then, for each rule, if the input record satisfies the rule's pattern, @command{awk} executes the rule's action. Otherwise, the rule does nothing for that input record. @item Rvalue A value that can appear on the right side of an assignment operator. In @command{awk}, essentially every expression has a value. These values are rvalues. @item Scalar A single value, be it a number or a string. Regular variables are scalars; arrays and functions are not. @item Search Path In @command{gawk}, a list of directories to search for @command{awk} program source files. In the shell, a list of directories to search for executable programs. @item Seed The initial value, or starting point, for a sequence of random numbers. @item @command{sed} See ``Stream Editor.'' @item Shell The command interpreter for Unix and POSIX-compliant systems. The shell works both interactively, and as a programming language for batch files, or shell scripts. @item Short-Circuit The nature of the @command{awk} logical operators @samp{&&} and @samp{||}. If the value of the entire expression is determinable from evaluating just the lefthand side of these operators, the righthand side is not evaluated. (@xref{Boolean Ops}.) @item Side Effect A side effect occurs when an expression has an effect aside from merely producing a value. Assignment expressions, increment and decrement expressions, and function calls have side effects. (@xref{Assignment Ops}.) @item Single-Precision An internal representation of numbers that can have fractional parts. Single-precision numbers keep track of fewer digits than do double-precision numbers, but operations on them are sometimes less expensive in terms of CPU time. This is the type used by some very old versions of @command{awk} to store numeric values. It is the C type @code{float}. @item Space The character generated by hitting the space bar on the keyboard. @item Special File A @value{FN} interpreted internally by @command{gawk}, instead of being handed directly to the underlying operating system---for example, @file{/dev/stderr}. (@xref{Special Files}.) @item Stream Editor A program that reads records from an input stream and processes them one or more at a time. This is in contrast with batch programs, which may expect to read their input files in entirety before starting to do anything, as well as with interactive programs which require input from the user. @item String A datum consisting of a sequence of characters, such as @samp{I am a string}. Constant strings are written with double quotes in the @command{awk} language and may contain escape sequences. (@xref{Escape Sequences}.) @item Tab The character generated by hitting the @kbd{TAB} key on the keyboard. It usually expands to up to eight spaces upon output. @item Text Domain A unique name that identifies an application. Used for grouping messages that are translated at runtime into the local language. @item Timestamp A value in the ``seconds since the epoch'' format used by Unix and POSIX systems. Used for the @command{gawk} functions @code{mktime}, @code{strftime}, and @code{systime}. See also ``Epoch'' and ``UTC.'' @cindex Linux @cindex GNU/Linux @cindex Unix @cindex BSD-based operating systems @cindex NetBSD @cindex FreeBSD @cindex OpenBSD @item Unix A computer operating system originally developed in the early 1970's at AT&T Bell Laboratories. It initially became popular in universities around the world and later moved into commercial environments as a software development system and network server system. There are many commercial versions of Unix, as well as several work-alike systems whose source code is freely available (such as GNU/Linux, NetBSD, FreeBSD, and OpenBSD). @item UTC The accepted abbreviation for ``Universal Coordinated Time.'' This is standard time in Greenwich, England, which is used as a reference time for day and date calculations. See also ``Epoch'' and ``GMT.'' @item Whitespace A sequence of space, TAB, or newline characters occurring inside an input record or a string. @end table @node Copying @unnumbered GNU General Public License @center Version 2, June 1991 @display Copyright @copyright{} 1989, 1991 Free Software Foundation, Inc. 59 Temple Place, Suite 330, Boston, MA 02111, USA Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. @end display @c fakenode --- for prepinfo @unnumberedsec Preamble The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software---to make sure the software is free for all its users. This General Public License applies to most of the Free Software Foundation's software and to any other program whose authors commit to using it. (Some other Free Software Foundation software is covered by the GNU Library General Public License instead.) You can apply it to your programs, too. When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things. To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the software, or if you modify it. For example, if you distribute copies of such a program, whether gratis or for a fee, you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights. We protect your rights with two steps: (1) copyright the software, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the software. Also, for each author's protection and ours, we want to make certain that everyone understands that there is no warranty for this free software. If the software is modified by someone else and passed on, we want its recipients to know that what they have is not the original, so that any problems introduced by others will not reflect on the original authors' reputations. Finally, any free program is threatened constantly by software patents. We wish to avoid the danger that redistributors of a free program will individually obtain patent licenses, in effect making the program proprietary. To prevent this, we have made it clear that any patent must be licensed for everyone's free use or not licensed at all. The precise terms and conditions for copying, distribution and modification follow. @ifnotinfo @c fakenode --- for prepinfo @unnumberedsec Terms and Conditions for Copying, Distribution and Modification @end ifnotinfo @ifinfo @center TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION @end ifinfo @enumerate 0 @item This License applies to any program or other work which contains a notice placed by the copyright holder saying it may be distributed under the terms of this General Public License. The ``Program'', below, refers to any such program or work, and a ``work based on the Program'' means either the Program or any derivative work under copyright law: that is to say, a work containing the Program or a portion of it, either verbatim or with modifications and/or translated into another language. (Hereinafter, translation is included without limitation in the term ``modification''.) Each licensee is addressed as ``you''. Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. The act of running the Program is not restricted, and the output from the Program is covered only if its contents constitute a work based on the Program (independent of having been made by running the Program). Whether that is true depends on what the Program does. @item You may copy and distribute verbatim copies of the Program's source code as you receive it, in any medium, provided that you conspicuously and appropriately publish on each copy an appropriate copyright notice and disclaimer of warranty; keep intact all the notices that refer to this License and to the absence of any warranty; and give any other recipients of the Program a copy of this License along with the Program. You may charge a fee for the physical act of transferring a copy, and you may at your option offer warranty protection in exchange for a fee. @item You may modify your copy or copies of the Program or any portion of it, thus forming a work based on the Program, and copy and distribute such modifications or work under the terms of Section 1 above, provided that you also meet all of these conditions: @enumerate a @item You must cause the modified files to carry prominent notices stating that you changed the files and the date of any change. @item You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License. @item If the modified program normally reads commands interactively when run, you must cause it, when started running for such interactive use in the most ordinary way, to print or display an announcement including an appropriate copyright notice and a notice that there is no warranty (or else, saying that you provide a warranty) and that users may redistribute the program under these conditions, and telling the user how to view a copy of this License. (Exception: if the Program itself is interactive but does not normally print such an announcement, your work based on the Program is not required to print an announcement.) @end enumerate These requirements apply to the modified work as a whole. If identifiable sections of that work are not derived from the Program, and can be reasonably considered independent and separate works in themselves, then this License, and its terms, do not apply to those sections when you distribute them as separate works. But when you distribute the same sections as part of a whole which is a work based on the Program, the distribution of the whole must be on the terms of this License, whose permissions for other licensees extend to the entire whole, and thus to each and every part regardless of who wrote it. Thus, it is not the intent of this section to claim rights or contest your rights to work written entirely by you; rather, the intent is to exercise the right to control the distribution of derivative or collective works based on the Program. In addition, mere aggregation of another work not based on the Program with the Program (or with a work based on the Program) on a volume of a storage or distribution medium does not bring the other work under the scope of this License. @item You may copy and distribute the Program (or a work based on it, under Section 2) in object code or executable form under the terms of Sections 1 and 2 above provided that you also do one of the following: @enumerate a @item Accompany it with the complete corresponding machine-readable source code, which must be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or, @item Accompany it with a written offer, valid for at least three years, to give any third party, for a charge no more than your cost of physically performing source distribution, a complete machine-readable copy of the corresponding source code, to be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or, @item Accompany it with the information you received as to the offer to distribute corresponding source code. (This alternative is allowed only for noncommercial distribution and only if you received the program in object code or executable form with such an offer, in accord with Subsection b above.) @end enumerate The source code for a work means the preferred form of the work for making modifications to it. For an executable work, complete source code means all the source code for all modules it contains, plus any associated interface definition files, plus the scripts used to control compilation and installation of the executable. However, as a special exception, the source code distributed need not include anything that is normally distributed (in either source or binary form) with the major components (compiler, kernel, and so on) of the operating system on which the executable runs, unless that component itself accompanies the executable. If distribution of executable or object code is made by offering access to copy from a designated place, then offering equivalent access to copy the source code from the same place counts as distribution of the source code, even though third parties are not compelled to copy the source along with the object code. @item You may not copy, modify, sublicense, or distribute the Program except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense or distribute the Program