\input texinfo @c -*-texinfo-*- @c %**start of header (This is for running Texinfo on a region.) @setfilename gawk.info @settitle The GAWK Manual @c @smallbook @c %**end of header (This is for running Texinfo on a region.) @ifinfo @synindex fn cp @synindex vr cp @end ifinfo @iftex @syncodeindex fn cp @syncodeindex vr cp @end iftex @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 @c ===> NOTE! <== @c Determine the edition number in *four* places by hand: @c 1. First ifinfo section 2. title page 3. copyright page 4. top node @c To find the locations, search for !!set @ifinfo This file documents @code{awk}, a program that you can use to select particular records in a file and perform operations upon them. This is Edition 0.15 of @cite{The GAWK Manual}, @* for the 2.15 version of the GNU implementation @* of AWK. Copyright (C) 1989, 1991, 1992, 1993 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. @ignore Permission is granted to process this file through TeX and print the results, provided the printed document carries copying permission notice identical to this one except for the removal of this paragraph (this paragraph not being relevant to the printed manual). @end ignore Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the Foundation. @end ifinfo @setchapternewpage odd @c !!set edition, date, version @titlepage @title The GAWK Manual @subtitle Edition 0.15 @subtitle April 1993 @author Diane Barlow Close @author Arnold D. Robbins @author Paul H. Rubin @author Richard Stallman @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 Copyright @copyright{} 1989, 1991, 1992, 1993 Free Software Foundation, Inc. @sp 2 @c !!set edition, date, version This is Edition 0.15 of @cite{The GAWK Manual}, @* for the 2.15 version of the GNU implementation @* of AWK. @sp 2 Published by the Free Software Foundation @* 675 Massachusetts Avenue @* Cambridge, MA 02139 USA @* Printed copies are available for $20 each. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the Foundation. @end titlepage @ifinfo @node Top, Preface, (dir), (dir) @comment node-name, next, previous, up @top General Introduction @c Preface or Licensing nodes should come right after the Top @c node, in `unnumbered' sections, then the chapter, `What is gawk'. This file documents @code{awk}, a program that you can use to select particular records in a file and perform operations upon them. @c !!set edition, date, version This is Edition 0.15 of @cite{The GAWK Manual}, @* for the 2.15 version of the GNU implementation @* of AWK. @end ifinfo @menu * Preface:: What you can do with @code{awk}; brief history and acknowledgements. * Copying:: Your right to copy and distribute @code{gawk}. * This Manual:: Using this manual. Includes sample input files that you can use. * Getting Started:: A basic introduction to using @code{awk}. How to run an @code{awk} program. Command line syntax. * Reading Files:: How to read files and manipulate fields. * Printing:: How to print using @code{awk}. Describes the @code{print} and @code{printf} statements. Also describes redirection of output. * One-liners:: Short, sample @code{awk} programs. * Patterns:: The various types of patterns explained in detail. * Actions:: The various types of actions are introduced here. Describes expressions and the various operators in detail. Also describes comparison expressions. * Expressions:: Expressions are the basic building blocks of statements. * Statements:: The various control statements are described in detail. * Arrays:: The description and use of arrays. Also includes array-oriented control statements. * Built-in:: The built-in functions are summarized here. * User-defined:: User-defined functions are described in detail. * Built-in Variables:: Built-in Variables * Command Line:: How to run @code{gawk}. * Language History:: The evolution of the @code{awk} language. * Installation:: Installing @code{gawk} under various operating systems. * Gawk Summary:: @code{gawk} Options and Language Summary. * Sample Program:: A sample @code{awk} program with a complete explanation. * Bugs:: Reporting Problems and Bugs. * Notes:: Something about the implementation of @code{gawk}. * Glossary:: An explanation of some unfamiliar terms. * Index:: @end menu @node Preface, Copying, Top, Top @comment node-name, next, previous, up @unnumbered Preface @iftex @cindex what is @code{awk} @end iftex If you are like many computer users, you would frequently like to make changes in various text files wherever certain patterns appear, or extract data from parts of certain lines while discarding the rest. To write a program to do this in a language such as C or Pascal is a time-consuming inconvenience that may take many lines of code. The job may be easier with @code{awk}. The @code{awk} utility interprets a special-purpose programming language that makes it possible to handle simple data-reformatting jobs easily with just a few lines of code. The GNU implementation of @code{awk} is called @code{gawk}; it is fully upward compatible with the System V Release 4 version of @code{awk}. @code{gawk} is also upward compatible with the @sc{posix} (draft) specification of the @code{awk} language. This means that all properly written @code{awk} programs should work with @code{gawk}. Thus, we usually don't distinguish between @code{gawk} and other @code{awk} implementations in this manual.@refill @cindex uses of @code{awk} This manual teaches you what @code{awk} does and how you can use @code{awk} effectively. You should already be familiar with basic system commands such as @code{ls}. Using @code{awk} you can: @refill @itemize @bullet @item manage small, personal databases @item generate reports @item validate data @item produce indexes, and perform other document preparation tasks @item even experiment with algorithms that can be adapted later to other computer languages @end itemize @iftex This manual has the difficult task of being both tutorial and 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 on-line Info version of the manual. @end iftex @menu * History:: The history of @code{gawk} and @code{awk}. Acknowledgements. @end menu @node History, , Preface, Preface @comment node-name, next, previous, up @unnumberedsec History of @code{awk} and @code{gawk} @cindex acronym @cindex history of @code{awk} The name @code{awk} comes from the initials of its designers: Alfred V. Aho, Peter J. Weinberger, and Brian W. Kernighan. The original version of @code{awk} was written in 1977. 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 generally available with System V Release 3.1. The version in System V Release 4 added some new features and also cleaned up the behavior in some of the ``dark corners'' of the language. The specification for @code{awk} in the @sc{posix} Command Language and Utilities standard further clarified the language based on feedback from both the @code{gawk} designers, and the original @code{awk} designers.@refill The GNU implementation, @code{gawk}, was written in 1986 by Paul Rubin and Jay Fenlason, with advice from Richard Stallman. John Woods contributed parts of the code as well. In 1988 and 1989, David Trueman, with help from Arnold Robbins, thoroughly reworked @code{gawk} for compatibility with the newer @code{awk}. Current development (1992) focuses on bug fixes, performance improvements, and standards compliance. We need to thank many people for their assistance in producing this manual. Jay Fenlason contributed many ideas and sample programs. Richard Mlynarik and Robert J. Chassell gave helpful comments on early drafts of this manual. The paper @cite{A Supplemental Document for @code{awk}} by John W. Pierce of the Chemistry Department at UC San Diego, pinpointed several issues relevant both to @code{awk} implementation and to this manual, that would otherwise have escaped us. David Trueman, Pat Rankin, and Michal Jaegermann also contributed sections of the manual.@refill The following people provided many helpful comments on this edition of the manual: Rick Adams, Michael Brennan, Rich Burridge, Diane Close, Christopher (``Topher'') Eliot, Michael Lijewski, Pat Rankin, Miriam Robbins, and Michal Jaegermann. Robert J. Chassell provided much valuable advice on the use of Texinfo. Finally, we would like to thank Brian Kernighan of Bell Labs for invaluable assistance during the testing and debugging of @code{gawk}, and for help in clarifying numerous points about the language.@refill @node Copying, This Manual, Preface, Top @unnumbered GNU GENERAL PUBLIC LICENSE @center Version 2, June 1991 @display Copyright @copyright{} 1989, 1991 Free Software Foundation, Inc. 675 Mass Ave, Cambridge, MA 02139, 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. @iftex @c fakenode --- for prepinfo @unnumberedsec TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION @end iftex @ifinfo @center TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION @end ifinfo @enumerate @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. 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(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. 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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. @iftex @c fakenode --- for prepinfo @heading NO WARRANTY @end iftex @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 @iftex @c fakenode --- for prepinfo @heading END OF TERMS AND CONDITIONS @end iftex @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 a brief idea of what it does.} Copyright (C) 19@var{yy} @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., 675 Mass Ave, Cambridge, MA 02139, 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) 19@var{yy} @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 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 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 Library General Public License instead of this License. @node This Manual, Getting Started, Copying, Top @chapter Using this Manual @cindex manual, using this @cindex using this manual @cindex language, @code{awk} @cindex program, @code{awk} @cindex @code{awk} language @cindex @code{awk} program The term @code{awk} refers to a particular program, and to the language you use to tell this program what to do. When we need to be careful, we call the program ``the @code{awk} utility'' and the language ``the @code{awk} language.'' The term @code{gawk} refers to a version of @code{awk} developed as part the GNU project. The purpose of this manual is to explain both the @code{awk} language and how to run the @code{awk} utility.@refill While concentrating on the features of @code{gawk}, the manual will also attempt to describe important differences between @code{gawk} and other @code{awk} implementations. In particular, any features that are not in the @sc{posix} standard for @code{awk} will be noted. @refill The term @dfn{@code{awk} program} refers to a program written by you in the @code{awk} programming language.@refill @xref{Getting Started, ,Getting Started with @code{awk}}, for the bare essentials you need to know to start using @code{awk}. Some useful ``one-liners'' are included to give you a feel for the @code{awk} language (@pxref{One-liners, ,Useful ``One-liners''}). @ignore @strong{I deleted four paragraphs here because they would confuse the beginner more than help him. They mention terms such as ``field,'' ``pattern,'' ``action,'' ``built-in function'' which the beginner doesn't know.} @strong{If you can find a way to introduce several of these concepts here, enough to give the reader a map of what is to follow, that might be useful. I'm not sure that can be done without taking up more space than ought to be used here. There may be no way to win.} @strong{ADR: I'd like to tackle this in phase 2 of my editing.} @end ignore A sample @code{awk} program has been provided for you (@pxref{Sample Program}).@refill If you find terms that you aren't familiar with, try looking them up in the glossary (@pxref{Glossary}).@refill The entire @code{awk} language is summarized for quick reference in @ref{Gawk Summary, ,@code{gawk} Summary}. Look there if you just need to refresh your memory about a particular feature.@refill Most of the time complete @code{awk} programs are used as examples, but in some of the more advanced sections, only the part of the @code{awk} program that illustrates the concept being described is shown.@refill @menu * Sample Data Files:: Sample data files for use in the @code{awk} programs illustrated in this manual. @end menu @node Sample Data Files, , This Manual, This Manual @section Data Files for the Examples @cindex input file, sample @cindex sample input file @cindex @file{BBS-list} file Many of the examples in this manual take their input from two sample data files. The first, called @file{BBS-list}, represents a list of computer bulletin board systems together with information about those systems. The second data file, called @file{inventory-shipped}, contains information about shipments on a monthly basis. Each line of these files is one @dfn{record}. In the file @file{BBS-list}, each record contains the name of a computer bulletin board, its phone number, the board's baud rate, 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 operates evening and weekend hours, only. A @samp{C} means the board operates only on weekends. @example 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 @end example @cindex @file{inventory-shipped} file The second data file, called @file{inventory-shipped}, represents information about shipments during the year. Each record contains the month of the year, 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 one year and 4 months of the next year.@refill @example 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 @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 @code{awk} (@xref{Misc File Ops, , , 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 manual. @end ifinfo @node Getting Started, Reading Files, This Manual, Top @chapter Getting Started with @code{awk} @cindex script, definition of @cindex rule, definition of @cindex program, definition of @cindex basic function of @code{gawk} The basic function of @code{awk} is to search files for lines (or other units of text) that contain certain patterns. When a line matches one of the patterns, @code{awk} performs specified actions on that line. @code{awk} keeps processing input lines in this way until the end of the input file is reached.@refill When you run @code{awk}, you specify an @code{awk} @dfn{program} which tells @code{awk} what to do. The program consists of a series of @dfn{rules}. (It may also contain @dfn{function definitions}, but that is an advanced feature, so we will ignore it for now. @xref{User-defined, ,User-defined Functions}.) Each rule specifies one pattern to search for, and one action to perform when that pattern is found. Syntactically, a rule consists of a pattern followed by an action. The action is enclosed in curly braces to separate it from the pattern. Rules are usually separated by newlines. Therefore, an @code{awk} program looks like this: @example @var{pattern} @{ @var{action} @} @var{pattern} @{ @var{action} @} @dots{} @end example @menu * Very Simple:: A very simple example. * Two Rules:: A less simple one-line example with two rules. * More Complex:: A more complex example. * Running gawk:: How to run @code{gawk} programs; includes command line syntax. * Comments:: Adding documentation to @code{gawk} programs. * Statements/Lines:: Subdividing or combining statements into lines. * When:: When to use @code{gawk} and when to use other things. @end menu @node Very Simple, Two Rules, Getting Started, Getting Started @section A Very Simple Example @cindex @samp{print $0} The following command runs a simple @code{awk} program that searches the input file @file{BBS-list} for the string of characters: @samp{foo}. (A string of characters is usually called, a @dfn{string}. The term @dfn{string} is perhaps 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 actual @code{awk} program. The slashes indicate that @samp{foo} is a pattern to search for. This type of pattern is called a @dfn{regular expression}, and is covered in more detail later (@pxref{Regexp, ,Regular Expressions as Patterns}). There are single-quotes around the @code{awk} program so that the shell won't interpret any of it as special shell characters.@refill Here is what this program prints: @example @group fooey 555-1234 2400/1200/300 B foot 555-6699 1200/300 B macfoo 555-6480 1200/300 A sabafoo 555-2127 1200/300 C @end group @end example @cindex action, default @cindex pattern, default @cindex default action @cindex default pattern In an @code{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. Thus, we could leave out the action (the @code{print} statement and the curly braces) in the above example, and the result would be the same: all lines matching the pattern @samp{foo} would be printed. By comparison, omitting the @code{print} statement but retaining the curly braces makes an empty action that does nothing; then no lines would be printed. @node Two Rules, More Complex, Very Simple, Getting Started @section An Example with Two Rules @cindex how @code{awk} works The @code{awk} utility reads the input files one line at a time. For each line, @code{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 @code{awk} program. If no patterns match, then no actions are run. After processing all the rules (perhaps none) that match the line, @code{awk} reads the next line (however, @pxref{Next Statement, ,The @code{next} Statement}). This continues until the end of the file is reached.@refill For example, the @code{awk} program: @example /12/ @{ print $0 @} /21/ @{ print $0 @} @end example @noindent contains two rules. 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 @code{awk} 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. If we run this program on our two sample data files, @file{BBS-list} and @file{inventory-shipped}, as shown here: @example awk '/12/ @{ print $0 @} /21/ @{ print $0 @}' BBS-list inventory-shipped @end example @noindent we get the following output: @example 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 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 sabafoo 555-2127 1200/300 C Jan 21 36 64 620 Apr 21 70 74 514 @end example @noindent Note how the line in @file{BBS-list} beginning with @samp{sabafoo} was printed twice, once for each rule. @node More Complex, Running gawk, Two Rules, Getting Started @comment node-name, next, previous, up @section A More Complex Example Here is an example to give you an idea of what typical @code{awk} programs do. This example shows how @code{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 '$5 == "Nov" @{ sum += $4 @} END @{ print sum @}' @end example 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). (In the C shell you would need to type a semicolon and then a backslash at the end of the first line; in a @sc{posix}-compliant shell, such as the Bourne shell or the Bourne-Again shell, you can type the example as shown.) The @w{@samp{ls -l}} part of this example is a command that gives you a listing of the files in a directory, including file size and date. Its output looks like this:@refill @example -rw-r--r-- 1 close 1933 Nov 7 13:05 Makefile -rw-r--r-- 1 close 10809 Nov 7 13:03 gawk.h -rw-r--r-- 1 close 983 Apr 13 12:14 gawk.tab.h -rw-r--r-- 1 close 31869 Jun 15 12:20 gawk.y -rw-r--r-- 1 close 22414 Nov 7 13:03 gawk1.c -rw-r--r-- 1 close 37455 Nov 7 13:03 gawk2.c -rw-r--r-- 1 close 27511 Dec 9 13:07 gawk3.c -rw-r--r-- 1 close 7989 Nov 7 13:03 gawk4.c @end example @noindent 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 contains the size of the file in bytes. The fifth, sixth, and seventh fields contain the month, day, and time, respectively, that the file was last modified. Finally, the eighth field contains the name of the file. The @code{$5 == "Nov"} in our @code{awk} program is an expression that tests whether the fifth field of the output from @w{@samp{ls -l}} matches the string @samp{Nov}. Each time a line has the string @samp{Nov} in its fifth field, the action @samp{@{ sum += $4 @}} is performed. This adds the fourth field (the file size) to the variable @code{sum}. As a result, when @code{awk} has finished reading all the input lines, @code{sum} is the sum of the sizes of files whose lines matched the pattern. (This works because @code{awk} variables are automatically initialized to zero.)@refill After the last line of output from @code{ls} has been processed, the @code{END} rule is executed, and the value of @code{sum} is printed. In this example, the value of @code{sum} would be 80600.@refill These more advanced @code{awk} techniques are covered in later sections (@pxref{Actions, ,Overview of Actions}). Before you can move on to more advanced @code{awk} programming, you have to know how @code{awk} interprets your input and displays your output. By manipulating fields and using @code{print} statements, you can produce some very useful and spectacular looking reports.@refill @node Running gawk, Comments, More Complex, Getting Started @section How to Run @code{awk} Programs @ignore Date: Mon, 26 Aug 91 09:48:10 +0200 From: gatech!vsoc07.cern.ch!matheys (Jean-Pol Matheys (CERN - ECP Division)) To: uunet.UU.NET!skeeve!arnold Subject: RE: status check The introduction of Chapter 2 (i.e. before 2.1) should include the whole of section 2.4 - it's better to tell people how to run awk programs before giving any examples ADR --- he's right. but for now, don't do this because the rest of the chapter would need some rewriting. @end ignore @cindex command line formats @cindex running @code{awk} programs There are several ways to run an @code{awk} program. If the program is short, it is easiest to include it in the command that runs @code{awk}, like this: @example awk '@var{program}' @var{input-file1} @var{input-file2} @dots{} @end example @noindent where @var{program} consists of a series of patterns and actions, as described earlier. 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 @menu * One-shot:: Running a short throw-away @code{awk} program. * Read Terminal:: Using no input files (input from terminal instead). * Long:: Putting permanent @code{awk} programs in files. * Executable Scripts:: Making self-contained @code{awk} programs. @end menu @node One-shot, Read Terminal, Running gawk, Running gawk @subsection One-shot Throw-away @code{awk} Programs Once you are familiar with @code{awk}, you will often type simple programs at the moment you want to use them. Then you can write the program as the first argument of the @code{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 quotes, why needed This command format instructs the shell to start @code{awk} and use the @var{program} to process records in the input file(s). There are single quotes around @var{program} so that the shell doesn't interpret any @code{awk} characters as special shell characters. They also cause the shell to treat all of @var{program} as a single argument for @code{awk} and allow @var{program} to be more than one line long.@refill This format is also useful for running short or medium-sized @code{awk} programs from shell scripts, because it avoids the need for a separate file for the @code{awk} program. A self-contained shell script is more reliable since there are no other files to misplace. @node Read Terminal, Long, One-shot, Running gawk @subsection Running @code{awk} without Input Files @cindex standard input @cindex input, standard You can also run @code{awk} without any input files. If you type the command line:@refill @example awk '@var{program}' @end example @noindent then @code{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{Control-d}. For example, if you execute this command: @example awk '/th/' @end example @noindent whatever you type next is taken as data for that @code{awk} program. If you go on to type the following data: @example Kathy Ben Tom Beth Seth Karen Thomas @kbd{Control-d} @end example @noindent then @code{awk} prints this output: @example Kathy Beth Seth @end example @noindent @cindex case sensitivity @cindex pattern, case sensitive as matching the pattern @samp{th}. Notice that it did not recognize @samp{Thomas} as matching the pattern. The @code{awk} language is @dfn{case sensitive}, and matches patterns exactly. (However, you can override this with the variable @code{IGNORECASE}. @xref{Case-sensitivity, ,Case-sensitivity in Matching}.) @node Long, Executable Scripts, Read Terminal, Running gawk @subsection Running Long Programs @cindex running long programs @cindex @samp{-f} option @cindex program file @cindex file, @code{awk} program Sometimes your @code{awk} programs can be very long. In this case it is more convenient to put the program into a separate file. To tell @code{awk} to use that file for its program, you type:@refill @example awk -f @var{source-file} @var{input-file1} @var{input-file2} @dots{} @end example The @samp{-f} instructs the @code{awk} utility to get the @code{awk} program from the file @var{source-file}. Any file name can be used for @var{source-file}. For example, you could put the program:@refill @example /th/ @end example @noindent into the file @file{th-prog}. Then this command: @example awk -f th-prog @end example @noindent does the same thing as this one: @example awk '/th/' @end example @noindent which was explained earlier (@pxref{Read Terminal, ,Running @code{awk} without Input Files}). Note that you don't usually need single quotes around the file name that you specify with @samp{-f}, because most file names don't contain any of the shell's special characters. Notice that in @file{th-prog}, the @code{awk} program did not have single quotes around it. The quotes are only needed for programs that are provided on the @code{awk} command line. If you want to identify your @code{awk} program files clearly as such, you can add the extension @file{.awk} to the file name. This doesn't affect the execution of the @code{awk} program, but it does make ``housekeeping'' easier. @node Executable Scripts, , Long, Running gawk @c node-name, next, previous, up @subsection Executable @code{awk} Programs @cindex executable scripts @cindex scripts, executable @cindex self contained programs @cindex program, self contained @cindex @samp{#!} Once you have learned @code{awk}, you may want to write self-contained @code{awk} scripts, using the @samp{#!} script mechanism. You can do this on many Unix systems @footnote{The @samp{#!} mechanism works on Unix systems derived from Berkeley Unix, System V Release 4, and some System V Release 3 systems.} (and someday on GNU).@refill For example, you could create a text file named @file{hello}, containing the following (where @samp{BEGIN} is a feature we have not yet discussed): @example #! /bin/awk -f # a sample awk program BEGIN @{ print "hello, world" @} @end example @noindent After making this file executable (with the @code{chmod} command), you can simply type: @example hello @end example @noindent at the shell, and the system will arrange to run @code{awk} @footnote{The line beginning with @samp{#!} lists the full pathname of an interpreter to be 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 pathname of the @code{awk} program. The rest of the argument list will either be options to @code{awk}, or data files, or both.} as if you had typed:@refill @example awk -f hello @end example @noindent Self-contained @code{awk} scripts are useful when you want to write a program which users can invoke without knowing that the program is written in @code{awk}. @cindex shell scripts @cindex scripts, shell If your system does not support the @samp{#!} mechanism, you can get a similar effect using a regular shell script. It would look something like this: @example : The colon makes sure this script is executed by the Bourne shell. awk '@var{program}' "$@@" @end example Using this technique, it is @emph{vital} to enclose the @var{program} in single quotes to protect it from interpretation by the shell. If you omit the quotes, only a shell wizard can predict the results. The @samp{"$@@"} causes the shell to forward all the command line arguments to the @code{awk} program, without interpretation. The first line, which starts with a colon, is used so that this shell script will work even if invoked by a user who uses the C shell. @c Someday: (See @cite{The Bourne Again Shell}, by ??.) @node Comments, Statements/Lines, Running gawk, Getting Started @section Comments in @code{awk} Programs @cindex @samp{#} @cindex comments @cindex use of comments @cindex documenting @code{awk} programs @cindex programs, documenting A @dfn{comment} is some text that is included in a program for the sake of human readers, and that is not really part of the program. Comments can explain what the program does, and how it works. Nearly all programming languages have provisions for comments, because programs are typically hard to understand without their extra help. In the @code{awk} language, a comment starts with the sharp sign character, @samp{#}, and continues to the end of the line. The @code{awk} language ignores the rest of a line following a sharp sign. For example, we could have put the following into @file{th-prog}:@refill @smallexample # This program finds records containing the pattern @samp{th}. This is how # you continue comments on additional lines. /th/ @end smallexample You can put comment lines into keyboard-composed throw-away @code{awk} programs also, but this usually isn't very useful; the purpose of a comment is to help you or another person understand the program at a later time.@refill @node Statements/Lines, When, Comments, Getting Started @section @code{awk} Statements versus Lines Most often, each line in an @code{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 But sometimes statements can be more than one line, and lines can contain several statements. You can split a statement into multiple lines by inserting a newline after any of the following:@refill @example , @{ ? : || && do else @end example @noindent A newline at any other point is considered the end of the statement. (Splitting lines after @samp{?} and @samp{:} is a minor @code{gawk} extension. The @samp{?} and @samp{:} referred to here is the three operand conditional expression described in @ref{Conditional Exp, ,Conditional Expressions}.)@refill @cindex backslash continuation @cindex continuation of lines 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{\}. This is allowed absolutely anywhere in the statement, even in the middle of a string or regular expression. For example: @example awk '/This program is too long, so continue it\ on the next line/ @{ print $1 @}' @end example @noindent We have generally not used backslash continuation in the sample programs in this manual. Since in @code{gawk} there is no limit on the length of a line, it is never strictly necessary; it just makes programs prettier. We have preferred to make them even more pretty by keeping the statements short. Backslash continuation is most useful when your @code{awk} program is in a separate source file, instead of typed in on the command line. You should also note that many @code{awk} implementations are more picky about where you may use backslash continuation. For maximal portability of your @code{awk} programs, it is best not to split your lines in the middle of a regular expression or a string.@refill @strong{Warning: backslash continuation does not work as described above with the C shell.} Continuation with backslash works for @code{awk} programs in files, and also for one-shot programs @emph{provided} you are using a @sc{posix}-compliant shell, such as the Bourne shell or the Bourne-again shell. But the C shell used on Berkeley Unix behaves differently! There, you must use two backslashes in a row, followed by a newline.@refill @cindex multiple statements on one line When @code{awk} statements within one rule are short, you might want to put more than one of them on a line. You do this by separating the statements with a semicolon, @samp{;}. This also applies to the rules themselves. Thus, the previous program could have been written:@refill @example /12/ @{ print $0 @} ; /21/ @{ print $0 @} @end example @noindent @strong{Note:} the requirement that rules on the same line must be separated with a semicolon is a recent change in the @code{awk} language; it was done for consistency with the treatment of statements within an action. @node When, , Statements/Lines, Getting Started @section When to Use @code{awk} @cindex when to use @code{awk} @cindex applications of @code{awk} You might wonder how @code{awk} might be useful for you. Using additional utility programs, more advanced patterns, field separators, arithmetic statements, and other selection criteria, you can produce much more complex output. The @code{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 @code{ls}. (@xref{More Complex, ,A More Complex Example}.) Programs written with @code{awk} are usually much smaller than they would be in other languages. This makes @code{awk} programs easy to compose and use. Often @code{awk} programs can be quickly composed at your terminal, used once, and thrown away. Since @code{awk} programs are interpreted, you can avoid the usually lengthy edit-compile-test-debug cycle of software development. Complex programs have been written in @code{awk}, including a complete retargetable assembler for 8-bit microprocessors (@pxref{Glossary}, for more information) and a microcode assembler for a special purpose Prolog computer. However, @code{awk}'s capabilities are strained by tasks of such complexity. If you find yourself writing @code{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 Lisp, 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 @code{awk} programs, but they are easier to maintain and usually run more efficiently.@refill @node Reading Files, Printing, Getting Started, Top @chapter Reading Input Files @cindex reading files @cindex input @cindex standard input @vindex FILENAME In the typical @code{awk} program, all input is read either from the standard input (by default the keyboard, but often a pipe from another command) or from files whose names you specify on the @code{awk} command line. If you specify input files, @code{awk} reads them in order, reading 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}).@refill The input is read in units called records, and processed by the rules one record at a time. By default, each record is one line. Each record is split automatically into fields, to make it more convenient for a rule to work on its parts. On rare occasions you will need to use the @code{getline} command, which can do explicit input from any number of files (@pxref{Getline, ,Explicit Input with @code{getline}}).@refill @menu * Records:: Controlling how data is split into records. * Fields:: An introduction to fields. * Non-Constant Fields:: Non-constant 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. * Close Input:: Closing an input file (so you can read from the beginning once more). @end menu @node Records, Fields, Reading Files, Reading Files @section How Input is Split into Records @cindex record separator The @code{awk} language divides its input into records and fields. Records are separated by a character called the @dfn{record separator}. By default, the record separator is the newline character, defining a record to be a single line of text.@refill @iftex @cindex changing the record separator @end iftex @vindex RS Sometimes you may want to use a different character to separate your records. You can use a different character by changing the built-in variable @code{RS}. The value of @code{RS} is a string that says how to separate records; the default value is @code{"\n"}, the string containing just a newline character. This is why records are, by default, single lines. @code{RS} can have any string as its value, but only the first character of the string is used as the record separator. The other characters are ignored. @code{RS} is exceptional in this regard; @code{awk} uses the full value of all its other built-in variables.@refill @ignore Someday this should be true! The value of @code{RS} is not limited to a one-character string. It can be any regular expression (@pxref{Regexp, ,Regular Expressions as Patterns}). 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, since it matches @code{RS}, is not part of either record.@refill @end ignore You can change the value of @code{RS} in the @code{awk} program with the assignment operator, @samp{=} (@pxref{Assignment Ops, ,Assignment Expressions}). The new record-separator character should be enclosed in quotation marks to make a string constant. 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 will be read with the proper separator. To do this, use the special @code{BEGIN} pattern (@pxref{BEGIN/END, ,@code{BEGIN} and @code{END} Special Patterns}). For example:@refill @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 @code{awk} program (the action with no pattern) prints each record. Since each @code{print} statement adds a newline at the end of its output, the effect of this @code{awk} program is to copy the input with each slash changed to a newline. Another way to change the record separator is on the command line, using the variable-assignment feature (@pxref{Command Line, ,Invoking @code{awk}}).@refill @example awk '@{ print $0 @}' RS="/" BBS-list @end example @noindent This sets @code{RS} to @samp{/} before processing @file{BBS-list}. 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}. @ignore @c merge the preceding paragraph and this stuff into one paragraph @c and put it in an `expert info' section. This produces correct behavior in the vast majority of cases, although the following (extreme) pipeline prints a surprising @samp{1}. (There is one field, consisting of a newline.) @example echo | awk 'BEGIN @{ RS = "a" @} ; @{ print NF @}' @end example @end ignore The empty string, @code{""} (a string of no characters), has a special meaning as the value of @code{RS}: it means that records are separated only by blank lines. @xref{Multiple Line, ,Multiple-Line Records}, for more details. @cindex number of records, @code{NR} or @code{FNR} @vindex NR @vindex FNR The @code{awk} utility 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 files. It starts at zero but is never automatically reset to zero. If you change the value of @code{RS} in the middle of an @code{awk} run, the new value is used to delimit subsequent records, but the record currently being processed (and records already processed) are not affected. @node Fields, Non-Constant Fields, Records, Reading Files @section Examining Fields @cindex examining fields @cindex fields @cindex accessing fields When @code{awk} reads an input record, the record is automatically separated or @dfn{parsed} by the interpreter into chunks called @dfn{fields}. By default, fields are separated by whitespace, like words in a line. Whitespace in @code{awk} means any string of one or more spaces and/or tabs; other characters such as newline, formfeed, and so on, that are considered whitespace by other languages are @emph{not} considered whitespace by @code{awk}.@refill 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 wish---but fields are what make simple @code{awk} programs so powerful. @cindex @code{$} (field operator) @cindex operators, @code{$} To refer to a field in an @code{awk} program, you use a dollar-sign, @samp{$}, 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. For example, suppose the following is a line of input:@refill @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{$7}, is @samp{example.}. Because there is no space between the @samp{e} and the @samp{.}, the period is considered part of the seventh field.@refill No matter how many fields there are, the last field in a record can be represented by @code{$NF}. So, in the example above, @code{$NF} would be the same as @code{$7}, which is @samp{example.}. Why this works is explained below (@pxref{Non-Constant Fields, ,Non-constant Field Numbers}). If you try to refer to a field beyond the last one, such as @code{$8} when the record has only 7 fields, you get the empty string.@refill @vindex NF @cindex number of fields, @code{NF} Plain @code{NF}, with no @samp{$}, is a built-in variable whose value is the number of fields in the current record. @code{$0}, which looks like an attempt to refer to the zeroth field, is a special case: it represents the whole input record. This is what you would use if you weren't interested in fields. Here are some more examples: @example awk '$1 ~ /foo/ @{ print $0 @}' BBS-list @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{Comparison Ops, ,Comparison Expressions}); it tests whether a string (here, the field @code{$1}) matches a given regular expression.@refill By contrast, the following example: @example awk '/foo/ @{ print $1, $NF @}' BBS-list @end example @noindent looks for @samp{foo} in @emph{the entire record} and prints the first field and the last field for each input record containing a match.@refill @node Non-Constant Fields, Changing Fields, Fields, Reading Files @section Non-constant Field Numbers The number of a field does not need to be a constant. Any expression in the @code{awk} language can be used after a @samp{$} 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:@refill @example awk '@{ print $NR @}' @end example @noindent Recall that @code{NR} is the number of records read so far: 1 in the first record, 2 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 $(2*2) @}' BBS-list @end example The @code{awk} language must evaluate the expression @code{(2*2)} and use its value as the number of the field to print. The @samp{*} sign represents multiplication, so the expression @code{2*2} evaluates to 4. The parentheses are used so that the multiplication is done before the @samp{$} 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}.@refill If the field number you compute is zero, you get the entire record. Thus, @code{$(2-2)} has the same value as @code{$0}. Negative field numbers are not allowed. The number of fields in the current record is stored in the built-in variable @code{NF} (@pxref{Built-in Variables}). The expression @code{$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, Field Separators, Non-Constant Fields, Reading Files @section Changing the Contents of a Field @cindex field, changing contents of @cindex changing contents of a field @cindex assignment to fields You can change the contents of a field as seen by @code{awk} within an @code{awk} program; this changes what @code{awk} perceives as the current input record. (The actual input is untouched: @code{awk} never modifies the input file.) Consider this example: @smallexample awk '@{ $3 = $2 - 10; print $2, $3 @}' inventory-shipped @end smallexample @noindent The @samp{-} sign represents subtraction, so this program reassigns field three, @code{$3}, to be the value of field two minus ten, @code{$2 - 10}. (@xref{Arithmetic Ops, ,Arithmetic Operators}.) Then field two, and the new value for field three, are printed. In order for this to work, the text in field @code{$2} must make sense as a number; the string of characters must be converted to a number in order for the computer to do arithmetic on it. The number resulting from the subtraction is converted back to a string of characters which then becomes field three. @xref{Conversion, ,Conversion of Strings and Numbers}.@refill When you change the value of a field (as perceived by @code{awk}), the text of the input record is recalculated to contain the new field where the old one was. Therefore, @code{$0} changes to reflect the altered field. Thus, @smallexample awk '@{ $2 = $2 - 10; print $0 @}' inventory-shipped @end smallexample @noindent prints a copy of the input file, with 10 subtracted from the second field of each line. You can also assign contents to fields that are out of range. For example: @smallexample awk '@{ $6 = ($5 + $4 + $3 + $2) ; print $6 @}' inventory-shipped @end smallexample @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 the internal @code{awk} copy of the current input record---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. This recomputation affects and is affected by several features not yet discussed, in particular, the @dfn{output field separator}, @code{OFS}, which is used to separate the fields (@pxref{Output Separators}), and @code{NF} (the number of fields; @pxref{Fields, ,Examining Fields}). For example, the value of @code{NF} is set to the number of the highest field you create.@refill Note, however, that merely @emph{referencing} an out-of-range field does @emph{not} change the value of either @code{$0} or @code{NF}. Referencing an out-of-range field merely produces a null string. For example:@refill @smallexample if ($(NF+1) != "") print "can't happen" else print "everything is normal" @end smallexample @noindent should print @samp{everything is normal}, because @code{NF+1} is certain to be out of range. (@xref{If Statement, ,The @code{if} Statement}, for more information about @code{awk}'s @code{if-else} statements.)@refill It is important to note that assigning to a field will change the value of @code{$0}, but will not change the value of @code{NF}, even when you assign the null string to a field. For example: @smallexample echo a b c d | awk '@{ OFS = ":"; $2 = "" ; print ; print NF @}' @end smallexample @noindent prints @smallexample a::c:d 4 @end smallexample @noindent The field is still there, it just has an empty value. You can tell because there are two colons in a row. @node Field Separators, Constant Size, Changing Fields, Reading Files @section Specifying how Fields are Separated @vindex FS @cindex fields, separating @cindex field separator, @code{FS} @cindex @samp{-F} option (This section is rather long; it describes one of the most fundamental operations in @code{awk}. If you are a novice with @code{awk}, we recommend that you re-read this section after you have studied the section on regular expressions, @ref{Regexp, ,Regular Expressions as Patterns}.) The way @code{awk} splits an input record into fields is controlled by the @dfn{field separator}, which is a single character or a regular expression. @code{awk} scans the input record for matches for the separator; the fields themselves are the text between the matches. For example, if the field separator is @samp{oo}, then the following line: @smallexample moo goo gai pan @end smallexample @noindent would be split into three fields: @samp{m}, @samp{@ g} and @samp{@ gai@ pan}. The field separator is represented by the built-in variable @code{FS}. Shell programmers take note! @code{awk} does not use the name @code{IFS} which is used by the shell.@refill You can change the value of @code{FS} in the @code{awk} program with the assignment operator, @samp{=} (@pxref{Assignment Ops, ,Assignment Expressions}). 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 will be read with the proper separator. To do this, use the special @code{BEGIN} pattern (@pxref{BEGIN/END, ,@code{BEGIN} and @code{END} Special Patterns}). For example, here we set the value of @code{FS} to the string @code{","}:@refill @smallexample awk 'BEGIN @{ FS = "," @} ; @{ print $2 @}' @end smallexample @noindent Given the input line, @smallexample John Q. Smith, 29 Oak St., Walamazoo, MI 42139 @end smallexample @noindent this @code{awk} program extracts the string @samp{@ 29 Oak St.}. @cindex field separator, choice of @cindex regular expressions as field separators Sometimes your input data will contain separator characters that don't separate fields the way you thought they would. For instance, the person's name in the example we've been using might have a title or suffix attached, such as @samp{John Q. Smith, LXIX}. From input containing such a name: @smallexample John Q. Smith, LXIX, 29 Oak St., Walamazoo, MI 42139 @end smallexample @noindent the previous sample program would extract @samp{@ LXIX}, instead of @samp{@ 29 Oak St.}. If you were expecting the program to print the address, you would be surprised. So choose your data layout and separator characters carefully to prevent such problems. As you know, by default, fields are separated by whitespace sequences (spaces and tabs), not by single spaces: two spaces in a row do not delimit an empty field. The default value of the field separator is a string @w{@code{" "}} containing a single space. If this value were interpreted 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 which does not follow these rules. More generally, the value of @code{FS} may be a string containing any regular expression. Then each match in the record for the regular expression separates fields. For example, the assignment:@refill @smallexample FS = ", \t" @end smallexample @noindent makes every area of an input line that consists of a comma followed by a space and a tab, into a field separator. (@samp{\t} stands for a tab.)@refill For a less trivial example of a regular expression, suppose you want single spaces to separate fields the way single commas were used above. You can set @code{FS} to @w{@code{"[@ ]"}}. This regular expression matches a single space and nothing else. @c the following index entry is an overfull hbox. --mew 30jan1992 @cindex field separator: on command line @cindex command line, setting @code{FS} on @code{FS} can be set on the command line. You use the @samp{-F} argument to do so. For example: @smallexample awk -F, '@var{program}' @var{input-files} @end smallexample @noindent sets @code{FS} to be the @samp{,} character. Notice that the argument uses a capital @samp{F}. Contrast this with @samp{-f}, which specifies a file containing an @code{awk} program. Case is significant in command options: the @samp{-F} and @samp{-f} options have nothing to do with each other. You can use both options at the same time to set the @code{FS} argument @emph{and} get an @code{awk} program from a file.@refill @c begin expert info The value used for the argument to @samp{-F} is processed in exactly the same way as assignments to the built-in variable @code{FS}. This means that if the field separator contains special characters, they must be escaped appropriately. For example, to use a @samp{\} as the field separator, you would have to type: @smallexample # same as FS = "\\" awk -F\\\\ '@dots{}' files @dots{} @end smallexample @noindent Since @samp{\} is used for quoting in the shell, @code{awk} will see @samp{-F\\}. Then @code{awk} processes the @samp{\\} for escape characters (@pxref{Constants, ,Constant Expressions}), finally yielding a single @samp{\} to be used for the field separator. @c end expert info As a special case, in compatibility mode (@pxref{Command Line, ,Invoking @code{awk}}), if the argument to @samp{-F} is @samp{t}, then @code{FS} is set to the tab character. (This is because if you type @samp{-F\t}, without the quotes, at the shell, the @samp{\} gets deleted, so @code{awk} figures that you really want your fields to be separated with tabs, and not @samp{t}s. Use @samp{-v FS="t"} on the command line if you really do want to separate your fields with @samp{t}s.)@refill For example, let's use an @code{awk} program file called @file{baud.awk} that contains the pattern @code{/300/}, and the action @samp{print $1}. Here is the program: @smallexample /300/ @{ print $1 @} @end smallexample 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:@refill @smallexample awk -F- -f baud.awk BBS-list @end smallexample @noindent It produces this output: @smallexample aardvark 555 alpo barfly 555 bites 555 camelot 555 core 555 fooey 555 foot 555 macfoo 555 sdace 555 sabafoo 555 @end smallexample @noindent Note the second line of output. If you check the original file, you will see that the second line looked like this: @smallexample alpo-net 555-3412 2400/1200/300 A @end smallexample 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. The following program searches the system password file, and prints the entries for users who have no password: @smallexample awk -F: '$2 == ""' /etc/passwd @end smallexample @noindent Here we use the @samp{-F} option on the command line to set the field separator. Note that fields in @file{/etc/passwd} are separated by colons. The second field represents a user's encrypted password, but if the field is empty, that user has no password. @c begin expert info According to the @sc{posix} standard, @code{awk} is supposed to behave as if each record is split into fields at the time that it is read. In particular, this means that you can change the value of @code{FS} after a record is read, but before any of the fields are referenced. The value of the fields (i.e. how they were split) should reflect the old value of @code{FS}, not the new one. However, many implementations of @code{awk} do not do this. Instead, they defer splitting the fields until a field reference actually happens, using the @emph{current} value of @code{FS}! This behavior can be difficult to diagnose. The following example illustrates the results of the two methods. (The @code{sed} command prints just the first line of @file{/etc/passwd}.) @smallexample sed 1q /etc/passwd | awk '@{ FS = ":" ; print $1 @}' @end smallexample @noindent will usually print @smallexample root @end smallexample @noindent on an incorrect implementation of @code{awk}, while @code{gawk} will print something like @smallexample root:nSijPlPhZZwgE:0:0:Root:/: @end smallexample @c end expert info @c begin expert info There is an important difference between the two cases of @samp{FS = @w{" "}} (a single blank) and @samp{FS = @w{"[ \t]+"}} (which is a regular expression matching one or more blanks or tabs). For both values of @code{FS}, fields are separated by runs of blanks and/or tabs. However, when the value of @code{FS} is @code{" "}, @code{awk} will strip leading and trailing whitespace from the record, and then decide where the fields are. For example, the following expression prints @samp{b}: @smallexample echo ' a b c d ' | awk '@{ print $2 @}' @end smallexample @noindent However, the following prints @samp{a}: @smallexample echo ' a b c d ' | awk 'BEGIN @{ FS = "[ \t]+" @} ; @{ print $2 @}' @end smallexample @noindent In this case, the first field is null. The stripping of leading and trailing whitespace also comes into play whenever @code{$0} is recomputed. For instance, this pipeline @smallexample echo ' a b c d' | awk '@{ print; $2 = $2; print @}' @end smallexample @noindent produces this output: @smallexample a b c d a b c d @end smallexample @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}. Since 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 end expert info The following table summarizes how fields are split, based on the value of @code{FS}. @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 single character} Fields are separated by each occurrence of the character. Multiple successive occurrences delimit empty fields, as do leading and trailing occurrences. @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. @end table @node Constant Size, Multiple Line, Field Separators, Reading Files @section Reading Fixed-width Data (This section discusses an advanced, experimental feature. If you are a novice @code{awk} user, you may wish to skip it on the first reading.) @code{gawk} 2.13 introduced a new facility for dealing with fixed-width fields with no distinctive field separator. Data of this nature arises typically in one of at least two ways: the input for old FORTRAN programs where numbers are run together, and 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, @code{awk}'s normal field splitting based on @code{FS} will not work well in this case. (Although a portable @code{awk} program can use a series of @code{substr} calls on @code{$0}, this is awkward and inefficient for a large number of fields.)@refill 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. The following data is the output of the @code{w} utility. It is useful to illustrate the use of @code{FIELDWIDTHS}. @smallexample 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 smallexample 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. (This program uses a number of @code{awk} features that haven't been introduced yet.)@refill @smallexample 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 smallexample Here is the result of running the program on the data: @smallexample 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 smallexample Another (possibly more practical) example of fixed-width input data would be the input from a deck of balloting cards. In some parts of the United States, voters make 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. Since a voter may choose not to vote on some issue, any column on the card may be empty. An @code{awk} program for processing such data could use the @code{FIELDWIDTHS} feature to simplify reading the data.@refill @c of course, getting gawk to run on a system with card readers is @c another story! This feature is still experimental, and will likely evolve over time. @node Multiple Line, Getline, Constant Size, Reading Files @section Multiple-Line Records @cindex multiple line records @cindex input, multiple line records @cindex reading files, multiple line records @cindex records, multiple line In some data bases, a single line cannot conveniently hold all the information in one entry. In such cases, you can use multi-line records. The first step in doing this is to choose your data format: when records are not defined as single lines, how do you want to define them? What should separate records? One technique is to use an unusual character or string to separate records. For example, you could use the formfeed character (written @code{\f} in @code{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.@refill @ignore Another technique is to have blank lines separate records. The string @code{"^\n+"} is a regular expression that matches any sequence of newlines starting at the beginning of a line---in other words, it matches a sequence of blank lines. If you set @code{RS} to this string, a record always ends at the first blank line encountered. In addition, a regular expression always matches the longest possible sequence when there is a choice. 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. @end ignore Another technique is to have blank lines separate records. By a special dispensation, a null string as the value of @code{RS} indicates that records are separated by one or more blank lines. If you set @code{RS} to the null string, a record always ends at the first blank line encountered. And 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. (End of file is also considered a record separator.)@refill @c !!! This use of `end of file' is confusing. Needs to be clarified. The second step is to separate the fields in the record. One way to do this is to put each field on a separate line: to do this, just set the variable @code{FS} to the string @code{"\n"}. (This simple regular expression matches a single newline.) Another way to separate fields is to divide each of the lines into fields in the normal manner. This happens by default as a result of a special feature: when @code{RS} is set to the null string, the newline character @emph{always} acts as a field separator. This is in addition to whatever field separations result from @code{FS}. The original motivation for this special exception was probably so that you get useful behavior in the default case (i.e., @w{@code{FS == " "}}). This feature can be a problem if you really don't want the newline character to separate fields, since 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, ,Built-in Functions for String Manipulation}).@refill @ignore Here are two ways to use records separated by blank lines and break each line into fields normally: @example awk 'BEGIN @{ RS = ""; FS = "[ \t\n]+" @} @{ print $1 @}' BBS-list @exdent @r{or} awk 'BEGIN @{ RS = "^\n+"; FS = "[ \t\n]+" @} @{ print $1 @}' BBS-list @end example @end ignore @ignore Here is how to use records separated by blank lines and break each line into fields normally: @example awk 'BEGIN @{ RS = ""; FS = "[ \t\n]+" @} ; @{ print $1 @}' BBS-list @end example @end ignore @node Getline, Close Input, Multiple Line, Reading Files @section Explicit Input with @code{getline} @findex getline @cindex input, explicit @cindex explicit input @cindex input, @code{getline} command @cindex reading files, @code{getline} command So far we have been getting our input files from @code{awk}'s main input stream---either the standard input (usually your terminal) or the files specified on the command line. The @code{awk} language has a special built-in command called @code{getline} that can be used to read input under your explicit control.@refill This command is quite complex and should @emph{not} be used by beginners. It is covered here because this is the chapter on input. 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 manual and have a good knowledge of how @code{awk} works. @vindex ERRNO @cindex differences: @code{gawk} and @code{awk} @code{getline} returns 1 if it finds a record, and 0 if the end of the file is encountered. 