is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance. @item You are not required to accept this License, since you have not signed it. However, nothing else grants you permission to modify or distribute the Program or its derivative works. These actions are prohibited by law if you do not accept this License. Therefore, by modifying or distributing the Program (or any work based on the Program), you indicate your acceptance of this License to do so, and all its terms and conditions for copying, distributing or modifying the Program or works based on it. @item Each time you redistribute the Program (or any work based on the Program), the recipient automatically receives a license from the original licensor to copy, distribute or modify the Program subject to these terms and conditions. You may not impose any further restrictions on the recipients' exercise of the rights granted herein. You are not responsible for enforcing compliance by third parties to this License. @item If, as a consequence of a court judgment or allegation of patent infringement or for any other reason (not limited to patent issues), conditions are imposed on you (whether by court order, agreement or otherwise) that contradict the conditions of this License, they do not excuse you from the conditions of this License. If you cannot distribute so as to satisfy simultaneously your obligations under this License and any other pertinent obligations, then as a consequence you may not distribute the Program at all. For example, if a patent license would not permit royalty-free redistribution of the Program by all those who receive copies directly or indirectly through you, then the only way you could satisfy both it and this License would be to refrain entirely from distribution of the Program. If any portion of this section is held invalid or unenforceable under any particular circumstance, the balance of the section is intended to apply and the section as a whole is intended to apply in other circumstances. It is not the purpose of this section to induce you to infringe any patents or other property right claims or to contest validity of any such claims; this section has the sole purpose of protecting the integrity of the free software distribution system, which is implemented by public license practices. Many people have made generous contributions to the wide range of software distributed through that system in reliance on consistent application of that system; it is up to the author/donor to decide if he or she is willing to distribute software through any other system and a licensee cannot impose that choice. This section is intended to make thoroughly clear what is believed to be a consequence of the rest of this License. @item If the distribution and/or use of the Program is restricted in certain countries either by patents or by copyrighted interfaces, the original copyright holder who places the Program under this License may add an explicit geographical distribution limitation excluding those countries, so that distribution is permitted only in or among countries not thus excluded. In such case, this License incorporates the limitation as if written in the body of this License. @item The Free Software Foundation may publish revised and/or new versions of the General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. Each version is given a distinguishing version number. If the Program specifies a version number of this License which applies to it and ``any later version'', you have the option of following the terms and conditions either of that version or of any later version published by the Free Software Foundation. If the Program does not specify a version number of this License, you may choose any version ever published by the Free Software Foundation. @item If you wish to incorporate parts of the Program into other free programs whose distribution conditions are different, write to the author to ask for permission. For software which is copyrighted by the Free Software Foundation, write to the Free Software Foundation; we sometimes make exceptions for this. Our decision will be guided by the two goals of preserving the free status of all derivatives of our free software and of promoting the sharing and reuse of software generally. @ifnotinfo @c fakenode --- for prepinfo @heading NO WARRANTY @end ifnotinfo @ifinfo @center NO WARRANTY @end ifinfo @item BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW@. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM ``AS IS'' WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE@. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU@. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION. @item IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. @end enumerate @ifnotinfo @c fakenode --- for prepinfo @heading END OF TERMS AND CONDITIONS @end ifnotinfo @ifinfo @center END OF TERMS AND CONDITIONS @end ifinfo @page @c fakenode --- for prepinfo @unnumberedsec How to Apply These Terms to Your New Programs If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms. To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the ``copyright'' line and a pointer to where the full notice is found. @smallexample @var{one line to give the program's name and an idea of what it does.} Copyright (C) @var{year} @var{name of author} This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE@. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111, USA. @end smallexample Also add information on how to contact you by electronic and paper mail. If the program is interactive, make it output a short notice like this when it starts in an interactive mode: @smallexample Gnomovision version 69, Copyright (C) @var{year} @var{name of author} Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'. This is free software, and you are welcome to redistribute it under certain conditions; type `show c' for details. @end smallexample The hypothetical commands @samp{show w} and @samp{show c} should show the appropriate parts of the General Public License. Of course, the commands you use may be called something other than @samp{show w} and @samp{show c}; they could even be mouse-clicks or menu items---whatever suits your program. You should also get your employer (if you work as a programmer) or your school, if any, to sign a ``copyright disclaimer'' for the program, if necessary. Here is a sample; alter the names: @smallexample @group Yoyodyne, Inc., hereby disclaims all copyright interest in the program `Gnomovision' (which makes passes at compilers) written by James Hacker. @var{signature of Ty Coon}, 1 April 1989 Ty Coon, President of Vice @end group @end smallexample This General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Lesser General Public License instead of this License. @node GNU Free Documentation License @unnumbered GNU Free Documentation License @cindex FDL (Free Documentation License) @cindex Free Documentation License (FDL) @cindex GNU Free Documentation License @center Version 1.2, November 2002 @display Copyright @copyright{} 2000,2001,2002 Free Software Foundation, Inc. 59 Temple Place, Suite 330, Boston, MA 02111-1307, USA Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. @end display @enumerate 0 @item PREAMBLE The purpose of this License is to make a manual, textbook, or other functional and useful document @dfn{free} in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others. This License is a kind of ``copyleft'', which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software. We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference. @item APPLICABILITY AND DEFINITIONS This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The ``Document'', below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as ``you''. You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law. A ``Modified Version'' of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language. A ``Secondary Section'' is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document's overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (Thus, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them. The ``Invariant Sections'' are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none. The ``Cover Texts'' are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may be at most 25 words. A ``Transparent'' copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, that is suitable for revising the document straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup, or absence of markup, has been arranged to thwart or discourage subsequent modification by readers is not Transparent. An image format is not Transparent if used for any substantial amount of text. A copy that is not ``Transparent'' is called ``Opaque''. Examples of suitable formats for Transparent copies include plain @sc{ascii} without markup, Texinfo input format, La@TeX{} input format, @acronym{SGML} or @acronym{XML} using a publicly available @acronym{DTD}, and standard-conforming simple @acronym{HTML}, PostScript or @acronym{PDF} designed for human modification. Examples of transparent image formats include @acronym{PNG}, @acronym{XCF} and @acronym{JPG}. 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It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document. @item MODIFICATIONS You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version: @enumerate A @item Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission. @item List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has fewer than five), unless they release you from this requirement. @item State on the Title page the name of the publisher of the Modified Version, as the publisher. @item Preserve all the copyright notices of the Document. @item Add an appropriate copyright notice for your modifications adjacent to the other copyright notices. @item Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below. @item Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document's license notice. @item Include an unaltered copy of this License. @item Preserve the section Entitled ``History'', Preserve its Title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. 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The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version. @item COMBINING DOCUMENTS You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers. The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work. In the combination, you must combine any sections Entitled ``History'' in the various original documents, forming one section Entitled ``History''; likewise combine any sections Entitled ``Acknowledgements'', and any sections Entitled ``Dedications''. You must delete all sections Entitled ``Endorsements.'' @item COLLECTIONS OF DOCUMENTS You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects. You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document. @item AGGREGATION WITH INDEPENDENT WORKS A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an ``aggregate'' if the copyright resulting from the compilation is not used to limit the legal rights of the compilation's users beyond what the individual works permit. When the Document is included an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document. If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document's Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate. @item TRANSLATION Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warrany Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail. If a section in the Document is Entitled ``Acknowledgements'', ``Dedications'', or ``History'', the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title. @item TERMINATION You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance. @item FUTURE REVISIONS OF THIS LICENSE The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See @uref{http://www.gnu.org/copyleft/}. Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License ``or any later version'' applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. @end enumerate @c fakenode --- for prepinfo @unnumberedsec ADDENDUM: How to use this License for your documents To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page: @smallexample @group Copyright (C) @var{year} @var{your name}. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''. @end group @end smallexample If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the ``with...Texts.'' line with this: @smallexample @group with the Invariant Sections being @var{list their titles}, with the Front-Cover Texts being @var{list}, and with the Back-Cover Texts being @var{list}. @end group @end smallexample If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation. If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software. @c Local Variables: @c ispell-local-pdict: "ispell-dict" @c End: @node Index @unnumbered Index @printindex cp @bye Unresolved Issues: ------------------ 1. From ADR. Robert J. Chassell points out that awk programs should have some indication of how to use them. It would be useful to perhaps have a "programming style" section of the manual that would include this and other tips. 2. The default AWKPATH search path should be configurable via `configure' The default and how this changes needs to be documented. Consistency issues: /.../ regexps are in @code, not @samp ".." strings are in @code, not @samp no @print before @dots values of expressions in the text (@code{x} has the value 15), should be in roman, not @code Use TAB and not tab Use ESC and not ESCAPE Use space and not blank to describe the space bar's character The term "blank" is thus basically reserved for "blank lines" etc. To make dark corners work, the @value{DARKCORNER} has to be outside closing `.' of a sentence and after (pxref{...}). This is a change from earlier versions. " " should have an @w{} around it Use "non-" only with language names or acronyms, or the words bug and option Use @command{ftp} when talking about anonymous ftp Use uppercase and lowercase, not "upper-case" and "lower-case" or "upper case" and "lower case" Use "single precision" and "double precision", not "single-precision" or "double-precision" Use alphanumeric, not alpha-numeric Use POSIX-compliant, not POSIX compliant Use --foo, not -Wfoo when describing long options Use "Bell Laboratories", but not "Bell Labs". Use "behavior" instead of "behaviour". Use "zeros" instead of "zeroes". Use "nonzero" not "non-zero". Use "runtime" not "run time" or "run-time". Use "command-line" not "command line". Use "online" not "on-line". Use "whitespace" not "white space". Use "Input/Output", not "input/output". Also "I/O", not "i/o". Use "lefthand"/"righthand", not "left-hand"/"right-hand". Use "workaround", not "work-around". Use "startup"/"cleanup", not "start-up"/"clean-up" Use @code{do}, and not @code{do}-@code{while}, except where actually discussing the do-while. Use "versus" in text and "vs." in index entries The words "a", "and", "as", "between", "for", "from", "in", "of", "on", "that", "the", "to", "with", and "without", should not be capitalized in @chapter, @section etc. "Into" and "How" should. Search for @dfn; make sure important items are also indexed. "e.g." should always be followed by a comma. "i.e." should always be followed by a comma. The numbers zero through ten should be spelled out, except when talking about file descriptor numbers. > 10 and < 0, it's ok to use numbers. In tables, put command-line options in @code, while in the text, put them in @option. When using @strong, use "Note:" or "Caution:" with colons and not exclamation points. Do not surround the paragraphs with @quotation ... @end quotation. For most cases, do NOT put a comma before "and", "or" or "but". But exercise taste with this rule. Don't show the awk command with a program in quotes when it's just the program. I.e. { .... } not awk '{ ... }' Do show it when showing command-line arguments, data files, etc, even if there is no output shown. Use numbered lists only to show a sequential series of steps. Use @code{xxx} for the xxx operator in indexing statements, not @samp. Date: Wed, 13 Apr 94 15:20:52 -0400 From: rms@gnu.org (Richard Stallman) To: gnu-prog@gnu.org Subject: A reminder: no pathnames in GNU It's a GNU convention to use the term "file name" for the name of a file, never "pathname". We use the term "path" for search paths, which are lists of file names. Using it for a single file name as well is potentially confusing to users. So please check any documentation you maintain, if you think you might have used "pathname". Note that "file name" should be two words when it appears as ordinary text. It's ok as one word when it's a metasyntactic variable, though. ------------------------ ORA uses filename, thus the macro. Suggestions: ------------ Enhance FIELDWIDTHS with some way to indicate "the rest of the record". E.g., a length of 0 or -1 or something. May be "n"? Make FIELDWIDTHS be an array? % Next edition: % 1. Talk about common extensions, those in nawk, gawk, mawk % 2. Use @code{foo} for variables and @code{foo()} for functions % 3. Standardize the error messages from the functions and programs % in Chapters 12 and 13. % 4. Nuke the BBS stuff and use something that won't be obsolete % 5. Reorg chapters 5 & 7 like so: %Chapter 5: % - Constants, Variables, and Conversions % + Constant Expressions % + Using Regular Expression Constants % + Variables % + Conversion of Strings and Numbers % - Operators % + Arithmetic Operators % + String Concatenation % + Assignment Expressions % + Increment and Decrement Operators % - Truth Values and Conditions % + True and False in Awk % + Boolean Expressions % + Conditional Expressions % - Function Calls % - Operator Precedence % %Chapter 7: % - Array Basics % + Introduction to Arrays % + Referring to an Array Element % + Assigning Array Elements % + Basic Array Example % + Scanning All Elements of an Array % - The delete Statement % - Using Numbers to Subscript Arrays % - Using Uninitialized Variables as Subscripts % - Multidimensional Arrays % + Scanning Multidimensional Arrays % - Sorting Array Values and Indices with gawk