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, @code{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. @table @code @item getline 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 you want to do some special processing @emph{right now} on the next record. Here's an example:@refill @example awk '@{ if (t = index($0, "/*")) @{ if (t > 1) tmp = substr($0, 1, t - 1) else tmp = "" u = index(substr($0, t + 2), "*/") while (u == 0) @{ getline t = -1 u = index($0, "*/") @} if (u <= length($0) - 2) $0 = tmp substr($0, t + u + 3) else $0 = tmp @} print $0 @}' @end example This @code{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, like searching for matches of a regular expression. (This program has a subtle problem---can you spot it?) @c the program to remove comments doesn't work if one @c comment ends and another begins on the same line. (Your @c idea for restart would be useful here). --- brennan@boeing.com This form of the @code{getline} command sets @code{NF} (the number of fields; @pxref{Fields, ,Examining Fields}), @code{NR} (the number of records read so far; @pxref{Records, ,How Input is Split into Records}), @code{FNR} (the number of records read from this input file), and the value of @code{$0}. @strong{Note:} the new value of @code{$0} is used in testing the patterns of any subsequent rules. The original value of @code{$0} that triggered the rule which 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, ,The @code{next} Statement}. @item getline @var{var} This form of @code{getline} reads a record into the variable @var{var}. This is useful when you want your program to read the next record from the current input file, but you don't want to subject the record to the normal input processing. For example, suppose the next line is a comment, or a special string, and you want to read it, but you must make certain that it won't trigger any rules. This version 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 @code{awk} never sees it. The following example swaps every two lines of input. For example, given: @example wan tew free phore @end example @noindent it outputs: @example tew wan phore free @end example @noindent Here's the program: @example @group awk '@{ if ((getline tmp) > 0) @{ print tmp print $0 @} else print $0 @}' @end group @end example The @code{getline} function 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.@refill @item getline < @var{file} @cindex input redirection @cindex redirection of input This form of the @code{getline} function takes its input from the file @var{file}. Here @var{file} is a string-valued expression that specifies the file name. @samp{< @var{file}} is called a @dfn{redirection} since it directs input to come from a different place. This form is useful if you want to read your input from a particular file, instead of from the main input stream. For example, the following program reads its input record from the file @file{foo.input} when it encounters a first field with a value equal to 10 in the current input file.@refill @example awk '@{ if ($1 == 10) @{ getline < "foo.input" print @} else print @}' @end example Since the main input stream is not used, the values of @code{NR} and @code{FNR} are not changed. But the record read is split into fields in the normal manner, so the values of @code{$0} and other fields are changed. So is the value of @code{NF}. This does not cause the record to be tested against all the patterns in the @code{awk} program, in the way that would happen if the record were read normally by the main processing loop of @code{awk}. However the new record is tested against any subsequent rules, just as when @code{getline} is used without a redirection. @item getline @var{var} < @var{file} This form of the @code{getline} function takes its input from the file @var{file} and puts 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}.@refill @example awk '@{ 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 from the data, from the second field on the @samp{@@include} line.@refill 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 Input, ,Closing Input Files and Pipes}.@refill One deficiency of this program is that it does not process nested @samp{@@include} statements the way a true macro preprocessor would. @item @var{command} | getline You can @dfn{pipe} the output of a command into @code{getline}. A pipe is simply a way to link the output of one program to the input of another. In this case, the string @var{command} is run as a shell command and its output is piped into @code{awk} to be used as input. This form of @code{getline} reads one record from the pipe. For example, the following program copies input to 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 awk '@{ if ($1 == "@@execute") @{ tmp = substr($0, 10) while ((tmp | getline) > 0) print close(tmp) @} else print @}' @end example @noindent 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. @xref{Close Input, ,Closing Input Files and Pipes}. Given the input: @example foo bar baz @@execute who bletch @end example @noindent the program might produce: @example foo bar baz hack ttyv0 Jul 13 14:22 hack ttyp0 Jul 13 14:23 (gnu:0) hack ttyp1 Jul 13 14:23 (gnu:0) hack ttyp2 Jul 13 14:23 (gnu:0) hack ttyp3 Jul 13 14:23 (gnu:0) bletch @end example @noindent Notice that this program ran the command @code{who} and printed the result. (If you try this program yourself, you will get different results, showing you 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. @item @var{command} | getline @var{var} The output of the command @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 @code{date} utility, and then prints it.@refill @example awk '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. @end table @node Close Input, , Getline, Reading Files @section Closing Input Files and Pipes @cindex closing input files and pipes @findex close If the same file name or the same shell command is used with @code{getline} more than once during the execution of an @code{awk} program, the file is opened (or the command is executed) only the first time. 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 in @code{getline}, another record is read from it, and so on. This implies that if you want to start reading the same file again from the beginning, or if you want to rerun a shell command (rather than reading more output from the command), you must take special steps. What you must do is use the @code{close} function, as follows: @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 exactly equal the string that was used to open the file or start the command---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 will reopen the file or rerun the command. @iftex @vindex ERRNO @cindex differences: @code{gawk} and @code{awk} @end iftex @code{close} returns a value of zero if the close succeeded. Otherwise, the value will be non-zero. In this case, @code{gawk} sets the variable @code{ERRNO} to a string describing the error that occurred. @node Printing, One-liners, Reading Files, Top @chapter Printing Output @cindex printing @cindex output One of the most common things that actions do is to output or @dfn{print} some or all of the input. For simple output, use the @code{print} statement. For fancier formatting use the @code{printf} statement. Both are described in this chapter. @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 @code{gawk}. @code{gawk} allows access to inherited file descriptors. @end menu @node Print, Print Examples, Printing, Printing @section The @code{print} Statement @cindex @code{print} statement The @code{print} statement does output with simple, standardized formatting. You specify only the strings or numbers to be printed, 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 optionally be enclosed in parentheses. The parentheses are necessary if any of the item expressions uses a relational operator; otherwise it could be confused with a redirection (@pxref{Redirection, ,Redirecting Output of @code{print} and @code{printf}}). The relational operators are @samp{==}, @samp{!=}, @samp{<}, @samp{>}, @samp{>=}, @samp{<=}, @samp{~} and @samp{!~} (@pxref{Comparison Ops, ,Comparison Expressions}).@refill The items printed can be constant strings or numbers, fields of the current record (such as @code{$1}), variables, or any @code{awk} expressions. The @code{print} statement is completely general for computing @emph{what} values to print. With two exceptions, you cannot specify @emph{how} to print them---how many columns, whether to use exponential notation or not, and so on. (@xref{Output Separators}, and @ref{OFMT, ,Controlling Numeric Output with @code{print}}.) For that, you need the @code{printf} statement (@pxref{Printf, ,Using @code{printf} Statements for Fancier Printing}).@refill 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 null, or empty, string. To print a fixed piece of text, use a string constant such as @w{@code{"Hello there"}} as one item. If you forget to use the double-quote characters, your text will be taken as an @code{awk} expression, and you will probably get an error. Keep in mind that a space is printed between any two items. Most often, each @code{print} statement makes one line of output. But it isn't limited to 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} can make any number of lines this way. @node Print Examples, Output Separators, Print, Printing @section Examples of @code{print} Statements Here is an example of printing a string that contains embedded newlines: @example awk 'BEGIN @{ print "line one\nline two\nline three" @}' @end example @noindent produces output like this: @example line one line two line three @end example Here is an example that prints the first two fields of each input record, with a space between them: @example awk '@{ print $1, $2 @}' inventory-shipped @end example @noindent Its output looks like this: @example Jan 13 Feb 15 Mar 15 @dots{} @end example 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 @code{awk} means to concatenate them. For example, without the comma: @example awk '@{ print $1 $2 @}' inventory-shipped @end example @noindent prints: @example @group Jan13 Feb15 Mar15 @dots{} @end group @end example Neither example's output makes much sense to someone unfamiliar with the file @file{inventory-shipped}. 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, ,@code{BEGIN} and @code{END} Special Patterns}) to force the headings to be printed only once: @example awk 'BEGIN @{ print "Month Crates" print "----- ------" @} @{ print $1, $2 @}' inventory-shipped @end example @noindent Did you already guess what happens? This program prints the following: @example @group Month Crates ----- ------ Jan 13 Feb 15 Mar 15 @dots{} @end group @end example @noindent The headings and the table data don't line up! We can fix this by printing some spaces between the two fields: @example awk 'BEGIN @{ print "Month Crates" print "----- ------" @} @{ print $1, " ", $2 @}' inventory-shipped @end example You can imagine that this way of lining up columns can get pretty complicated when you have many columns to fix. Counting spaces for two or three columns can be simple, but more than this and you can get ``lost'' quite easily. This is why the @code{printf} statement was created (@pxref{Printf, ,Using @code{printf} Statements for Fancier Printing}); one of its specialties is lining up columns of data.@refill @node Output Separators, OFMT, Print Examples, Printing @section Output Separators @cindex output field separator, @code{OFS} @vindex OFS @vindex ORS @cindex output record separator, @code{ORS} 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. But they do not have to be spaces; a single space is only the default. You can specify any string of characters to use 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, just a single space.@refill 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}. The built-in variable @code{ORS} specifies this string. The initial value of the variable is the string @code{"\n"} containing a newline character; thus, normally each @code{print} statement makes a separate line. You can change how output fields and records are separated by assigning new values to the variables @code{OFS} and/or @code{ORS}. The usual place to do this is in the @code{BEGIN} rule (@pxref{BEGIN/END, ,@code{BEGIN} and @code{END} Special Patterns}), so that it happens before any input is processed. You may also do this with assignments on the command line, before the names of your input files.@refill The following example prints the first and second fields of each input record separated by a semicolon, with a blank line added after each line:@refill @example @group awk 'BEGIN @{ OFS = ";"; ORS = "\n\n" @} @{ print $1, $2 @}' BBS-list @end group @end example If the value of @code{ORS} does not contain a newline, all your output will be run together on a single line, unless you output newlines some other way. @node OFMT, Printf, Output Separators, Printing @section Controlling Numeric Output with @code{print} @vindex OFMT When you use the @code{print} statement to print numeric values, @code{awk} internally converts the number to a string of characters, and prints that string. @code{awk} uses the @code{sprintf} function to do this conversion. 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 that numbers can be formatted. The different format specifications are discussed more fully in @ref{Printf, ,Using @code{printf} Statements for Fancier Printing}.@refill 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. By supplying different format specifications as the value of @code{OFMT}, you can change how @code{print} will print your numbers. As a brief example: @example @group awk 'BEGIN @{ OFMT = "%d" # print numbers as integers print 17.23 @}' @end group @end example @noindent will print @samp{17}. @node Printf, Redirection, OFMT, Printing @section Using @code{printf} Statements for Fancier Printing @cindex formatted output @cindex output, formatted If you want more precise control over the output format than @code{print} gives you, use @code{printf}. With @code{printf} you can specify the width to use for each item, and you can specify various stylistic choices for numbers (such as what radix to use, whether to print an exponent, whether to print a sign, and how many digits to print after the decimal point). You do this by specifying a string, called the @dfn{format string}, which 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, Control Letters, Printf, Printf @subsection Introduction to the @code{printf} Statement @cindex @code{printf} statement, syntax of The @code{printf} statement looks like this:@refill @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 uses a relational operator; otherwise it could be confused with a redirection (@pxref{Redirection, ,Redirecting Output of @code{print} and @code{printf}}). The relational operators are @samp{==}, @samp{!=}, @samp{<}, @samp{>}, @samp{>=}, @samp{<=}, @samp{~} and @samp{!~} (@pxref{Comparison Ops, ,Comparison Expressions}).@refill @cindex format string The difference between @code{printf} and @code{print} is the argument @var{format}. 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 the same as in the @sc{ansi} C library function @code{printf}. Most of @var{format} is text to be output verbatim. Scattered among this text are @dfn{format specifiers}, one per item. Each format specifier says to output the next item at that place in the format.@refill The @code{printf} statement does not automatically append a newline to its output. It outputs only what the format specifies. So if you want a newline, you must include one in the format. The output separator variables @code{OFS} and @code{ORS} have no effect on @code{printf} statements.@refill @node Control Letters, Format Modifiers, Basic Printf, Printf @subsection Format-Control Letters @cindex @code{printf}, format-control characters @cindex format specifier 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. (If you actually want to output a @samp{%}, write @samp{%%}.) The format-control letter specifies what kind of value to print. The rest of the format specifier is made up of optional @dfn{modifiers} which are parameters such as the field width to use.@refill Here is a list of the format-control letters: @table @samp @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 This prints a decimal integer. @item i This also prints a decimal integer. @item e This prints a number in scientific (exponential) notation. For example, @example printf "%4.3e", 1950 @end example @noindent prints @samp{1.950e+03}, with a total of four significant figures of which three follow the decimal point. The @samp{4.3} are @dfn{modifiers}, discussed below. @item f This prints a number in floating point notation. @item g This prints a number in either scientific notation or floating point notation, whichever uses fewer characters. @ignore From: gatech!ames!elroy!cit-vax!EQL.Caltech.Edu!rankin (Pat Rankin) In the description of printf formats (p.43), the information for %g is incorrect (mainly, it's too much of an oversimplification). It's wrong in the AWK book too, and in the gawk man page. I suggested to David Trueman before 2.13 was released that the latter be revised, so that it matched gawk's behavior (rather than trying to change gawk to match the docs ;-). The documented description is nice and simple, but it doesn't match the actual underlying behavior of %g in the various C run-time libraries that gawk relies on. The precision value for g format is different than for f and e formats, so it's inaccurate to say 'g' is the shorter of 'e' or 'f'. For 'g', precision represents the number of significant digits rather than the number of decimal places, and it has special rules about how to format numbers with range between 10E-1 and 10E-4. All in all, it's pretty messy, and I had to add that clumsy GFMT_WORKAROUND code because the VMS run-time library doesn't conform to the ANSI-C specifications. @end ignore @item o This prints an unsigned octal integer. @item s This prints a string. @item x This prints an unsigned hexadecimal integer. @item X This prints an unsigned hexadecimal integer. However, for the values 10 through 15, it uses the letters @samp{A} through @samp{F} instead of @samp{a} through @samp{f}. @item % This isn't really a format-control letter, but it does have a meaning when used after a @samp{%}: the sequence @samp{%%} outputs one @samp{%}. It does not consume an argument. @end table @node Format Modifiers, Printf Examples, Control Letters, Printf @subsection Modifiers for @code{printf} Formats @cindex @code{printf}, modifiers @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 and how much space it gets. The modifiers come between the @samp{%} and the format-control letter. Here are the possible modifiers, in the order in which they may appear: @table @samp @item - The minus sign, used before the width modifier, 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 }. @item @var{width} This is a number representing the desired width of a field. Inserting any number between the @samp{%} sign and the format control character forces the field to be expanded 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{ 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, @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} This is a number that specifies the precision to use when printing. This specifies the number of digits you want printed to the right of the decimal point. For a string, it specifies the maximum number of characters from the string that should be printed. @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, you pass them in the argument list. For example:@refill @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>}}. (We have used the bullet symbol ``@bullet{}'' to represent a space, to clearly show you that there are two spaces in the output.)@refill Earlier versions of @code{awk} did not support this capability. You may simulate it by using concatenation to build up the format string, like so:@refill @example w = 5 p = 3 s = "abcdefg" printf "<%" w "." p "s>\n", s @end example @noindent This is not particularly easy to read, however. @node Printf Examples, , Format Modifiers, Printf @subsection Examples of Using @code{printf} Here is how to use @code{printf} to make an aligned table: @example awk '@{ printf "%-10s %s\n", $1, $2 @}' BBS-list @end example @noindent prints the names of bulletin boards (@code{$1}) of the file @file{BBS-list} as a string of 10 characters, left justified. It also prints the phone numbers (@code{$2}) afterward on the line. This produces an aligned two-column table of names and phone numbers:@refill @example @group aardvark 555-5553 alpo-net 555-3412 barfly 555-7685 bites 555-1675 camelot 555-0542 core 555-2912 fooey 555-1234 foot 555-6699 macfoo 555-6480 sdace 555-3430 sabafoo 555-2127 @end group @end example Did you notice that we did not specify that the phone numbers be printed as numbers? They had to be printed as strings because the numbers are separated by a dash. This dash would be interpreted as a minus sign if we had tried to print the phone numbers as numbers. This would have led to some pretty confusing results. We did not specify a width for the phone numbers because they are the last things on their lines. We don't need to put spaces after them. We could make our table look even nicer by adding headings to the tops of the columns. To do this, use the @code{BEGIN} pattern (@pxref{BEGIN/END, ,@code{BEGIN} and @code{END} Special Patterns}) to force the header to be printed only once, at the beginning of the @code{awk} program:@refill @example @group awk 'BEGIN @{ print "Name Number" print "---- ------" @} @{ printf "%-10s %s\n", $1, $2 @}' BBS-list @end group @end example Did you notice that we mixed @code{print} and @code{printf} statements in the above example? We could have used just @code{printf} statements to get the same results: @example @group awk 'BEGIN @{ printf "%-10s %s\n", "Name", "Number" printf "%-10s %s\n", "----", "------" @} @{ printf "%-10s %s\n", $1, $2 @}' BBS-list @end group @end example @noindent By outputting each column heading with the same format specification used for the elements of the column, we have made sure 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 See if you can use the @code{printf} statement to line up the headings and table data for our @file{inventory-shipped} example covered earlier in the section on the @code{print} statement (@pxref{Print, ,The @code{print} Statement}).@refill @node Redirection, Special Files, Printf, Printing @section Redirecting Output of @code{print} and @code{printf} @cindex output redirection @cindex redirection of output So far we have been dealing only with output that prints to the standard output, usually your terminal. Both @code{print} and @code{printf} can also send their output to other places. This is called @dfn{redirection}.@refill A redirection appears after the @code{print} or @code{printf} statement. Redirections in @code{awk} are written just like redirections in shell commands, except that they are written inside the @code{awk} program. @menu * File/Pipe Redirection:: Redirecting Output to Files and Pipes. * Close Output:: How to close output files and pipes. @end menu @node File/Pipe Redirection, Close Output, Redirection, Redirection @subsection Redirecting Output to Files and Pipes Here are the three forms of output redirection. They are all shown for the @code{print} statement, but they work identically for @code{printf} also.@refill @table @code @item print @var{items} > @var{output-file} This type of redirection prints the items onto the output file @var{output-file}. The file name @var{output-file} can be any expression. Its value is changed to a string and then used as a file name (@pxref{Expressions, ,Expressions as Action Statements}).@refill When this type of redirection is used, the @var{output-file} is erased before the first output is written to it. Subsequent writes do not erase @var{output-file}, but append to it. If @var{output-file} does not exist, then it is created.@refill For example, here is how one @code{awk} program can write a list of BBS names to a file @file{name-list} and a list of phone numbers to a file @file{phone-list}. Each output file contains one name or number per line. @smallexample awk '@{ print $2 > "phone-list" print $1 > "name-list" @}' BBS-list @end smallexample @item print @var{items} >> @var{output-file} This type of redirection prints the items onto the output file @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 @code{awk} output is appended to the file. @cindex pipes for output @cindex output, piping @item print @var{items} | @var{command} It is also possible to send output through a @dfn{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}.@refill The redirection argument @var{command} is actually an @code{awk} expression. Its value is converted to a string, whose contents give the shell command to be run. For example, this produces two files, one unsorted list of BBS names and one list sorted in reverse alphabetical order: @smallexample awk '@{ print $1 > "names.unsorted" print $1 | "sort -r > names.sorted" @}' BBS-list @end smallexample Here the unsorted list is written with an ordinary redirection while the sorted list is written by piping through the @code{sort} utility. Here is an example that uses redirection to mail a message to a mailing list @samp{bug-system}. This might be useful when trouble is encountered in an @code{awk} script run periodically for system maintenance. @smallexample report = "mail bug-system" print "Awk script failed:", $0 | report print "at record number", FNR, "of", FILENAME | report close(report) @end smallexample We call the @code{close} function 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 Output, ,Closing Output Files and Pipes}, for more information on this. 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, since @code{awk} requires you to spell the string value identically every time. @end table Redirecting output using @samp{>}, @samp{>>}, or @samp{|} asks the system to open a file or pipe only if the particular @var{file} or @var{command} you've specified has not already been written to by your program, or if it has been closed since it was last written to.@refill @node Close Output, , File/Pipe Redirection, Redirection @subsection Closing Output Files and Pipes @cindex closing output files and pipes @findex close When a file or pipe is opened, the file name or command associated with it is remembered by @code{awk} and subsequent writes to the same file or command are appended to the previous writes. The file or pipe stays open until @code{awk} exits. This is usually convenient. Sometimes there is a reason to close an output file or pipe earlier than that. To do this, use the @code{close} function, as follows: @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 exactly equal the string used to open the file or pipe to begin with---for example, if you open a pipe with this: @example print $1 | "sort -r > names.sorted" @end example @noindent then you must close it with this: @example close("sort -r > names.sorted") @end example Here are some reasons why you might need to close an output file: @itemize @bullet @item To write a file and read it back later on in the same @code{awk} program. Close the file when you are finished writing it; then you can start reading it with @code{getline} (@pxref{Getline, ,Explicit Input with @code{getline}}).@refill @item To write numerous files, successively, in the same @code{awk} program. If you don't close the files, eventually you may exceed a system limit on the number of open files in one process. So close each one when you are finished writing it. @item To make a command finish. When you redirect output 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 you redirect output to the @code{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 you pipe output to the @code{mail} program. If you output several lines redirected to this pipe without closing it, they make a single message of several lines. By contrast, if you close the pipe after each line of output, then each line makes a separate message. @end itemize @iftex @vindex ERRNO @cindex differences: @code{gawk} and @code{awk} @end iftex @code{close} returns a value of zero if the close succeeded. Otherwise, the value will be non-zero. In this case, @code{gawk} sets the variable @code{ERRNO} to a string describing the error that occurred. @node Special Files, , Redirection, Printing @section Standard I/O Streams @cindex standard input @cindex standard output @cindex standard error output @cindex 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, terminal input and output, but they are often redirected with the shell, via the @samp{<}, @samp{<<}, @samp{>}, @samp{>>}, @samp{>&} and @samp{|} operators. Standard error is used only for writing error messages; the reason we have two separate streams, standard output and standard error, is so that they can be redirected separately. @iftex @cindex differences: @code{gawk} and @code{awk} @end iftex In other implementations of @code{awk}, the only way to write an error message to standard error in an @code{awk} program is as follows: @smallexample print "Serious error detected!\n" | "cat 1>&2" @end smallexample @noindent This works by opening a pipeline to a shell command which can access the standard error stream which it inherits from the @code{awk} process. This is far from elegant, and is also inefficient, since it requires a separate process. So people writing @code{awk} programs have often neglected to do this. Instead, they have sent the error messages to the terminal, like this: @smallexample @group NF != 4 @{ printf("line %d skipped: doesn't have 4 fields\n", FNR) > "/dev/tty" @} @end group @end smallexample @noindent This has the same effect most of the time, but not always: although the standard error stream is usually the terminal, it can be redirected, and when that happens, writing to the terminal is not correct. In fact, if @code{awk} is run from a background job, it may not have a terminal at all. Then opening @file{/dev/tty} will fail. @code{gawk} provides special file names for accessing the three standard streams. When you redirect input or output in @code{gawk}, if the file name matches one of these special names, then @code{gawk} directly uses the stream it stands for. @cindex @file{/dev/stdin} @cindex @file{/dev/stdout} @cindex @file{/dev/stderr} @cindex @file{/dev/fd/} @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 have been opened by the program initiating the @code{awk} execution (typically the shell). Unless you take special pains, only descriptors 0, 1 and 2 are available. @end table The file names @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, but they are more self-explanatory. The proper way to write an error message in a @code{gawk} program is to use @file{/dev/stderr}, like this: @smallexample NF != 4 @{ printf("line %d skipped: doesn't have 4 fields\n", FNR) > "/dev/stderr" @} @end smallexample @code{gawk} also provides special file names that give access to information about the running @code{gawk} process. Each of these ``files'' provides a single record of information. To read them more than once, you must first close them with the @code{close} function (@pxref{Close Input, ,Closing Input Files and Pipes}). The filenames are: @cindex @file{/dev/pid} @cindex @file{/dev/pgrpid} @cindex @file{/dev/ppid} @cindex @file{/dev/user} @table @file @item /dev/pid Reading this file returns the process ID of the current process, in decimal, terminated with a newline. @item /dev/ppid Reading this file returns the parent process ID of the current process, in decimal, terminated with a newline. @item /dev/pgrpid Reading this file returns the process group ID of the current process, in decimal, terminated with a newline. @item /dev/user Reading this file returns a single record terminated with a newline. The fields are separated with blanks. The fields represent the following information: @table @code @item $1 The value of the @code{getuid} system call. @item $2 The value of the @code{geteuid} system call. @item $3 The value of the @code{getgid} system call. @item $4 The value of the @code{getegid} system call. @end table If there are any additional fields, they are the group IDs returned by @code{getgroups} system call. (Multiple groups may not be supported on all systems.)@refill @end table These special file names may be used on the command line as data files, as well as for I/O redirections within an @code{awk} program. They may not be used as source files with the @samp{-f} option. Recognition of these special file names is disabled if @code{gawk} is in compatibility mode (@pxref{Command Line, ,Invoking @code{awk}}). @quotation @strong{Caution}: Unless your system actually has a @file{/dev/fd} directory (or any of the other above listed special files), the interpretation of these file names is done by @code{gawk} itself. For example, using @samp{/dev/fd/4} for output will actually write on file descriptor 4, and not on a new file descriptor that was @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. If you do close one of these files, unpredictable behavior will result. @end quotation @node One-liners, Patterns, Printing, Top @chapter Useful ``One-liners'' @cindex one-liners Useful @code{awk} programs are often short, just a line or two. Here 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 manual to become an @code{awk} expert! @c Per suggestions from Michal Jaegermann @ifinfo Since you are reading this in Info, each line of the example code is enclosed in quotes, to represent text that you would type literally. The examples themselves represent shell commands that use single quotes to keep the shell from interpreting the contents of the program. When reading the examples, focus on the text between the open and close quotes. @end ifinfo @table @code @item awk '@{ if (NF > max) max = NF @} @itemx @ @ @ @ @ END @{ print max @}' This program prints the maximum number of fields on any input line. @item awk 'length($0) > 80' This program prints every line longer than 80 characters. The sole rule has a relational expression as its pattern, and has no action (so the default action, printing the record, is used). @item awk 'NF > 0' This program prints every line that has at least one field. 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 deleted). @item awk '@{ if (NF > 0) print @}' This program also prints every line that has at least one field. Here we allow the rule to match every line, then decide in the action whether to print. @item awk@ 'BEGIN@ @{@ for (i = 1; i <= 7; i++) @itemx @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ print int(101 * rand()) @}' This program prints 7 random numbers from 0 to 100, inclusive. @item ls -l @var{files} | awk '@{ x += $4 @} ; END @{ print "total bytes: " x @}' This program prints the total number of bytes used by @var{files}. @item expand@ @var{file}@ |@ awk@ '@{ if (x < length()) x = length() @} @itemx @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ END @{ print "maximum line length is " x @}' This program prints the maximum line length of @var{file}. The input is piped through the @code{expand} program to change tabs into spaces, so the widths compared are actually the right-margin columns. @item awk 'BEGIN @{ FS = ":" @} @itemx @ @ @ @ @ @{ print $1 | "sort" @}' /etc/passwd This program prints a sorted list of the login names of all users. @item awk '@{ nlines++ @} @itemx @ @ @ @ @ END@ @{ print nlines @}' This programs counts lines in a file. @item awk 'END @{ print NR @}' This program also counts lines in a file, but lets @code{awk} do the work. @item awk '@{ print NR, $0 @}' This program adds line numbers to all its input files, similar to @samp{cat -n}. @end table @node Patterns, Actions, One-liners, Top @chapter Patterns @cindex pattern, definition of Patterns in @code{awk} control the execution of rules: a rule is executed when its pattern matches the current input record. This chapter tells all about how to write patterns. @menu * Kinds of Patterns:: A list of all kinds of patterns. The following subsections describe them in detail. * Regexp:: Regular expressions such as @samp{/foo/}. * Comparison Patterns:: Comparison expressions such as @code{$1 > 10}. * Boolean Patterns:: Combining comparison expressions. * 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 @node Kinds of Patterns, Regexp, Patterns, Patterns @section Kinds of Patterns @cindex patterns, types of Here is a summary of the types of patterns supported in @code{awk}. @c At the next rewrite, check to see that this order matches the @c order in the text. It might not matter to a reader, but it's good @c style. Also, it might be nice to mention all the topics of sections @c that follow in this list; that way people can scan and know when to @c expect a specific topic. Specifically please also make an entry @c for Boolean operators as patterns in the right place. --mew @table @code @item /@var{regular expression}/ A regular expression as a pattern. It matches when the text of the input record fits the regular expression. (@xref{Regexp, ,Regular Expressions as Patterns}.)@refill @item @var{expression} A single expression. It matches when its value, converted to a number, is nonzero (if a number) or nonnull (if a string). (@xref{Expression Patterns, ,Expressions as Patterns}.)@refill @item @var{pat1}, @var{pat2} A pair of patterns separated by a comma, specifying a range of records. (@xref{Ranges, ,Specifying Record Ranges with Patterns}.) @item BEGIN @itemx END Special patterns to supply start-up or clean-up information to @code{awk}. (@xref{BEGIN/END, ,@code{BEGIN} and @code{END} Special Patterns}.) @item @var{null} The empty pattern matches every input record. (@xref{Empty, ,The Empty Pattern}.)@refill @end table @node Regexp, Comparison Patterns, Kinds of Patterns, Patterns @section Regular Expressions as Patterns @cindex pattern, regular expressions @cindex regexp @cindex regular expressions as patterns A @dfn{regular expression}, or @dfn{regexp}, is a way of describing a class of strings. A regular expression enclosed in slashes (@samp{/}) is an @code{awk} pattern that matches every input record whose text belongs to that class. 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 @samp{foo}. Other kinds of regexps let you specify more complicated classes of strings. @menu * Regexp Usage:: How to Use Regular Expressions * Regexp Operators:: Regular Expression Operators * Case-sensitivity:: How to do case-insensitive matching. @end menu @node Regexp Usage, Regexp Operators, Regexp, Regexp @subsection How to Use Regular Expressions A regular expression can be used as a pattern by enclosing it in slashes. Then the regular expression is matched against the entire text of each record. (Normally, it only needs to match some part of the text in order to succeed.) For example, this prints the second field of each record that contains @samp{foo} anywhere: @example awk '/foo/ @{ print $2 @}' BBS-list @end example @cindex regular expression matching operators @cindex string-matching operators @cindex operators, string-matching @cindex operators, regexp matching @cindex regexp search operators Regular expressions can also be used in comparison expressions. Then you can specify the string to match against; it need not be the entire current input record. These comparison expressions can be used as patterns or in @code{if}, @code{while}, @code{for}, and @code{do} statements. @table @code @item @var{exp} ~ /@var{regexp}/ This is true if the expression @var{exp} (taken as a character string) is matched by @var{regexp}. The following example matches, or selects, all input records with the upper-case letter @samp{J} somewhere in the first field:@refill @example awk '$1 ~ /J/' inventory-shipped @end example So does this: @example awk '@{ if ($1 ~ /J/) print @}' inventory-shipped @end example @item @var{exp} !~ /@var{regexp}/ This is true if the expression @var{exp} (taken as a character string) is @emph{not} matched by @var{regexp}. The following example matches, or selects, all input records whose first field @emph{does not} contain the upper-case letter @samp{J}:@refill @example awk '$1 !~ /J/' inventory-shipped @end example @end table @cindex computed regular expressions @cindex regular expressions, computed @cindex dynamic regular expressions The right hand side of a @samp{~} or @samp{!~} operator need not be a constant regexp (i.e., a string of characters between slashes). It may be any expression. The expression is evaluated, and converted if necessary to a string; the contents of the string are used as the regexp. A regexp that is computed in this way is called a @dfn{dynamic regexp}. For example: @example identifier_regexp = "[A-Za-z_][A-Za-z_0-9]+" $0 ~ identifier_regexp @end example @noindent sets @code{identifier_regexp} to a regexp that describes @code{awk} variable names, and tests if the input record matches this regexp. @node Regexp Operators, Case-sensitivity, Regexp Usage, Regexp @subsection Regular Expression Operators @cindex metacharacters @cindex regular expression metacharacters You can combine regular expressions with the following characters, called @dfn{regular expression operators}, or @dfn{metacharacters}, to increase the power and versatility of regular expressions. Here is a table of metacharacters. All characters not listed in the table stand for themselves. @table @code @item ^ This matches the beginning of the string or the beginning of a line within the string. For example: @example ^@@chapter @end example @noindent matches the @samp{@@chapter} at the beginning of a string, and can be used to identify chapter beginnings in Texinfo source files. @item $ This is similar to @samp{^}, but it matches only at the end of a string or the end of a line within the string. For example: @example p$ @end example @noindent matches a record that ends with a @samp{p}. @item . This matches any single character except a newline. For example: @example .P @end example @noindent matches any single character followed by a @samp{P} in a string. Using concatenation we can make regular expressions like @samp{U.A}, which matches any three-character sequence that begins with @samp{U} and ends with @samp{A}. @item [@dots{}] This is called a @dfn{character set}. It matches any one of the characters that are enclosed in the square brackets. For example: @example [MVX] @end example @noindent matches any one of the characters @samp{M}, @samp{V}, or @samp{X} in a string.@refill Ranges of characters are indicated by using a hyphen between the beginning and ending characters, and enclosing the whole thing in brackets. For example:@refill @example [0-9] @end example @noindent matches any digit. To include the character @samp{\}, @samp{]}, @samp{-} or @samp{^} in a character set, put a @samp{\} in front of it. For example: @example [d\]] @end example @noindent matches either @samp{d}, or @samp{]}.@refill This treatment of @samp{\} is compatible with other @code{awk} implementations, and is also mandated by the @sc{posix} Command Language and Utilities standard. The regular expressions in @code{awk} are a superset of the @sc{posix} specification for Extended Regular Expressions (EREs). @sc{posix} EREs are based on the regular expressions accepted by the traditional @code{egrep} utility. In @code{egrep} syntax, backslash is not syntactically special within square brackets. This means that special tricks have to be used to represent the characters @samp{]}, @samp{-} and @samp{^} as members of a character set. In @code{egrep} syntax, to match @samp{-}, write it as @samp{---}, which is a range containing only @w{@samp{-}.} You may also give @samp{-} as the first or last character in the set. To match @samp{^}, put it anywhere except as the first character of a set. To match a @samp{]}, make it the first character in the set. For example:@refill @example []d^] @end example @noindent matches either @samp{]}, @samp{d} or @samp{^}.@refill @item [^ @dots{}] This is a @dfn{complemented character set}. The first character after the @samp{[} @emph{must} be a @samp{^}. It matches any characters @emph{except} those in the square brackets (or newline). For example: @example [^0-9] @end example @noindent matches any character that is not a digit. @item | This is the @dfn{alternation operator} and it is used to specify alternatives. For example: @example ^P|[0-9] @end example @noindent matches any string that matches either @samp{^P} or @samp{[0-9]}. This means it matches any string that contains a digit or starts with @samp{P}. The alternation applies to the largest possible regexps on either side. @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{|}. @item * This symbol means that the preceding regular expression is to be repeated as many times as possible to find a match. For example: @example ph* @end example @noindent applies the @samp{*} symbol to the preceding @samp{h} and looks for matches to one @samp{p} followed by any number of @samp{h}s. This will also match just @samp{p} if no @samp{h}s are present. The @samp{*} repeats the @emph{smallest} possible preceding expression. (Use parentheses if you wish to repeat a larger expression.) It finds as many repetitions as possible. For example: @example awk '/\(c[ad][ad]*r x\)/ @{ print @}' sample @end example @noindent prints every record in the input containing a string of the form @samp{(car x)}, @samp{(cdr x)}, @samp{(cadr x)}, and so on.@refill @item + This symbol is similar to @samp{*}, but the preceding expression must be matched at least once. This means that: @example wh+y @end example @noindent would match @samp{why} and @samp{whhy} but not @samp{wy}, whereas @samp{wh*y} would match all three of these strings. This is a simpler way of writing the last @samp{*} example: @example awk '/\(c[ad]+r x\)/ @{ print @}' sample @end example @item ? This symbol is similar to @samp{*}, but the preceding expression can be matched once or not at all. For example: @example fe?d @end example @noindent will match @samp{fed} and @samp{fd}, but nothing else.@refill @item \ This is used to suppress the special meaning of a character when matching. For example: @example \$ @end example @noindent matches the character @samp{$}. The escape sequences used for string constants (@pxref{Constants, ,Constant Expressions}) are valid in regular expressions as well; they are also introduced by a @samp{\}.@refill @end table In regular expressions, the @samp{*}, @samp{+}, and @samp{?} operators have the highest precedence, followed by concatenation, and finally by @samp{|}. As in arithmetic, parentheses can change how operators are grouped.@refill @node Case-sensitivity, , Regexp Operators, Regexp @subsection Case-sensitivity in Matching 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 lower case @samp{w} and not an upper case @samp{W}. The simplest way to do a case-independent match is to use a character set: @samp{[Ww]}. However, this can be cumbersome if you need to use it often; and it can make the regular expressions harder for humans to read. There are two other alternatives that you might prefer. One way to do 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, ,Built-in Functions for String Manipulation}). For example:@refill @example tolower($1) ~ /foo/ @{ @dots{} @} @end example @noindent converts the first field to lower case before matching against it. Another method is to set the variable @code{IGNORECASE} to a nonzero value (@pxref{Built-in Variables}). When @code{IGNORECASE} is not zero, @emph{all} regexp operations ignore case. Changing the value of @code{IGNORECASE} dynamically controls the case sensitivity of your 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 way to set @code{IGNORECASE} just for the pattern of a particular rule. To do this, you must use character sets or @code{tolower}. However, one thing you can do only with @code{IGNORECASE} is turn case-sensitivity on or off dynamically for all the rules at once.@refill @code{IGNORECASE} can be set on the command line, or in a @code{BEGIN} rule. Setting @code{IGNORECASE} from the command line is a way to make a program case-insensitive without having to edit it. The value of @code{IGNORECASE} has no effect if @code{gawk} is in compatibility mode (@pxref{Command Line, ,Invoking @code{awk}}). Case is always significant in compatibility mode.@refill @node Comparison Patterns, Boolean Patterns, Regexp, Patterns @section Comparison Expressions as Patterns @cindex comparison expressions as patterns @cindex pattern, comparison expressions @cindex relational operators @cindex operators, relational @dfn{Comparison patterns} test relationships such as equality between two strings or numbers. They are a special case of expression patterns (@pxref{Expression Patterns, ,Expressions as Patterns}). They are written with @dfn{relational operators}, which are a superset of those in C. Here is a table of them:@refill @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 @var{x} matches the regular expression described by @var{y}. @item @var{x} !~ @var{y} True if @var{x} does not match the regular expression described by @var{y}. @end table The operands of a relational operator are compared as numbers if they are both numbers. Otherwise they are converted to, and compared as, strings (@pxref{Conversion, ,Conversion of Strings and Numbers}, for the detailed rules). Strings are compared by comparing the first character of each, then the second character of each, and so on, until there is a difference. If the two strings are equal until the shorter one runs out, the shorter one is considered to be less than the longer one. Thus, @code{"10"} is less than @code{"9"}, and @code{"abc"} is less than @code{"abcd"}.@refill 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{Regexp Usage, ,How to Use Regular Expressions}).@refill The following example prints the second field of each input record whose first field is precisely @samp{foo}. @example awk '$1 == "foo" @{ print $2 @}' BBS-list @end example @noindent Contrast this with the following regular expression match, which would accept any record with a first field that contains @samp{foo}: @example awk '$1 ~ "foo" @{ print $2 @}' BBS-list @end example @noindent or, equivalently, this one: @example awk '$1 ~ /foo/ @{ print $2 @}' BBS-list @end example @node Boolean Patterns, Expression Patterns, Comparison Patterns, Patterns @section Boolean Operators and Patterns @cindex patterns, boolean @cindex boolean patterns A @dfn{boolean pattern} is an expression which combines other patterns using the @dfn{boolean operators} ``or'' (@samp{||}), ``and'' (@samp{&&}), and ``not'' (@samp{!}). Whether the boolean pattern matches an input record depends on whether its subpatterns match. For example, the following command prints all records in the input file @file{BBS-list} that contain both @samp{2400} and @samp{foo}.@refill @example awk '/2400/ && /foo/' BBS-list @end example The following command prints all records in the input file @file{BBS-list} that contain @emph{either} @samp{2400} or @samp{foo}, or both.@refill @example awk '/2400/ || /foo/' BBS-list @end example The following command prints all records in the input file @file{BBS-list} that do @emph{not} contain the string @samp{foo}. @example awk '! /foo/' BBS-list @end example Note that boolean patterns are a special case of expression patterns (@pxref{Expression Patterns, ,Expressions as Patterns}); they are expressions that use the boolean operators. @xref{Boolean Ops, ,Boolean Expressions}, for complete information on the boolean operators.@refill The subpatterns of a boolean pattern can be constant regular expressions, comparisons, or any other @code{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 Expression Patterns, Ranges, Boolean Patterns, Patterns @section Expressions as Patterns Any @code{awk} expression is also valid as an @code{awk} pattern. Then the pattern ``matches'' if the expression's value is nonzero (if a number) or nonnull (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 only on what has happened so far in the execution of the @code{awk} program, but that may still be useful. Comparison patterns are actually a special case of this. For example, the expression @code{$5 == "foo"} has the value 1 when the value of @code{$5} equals @code{"foo"}, and 0 otherwise; therefore, this expression as a pattern matches when the two values are equal. Boolean patterns are also special cases of expression patterns. A constant regexp as a pattern is also a special case of an expression pattern. @code{/foo/} as an expression has the value 1 if @samp{foo} appears in the current input record; thus, as a pattern, @code{/foo/} matches any record containing @samp{foo}. Other implementations of @code{awk} that are not yet @sc{posix} compliant are less general than @code{gawk}: they allow comparison expressions, and boolean combinations thereof (optionally with parentheses), but not necessarily other kinds of expressions. @node Ranges, BEGIN/END, Expression Patterns, Patterns @section Specifying Record Ranges with Patterns @cindex range pattern @cindex patterns, range A @dfn{range pattern} is made of two patterns separated by a comma, of the form @code{@var{begpat}, @var{endpat}}. It matches ranges of consecutive input records. The first pattern @var{begpat} controls where the range begins, and the second one @var{endpat} controls where it ends. For example,@refill @example awk '$1 == "on", $1 == "off"' @end example @noindent prints every record 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 becomes @dfn{turned on}. The range pattern matches this record. As long as it stays turned on, it automatically matches every input record read. It also matches @var{endpat} against every input record; when that succeeds, the range pattern is turned off again for the following record. Now it goes back to checking @var{begpat} against each record. 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. It is possible for a pattern to be turned both on and off by the same record, if both conditions are satisfied by that record. Then the action is executed for just that record. @node BEGIN/END, Empty, Ranges, Patterns @section @code{BEGIN} and @code{END} Special Patterns @cindex @code{BEGIN} special pattern @cindex patterns, @code{BEGIN} @cindex @code{END} special pattern @cindex patterns, @code{END} @code{BEGIN} and @code{END} are special patterns. They are not used to match input records. Rather, they are used for supplying start-up or clean-up information to your @code{awk} script. A @code{BEGIN} rule is executed, once, before the first input record has been read. An @code{END} rule is executed, once, after all the input has been read. For example:@refill @example awk 'BEGIN @{ print "Analysis of `foo'" @} /foo/ @{ ++foobar @} END @{ print "`foo' appears " foobar " times." @}' BBS-list @end example 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{foobar} to zero, as @code{awk} does this for us automatically (@pxref{Variables}). The second rule increments the variable @code{foobar} every time a record containing the pattern @samp{foo} is read. The @code{END} rule prints the value of @code{foobar} at the end of the run.@refill 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 @code{awk} program may have multiple @code{BEGIN} and/or @code{END} rules. They are executed in the order they appear, all the @code{BEGIN} rules at start-up and all the @code{END} rules at termination. Multiple @code{BEGIN} and @code{END} sections are useful for writing library functions, since each library can have its own @code{BEGIN} or @code{END} rule to do its own initialization and/or cleanup. Note that 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 to write such rules in library files so that the order in which they are executed doesn't matter. @xref{Command Line, ,Invoking @code{awk}}, for more information on using library functions. If an @code{awk} program only has a @code{BEGIN} rule, and no other rules, then the program exits after the @code{BEGIN} rule has been run. (Older versions of @code{awk} used to keep reading and ignoring input until end of file was seen.) However, if an @code{END} rule exists as well, then the input will be read, even if there are no other rules in the program. This is necessary in case the @code{END} rule checks the @code{NR} variable. @code{BEGIN} and @code{END} rules must have actions; there is no default action for these rules since there is no current record when they run. @node Empty, , BEGIN/END, Patterns @comment node-name, next, previous, up @section The Empty Pattern @cindex empty pattern @cindex pattern, empty An empty pattern is considered to match @emph{every} input record. For example, the program:@refill @example awk '@{ print $1 @}' BBS-list @end example @noindent prints the first field of every record. @node Actions, Expressions, Patterns, Top @chapter Overview of Actions @cindex action, definition of @cindex curly braces @cindex action, curly braces @cindex action, separating statements An @code{awk} program or script consists of a series of rules and function definitions, interspersed. (Functions are described later. @xref{User-defined, ,User-defined Functions}.) A rule contains a pattern and an action, either of which may be omitted. The purpose of the @dfn{action} is to tell @code{awk} what to do once a match for the pattern is found. Thus, the entire program looks somewhat 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 An action consists of one or more @code{awk} @dfn{statements}, enclosed in curly braces (@samp{@{} and @samp{@}}). Each statement specifies one thing to be done. 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 even 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 @}}.) Here are the kinds of statements supported in @code{awk}: @itemize @bullet @item Expressions, which can call functions or assign values to variables (@pxref{Expressions, ,Expressions as Action Statements}). Executing this kind of statement simply computes the value of the expression and then ignores it. This is useful when the expression has side effects (@pxref{Assignment Ops, ,Assignment Expressions}).@refill @item Control statements, which specify the control flow of @code{awk} programs. The @code{awk} language gives you C-like constructs (@code{if}, @code{for}, @code{while}, and so on) as well as a few special ones (@pxref{Statements, ,Control Statements in Actions}).@refill @item Compound statements, which 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 control, using the @code{getline} command (@pxref{Getline, ,Explicit Input with @code{getline}}), and the @code{next} statement (@pxref{Next Statement, ,The @code{next} Statement}). @item Output statements, @code{print} and @code{printf}. @xref{Printing, ,Printing Output}.@refill @item Deletion statements, for deleting array elements. @xref{Delete, ,The @code{delete} Statement}.@refill @end itemize @iftex The next two chapters cover in detail expressions and control statements, respectively. We go on to treat arrays and built-in functions, both of which are used in expressions. Then we proceed to discuss how to define your own functions. @end iftex @node Expressions, Statements, Actions, Top @chapter Expressions as Action Statements @cindex expression Expressions are the basic building block of @code{awk} actions. An expression evaluates to a value, which you can print, test, store in a variable or pass to a function. But beyond that, an expression can assign a new value to a variable or a field, with an assignment operator. An expression can serve as a statement on its own. Most other kinds of statements contain one or more expressions which specify data to be operated on. As in other languages, expressions in @code{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. * Variables:: Variables give names to values for later use. * Arithmetic Ops:: Arithmetic operations (@samp{+}, @samp{-}, etc.) * Concatenation:: Concatenating strings. * Comparison Ops:: Comparison of numbers and strings with @samp{<}, etc. * Boolean Ops:: Combining comparison expressions using boolean operators @samp{||} (``or''), @samp{&&} (``and'') and @samp{!} (``not''). * Assignment Ops:: Changing the value of a variable or a field. * Increment Ops:: Incrementing the numeric value of a variable. * Conversion:: The conversion of strings to numbers and vice versa. * Values:: The whole truth about numbers and strings. * 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, Variables, Expressions, Expressions @section Constant Expressions @cindex constants, types of @cindex string constants The simplest type of expression is the @dfn{constant}, which always has the same value. There are three types of constants: numeric constants, string constants, and regular expression constants. @cindex numeric constant @cindex numeric value A @dfn{numeric constant} stands for a number. This number can be an integer, a decimal fraction, or a number in scientific (exponential) notation. Note that all numeric values are represented within @code{awk} in double-precision floating point. Here are some examples of numeric constants, which all have the same value: @example 105 1.05e+2 1050e-1 @end example A string constant consists of a sequence of characters enclosed in double-quote marks. For example: @example "parrot" @end example @noindent @iftex @cindex differences between @code{gawk} and @code{awk} @end iftex represents the string whose contents are @samp{parrot}. Strings in @code{gawk} can be of any length and they can contain all the possible 8-bit ASCII characters including ASCII NUL. Other @code{awk} implementations may have difficulty with some character codes.@refill @cindex escape sequence notation Some characters cannot be included literally in a string constant. You represent them instead 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. Since a plain double-quote would end the string, you must use @samp{\"} to represent a single double-quote character as a part of the string. The backslash character itself is another character that cannot be included normally; you write @samp{\\} to put one backslash in the string. Thus, the string whose contents are the two characters @samp{"\} must be written @code{"\"\\"}. Another use of backslash is to represent unprintable characters such as newline. While there is nothing to stop you from writing most of these characters directly in a string constant, they may look ugly. Here is a table of all the escape sequences used in @code{awk}: @table @code @item \\ Represents a literal backslash, @samp{\}. @item \a Represents the ``alert'' character, control-g, ASCII code 7. @item \b Represents a backspace, control-h, ASCII code 8. @item \f Represents a formfeed, control-l, ASCII code 12. @item \n Represents a newline, control-j, ASCII code 10. @item \r Represents a carriage return, control-m, ASCII code 13. @item \t Represents a horizontal tab, control-i, ASCII code 9. @item \v Represents a vertical tab, control-k, ASCII code 11. @item \@var{nnn} Represents the octal value @var{nnn}, where @var{nnn} are one to three digits between 0 and 7. For example, the code for the ASCII ESC (escape) character is @samp{\033}.@refill @item \x@var{hh}@dots{} Represents the hexadecimal value @var{hh}, where @var{hh} are hexadecimal digits (@samp{0} through @samp{9} and either @samp{A} through @samp{F} or @samp{a} through @samp{f}). Like the same construct in @sc{ansi} C, the escape sequence continues until the first non-hexadecimal digit is seen. However, using more than two hexadecimal digits produces undefined results. (The @samp{\x} escape sequence is not allowed in @sc{posix} @code{awk}.)@refill @end table A @dfn{constant regexp} is a regular expression description enclosed in slashes, such as @code{/^beginning and end$/}. Most regexps used in @code{awk} programs are constant, but the @samp{~} and @samp{!~} operators can also match computed or ``dynamic'' regexps (@pxref{Regexp Usage, ,How to Use Regular Expressions}).@refill Constant regexps may be used like simple expressions. When a constant regexp is not on the right hand side of the @samp{~} or @samp{!~} operators, it has the same meaning as if it appeared in a pattern, i.e. @samp{($0 ~ /foo/)} (@pxref{Expression Patterns, ,Expressions as Patterns}). This means that the two code segments,@refill @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 legal, but does not do what the user intended:@refill @example if (/foo/ ~ $1) print "found foo" @end example This code is ``obviously'' testing @code{$1} for a match against the regexp @code{/foo/}. But in fact, the expression @code{(/foo/ ~ $1)} actually means @code{(($0 ~ /foo/) ~ $1)}. In other words, first match the input record against the regexp @code{/foo/}. The result will be either a 0 or a 1, depending upon the success or failure of the match. Then match that result against the first field in the record.@refill Since it is unlikely that you would ever really wish to make this kind of test, @code{gawk} will issue a warning when it sees this construct in a program.@refill Another consequence of this rule is that the assignment statement @example matches = /foo/ @end example @noindent will assign either 0 or 1 to the variable @code{matches}, depending upon the contents of the current input record. Constant regular expressions are also used as the first argument for the @code{sub} and @code{gsub} functions (@pxref{String Functions, ,Built-in Functions for String Manipulation}).@refill This feature of the language was never well documented until the @sc{posix} specification. You may be wondering, when is @example $1 ~ /foo/ @{ @dots{} @} @end example @noindent preferable to @example $1 ~ "foo" @{ @dots{} @} @end example Since the right-hand sides of both @samp{~} operators are constants, it is more efficient to use the @samp{/foo/} form: @code{awk} can note that you have supplied a regexp and store it internally in a form that makes pattern matching more efficient. In the second form, @code{awk} must first convert the string into this internal form, and then perform the pattern matching. The first form is also better style; it shows clearly that you intend a regexp match. @node Variables, Arithmetic Ops, Constants, Expressions @section Variables @cindex variables, user-defined @cindex user-defined variables @c there should be more than one subsection, ideally. Not a big deal. @c But usually there are supposed to be at least two. One way to get @c around this is to write the info in the subsection as the info in the @c section itself and not have any subsections.. --mew Variables let you give names to values and refer to them later. You have already seen variables in many of the examples. The name of a variable must be a sequence of letters, digits and underscores, but 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} and @dfn{increment operators}. @xref{Assignment Ops, ,Assignment Expressions}. 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 them. These built-in variables can be used and assigned just like all other variables, but their values are also used or changed automatically by @code{awk}. Each built-in variable's name is made entirely of upper case letters. Variables in @code{awk} can be assigned either numeric or string values. By default, variables are initialized to the null string, which is effectively zero if converted to a number. There is no need to ``initialize'' each variable explicitly in @code{awk}, the way you would in C or most other traditional languages. @menu * 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 Assignment Options, , Variables, Variables @subsection Assigning Variables on the Command Line You can set any @code{awk} variable by including a @dfn{variable assignment} among the arguments on the command line when you invoke @code{awk} (@pxref{Command Line, ,Invoking @code{awk}}). Such an assignment has this form:@refill @example @var{variable}=@var{text} @end example @noindent With it, you can set a variable either at the beginning of the @code{awk} run or in between input files. If you precede the assignment with the @samp{-v} option, like this: @example -v @var{variable}=@var{text} @end example @noindent then the variable is set at the very beginning, before even the @code{BEGIN} rules are run. The @samp{-v} option and its assignment must precede all the file name arguments, as well as the program text. 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 4. 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 2, so that the second field is printed in lines from @file{BBS-list}. Command line arguments are made available for explicit examination by the @code{awk} program in an array named @code{ARGV} (@pxref{Built-in Variables}).@refill @code{awk} processes the values of command line assignments for escape sequences (@pxref{Constants, ,Constant Expressions}). @node Arithmetic Ops, Concatenation, Variables, Expressions @section Arithmetic Operators @cindex arithmetic operators @cindex operators, arithmetic @cindex addition @cindex subtraction @cindex multiplication @cindex division @cindex remainder @cindex quotient @cindex exponentiation The @code{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. This example divides field three by field four, adds field two, stores the result into field one, and prints the resulting altered input record: @example awk '@{ $1 = $2 + $3 / $4; print @}' inventory-shipped @end example The arithmetic operators in @code{awk} are: @table @code @item @var{x} + @var{y} Addition. @item @var{x} - @var{y} Subtraction. @item - @var{x} Negation. @item + @var{x} Unary plus. No real effect on the expression. @item @var{x} * @var{y} Multiplication. @item @var{x} / @var{y} Division. Since all numbers in @code{awk} are double-precision floating point, the result is not rounded to an integer: @code{3 / 4} has the value 0.75. @item @var{x} % @var{y} @iftex @cindex differences between @code{gawk} and @code{awk} @end iftex Remainder. The quotient is rounded toward zero to an integer, multiplied by @var{y} and 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 @code{awk} implementations, the signedness of the remainder may be machine dependent. @item @var{x} ^ @var{y} @itemx @var{x} ** @var{y} Exponentiation: @var{x} raised to the @var{y} power. @code{2 ^ 3} has the value 8. The character sequence @samp{**} is equivalent to @samp{^}. (The @sc{posix} standard only specifies the use of @samp{^} for exponentiation.) @end table @node Concatenation, Comparison Ops, Arithmetic Ops, Expressions @section String Concatenation @cindex string operators @cindex operators, string @cindex concatenation 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 @end example @noindent produces, for the first record in @file{BBS-list}: @example Field number one: aardvark @end example Without the space in the string constant after the @samp{:}, the line would run together. For example: @example awk '@{ print "Field number one:" $1 @}' BBS-list @end example @noindent produces, for the first record in @file{BBS-list}: @example Field number one:aardvark @end example Since string concatenation does not have an explicit operator, it is often necessary to insure that it happens where you want it to by enclosing the items to be concatenated in parentheses. 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 We recommend you use parentheses around concatenation in all but the most common contexts (such as in the right-hand operand of @samp{=}). @ignore @code{gawk} actually now allows a concatenation on the right hand side of a @code{>} redirection, but other @code{awk}s don't. So for now we won't mention that fact. @end ignore @node Comparison Ops, Boolean Ops, Concatenation, Expressions @section Comparison Expressions @cindex comparison expressions @cindex expressions, comparison @cindex relational operators @cindex operators, relational @cindex regexp operators @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: @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 array @var{array} has an element with the subscript @var{subscript}. @end table Comparison expressions have the value 1 if true and 0 if false. The rules @code{gawk} uses for performing comparisons are based on those in draft 11.2 of the @sc{posix} standard. The @sc{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"}}. @vindex CONVFMT When performing a relational operation, @code{gawk} considers the type of an operand to be the type it received on its last @emph{assignment}, rather than the type of its last @emph{use} (@pxref{Values, ,Numeric and String Values}). This type is @emph{unknown} when the operand is from an ``external'' source: field variables, command line arguments, array elements resulting from a @code{split} operation, and the value of an @code{ENVIRON} element. In this case only, if the operand is a numeric string, then it is considered to be of both string type and numeric type. If at least one operand of a comparison is of string type only, then a string comparison is performed. Any numeric operand will be converted to a string using the value of @code{CONVFMT} (@pxref{Conversion, ,Conversion of Strings and Numbers}). If one operand of a comparison is numeric, and the other operand is either numeric or both numeric and string, then @code{gawk} does a numeric comparison. If both operands have both types, then the comparison is numeric. 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"}.@refill Here are some sample expressions, how @code{gawk} compares them, and 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) @end table @example echo 1e2 3 | awk '@{ print ($1 < $2) ? "true" : "false" @}' @end example @noindent prints @samp{false} since both @code{$1} and @code{$2} are numeric strings and thus have both string and numeric types, thus 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 $1 == "foo" @end example @noindent has the value of 1, or is true, if the first field of the current input record is precisely @samp{foo}. By contrast, @example $1 ~ /foo/ @end example @noindent has the value 1 if the first field contains @samp{foo}, such as @samp{foobar}. The right hand operand of the @samp{~} and @samp{!~} operators may be either a constant regexp (@code{/@dots{}/}), or it may be an ordinary expression, in which case the value of the expression as a string is a dynamic regexp (@pxref{Regexp Usage, ,How to Use Regular Expressions}). @cindex regexp as expression In very recent implementations of @code{awk}, a constant regular expression in slashes by itself is also an expression. The regexp @code{/@var{regexp}/} is an abbreviation for this comparison expression: @example $0 ~ /@var{regexp}/ @end example In some contexts it may be necessary to write parentheses around the regexp to avoid confusing the @code{gawk} parser. For example, @code{(/x/ - /y/) > threshold} is not allowed, but @code{((/x/) - (/y/)) > threshold} parses properly. One special place where @code{/foo/} is @emph{not} an abbreviation for @code{$0 ~ /foo/} is when it is the right-hand operand of @samp{~} or @samp{!~}! @xref{Constants, ,Constant Expressions}, where this is discussed in more detail. @node Boolean Ops, Assignment Ops, Comparison Ops, Expressions @section Boolean Expressions @cindex expressions, boolean @cindex boolean expressions @cindex operators, boolean @cindex boolean operators @cindex logical operations @cindex and operator @cindex or operator @cindex not operator 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 of the boolean expression is computed by combining the truth values of the component expressions. 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. They have numeric values (1 if true, 0 if false), which come into play if the result of the boolean expression is stored in a variable, or used in arithmetic.@refill In addition, every boolean expression is also a valid boolean pattern, so you can use it as a pattern to control the execution of rules. Here are descriptions of the three boolean operators, with an example of each. It may be instructive to compare these examples with the analogous examples of boolean patterns (@pxref{Boolean Patterns, ,Boolean Operators and Patterns}), which use the same boolean operators in patterns instead of expressions.@refill @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}.@refill @smallexample if ($0 ~ /2400/ && $0 ~ /foo/) print @end smallexample 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 @code{$0 ~ /foo/ && ($2 == bar++)}, the variable @code{bar} is not incremented if there is no @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 command prints all records in the input file @file{BBS-list} that contain @emph{either} @samp{2400} or @samp{foo}, or both.@refill @smallexample awk '@{ if ($0 ~ /2400/ || $0 ~ /foo/) print @}' BBS-list @end smallexample 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 all records in the input file @file{BBS-list} that do @emph{not} contain the string @samp{foo}. @smallexample awk '@{ if (! ($0 ~ /foo/)) print @}' BBS-list @end smallexample @end table @node Assignment Ops, Increment Ops, Boolean Ops, Expressions @section Assignment Expressions @cindex assignment operators @cindex operators, assignment @cindex expressions, assignment An @dfn{assignment} is an expression that stores a new value into a variable. For example, let's assign the value 1 to the variable @code{z}:@refill @example z = 1 @end example After this expression is executed, the variable @code{z} has the value 1. Whatever old value @code{z} had before the assignment is forgotten. Assignments can store string values also. For example, this would store 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 (This also illustrates concatenation of strings.) The @samp{=} sign is called an @dfn{assignment operator}. It is the simplest assignment operator because the value of the right-hand operand is stored unchanged. @cindex side effect Most operators (addition, concatenation, and so on) have no effect except to compute a value. If you ignore the value, you might as well not use the operator. An assignment operator is different; it does produce a value, but even if you ignore the value, the assignment still makes itself felt through the alteration of the variable. We call this a @dfn{side effect}. @cindex lvalue The left-hand operand of an assignment need not be a variable (@pxref{Variables}); it can also be a field (@pxref{Changing Fields, ,Changing the Contents of a Field}) or an array element (@pxref{Arrays, ,Arrays in @code{awk}}). These are all called @dfn{lvalues}, which means they can appear on the left-hand side of an assignment operator. The right-hand operand may be any expression; it produces the new value which the assignment stores in the specified variable, field or array element.@refill It is important to note that variables do @emph{not} have permanent types. The type of a variable 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. An assignment is an expression, so it has a value: the same value that is assigned. Thus, @code{z = 1} as an expression has the value 1. One consequence of this is that you can write multiple assignments together: @example x = y = z = 0 @end example @noindent stores the value 0 in all three variables. It does this because the value of @code{z = 0}, which is 0, is stored into @code{y}, and then the value of @code{y = z = 0}, which is 0, is stored into @code{x}. You can use an assignment anywhere an expression is called for. For example, it is valid to write @code{x != (y = 1)} to set @code{y} to 1 and then test whether @code{x} equals 1. But this style tends to make programs hard to read; except in a one-shot program, you should rewrite it to get rid of such nesting of assignments. This is never very hard. 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 right-hand value to the old value of the variable. Thus, the following assignment adds 5 to the value of @code{foo}: @example foo += 5 @end example @noindent This is precisely equivalent to the following: @example foo = foo + 5 @end example @noindent Use whichever one makes the meaning of your program clearer. Here is a table of the arithmetic assignment operators. In each case, the right-hand operand is an expression whose value is converted to a number. @table @code @item @var{lvalue} += @var{increment} Adds @var{increment} to the value of @var{lvalue} to make the new 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{quotient} Divides the value of @var{lvalue} by @var{quotient}. @item @var{lvalue} %= @var{modulus} Sets @var{lvalue} to its remainder by @var{modulus}. @item @var{lvalue} ^= @var{power} @itemx @var{lvalue} **= @var{power} Raises @var{lvalue} to the power @var{power}. (Only the @code{^=} operator is specified by @sc{posix}.) @end table @ignore From: gatech!ames!elroy!cit-vax!EQL.Caltech.Edu!rankin (Pat Rankin) In the discussion of assignment operators, it states that ``foo += 5'' "is precisely equivalent to" ``foo = foo + 5'' (p.77). That may be true for simple variables, but it's not true for expressions with side effects, like array references. For proof, try 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] } I suspect that the original statement is simply untrue--that '+=' is more efficient in all cases. ADR --- Try to add something about this here for the next go 'round. @end ignore @node Increment Ops, Conversion, Assignment Ops, Expressions @section Increment Operators @cindex increment operators @cindex operators, increment @dfn{Increment operators} increase or decrease the value of a variable by 1. You could do the same thing with an assignment operator, so the increment operators add no power to the @code{awk} language; but they are convenient abbreviations for something very common. The operator to add 1 is written @samp{++}. It can be used to increment a variable either before or after taking its value. To pre-increment a variable @var{v}, write @code{++@var{v}}. This adds 1 to the value of @var{v} and that new value is also the value of this expression. The assignment expression @code{@var{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 4, then the expression @code{foo++} has the value 4, but it changes the value of @code{foo} to 5. The post-increment @code{foo++} is nearly equivalent to writing @code{(foo += 1) - 1}. It is not perfectly equivalent because all numbers in @code{awk} are floating point: in floating point, @code{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 a trillion). Any lvalue can be incremented. Fields and array elements are incremented just like variables. (Use @samp{$(i++)} when you wish 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{$}.) @c expert information in the last parenthetical remark The decrement operator @samp{--} works just like @samp{++} except that it subtracts 1 instead of adding. Like @samp{++}, it can be used before the lvalue to pre-decrement or after it to post-decrement. Here is a summary of increment and decrement expressions. @table @code @item ++@var{lvalue} This expression increments @var{lvalue} and the new value becomes the value of this expression. @item @var{lvalue}++ This expression causes the contents of @var{lvalue} to be incremented. The value of the expression is the @emph{old} value of @var{lvalue}. @item --@var{lvalue} Like @code{++@var{lvalue}}, but instead of adding, it subtracts. It decrements @var{lvalue} and delivers the value that results. @item @var{lvalue}-- Like @code{@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 @node Conversion, Values, Increment Ops, Expressions @section Conversion of Strings and Numbers @cindex conversion of strings and numbers Strings are converted to numbers, and numbers to strings, if the context of the @code{awk} program demands it. For example, if the value of either @code{foo} or @code{bar} in the expression @code{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 this:@refill @example two = 2; three = 3 print (two three) + 4 @end example @noindent This eventually prints the (numeric) value 27. The numeric values of the variables @code{two} and @code{three} are converted to strings and concatenated together, and the resulting string is converted back to the number 23, to which 4 is then added. If, for some reason, you need to force a number to be converted to a string, concatenate the null string 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 a 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 are converted to zero. @vindex CONVFMT The exact manner in which numbers are converted into strings is controlled by the @code{awk} built-in variable @code{CONVFMT} (@pxref{Built-in Variables}). Numbers are converted using a special version of the @code{sprintf} function (@pxref{Built-in, ,Built-in Functions}) with @code{CONVFMT} as the format specifier.@refill @code{CONVFMT}'s default value is @code{"%.6g"}, which prints a value with at least six significant digits. For some applications you will want to change it to specify more precision. Double precision on most modern machines gives you 16 or 17 decimal digits of precision. Strange results can happen 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, all numbers will be converted to the same constant string.@refill 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"}. @ignore For the 2.14 version, describe the ``stickyness'' of conversions. Right now the manual assumes everywhere that variables are either numbers or strings; in fact both kinds of values may be valid. If both happen to be valid, a conversion isn't necessary and isn't done. Revising the manual to be consistent with this, though, is too big a job to tackle at the moment. 7/92: This has sort of been done, only the section isn't completely right! What to do? 7/92: Pretty much fixed, at least for the short term, thanks to text from David. @end ignore @vindex OFMT Prior to the @sc{posix} standard, @code{awk} specified that the value of @code{OFMT} was used 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 conversions 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 @code{awk} programs will not change their behavior. However, this use of @code{OFMT} is something to keep in mind if you must port your program to other implementations of @code{awk}; we recommend that instead of changing your programs, you just port @code{gawk} itself!@refill @node Values, Conditional Exp, Conversion, Expressions @section Numeric and String Values @cindex conversion of strings and numbers Through most of this manual, we present @code{awk} values (such as constants, fields, or variables) as @emph{either} numbers @emph{or} strings. This is a convenient way to think about them, since typically they are used in only one way, or the other. In truth though, @code{awk} values can be @emph{both} string and numeric, at the same time. Internally, @code{awk} represents values with a string, a (floating point) number, and an indication that one, the other, or both representations of the value are valid. Keeping track of both kinds of values is important for execution efficiency: a variable can acquire a string value the first time it is used as a string, and then that string value can be used until the variable is assigned a new value. Thus, if a variable with only a numeric value is used in several concatenations in a row, it only has to be given a string representation once. The numeric value remains valid, so that no conversion back to a number is necessary if the variable is later used in an arithmetic expression. Tracking both kinds of values is also important for precise numerical calculations. Consider the following: @smallexample a = 123.321 CONVFMT = "%3.1f" b = a " is a number" c = a + 1.654 @end smallexample @noindent The variable @code{a} receives a string value in the concatenation and assignment to @code{b}. The string value of @code{a} is @code{"123.3"}. If the numeric value was lost when it was converted to a string, then the numeric use of @code{a} in the last statement would lose information. @code{c} would be assigned the value 124.954 instead of 124.975. Such errors accumulate rapidly, and very adversely affect numeric computations.@refill Once a numeric value acquires a corresponding string value, it stays valid until a new assignment is made. If @code{CONVFMT} (@pxref{Conversion, ,Conversion of Strings and Numbers}) changes in the meantime, the old string value will still be used. For example:@refill @smallexample BEGIN @{ CONVFMT = "%2.2f" a = 123.456 b = a "" # force `a' to have string value too printf "a = %s\n", a CONVFMT = "%.6g" printf "a = %s\n", a a += 0 # make `a' numeric only again printf "a = %s\n", a # use `a' as string @} @end smallexample @noindent This program prints @samp{a = 123.46} twice, and then prints @samp{a = 123.456}. @xref{Conversion, ,Conversion of Strings and Numbers}, for the rules that specify how string values are made from numeric values. @node Conditional Exp, Function Calls, Values, Expressions @section Conditional Expressions @cindex conditional expression @cindex expression, conditional A @dfn{conditional expression} is a special kind of expression with three operands. It allows you to use one expression's value to select one of two other expressions. The conditional expression looks the same as in the C language: @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 and 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.@refill For example, this expression produces the absolute value of @code{x}: @example x > 0 ? x : -x @end example Each time the conditional expression is computed, exactly one of @var{if-true-exp} and @var{if-false-exp} is computed; the other is ignored. This is important when the expressions contain 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 one or the other of the two increment expressions is executed, and the other is not. @node Function Calls, Precedence, Conditional Exp, Expressions @section Function Calls @cindex function call @cindex calling a function A @dfn{function} is a name for a particular calculation. Because it has a name, you can ask for it by name at any point in the program. For example, the function @code{sqrt} computes the square root of a number. A fixed set of functions are @dfn{built-in}, which means they are available in every @code{awk} program. The @code{sqrt} function is one of these. @xref{Built-in, ,Built-in Functions}, for a list of built-in functions and their descriptions. In addition, you can define your own functions in the program for use elsewhere in the same program. @xref{User-defined, ,User-defined Functions}, for how to do this.@refill @cindex arguments in function call The way to use a function is with a @dfn{function call} expression, which consists of the function name followed by a list of @dfn{arguments} in parentheses. The arguments are expressions which give the raw materials for the calculation that the function will do. When there is more than one argument, they are separated by commas. If there are no arguments, write just @samp{()} after the function name. Here are some examples: @example sqrt(x^2 + y^2) # @r{One argument} atan2(y, x) # @r{Two arguments} rand() # @r{No arguments} @end example @strong{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, and space would make the expression look like concatenation of a variable with an expression inside parentheses. Space before the parenthesis is harmless with built-in functions, 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 to take the square root of: @example sqrt(@var{argument}) @end example Some of the built-in functions allow you to omit the final argument. If you do so, they use a reasonable default. @xref{Built-in, ,Built-in Functions}, for full details. If arguments are omitted in calls to user-defined functions, then those arguments are treated as local variables, initialized to the null string (@pxref{User-defined, ,User-defined Functions}).@refill 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 @code{sqrt(@var{argument})} is the square root of the argument. A function can also have side effects, such as assigning the values of certain variables or doing I/O. Here is a command to read numbers, one number per line, and print the square root of each one: @example awk '@{ print "The square root of", $1, "is", sqrt($1) @}' @end example @node Precedence, , Function Calls, Expressions @section Operator Precedence (How Operators Nest) @cindex precedence @cindex operator precedence @dfn{Operator precedence} determines how operators are grouped, when different operators appear close by in one expression. For example, @samp{*} has higher precedence than @samp{+}; thus, @code{a + b * c} means to multiply @code{b} and @code{c}, and then add @code{a} to the product (i.e., @code{a + (b * c)}). You can overrule the precedence of the operators by using parentheses. You can think of the precedence rules as saying where the parentheses are assumed if you do not write parentheses yourself. In fact, it is wise to always use parentheses whenever you have an unusual combination of operators, because other people who read the program may not remember what the precedence is in this case. You might forget, too; then you could make a mistake. Explicit parentheses will help prevent any such mistake. When operators of equal precedence are used together, the leftmost operator groups first, except for the assignment, conditional and exponentiation operators, which group in the opposite order. Thus, @code{a - b + c} groups as @code{(a - b) + c}; @code{a = b = c} groups as @code{a = (b = c)}.@refill The precedence of prefix unary operators does not matter as long as only unary operators are involved, because there is only one way to parse them---innermost first. Thus, @code{$++i} means @code{$(++i)} and @code{++$x} means @code{++($x)}. However, when another operator follows the operand, then the precedence of the unary operators can matter. Thus, @code{$x^2} means @code{($x)^2}, but @code{-x^2} means @code{-(x^2)}, because @samp{-} has lower precedence than @samp{^} while @samp{$} has higher precedence. Here is a table of the operators of @code{awk}, in order of increasing precedence: @table @asis @item assignment @samp{=}, @samp{+=}, @samp{-=}, @samp{*=}, @samp{/=}, @samp{%=}, @samp{^=}, @samp{**=}. These operators group right-to-left. (The @samp{**=} operator is not specified by @sc{posix}.) @item conditional @samp{?:}. This operator groups right-to-left. @item logical ``or''. @samp{||}. @item logical ``and''. @samp{&&}. @item array membership @samp{in}. @item matching @samp{~}, @samp{!~}. @item 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. The relational operators are @samp{<}, @samp{<=}, @samp{==}, @samp{!=}, @samp{>=} and @samp{>}. The I/O redirection operators are @samp{<}, @samp{>}, @samp{>>} and @samp{|}. Note that 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 which 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. @item concatenation No special token is used to indicate concatenation. The operands are simply written side by side. @item add, subtract @samp{+}, @samp{-}. @item multiply, divide, mod @samp{*}, @samp{/}, @samp{%}. @item unary plus, minus, ``not'' @samp{+}, @samp{-}, @samp{!}. @item exponentiation @samp{^}, @samp{**}. These operators group right-to-left. (The @samp{**} operator is not specified by @sc{posix}.) @item increment, decrement @samp{++}, @samp{--}. @item field @samp{$}. @end table @node Statements, Arrays, Expressions, Top @chapter Control Statements in Actions @cindex control statement @dfn{Control statements} such as @code{if}, @code{while}, and so on control the flow of execution in @code{awk} programs. Most of the control statements in @code{awk} are patterned on similar statements in C. 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 which may or may not be executed. The contained statement is called the @dfn{body}. If you want to include more than one statement in the body, group them into a single compound statement with curly braces, separating them with newlines or semicolons. @menu * If Statement:: Conditionally execute some @code{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. * 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. * Next File Statement:: Stop processing the current file. * Exit Statement:: Stop execution of @code{awk}. @end menu @node If Statement, While Statement, Statements, Statements @section The @code{if} Statement @cindex @code{if} statement The @code{if}-@code{else} statement is @code{awk}'s decision-making statement. It looks like this:@refill @example if (@var{condition}) @var{then-body} @r{[}else @var{else-body}@r{]} @end example @noindent @var{condition} is an expression that controls what the rest of the statement will do. If @var{condition} is true, @var{then-body} is executed; otherwise, @var{else-body} is executed (assuming that the @code{else} clause is present). The @code{else} part of the statement is optional. The condition is considered false if its value is zero or the null string, and true otherwise.@refill Here is an example: @example if (x % 2 == 0) print "x is even" else print "x is odd" @end example In this example, if the expression @code{x % 2 == 0} is true (that is, the value of @code{x} is divisible by 2), then the first @code{print} statement is executed, otherwise the second @code{print} statement is performed.@refill If the @code{else} 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 @code{else}. To illustrate this, let's rewrite the previous example: @example awk '@{ if (x % 2 == 0) print "x is even"; else print "x is odd" @}' @end example @noindent If you forget the @samp{;}, @code{awk} won't be able to parse the statement, and you will get a syntax error. We would not actually write this example this way, because a human reader might fail to see the @code{else} if it were not the first thing on its line. @node While Statement, Do Statement, If Statement, Statements @section The @code{while} Statement @cindex @code{while} statement @cindex loop @cindex body of a loop In programming, a @dfn{loop} means a part of a program that is (or at least can be) executed two or more times in succession. The @code{while} statement is the simplest looping statement in @code{awk}. It repeatedly executes a statement as long as a condition is true. It looks like this: @example while (@var{condition}) @var{body} @end example @noindent Here @var{body} is a statement that we call 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 @var{condition}. If @var{condition} is true, it executes the statement @var{body}. (@var{condition} is true when the value is not zero and not a null string.) 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 @var{condition} is no longer true. If @var{condition} is initially false, the body of the loop is never executed.@refill This example prints the first three fields of each record, one per line. @example awk '@{ i = 1 while (i <= 3) @{ print $i i++ @} @}' @end example @noindent Here the body of the loop is a compound statement enclosed in braces, containing two statements. The loop works like this: first, the value of @code{i} is set to 1. Then, the @code{while} tests whether @code{i} is less than or equal to three. This is the case when @code{i} equals one, so the @code{i}-th field is printed. Then the @code{i++} increments the value of @code{i} and the loop repeats. The loop terminates when @code{i} reaches 4. As you can see, a newline is not required between the condition and the body; but using one makes the program clearer unless the body is a compound statement or is very simple. The newline after the open-brace that begins the compound statement is not required either, but the program would be hard to read without it. @node Do Statement, For Statement, While Statement, Statements @section The @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, then repeats @var{body} as long as @var{condition} is true. It looks like this: @example do @var{body} while (@var{condition}) @end example Even if @var{condition} is false at the start, @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 @var{condition} is false to begin with. Here is an example of a @code{do} statement: @example awk '@{ i = 1 do @{ print $0 i++ @} while (i <= 10) @}' @end example @noindent prints each input record ten times. It isn't a very realistic example, since in this case an ordinary @code{while} would do just as well. But this reflects actual experience; there is only occasionally a real use for a @code{do} statement.@refill @node For Statement, Break Statement, Do Statement, Statements @section 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:@refill @example for (@var{initialization}; @var{condition}; @var{increment}) @var{body} @end example @noindent This statement starts by executing @var{initialization}. Then, as long as @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 1 to it, and @var{condition} compares it against the desired number of iterations. Here is an example of a @code{for} statement: @example @group awk '@{ for (i = 1; i <= 3; i++) print $i @}' @end group @end example @noindent This prints the first three fields of each input record, one field per line. In the @code{for} statement, @var{body} stands for any statement, but @var{initialization}, @var{condition} and @var{increment} are just expressions. You cannot set more than one variable in the @var{initialization} part unless you use a multiple assignment statement such as @code{x = y = 0}, which is possible only if all the initial values are equal. (But you can initialize additional variables by writing their assignments as separate statements preceding the @code{for} loop.) The same is true of the @var{increment} part; to increment additional variables, you must write separate statements at the end of the loop. The C compound expression, using C's comma operator, would be useful in this context, but it is not supported in @code{awk}. Most often, @var{increment} is an increment expression, as in the example above. But this is not required; it can be any expression whatever. For example, this statement prints all the powers of 2 between 1 and 100: @example for (i = 1; i <= 100; i *= 2) print i @end example Any of the three expressions in the parentheses following the @code{for} may be omitted if there is nothing to be done there. Thus, @w{@samp{for (;x > 0;)}} is equivalent to @w{@samp{while (x > 0)}}. If the @var{condition} is omitted, it is treated as @var{true}, effectively yielding an @dfn{infinite loop} (i.e., a loop that will never terminate).@refill 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 @noindent The only exception is when the @code{continue} statement (@pxref{Continue Statement, ,The @code{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.@refill 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{Arrays, ,Arrays in @code{awk}}, for more information on this version of the @code{for} loop. The @code{awk} language has a @code{for} statement in addition to a @code{while} statement because often a @code{for} loop is 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. The next section has more complicated examples of @code{for} loops. @node Break Statement, Continue Statement, For Statement, Statements @section 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}-@code{while} loop that encloses it. The following example finds the smallest divisor of any integer, and also identifies prime numbers:@refill @smallexample awk '# 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 smallexample When the remainder is zero in the first @code{if} statement, @code{awk} immediately @dfn{breaks out} of the containing @code{for} loop. This means that @code{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 @code{awk} program. @xref{Exit Statement, ,The @code{exit} Statement}.)@refill Here is another program equivalent to the previous one. It illustrates how the @var{condition} of a @code{for} or @code{while} could just as well be replaced with a @code{break} inside an @code{if}: @smallexample @group awk '# 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 group @end smallexample @node Continue Statement, Next Statement, Break Statement, Statements @section The @code{continue} Statement @cindex @code{continue} statement The @code{continue} statement, like @code{break}, is used only inside @code{for}, @code{while}, and @code{do}-@code{while} 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. Here is an example:@refill @example # print names that don't contain the string "ignore" # first, save the text of each line @{ names[NR] = $0 @} # print what we're interested in END @{ for (x in names) @{ if (names[x] ~ /ignore/) continue print names[x] @} @} @end example If one of the input records contains the string @samp{ignore}, this example skips the print statement for that record, and continues back to the first statement in the loop. This is not a practical example of @code{continue}, since it would be just as easy to write the loop like this: @example for (x in names) if (names[x] !~ /ignore/) print names[x] @end example @ignore from brennan@boeing.com: page 90, section 9.6. The example is too artificial as the one line program !/ignore/ does the same thing. @end ignore @c ADR --- he's right, but don't worry about this for now The @code{continue} statement in a @code{for} loop directs @code{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:@refill @example awk '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. Since the increment @code{x++} is not skipped, @code{x} does not remain stuck at 5. Contrast the @code{for} loop above with the @code{while} loop: @example awk '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} gets to 5. As described above, the @code{continue} statement has no meaning when used outside the body of a loop. However, although it was never documented, historical implementations of @code{awk} have treated the @code{continue} statement outside of a loop as if it were a @code{next} statement (@pxref{Next Statement, ,The @code{next} Statement}). By default, @code{gawk} silently supports this usage. However, if @samp{-W posix} has been specified on the command line (@pxref{Command Line, ,Invoking @code{awk}}), it will be treated as an error, since the @sc{posix} standard specifies that @code{continue} should only be used inside the body of a loop.@refill @node Next Statement, Next File Statement, Continue Statement, Statements @section The @code{next} Statement @cindex @code{next} statement The @code{next} statement forces @code{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. The rest of the current rule's action is not executed either. Contrast this with the effect of the @code{getline} function (@pxref{Getline, ,Explicit Input with @code{getline}}). That too causes @code{awk} to read the next record immediately, but it does not alter the flow of control in any way. So the rest of the current action executes with a new input record. At the highest level, @code{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, if your @code{awk} program works only on records with four fields, and you don't want it to fail when given bad input, you might use this rule near the beginning of the program: @smallexample NF != 4 @{ printf("line %d skipped: doesn't have 4 fields", FNR) > "/dev/stderr" next @} @end smallexample @noindent so that the following rules will not see the bad record. The error message is redirected to the standard error output stream, as error messages should be. @xref{Special Files, ,Standard I/O Streams}. According to the @sc{posix} standard, the behavior is undefined if the @code{next} statement is used in a @code{BEGIN} or @code{END} rule. @code{gawk} will treat it as a syntax error. If the @code{next} statement causes the end of the input to be reached, then the code in the @code{END} rules, if any, will be executed. @xref{BEGIN/END, ,@code{BEGIN} and @code{END} Special Patterns}. @node Next File Statement, Exit Statement, Next Statement, Statements @section The @code{next file} Statement @cindex @code{next file} statement The @code{next file} statement is similar to the @code{next} statement. However, instead of abandoning processing of the current record, the @code{next file} statement instructs @code{awk} to stop processing the current data file. Upon execution of the @code{next file} statement, @code{FILENAME} is updated to the name of the next data file listed on the command line, @code{FNR} is reset to 1, and processing starts over with the first rule in the progam. @xref{Built-in Variables}. If the @code{next file} statement causes the end of the input to be reached, then the code in the @code{END} rules, if any, will be executed. @xref{BEGIN/END, ,@code{BEGIN} and @code{END} Special Patterns}. The @code{next file} statement is a @code{gawk} extension; it is not (currently) available in any other @code{awk} implementation. You can simulate its behavior by creating a library file named @file{nextfile.awk}, with the following contents. (This sample program uses user-defined functions, a feature that has not been presented yet. @xref{User-defined, ,User-defined Functions}, for more information.)@refill @smallexample # nextfile --- function to skip remaining records in current file # this should be read in before the "main" awk program function nextfile() @{ _abandon_ = FILENAME; next @} _abandon_ == FILENAME && FNR > 1 @{ next @} _abandon_ == FILENAME && FNR == 1 @{ _abandon_ = "" @} @end smallexample The @code{nextfile} function simply sets a ``private'' variable@footnote{Since all variables in @code{awk} are global, this program uses the common practice of prefixing the variable name with an underscore. In fact, it also suffixes the variable name with an underscore, as extra insurance against using a variable name that might be used in some other library file.} to the name of the current data file, and then retrieves the next record. Since this file is read before the main @code{awk} program, the rules that follows the function definition will be executed before the rules in the main program. The first rule continues to skip records as long as the name of the input file has not changed, and this is not the first record in the file. This rule is sufficient most of the time. But what if the @emph{same} data file is named twice in a row on the command line? This rule would not process the data file the second time. The second rule catches this case: If the data file name is what was being skipped, but @code{FNR} is 1, then this is the second time the file is being processed, and it should not be skipped. The @code{next file} statement would be useful if you have many data files to process, and due to the nature of the data, you expect that you would not want to process every record in the file. In order to move on to the next data file, you would have to continue scanning the unwanted records (as described above). The @code{next file} statement accomplishes this much more efficiently. @ignore Would it make sense down the road to nuke `next file' in favor of semantics that would make this work? function nextfile() { ARGIND++ ; next } @end ignore @node Exit Statement, , Next File Statement, Statements @section The @code{exit} Statement @cindex @code{exit} statement The @code{exit} statement causes @code{awk} to immediately stop executing the current rule and to stop processing input; any remaining input is ignored.@refill If 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, it is executed (@pxref{BEGIN/END, ,@code{BEGIN} and @code{END} Special Patterns}). 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 part of an ordinary rule (that is, not part of a @code{BEGIN} or @code{END} rule) stops the execution of any further automatic rules, but the @code{END} rule is executed if there is one. If you do not want the @code{END} rule to do its job in this case, you can set a variable to nonzero before the @code{exit} statement, and check that variable in the @code{END} rule. If an argument is supplied to @code{exit}, its value is used as the exit status code for the @code{awk} process. If no argument is supplied, @code{exit} returns status zero (success).@refill For example, let's say you've discovered an error condition you really don't know how to handle. Conventionally, programs report this by exiting with a nonzero status. Your @code{awk} program can do this using an @code{exit} statement with a nonzero argument. Here's an example of this:@refill @example @group BEGIN @{ if (("date" | getline date_now) < 0) @{ print "Can't get system date" > "/dev/stderr" exit 4 @} @} @end group @end example @node Arrays, Built-in, Statements, Top @chapter Arrays in @code{awk} 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. Each array has a name, which looks like a variable name, but must not be in use as a variable name in the same @code{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 @code{awk}. * Multi-dimensional:: Emulating multi-dimensional arrays in @code{awk}. * Multi-scanning:: Scanning multi-dimensional arrays. @end menu @node Array Intro, Reference to Elements, Arrays, Arrays @section Introduction to Arrays @cindex arrays The @code{awk} language has one-dimensional @dfn{arrays} for storing groups of related strings or numbers. Every @code{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 you cannot use one name in both ways (as an array and as a variable) in one @code{awk} program. Arrays in @code{awk} superficially resemble arrays in other programming languages; but there are fundamental differences. In @code{awk}, you don't need to specify the size of an array before you start to use it. Additionally, any number or string in @code{awk} may be used as an array index. In most other languages, you have to @dfn{declare} an array and specify how many elements or components it contains. In such languages, the declaration causes a contiguous block of memory to be allocated for that many elements. An index in the array must be a positive integer; for example, the index 0 specifies the first element in the array, which is actually stored at the beginning of the block of memory. Index 1 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 for only as many elements as you declared. A contiguous array of four elements might look like this, conceptually, if the element values are @code{8}, @code{"foo"}, @code{""} and @code{30}:@refill @example +---------+---------+--------+---------+ | 8 | "foo" | "" | 30 | @r{value} +---------+---------+--------+---------+ 0 1 2 3 @r{index} @end example @noindent Only the values are stored; the indices are implicit from the order of the values. @code{8} is the value at index 0, because @code{8} appears in the position with 0 elements before it. @cindex arrays, definition of @cindex associative arrays Arrays in @code{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} 4 @r{Value} 30 @r{Element} 2 @r{Value} "foo" @r{Element} 1 @r{Value} 8 @r{Element} 3 @r{Value} "" @end example @noindent We have shown the pairs in jumbled order because their order is irrelevant. One advantage of an associative array is that new pairs can be added at any time. For example, suppose we add to the above array a tenth element whose value is @w{@code{"number ten"}}. The result is this: @example @r{Element} 10 @r{Value} "number ten" @r{Element} 4 @r{Value} 30 @r{Element} 2 @r{Value} "foo" @r{Element} 1 @r{Value} 8 @r{Element} 3 @r{Value} "" @end example @noindent Now the array is @dfn{sparse} (i.e., some indices are missing): it has elements 1--4 and 10, but doesn't have elements 5, 6, 7, 8, or 9.@refill 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, here is an array which translates words from English into 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 1 in both spelled-out and numeric form---thus illustrating that a single array can have both numbers and strings as indices. When @code{awk} creates an array for you, e.g., with the @code{split} built-in function, that array's indices are consecutive integers starting at 1. (@xref{String Functions, ,Built-in Functions for String Manipulation}.) @node Reference to Elements, Assigning Elements, Array Intro, Arrays @section Referring to an Array Element @cindex array reference @cindex element of array @cindex reference to array The principal way of using an array is to refer to one of its elements. An array reference is an expression which looks like this: @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 element of the array that you want. 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 4.3. If you refer to an array element that has no recorded value, the value of the reference is @code{""}, the null string. This includes elements to which you have not assigned any value, and elements that have been deleted (@pxref{Delete, ,The @code{delete} Statement}). 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 @code{awk}). @cindex arrays, presence of elements You can find out if an element exists in an array at a certain index with the expression: @example @var{index} in @var{array} @end example @noindent This expression tests whether or not the particular index exists, without the side effect of creating that element if it is not present. The expression has the value 1 (true) if @code{@var{array}[@var{index}]} exists, and 0 (false) if it does not exist.@refill For example, to test whether the array @code{frequencies} contains the index @code{"2"}, you could write this statement:@refill @smallexample if ("2" in frequencies) print "Subscript \"2\" is present." @end smallexample Note that this is @emph{not} a test of whether or not the array @code{frequencies} contains an element whose @emph{value} is @code{"2"}. (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 would do so:@refill @smallexample if (frequencies["2"] != "") print "Subscript \"2\" is present." @end smallexample @node Assigning Elements, Array Example, Reference to Elements, Arrays @section Assigning Array Elements @cindex array assignment @cindex element assignment Array elements are lvalues: they can be assigned values just like @code{awk} variables: @example @var{array}[@var{subscript}] = @var{value} @end example @noindent Here @var{array} is the name of your array. The expression @var{subscript} is the index of the element of the array that you want to assign a value. The expression @var{value} is the value you are assigning to that element of the array.@refill @node Array Example, Scanning an Array, Assigning Elements, Arrays @section Basic Example of an Array 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, however, when they are first read: they are scrambled. This program sorts the lines by making an array using the line numbers as subscripts. It then prints out the lines in sorted order of their numbers. It is a very simple program, and gets confused if it encounters repeated numbers, gaps, or lines that don't begin with a number.@refill @example @{ if ($1 > max) max = $1 arr[$1] = $0 @} END @{ for (x = 1; x <= max; x++) print arr[x] @} @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 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. @end example @noindent its output is this: @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: @example END @{ for (x = 1; x <= max; x++) if (x in arr) print arr[x] @} @end example @node Scanning an Array, Delete, Array Example, Arrays @section Scanning all Elements of an Array @cindex @code{for (x in @dots{})} @cindex arrays, special @code{for} statement @cindex scanning an array In programs that use arrays, often you need 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: the largest index is one less than the length of the array, and you can find all the valid indices by counting from zero up to that value. This technique won't do the job in @code{awk}, since any number or string may be an array index. So @code{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 This loop executes @var{body} once for each different value that your program has previously used as an index in @var{array}, with the variable @var{var} set to that index.@refill Here is a program that 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 1 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{Built-in, ,Built-in Functions}, for more information on the built-in function @code{length}. @smallexample # 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 smallexample @noindent @xref{Sample Program}, for a more detailed example of this type. The order in which elements of the array are accessed by this statement is determined by the internal arrangement of the array elements within @code{awk} and cannot be controlled or changed. This can lead to problems if new elements are added to @var{array} by statements in @var{body}; you cannot predict whether or not the @code{for} loop will reach them. Similarly, changing @var{var} inside the loop can produce strange results. It is best to avoid such things.@refill @node Delete, Numeric Array Subscripts, Scanning an Array, Arrays @section The @code{delete} Statement @cindex @code{delete} statement @cindex deleting elements of arrays @cindex removing elements of arrays @cindex arrays, deleting an element You can remove an individual element of an array using the @code{delete} statement: @example delete @var{array}[@var{index}] @end example You can not refer to an array element after it has been deleted; it is as if you had never referred to it and had never given it any value. You can no longer obtain any value the element once had. Here 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}. If you delete an element, a subsequent @code{for} statement to scan the array will not report that element, and the @code{in} operator to check for the presence of that element will return 0: @example delete foo[4] if (4 in foo) print "This will never be printed" @end example It is not an error to delete an element which does not exist. @node Numeric Array Subscripts, Multi-dimensional, Delete, Arrays @section Using Numbers to Subscript Arrays An important aspect of arrays to remember is that array subscripts are @emph{always} strings. If you use a numeric value as a subscript, it will be converted to a string value before it is used for subscripting (@pxref{Conversion, ,Conversion of Strings and Numbers}). @cindex conversions, during subscripting @cindex numbers, used as subscripts @vindex CONVFMT This means that the value of the @code{CONVFMT} can potentially affect how your program accesses elements of an array. For example: @example a = b = 12.153 data[a] = 1 CONVFMT = "%2.2f" if (b in data) printf "%s is in data", b else printf "%s is not in data", b @end example @noindent should print @samp{12.15 is not in data}. The first statement gives both @code{a} and @code{b} the same numeric value. Assigning to @code{data[a]} first gives @code{a} the string value @code{"12.153"} (using the default conversion value of @code{CONVFMT}, @code{"%.6g"}), and then assigns 1 to @code{data["12.153"]}. The program then changes the value of @code{CONVFMT}. The test @samp{(b in data)} forces @code{b} to be converted to a string, this time @code{"12.15"}, since 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"}.@refill According to the rules for conversions (@pxref{Conversion, ,Conversion of Strings and Numbers}), 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@refill @example for (i = 1; i <= maxsub; i++) @i{do something with} array[i] @end example @noindent will work, no matter what the value of @code{CONVFMT}. Like many things in @code{awk}, the majority of the time things work as you would expect them to work. But it is useful to have a precise knowledge of the actual rules, since sometimes they can have a subtle effect on your programs. @node Multi-dimensional, Multi-scanning, Numeric Array Subscripts, Arrays @section Multi-dimensional Arrays @c the following index entry is an overfull hbox. --mew 30jan1992 @cindex subscripts in arrays @cindex arrays, multi-dimensional subscripts @cindex multi-dimensional subscripts A multi-dimensional array is an array in which an element is identified by a sequence of indices, not a single index. For example, a two-dimensional array requires two indices. The usual way (in most languages, including @code{awk}) to refer to an element of a two-dimensional array named @code{grid} is with @code{grid[@var{x},@var{y}]}. @vindex SUBSEP Multi-dimensional arrays are supported in @code{awk} through concatenation of indices into one string. What happens is that @code{awk} converts the indices into strings (@pxref{Conversion, ,Conversion of Strings and Numbers}) 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}.@refill For example, suppose we evaluate the expression @code{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"}.@refill Once the element's value is stored, @code{awk} has no record of whether it was stored with a single index or a sequence of indices. The two expressions @code{foo[5,12]} and @w{@code{foo[5 SUBSEP 12]}} always have the same value. The default value of @code{SUBSEP} is the string @code{"\034"}, which contains a nonprinting character that is unlikely to appear in an @code{awk} program or in the input data. The usefulness of choosing an unlikely character comes from the fact that index values that contain a string matching @code{SUBSEP} lead to combined strings that are ambiguous. Suppose that @code{SUBSEP} were @code{"@@"}; then @w{@code{foo["a@@b", "c"]}} and @w{@code{foo["a", "b@@c"]}} would be indistinguishable because both would actually be stored as @code{foo["a@@b@@c"]}. Because @code{SUBSEP} is @code{"\034"}, such confusion can arise only when an index contains the character with ASCII code 034, which is a rare event.@refill You can test whether a particular index-sequence exists in a ``multi-dimensional'' array with the same operator @code{in} used for single dimensional arrays. Instead of a single index as the left-hand operand, write the whole sequence of indices, separated by commas, in parentheses:@refill @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 awk '@{ 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 @group 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 group @end example @noindent it produces: @example @group 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 group @end example @node Multi-scanning, , Multi-dimensional, Arrays @section Scanning Multi-dimensional Arrays There is no special @code{for} statement for scanning a ``multi-dimensional'' array; there cannot be one, because in truth there are no multi-dimensional arrays or elements; there is only a multi-dimensional @emph{way of accessing} an array. However, if your program has an array that is always accessed as multi-dimensional, you can get the effect of scanning it by combining the scanning @code{for} statement (@pxref{Scanning an Array, ,Scanning all Elements of an Array}) with the @code{split} built-in function (@pxref{String Functions, ,Built-in Functions for String Manipulation}). It works like this:@refill @example for (combined in @var{array}) @{ split(combined, separate, SUBSEP) @dots{} @} @end example @noindent This finds 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 split-out indices become the elements of the array @code{separate}. Thus, suppose you have previously stored in @code{@var{array}[1, "foo"]}; then an element with index @code{"1\034foo"} exists in @var{array}. (Recall that the default value of @code{SUBSEP} contains the character with code 034.) Sooner or later the @code{for} statement will find that index and do an iteration with @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 of this is to set @code{separate[1]} to 1 and @code{separate[2]} to @code{"foo"}. Presto, the original sequence of separate indices has been recovered. @node Built-in, User-defined, Arrays, Top @chapter Built-in Functions @cindex built-in functions @dfn{Built-in} functions are functions that are always available for your @code{awk} program to call. This chapter defines all the built-in functions in @code{awk}; some of them are mentioned in other sections, but they are summarized here for your convenience. (You can also define new functions yourself. @xref{User-defined, ,User-defined Functions}.) @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 time stamps. @end menu @node Calling Built-in, Numeric Functions, Built-in, Built-in @section Calling Built-in Functions To call a built-in function, write the name of the function followed by arguments in parentheses. For example, @code{atan2(y + z, 1)} is a call to the function @code{atan2}, with two arguments. Whitespace is ignored between the built-in function name and the open-parenthesis, but we recommend that you avoid using whitespace there. User-defined functions do not permit whitespace in this way, and you will find it easier to avoid mistakes by following a simple convention which always works: no whitespace after a function name. Each built-in function accepts a certain number of arguments. In most cases, any extra arguments given to built-in functions are ignored. The defaults for omitted arguments vary from function to function and are described under the individual functions. When a function is called, expressions that create the function's actual parameters are evaluated completely before the function call is performed. For example, in the code fragment: @example i = 4 j = sqrt(i++) @end example @noindent the variable @code{i} is set to 5 before @code{sqrt} is called with a value of 4 for its actual parameter. @node Numeric Functions, String Functions, Calling Built-in, Built-in @section Numeric Built-in Functions @c I didn't make all the examples small because a couple of them were @c short already. --mew 29jan1992 Here is a full list of built-in functions that work with numbers: @table @code @item int(@var{x}) This gives you the integer part of @var{x}, truncated toward 0. This produces the nearest integer to @var{x}, located between @var{x} and 0. 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.@refill @item sqrt(@var{x}) This gives you the positive square root of @var{x}. It reports an error if @var{x} is negative. Thus, @code{sqrt(4)} is 2.@refill @item exp(@var{x}) This gives you the exponential of @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.@refill @item log(@var{x}) This gives you the natural logarithm of @var{x}, if @var{x} is positive; otherwise, it reports an error.@refill @item sin(@var{x}) This gives you the sine of @var{x}, with @var{x} in radians. @item cos(@var{x}) This gives you the cosine of @var{x}, with @var{x} in radians. @item atan2(@var{y}, @var{x}) This gives you the arctangent of @code{@var{y} / @var{x}} in radians. @item rand() This gives you a random number. The values of @code{rand} are uniformly-distributed between 0 and 1. The value is never 0 and never 1. Often you want random integers instead. Here is a user-defined function you can use to obtain a random nonnegative integer less than @var{n}: @example function randint(n) @{ return int(n * rand()) @} @end example @noindent The multiplication produces a random real number greater than 0 and less than @var{n}. We then make it an integer (using @code{int}) between 0 and @code{@var{n} @minus{} 1}. Here is an example where a similar function is used to produce random integers between 1 and @var{n}. Note that this program will print a new random number for each input record. @smallexample awk ' # 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 smallexample @strong{Note:} @code{rand} starts generating numbers from the same point, or @dfn{seed}, each time you run @code{awk}. This means that a program will produce the same results each time you run it. The numbers are random within one @code{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 will be different in each run. To do this, use @code{srand}. @item srand(@var{x}) The function @code{srand} sets the starting point, or @dfn{seed}, for generating random numbers to the value @var{x}. Each seed value leads to a particular sequence of ``random'' numbers. Thus, if you set the seed to the same value a second time, you will get the same sequence of ``random'' numbers again. If you omit the argument @var{x}, as in @code{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 for use in consistently reproducing sequences of random numbers. @end table @node String Functions, I/O Functions, Numeric Functions, Built-in @section Built-in Functions for String Manipulation The functions in this section look at or change the text of one or more strings. @table @code @item index(@var{in}, @var{find}) @findex match 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}. For example:@refill @smallexample awk 'BEGIN @{ print index("peanut", "an") @}' @end smallexample @noindent prints @samp{3}. If @var{find} is not found, @code{index} returns 0. (Remember that string indices in @code{awk} start at 1.) @item length(@var{string}) @findex length This gives you 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. How? Well, 15 * 35 = 525, and 525 is then converted to the string @samp{"525"}, which has three characters. If no argument is supplied, @code{length} returns the length of @code{$0}. In older versions of @code{awk}, you could call the @code{length} function without any parentheses. Doing so is marked as ``deprecated'' in the @sc{posix} standard. This means that while you can do this in your programs, it is a feature that can eventually be removed from a future version of the standard. Therefore, for maximal portability of your @code{awk} programs you should always supply the parentheses. @item match(@var{string}, @var{regexp}) @findex match The @code{match} function searches the string, @var{string}, for the longest, leftmost substring matched by the regular expression, @var{regexp}. It returns the character position, or @dfn{index}, of where that substring begins (1, if it starts at the beginning of @var{string}). If no match if found, it returns 0. @vindex RSTART @vindex RLENGTH 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 0, and @code{RLENGTH} to @minus{}1. For example: @smallexample awk '@{ if ($1 == "FIND") regex = $2 else @{ where = match($0, regex) if (where) print "Match of", regex, "found at", where, "in", $0 @} @}' @end smallexample @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, given: @smallexample FIND fo*bar My program was a foobar But none of it would doobar FIND Melvin JF+KM This line is property of The Reality Engineering Co. This file created by Melvin. @end smallexample @noindent @code{awk} prints: @smallexample Match of fo*bar found at 18 in My program was a foobar Match of Melvin found at 26 in This file created by Melvin. @end smallexample @item split(@var{string}, @var{array}, @var{fieldsep}) @findex split This 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 the @var{fieldsep} is omitted, the value of @code{FS} is used. @code{split} returns the number of elements created.@refill The @code{split} function, then, splits strings into pieces in a manner similar to the way input lines are split into fields. For example: @smallexample split("auto-da-fe", a, "-") @end smallexample @noindent splits the string @samp{auto-da-fe} into three fields using @samp{-} as the separator. It sets the contents of the array @code{a} as follows: @smallexample a[1] = "auto" a[2] = "da" a[3] = "fe" @end smallexample @noindent The value returned by this call to @code{split} is 3. As with input field-splitting, when the value of @var{fieldsep} is @code{" "}, leading and trailing whitespace is ignored, and the elements are separated by runs of whitespace. @item sprintf(@var{format}, @var{expression1},@dots{}) @findex sprintf This returns (without printing) the string that @code{printf} would have printed out with the same arguments (@pxref{Printf, ,Using @code{printf} Statements for Fancier Printing}). For example:@refill @smallexample sprintf("pi = %.2f (approx.)", 22/7) @end smallexample @noindent returns the string @w{@code{"pi = 3.14 (approx.)"}}. @item sub(@var{regexp}, @var{replacement}, @var{target}) @findex sub The @code{sub} function alters the value of @var{target}. It searches this value, which should be a string, for the leftmost substring matched by the regular expression, @var{regexp}, extending this match as far as possible. Then the entire string is changed by replacing the matched text with @var{replacement}. The modified string becomes the new value of @var{target}. 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 reference, 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}. For example:@refill @smallexample str = "water, water, everywhere" sub(/at/, "ith", str) @end smallexample @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:@refill @smallexample awk '@{ sub(/candidate/, "& and his wife"); print @}' @end smallexample @noindent changes the first occurrence of @samp{candidate} to @samp{candidate and his wife} on each input line. Here is another example: @smallexample awk 'BEGIN @{ str = "daabaaa" sub(/a*/, "c&c", str) print str @}' @end smallexample @noindent prints @samp{dcaacbaaa}. This show how @samp{&} can represent a non-constant string, and also illustrates the ``leftmost, longest'' rule. 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, here is how to replace the first @samp{|} on each line with an @samp{&}:@refill @smallexample awk '@{ sub(/\|/, "\\&"); print @}' @end smallexample @strong{Note:} as mentioned above, the third argument to @code{sub} must be an lvalue. Some versions of @code{awk} allow the third argument to be an expression which is not an lvalue. In such a case, @code{sub} would still search for the pattern and return 0 or 1, but the result of the substitution (if any) would be thrown away because there is no place to put it. Such versions of @code{awk} accept expressions like this:@refill @smallexample sub(/USA/, "United States", "the USA and Canada") @end smallexample @noindent But that is considered erroneous in @code{gawk}. @item gsub(@var{regexp}, @var{replacement}, @var{target}) @findex gsub 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:@refill @smallexample awk '@{ gsub(/Britain/, "United Kingdom"); print @}' @end smallexample @noindent replaces all occurrences of the string @samp{Britain} with @samp{United Kingdom} for all input records.@refill The @code{gsub} function returns the number of substitutions made. If the variable to be searched and altered, @var{target}, is omitted, then the entire input record, @code{$0}, is used.@refill As in @code{sub}, the characters @samp{&} and @samp{\} are special, and the third argument must be an lvalue. @item substr(@var{string}, @var{start}, @var{length}) @findex substr 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. For example, @code{substr("washington", 5, 3)} returns @code{"ing"}.@refill 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"}. This is also the case if @var{length} is greater than the number of characters remaining in the string, counting from character number @var{start}. @item tolower(@var{string}) @findex tolower This returns a copy of @var{string}, with each upper-case character in the string replaced with its corresponding lower-case character. Nonalphabetic characters are left unchanged. For example, @code{tolower("MiXeD cAsE 123")} returns @code{"mixed case 123"}. @item toupper(@var{string}) @findex toupper This returns a copy of @var{string}, with each lower-case character in the string replaced with its corresponding upper-case character. Nonalphabetic characters are left unchanged. For example, @code{toupper("MiXeD cAsE 123")} returns @code{"MIXED CASE 123"}. @end table @node I/O Functions, Time Functions, String Functions, Built-in @section Built-in Functions for Input/Output @table @code @item close(@var{filename}) Close the file @var{filename}, for input or output. The argument may alternatively be a shell command that was used for redirecting to or from a pipe; then the pipe is closed. @xref{Close Input, ,Closing Input Files and Pipes}, regarding closing input files and pipes. @xref{Close Output, ,Closing Output Files and Pipes}, regarding closing output files and pipes.@refill @item system(@var{command}) @findex system @c the following index entry is an overfull hbox. --mew 30jan1992 @cindex interaction, @code{awk} and other programs The system function allows the user to execute operating system commands and then return to the @code{awk} program. The @code{system} function executes the command given by the string @var{command}. It returns, as its value, the status returned by the command that was executed. For example, if the following fragment of code is put in your @code{awk} program: @smallexample END @{ system("mail -s 'awk run done' operator < /dev/null") @} @end smallexample @noindent the system operator will be sent mail when the @code{awk} program finishes processing input and begins its end-of-input processing. Note that much the same result can be obtained by redirecting @code{print} or @code{printf} into a pipe. However, if your @code{awk} program is interactive, @code{system} is useful for cranking up large self-contained programs, such as a shell or an editor.@refill 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 Controlling Output Buffering with @code{system} @cindex flushing buffers @cindex buffers, flushing @cindex buffering output @cindex output, buffering Many utility programs will @dfn{buffer} their output; they save information to be written to a disk file or terminal in memory, until there is enough to be written in one operation. 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. You can do this from your @code{awk} program by calling @code{system} with a null string as its argument: @example system("") # flush output @end example @noindent @code{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 @code{gawk}, this idiom is not only useful, it is efficient. While this idiom should work with other @code{awk} implementations, it will not necessarily avoid starting an unnecessary shell. @ignore Need a better explanation, perhaps in a separate paragraph. Explain that for awk 'BEGIN { print "hi" system("echo hello") print "howdy" }' that the output had better be hi hello howdy and not hello hi howdy which it would be if awk did not flush its buffers before calling system. @end ignore @node Time Functions, , I/O Functions, Built-in @section Functions for Dealing with Time Stamps @cindex time stamps @cindex time of day A common use for @code{awk} programs is the processing of log files. Log files often contain time stamp information, indicating when a particular log record was written. Many programs log their time stamp in the form returned by the @code{time} system call, which is the number of seconds since a particular epoch. On @sc{posix} systems, it is the number of seconds since Midnight, January 1, 1970, @sc{utc}. In order to make it easier to process such log files, and to easily produce useful reports, @code{gawk} provides two functions for working with time stamps. Both of these are @code{gawk} extensions; they are not specified in the @sc{posix} standard, nor are they in any other known version of @code{awk}. @table @code @item systime() @findex systime This function returns the current time as the number of seconds since the system epoch. On @sc{posix} systems, this is the number of seconds since Midnight, January 1, 1970, @sc{utc}. It may be a different number on other systems. @item strftime(@var{format}, @var{timestamp}) @findex strftime This function returns a string. It is similar to the function of the same name in the @sc{ansi} C standard library. The time specified by @var{timestamp} is used to produce a string, based on the contents of the @var{format} string. @end table The @code{systime} function allows you to compare a time stamp 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. The @code{strftime} function allows you to easily turn a time stamp into human-readable information. It is similar in nature to the @code{sprintf} function, copying non-format specification characters verbatim to the returned string, and substituting date and time values for format specifications in the @var{format} string. If no @var{timestamp} argument is supplied, @code{gawk} will use the current time of day as the time stamp.@refill @code{strftime} is guaranteed by the @sc{ansi} C standard 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. @item %d The day of the month as a decimal number (01--31). @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 %p The locale's equivalent of the AM/PM designations associated with a 12-hour clock. @item %S The second as a decimal number (00--61). (Occasionally there are minutes in a year with one or two leap seconds, which is why the seconds can go from 0 all the way to 61.) @item %U The week number of the year (the first Sunday as the first day of week 1) as a decimal number (00--53). @item %w The weekday as a decimal number (0--6). Sunday is day 0. @item %W The week number of the year (the first Monday as the first day of week 1) as a decimal number (00--53). @item %x The locale's ``appropriate'' date representation. @item %X The locale's ``appropriate'' time representation. @item %y The year without century as a decimal number (00--99). @item %Y The year with century as a decimal number. @item %Z The time zone name or abbreviation, or no characters if no time zone is determinable. @item %% A literal @samp{%}. @end table @c The parenthetical remark here should really be a footnote, but @c it gave formatting problems at the FSF. So for now put it in @c parentheses. If a conversion specifier is not one of the above, the behavior is undefined. (This is because the @sc{ansi} standard for C leaves the behavior of the C version of @code{strftime} undefined, and @code{gawk} will use the system's version of @code{strftime} if it's there. Typically, the conversion specifier will either not appear in the returned string, or it will appear literally.) 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 would be ``9/4/91''. In many countries in Europe, however, it would be 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 @sc{ansi} 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 shipped with @code{gawk} for systems that are not yet fully @sc{ansi}-compliant. If that version is used to compile @code{gawk} (@pxref{Installation, ,Installing @code{gawk}}), then the following additional format specifications are available:@refill @table @code @item %D Equivalent to specifying @samp{%m/%d/%y}. @item %e The day of the month, padded with a blank if it is only one digit. @item %h Equivalent to @samp{%b}, above. @item %n A newline character (ASCII LF). @item %r Equivalent to specifying @samp{%I:%M:%S %p}. @item %R Equivalent to specifying @samp{%H:%M}. @item %T Equivalent to specifying @samp{%H:%M:%S}. @item %t A TAB character. @item %k is replaced by the hour (24-hour clock) as a decimal number (0-23). Single digit numbers are padded with a blank. @item %l is replaced by the hour (12-hour clock) as a decimal number (1-12). Single digit numbers are padded with a blank. @item %C The century, as a number between 00 and 99. @item %u is replaced by the weekday as a decimal number [1 (Monday)--7]. @item %V is replaced by the week number of the year (the first Monday as the first day of week 1) 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 1, otherwise it is week 53 of the previous year and the next week is week 1).@refill @item %Ec %EC %Ex %Ey %EY %Od %Oe %OH %OI @itemx %Om %OM %OS %Ou %OU %OV %Ow %OW %Oy These are ``alternate representations'' for the specifications that use only the second letter (@samp{%c}, @samp{%C}, and so on). They are recognized, but their normal representations are used. (These facilitate compliance with the @sc{posix} @code{date} utility.)@refill @item %v The date in VMS format (e.g. 20-JUN-1991). @end table Here are two examples that use @code{strftime}. The first is an @code{awk} version of the C @code{ctime} function. (This is a user defined function, which we have not discussed yet. @xref{User-defined, ,User-defined Functions}, for more information.) @smallexample # ctime.awk # # awk version of C ctime(3) function function ctime(ts, format) @{ format = "%a %b %e %H:%M:%S %Z %Y" if (ts == 0) ts = systime() # use current time as default return strftime(format, ts) @} @end smallexample This next example is an @code{awk} implementation of the @sc{posix} @code{date} utility. Normally, the @code{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{+}, @code{date} will copy non-format specifier characters to the standard output, and will interpret the current time according to the format specifiers in the string. For example: @smallexample date '+Today is %A, %B %d, %Y.' @end smallexample @noindent might print @smallexample Today is Thursday, July 11, 1991. @end smallexample Here is the @code{awk} version of the @code{date} utility. @smallexample #! /usr/bin/gawk -f # # date --- implement the P1003.2 Draft 11 'date' command # # Bug: does not recognize the -u argument. BEGIN \ @{ format = "%a %b %e %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 smallexample @node User-defined, Built-in Variables, Built-in, Top @chapter User-defined Functions @cindex user-defined functions @cindex functions, user-defined Complicated @code{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---to tell @code{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. @end menu @node Definition Syntax, Function Example, User-defined, User-defined @section Syntax of Function Definitions @cindex defining functions @cindex function definition Definitions of functions can appear anywhere between the rules of the @code{awk} program. Thus, the general form of an @code{awk} program is extended to include sequences of rules @emph{and} user-defined function definitions. The definition of a function named @var{name} looks like this: @example function @var{name} (@var{parameter-list}) @{ @var{body-of-function} @} @end example @noindent @var{name} is the name of the function to be defined. A valid function name is like a valid variable name: a sequence of letters, digits and underscores, not starting with a digit. Functions share the same pool of names as variables and arrays. @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 null string. The @var{body-of-function} consists of @code{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, 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. 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 locals. By convention, you should write an extra space between the arguments and the locals, so other people can follow how your function is supposed to be used. 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 @code{awk} program can be referenced or set normally in the function definition. The arguments and local variables last only as long as the function body is executing. Once the body finishes, the shadowed variables come back. The function body can contain expressions which 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}. There is no need in @code{awk} to put the definition of a function before all uses of the function. This is because @code{awk} reads the entire program before starting to execute any of it. In many @code{awk} implementations, the keyword @code{function} may be abbreviated @code{func}. However, @sc{posix} only specifies the use of the keyword @code{function}. This actually has some practical implications. If @code{gawk} is in @sc{posix}-compatibility mode (@pxref{Command Line, ,Invoking @code{awk}}), then the following statement will @emph{not} define a function:@refill @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}, and based on the truth value of the result, executes the corresponding action. This is probably not what was desired. (@code{awk} accepts this input as syntactically valid, since functions may be used before they are defined in @code{awk} programs.) @node Function Example, Function Caveats, Definition Syntax, User-defined @section Function Definition Example 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 @code{awk} rule which 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: @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 Here is a rather contrived example of a recursive function. It prints a string backwards: @example function rev (str, len) @{ if (len == 0) @{ printf "\n" return @} printf "%c", substr(str, len, 1) rev(str, len - 1) @} @end example @node Function Caveats, Return Statement, Function Example, User-defined @section Calling User-defined Functions @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. What you write in the call for the arguments are @code{awk} expressions; 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 @quotation @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, @code{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. @end quotation @cindex call by value When a function is called, it is given a @emph{copy} of the values of its arguments. This is called @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 this 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. In particular, if @code{myfunc} does this: @example function myfunc (win) @{ print win win = "zzz" print win @} @end example @noindent to change its first argument variable @code{win}, this @emph{does 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{win} 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 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. @ifinfo This can be @strong{very} dangerous if you do not watch what you are doing. For example:@refill @end ifinfo @iftex @emph{This can be very dangerous if you do not watch what you are doing.} For example:@refill @end iftex @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 calling @code{changeit} stores @code{"two"} in the second element of @code{a}. @node Return Statement, , Function Caveats, User-defined @section The @code{return} Statement @cindex @code{return} statement The body of a user-defined function can contain a @code{return} statement. This statement returns control to the rest of the @code{awk} program. It can also be used to return a value for use in the rest of the @code{awk} program. It looks like this:@refill @example return @var{expression} @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. @code{awk} will not warn you if you use the return value of such a function; you will simply get unpredictable or unexpected results. Here is an example of a user-defined function that returns a value for the largest number among the elements of an array:@refill @example @group function maxelt (vec, i, ret) @{ for (i in vec) @{ if (ret == "" || vec[i] > ret) ret = vec[i] @} return ret @} @end group @end example @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 two or three arguments to @code{maxelt}, the results would be strange. The extra space before @code{i} in the function parameter list is to indicate that @code{i} and @code{ret} are not supposed to be arguments. This is a convention which you should follow when you define functions. Here is a program that uses our @code{maxelt} function. It loads an array, calls @code{maxelt}, and then reports the maximum number in that array:@refill @example @group awk ' function maxelt (vec, i, ret) @{ for (i in vec) @{ if (ret == "" || vec[i] > ret) ret = vec[i] @} return ret @} @end group @group # Load all fields of each record into nums. @{ for(i = 1; i <= NF; i++) nums[NR, i] = $i @} END @{ print maxelt(nums) @}' @end group @end example Given the following input: @example @group 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 group @end example @noindent our program tells us (predictably) that: @example 99385 @end example @noindent is the largest number in our array. @node Built-in Variables, Command Line, User-defined, Top @chapter Built-in Variables @cindex built-in variables Most @code{awk} variables are available for you to use for your own purposes; they never change except when your program assigns values to them, and never affect anything except when your program examines them. A few variables have special built-in meanings. Some of them @code{awk} examines automatically, so that they enable you to tell @code{awk} how to do certain things. Others are set automatically by @code{awk}, so that they carry information from the internal workings of @code{awk} to your program. This chapter documents all the built-in variables of @code{gawk}. Most of them are also documented in the chapters where their areas of activity are described. @menu * User-modified:: Built-in variables that you change to control @code{awk}. * Auto-set:: Built-in variables where @code{awk} gives you information. @end menu @node User-modified, Auto-set, Built-in Variables, Built-in Variables @section Built-in Variables that Control @code{awk} @cindex built-in variables, user modifiable This is a list of the variables which you can change to control how @code{awk} does certain things. @table @code @iftex @vindex CONVFMT @end iftex @item CONVFMT This string is used by @code{awk} to control conversion of numbers to strings (@pxref{Conversion, ,Conversion of Strings and Numbers}). It works by being passed, in effect, as the first argument to the @code{sprintf} function. Its default value is @code{"%.6g"}. @code{CONVFMT} was introduced by the @sc{posix} standard.@refill @iftex @vindex FIELDWIDTHS @end iftex @item FIELDWIDTHS This is a space separated list of columns that tells @code{gawk} how to manage input with fixed, columnar boundaries. It is an experimental feature that is still evolving. Assigning to @code{FIELDWIDTHS} overrides the use of @code{FS} for field splitting. @xref{Constant Size, ,Reading Fixed-width Data}, for more information.@refill If @code{gawk} is in compatibility mode (@pxref{Command Line, ,Invoking @code{awk}}), then @code{FIELDWIDTHS} has no special meaning, and field splitting operations are done based exclusively on the value of @code{FS}.@refill @iftex @vindex FS @end iftex @item FS @code{FS} is the input field separator (@pxref{Field Separators, ,Specifying how Fields are Separated}). The value is a single-character string or a multi-character regular expression that matches the separations between fields in an input record.@refill The default value is @w{@code{" "}}, a string consisting of a single space. As a special exception, this value actually means that any sequence of spaces and tabs is a single separator. It also causes spaces and tabs at the beginning or end of a line to be ignored. You can set the value of @code{FS} on the command line using the @samp{-F} option: @example awk -F, '@var{program}' @var{input-files} @end example If @code{gawk} is using @code{FIELDWIDTHS} for field-splitting, assigning a value to @code{FS} will cause @code{gawk} to return to the normal, regexp-based, field splitting. @item IGNORECASE @iftex @vindex IGNORECASE @end iftex If @code{IGNORECASE} is nonzero, then @emph{all} regular expression matching is done in a case-independent fashion. In particular, regexp matching with @samp{~} and @samp{!~}, and the @code{gsub} @code{index}, @code{match}, @code{split} and @code{sub} functions all ignore case when doing their particular regexp operations. @strong{Note:} since field splitting with the value of the @code{FS} variable is also a regular expression operation, that too is done with case ignored. @xref{Case-sensitivity, ,Case-sensitivity in Matching}. If @code{gawk} is in compatibility mode (@pxref{Command Line, ,Invoking @code{awk}}), then @code{IGNORECASE} has no special meaning, and regexp operations are always case-sensitive.@refill @item OFMT @iftex @vindex OFMT @end iftex This string is used by @code{awk} to control conversion of numbers to strings (@pxref{Conversion, ,Conversion of Strings and Numbers}) for printing with the @code{print} statement. It works by being passed, in effect, as the first argument to the @code{sprintf} function. Its default value is @code{"%.6g"}. Earlier versions of @code{awk} also used @code{OFMT} to specify the format for converting numbers to strings in general expressions; this has been taken over by @code{CONVFMT}.@refill @item OFS @iftex @vindex OFS @end iftex This is the output field separator (@pxref{Output Separators}). It is output between the fields output by a @code{print} statement. Its default value is @w{@code{" "}}, a string consisting of a single space. @item ORS @iftex @vindex ORS @end iftex This is the output record separator. It is output at the end of every @code{print} statement. Its default value is a string containing a single newline character, which could be written as @code{"\n"}. (@xref{Output Separators}.)@refill @item RS @iftex @vindex RS @end iftex This is @code{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. (@xref{Records, ,How Input is Split into Records}.)@refill @item SUBSEP @iftex @vindex SUBSEP @end iftex @code{SUBSEP} is the subscript separator. It has the default value of @code{"\034"}, and is used to separate the parts of the name of a multi-dimensional array. Thus, if you access @code{foo[12,3]}, it really accesses @code{foo["12\0343"]} (@pxref{Multi-dimensional, ,Multi-dimensional Arrays}).@refill @end table @node Auto-set, , User-modified, Built-in Variables @section Built-in Variables that Convey Information This is a list of the variables that are set automatically by @code{awk} on certain occasions so as to provide information to your program. @table @code @item ARGC @itemx ARGV @iftex @vindex ARGC @vindex ARGV @end iftex The command-line arguments available to @code{awk} programs are stored in an array called @code{ARGV}. @code{ARGC} is the number of command-line arguments present. @xref{Command Line, ,Invoking @code{awk}}. @code{ARGV} is indexed from zero to @w{@code{ARGC - 1}}. For example:@refill @example awk 'BEGIN @{ for (i = 0; i < ARGC; i++) print ARGV[i] @}' inventory-shipped BBS-list @end example @noindent In this example, @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 3, one more than the index of the last element in @code{ARGV} since the elements are numbered from zero.@refill The names @code{ARGC} and @code{ARGV}, as well the convention of indexing the array from 0 to @w{@code{ARGC - 1}}, are derived from the C language's method of accessing command line arguments.@refill Notice that the @code{awk} program is not entered in @code{ARGV}. The other special command line options, with their arguments, are also not entered. But variable assignments on the command line @emph{are} treated as arguments, and do show up in the @code{ARGV} array. Your program can alter @code{ARGC} and the elements of @code{ARGV}. Each time @code{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, your program can change which files are read. You can use @code{"-"} to represent the standard input. By storing additional elements and incrementing @code{ARGC} you can cause additional files to be read. If you decrease the value of @code{ARGC}, that eliminates input files from the end of the list. By recording the old value of @code{ARGC} elsewhere, your program can treat the eliminated arguments as something other than file names. 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, @code{awk} ignores file names that have been replaced with the null string. @ignore see getopt.awk in the examples... @end ignore @item ARGIND @vindex ARGIND The index in @code{ARGV} of the current file being processed. Every time @code{gawk} opens a new data file for processing, it sets @code{ARGIND} to the index in @code{ARGV} of the file name. Thus, the condition @samp{FILENAME == ARGV[ARGIND]} is always true. This variable is useful in file processing; it allows you to tell how far along you are in the list of data files, and to distinguish between multiple successive instances of the same filename on the command line. While you can change the value of @code{ARGIND} within your @code{awk} program, @code{gawk} will automatically set it to a new value when the next file is opened. This variable is a @code{gawk} extension; in other @code{awk} implementations it is not special. @item ENVIRON @vindex ENVIRON This is an array that contains the values of the environment. The array indices are the environment variable names; the values are the values of the particular environment variables. For example, @code{ENVIRON["HOME"]} might be @file{/u/close}. Changing this array does not affect the environment passed on to any programs that @code{awk} may spawn via redirection or the @code{system} function. (In a future version of @code{gawk}, it may do so.) Some operating systems may not have environment variables. On such systems, the array @code{ENVIRON} is empty. @item ERRNO @iftex @vindex ERRNO @end iftex If a system error occurs either doing a redirection for @code{getline}, during a read for @code{getline}, or during a @code{close} operation, then @code{ERRNO} will contain a string describing the error. This variable is a @code{gawk} extension; in other @code{awk} implementations it is not special. @item FILENAME @iftex @vindex FILENAME @end iftex This is the name of the file that @code{awk} is currently reading. If @code{awk} is reading from the standard input (in other words, there are no files listed on the command line), @code{FILENAME} is set to @code{"-"}. @code{FILENAME} is changed each time a new file is read (@pxref{Reading Files, ,Reading Input Files}).@refill @item FNR @iftex @vindex FNR @end iftex @code{FNR} is the current record number in the current file. @code{FNR} is incremented each time a new record is read (@pxref{Getline, ,Explicit Input with @code{getline}}). It is reinitialized to 0 each time a new input file is started.@refill @item NF @iftex @vindex NF @end iftex @code{NF} is 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, ,Examining Fields}).@refill @item NR @iftex @vindex NR @end iftex This is the number of input records @code{awk} has processed since the beginning of the program's execution. (@pxref{Records, ,How Input is Split into Records}). @code{NR} is set each time a new record is read.@refill @item RLENGTH @iftex @vindex RLENGTH @end iftex @code{RLENGTH} is the length of the substring matched by the @code{match} function (@pxref{String Functions, ,Built-in Functions for String Manipulation}). @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 was found.@refill @item RSTART @iftex @vindex RSTART @end iftex @code{RSTART} is the start-index in characters of the substring matched by the @code{match} function (@pxref{String Functions, ,Built-in Functions for String Manipulation}). @code{RSTART} is set by invoking the @code{match} function. Its value is the position of the string where the matched substring starts, or 0 if no match was found.@refill @end table @node Command Line, Language History, Built-in Variables, Top @c node-name, next, previous, up @chapter Invoking @code{awk} @cindex command line @cindex invocation of @code{gawk} @cindex arguments, command line @cindex options, command line @cindex long options @cindex options, long There are two ways to run @code{awk}: with an explicit program, or with one or more program files. Here are templates for both of them; items enclosed in @samp{@r{[}@dots{}@r{]}} in these templates are optional. Besides traditional one-letter @sc{posix}-style options, @code{gawk} also supports GNU long named options. @example awk @r{[@var{POSIX or GNU style options}]} -f progfile @r{[@code{--}]} @var{file} @dots{} awk @r{[@var{POSIX or GNU style options}]} @r{[@code{--}]} '@var{program}' @var{file} @dots{} @end example @menu * Options:: Command line options and their meanings. * Other Arguments:: Input file names and variable assignments. * AWKPATH Variable:: Searching directories for @code{awk} programs. * Obsolete:: Obsolete Options and/or features. * Undocumented:: Undocumented Options and Features. @end menu @node Options, Other Arguments, Command Line, Command Line @section Command Line Options Options begin with a minus sign, and consist of a single character. GNU style long named options consist of two minus signs and a keyword that can be abbreviated if the abbreviation allows the option to be uniquely identified. If the option takes an argument, then the keyword is immediately followed by an equals sign (@samp{=}) and the argument's value. For brevity, the discussion below only refers to the traditional short options; however the long and short options are interchangeable in all contexts. Each long named option for @code{gawk} has a corresponding @sc{posix}-style option. The options and their meanings are as follows: @table @code @item -F @var{fs} @itemx --field-separator=@var{fs} @iftex @cindex @code{-F} option @end iftex @cindex @code{--field-separator} option Sets the @code{FS} variable to @var{fs} (@pxref{Field Separators, ,Specifying how Fields are Separated}).@refill @item -f @var{source-file} @itemx --file=@var{source-file} @iftex @cindex @code{-f} option @end iftex @cindex @code{--file} option Indicates that the @code{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 @samp{-v} option @cindex @code{--assign} option 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 (see below for a fuller explanation). The @samp{-v} option can only set one variable, but you can use it more than once, setting another variable each time, like this: @samp{@w{-v foo=1} @w{-v bar=2}}. @item -W @var{gawk-opt} @cindex @samp{-W} option Following the @sc{posix} standard, options that are implementation specific are supplied as arguments to the @samp{-W} option. With @code{gawk}, these arguments may be separated by commas, or quoted and separated by whitespace. Case is ignored when processing these options. These options also have corresponding GNU style long named options. The following @code{gawk}-specific options are available: @table @code @item -W compat @itemx --compat @cindex @code{--compat} option Specifies @dfn{compatibility mode}, in which the GNU extensions in @code{gawk} are disabled, so that @code{gawk} behaves just like Unix @code{awk}. @xref{POSIX/GNU, ,Extensions in @code{gawk} not in POSIX @code{awk}}, which summarizes the extensions. Also see @ref{Compatibility Mode, ,Downward Compatibility and Debugging}.@refill @item -W copyleft @itemx -W copyright @itemx --copyleft @itemx --copyright @cindex @code{--copyleft} option @cindex @code{--copyright} option Print the short version of the General Public License. This option may disappear in a future version of @code{gawk}. @item -W help @itemx -W usage @itemx --help @itemx --usage @cindex @code{--help} option @cindex @code{--usage} option Print a ``usage'' message summarizing the short and long style options that @code{gawk} accepts, and then exit. @item -W lint @itemx --lint @cindex @code{--lint} option Provide warnings about constructs that are dubious or non-portable to other @code{awk} implementations. Some warnings are issued when @code{gawk} first reads your program. Others are issued at run-time, as your program executes. @item -W posix @itemx --posix @cindex @code{--posix} option Operate in strict @sc{posix} mode. This disables all @code{gawk} extensions (just like @code{-W compat}), and adds the following additional restrictions: @itemize @bullet{} @item @code{\x} escape sequences are not recognized (@pxref{Constants, ,Constant Expressions}).@refill @item The synonym @code{func} for the keyword @code{function} is not recognized (@pxref{Definition Syntax, ,Syntax of Function Definitions}). @item The operators @samp{**} and @samp{**=} cannot be used in place of @samp{^} and @samp{^=} (@pxref{Arithmetic Ops, ,Arithmetic Operators}, and also @pxref{Assignment Ops, ,Assignment Expressions}).@refill @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, ,Specifying how Fields are Separated}).@refill @end itemize Although you can supply both @samp{-W compat} and @samp{-W posix} on the command line, @samp{-W posix} will take precedence. @item -W source=@var{program-text} @itemx --source=@var{program-text} @cindex @code{--source} option Program source code is taken from the @var{program-text}. This option allows you to mix @code{awk} source code in files with program source code that you would enter on the command line. This is particularly useful when you have library functions that you wish to use from your command line programs (@pxref{AWKPATH Variable, ,The @code{AWKPATH} Environment Variable}). @item -W version @itemx --version @cindex @code{--version} option Prints version information for this particular copy of @code{gawk}. This is so you can determine if your copy of @code{gawk} is up to date with respect to whatever the Free Software Foundation is currently distributing. This option may disappear in a future version of @code{gawk}. @end table @item -- 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 @samp{--} follows the @sc{posix} argument parsing conventions. This is useful if you have file names that start with @samp{-}, or in shell scripts, if you have file names that will be specified by the user which could start with @samp{-}. @end table Any other options are flagged as invalid with a warning message, but are otherwise ignored. In compatibility mode, as a special case, if the value of @var{fs} supplied to the @samp{-F} option is @samp{t}, then @code{FS} is set to the tab character (@code{"\t"}). This is only true for @samp{-W compat}, and not for @samp{-W posix} (@pxref{Field Separators, ,Specifying how Fields are Separated}).@refill If the @samp{-f} option is @emph{not} used, then the first non-option command line argument is expected to be the program text. The @samp{-f} option may be used more than once on the command line. If it is, @code{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 @code{awk} functions. Useful functions can be written once, and then retrieved from a standard place, instead of having to be included into each individual program. You can still type in a program at the terminal and use library functions, by specifying @samp{-f /dev/tty}. @code{awk} will read a file from the terminal to use as part of the @code{awk} program. After typing your program, type @kbd{Control-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 @code{awk} mechanisms to mix source file and command line @code{awk} programs, @code{gawk} provides the @samp{--source} option. This does not require you to pre-empt the standard input for your source code, and allows you to easily mix command line and library source code (@pxref{AWKPATH Variable, ,The @code{AWKPATH} Environment Variable}). If no @samp{-f} or @samp{--source} option is specified, then @code{gawk} will use the first non-option command line argument as the text of the program source code. @node Other Arguments, AWKPATH Variable, Options, Command Line @section Other Command Line Arguments 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}}, means to assign the value @var{value} to the variable @var{var}---it does not specify a file at all. @vindex ARGV All these arguments are made available to your @code{awk} program in the @code{ARGV} array (@pxref{Built-in Variables}). Command line options and the program text (if present) are omitted from the @code{ARGV} array. All other arguments, including variable assignments, are included. The distinction between file name arguments and variable-assignment arguments is made when @code{awk} is about to open the next input file. At that point in execution, it checks the ``file name'' to see whether it is really a variable assignment; if so, @code{awk} sets the variable instead of reading a file. Therefore, the variables actually receive the specified 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, ,@code{BEGIN} and @code{END} Special Patterns}), since such rules are run before @code{awk} begins scanning the argument list. The values given on the command line are processed for escape sequences (@pxref{Constants, ,Constant Expressions}).@refill In some earlier implementations of @code{awk}, when a variable assignment occurred before any file names, the assignment would happen @emph{before} the @code{BEGIN} rule was executed. Some applications came to depend upon this ``feature.'' When @code{awk} was changed to be more consistent, the @samp{-v} option was added to accommodate applications that depended upon this 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 data files. It is also useful for controlling state if multiple passes are needed over a data file. For example:@refill @cindex multiple passes over data @cindex passes, multiple @smallexample awk 'pass == 1 @{ @var{pass 1 stuff} @} pass == 2 @{ @var{pass 2 stuff} @}' pass=1 datafile pass=2 datafile @end smallexample Given the variable assignment feature, the @samp{-F} option is not strictly necessary. It remains for historical compatibility. @node AWKPATH Variable, Obsolete, Other Arguments, Command Line @section The @code{AWKPATH} Environment Variable @cindex @code{AWKPATH} environment variable @cindex search path @cindex directory search @cindex path, search @iftex @cindex differences between @code{gawk} and @code{awk} @end iftex The previous section described how @code{awk} program files can be named on the command line with the @samp{-f} option. In some @code{awk} implementations, you must supply a precise path name for each program file, unless the file is in the current directory. But in @code{gawk}, if the file name supplied in the @samp{-f} option does not contain a @samp{/}, then @code{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 actually a string consisting of directory names separated by colons. @code{gawk} gets its search path from the @code{AWKPATH} environment variable. If that variable does not exist, @code{gawk} uses the default path, which is @samp{.:/usr/lib/awk:/usr/local/lib/awk}. (Programs written by system administrators should use an @code{AWKPATH} variable that does not include the current directory, @samp{.}.)@refill The search path feature is particularly useful for building up libraries of useful @code{awk} functions. The library files can be placed in a standard directory that is in the default path, and then specified on the command line with a short file name. Otherwise, the full file name would have to be typed for each file. By combining the @samp{--source} and @samp{-f} options, your command line @code{awk} programs can use facilities in @code{awk} library files. Path searching is not done if @code{gawk} is in compatibility mode. This is true for both @samp{-W compat} and @samp{-W posix}. @xref{Options, ,Command Line 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 writing @file{.} as an entry 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}.... @node Obsolete, Undocumented, AWKPATH Variable, Command Line @section Obsolete Options and/or Features @cindex deprecated options @cindex obsolete options @cindex deprecated features @cindex obsolete features This section describes features and/or command line options from the previous release of @code{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! For version 2.15 of @code{gawk}, the following command line options from version 2.11.1 are no longer recognized. @table @samp @ignore @item -nostalgia Use @samp{-W nostalgia} instead. @end ignore @item -c Use @samp{-W compat} instead. @item -V Use @samp{-W version} instead. @item -C Use @samp{-W copyright} instead. @item -a @itemx -e These options produce an ``unrecognized option'' error message but have no effect on the execution of @code{gawk}. The @sc{posix} standard now specifies traditional @code{awk} regular expressions for the @code{awk} utility. @end table The public-domain version of @code{strftime} that is distributed with @code{gawk} changed for the 2.14 release. The @samp{%V} conversion specifier that used to generate the date in VMS format was changed to @samp{%v}. This is because the @sc{posix} standard for the @code{date} utility now specifies a @samp{%V} conversion specifier. @xref{Time Functions, ,Functions for Dealing with Time Stamps}, for details. @node Undocumented, , Obsolete, Command Line @section Undocumented Options and Features This section intentionally left blank. @c Read The Source, Luke! @ignore @c If these came out in the Info file or TeX manual, then they wouldn't @c be undocumented, would they? @code{gawk} has one undocumented option: @table @samp @item -W 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 @code{awk}, and by a t--shirt. @end table Early versions of @code{awk} used to not require any separator (either a newline or @samp{;}) between the rules in @code{awk} programs. Thus, it was common to see one-line programs like: @example awk '@{ sum += $1 @} END @{ print sum @}' @end example @code{gawk} actually supports this, but it is purposely undocumented since 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, ,@code{awk} Statements versus Lines}, for a fuller explanation.@refill As an accident of the implementation of the original Unix @code{awk}, if a built-in function used @code{$0} as its default argument, it was possible to call that function without the parentheses. In particular, it was common practice to use the @code{length} function in this fashion. For example, the pipeline: @example echo abcdef | awk '@{ print length @}' @end example @noindent would print @samp{6}. For backwards compatibility with old programs, @code{gawk} supports this usage, but only for the @code{length} function. New programs should @emph{not} call the @code{length} function this way. In particular, this usage will not be portable to other @sc{posix} compliant versions of @code{awk}. It is also poor style. @end ignore @node Language History, Installation, Command Line, Top @chapter The Evolution of the @code{awk} Language This manual describes the GNU implementation of @code{awk}, which is patterned after the @sc{posix} specification. Many @code{awk} users are only familiar with the original @code{awk} implementation in Version 7 Unix, which is also the basis for the version in Berkeley Unix (through 4.3--Reno). This chapter briefly describes the evolution of the @code{awk} language. @menu * V7/S5R3.1:: The major changes between V7 and System V Release 3.1. * S5R4:: Minor changes between System V Releases 3.1 and 4. * POSIX:: New features from the @sc{posix} standard. * POSIX/GNU:: The extensions in @code{gawk} not in @sc{posix} @code{awk}. @end menu @node V7/S5R3.1, S5R4, Language History, Language History @section Major Changes between V7 and S5R3.1 The @code{awk} language evolved considerably between the release of Version 7 Unix (1978) and the new version first made widely available in System V Release 3.1 (1987). This section summarizes the changes, with cross-references to further details. @itemize @bullet @item The requirement for @samp{;} to separate rules on a line (@pxref{Statements/Lines, ,@code{awk} Statements versus Lines}). @item User-defined functions, and the @code{return} statement (@pxref{User-defined, ,User-defined Functions}). @item The @code{delete} statement (@pxref{Delete, ,The @code{delete} Statement}). @item The @code{do}-@code{while} statement (@pxref{Do Statement, ,The @code{do}-@code{while} Statement}).@refill @item The built-in functions @code{atan2}, @code{cos}, @code{sin}, @code{rand} and @code{srand} (@pxref{Numeric Functions, ,Numeric Built-in Functions}). @item The built-in functions @code{gsub}, @code{sub}, and @code{match} (@pxref{String Functions, ,Built-in Functions for String Manipulation}). @item The built-in functions @code{close}, which closes an open file, and @code{system}, which allows the user to execute operating system commands (@pxref{I/O Functions, ,Built-in Functions for Input/Output}).@refill @c Does the above verbiage prevents an overfull hbox? --mew, rjc 24jan1992 @item The @code{ARGC}, @code{ARGV}, @code{FNR}, @code{RLENGTH}, @code{RSTART}, and @code{SUBSEP} built-in variables (@pxref{Built-in Variables}). @item The conditional expression using the operators @samp{?} and @samp{:} (@pxref{Conditional Exp, ,Conditional Expressions}).@refill @item The exponentiation operator @samp{^} (@pxref{Arithmetic Ops, ,Arithmetic Operators}) and its assignment operator form @samp{^=} (@pxref{Assignment Ops, ,Assignment Expressions}).@refill @item C-compatible operator precedence, which breaks some old @code{awk} programs (@pxref{Precedence, ,Operator Precedence (How Operators Nest)}). @item Regexps as the value of @code{FS} (@pxref{Field Separators, ,Specifying how Fields are Separated}), and as the third argument to the @code{split} function (@pxref{String Functions, ,Built-in Functions for String Manipulation}).@refill @item Dynamic regexps as operands of the @samp{~} and @samp{!~} operators (@pxref{Regexp Usage, ,How to Use Regular Expressions}). @item Escape sequences (@pxref{Constants, ,Constant Expressions}) in regexps.@refill @item The escape sequences @samp{\b}, @samp{\f}, and @samp{\r} (@pxref{Constants, ,Constant Expressions}). @item Redirection of input for the @code{getline} function (@pxref{Getline, ,Explicit Input with @code{getline}}).@refill @item Multiple @code{BEGIN} and @code{END} rules (@pxref{BEGIN/END, ,@code{BEGIN} and @code{END} Special Patterns}).@refill @item Simulated multi-dimensional arrays (@pxref{Multi-dimensional, ,Multi-dimensional Arrays}).@refill @end itemize @node S5R4, POSIX, V7/S5R3.1, Language History @section Changes between S5R3.1 and S5R4 The System V Release 4 version of Unix @code{awk} added these features (some of which originated in @code{gawk}): @itemize @bullet @item The @code{ENVIRON} variable (@pxref{Built-in Variables}). @item Multiple @samp{-f} options on the command line (@pxref{Command Line, ,Invoking @code{awk}}).@refill @item The @samp{-v} option for assigning variables before program execution begins (@pxref{Command Line, ,Invoking @code{awk}}).@refill @item The @samp{--} option for terminating command line options. @item The @samp{\a}, @samp{\v}, and @samp{\x} escape sequences (@pxref{Constants, ,Constant Expressions}).@refill @item A defined return value for the @code{srand} built-in function (@pxref{Numeric Functions, ,Numeric Built-in Functions}). @item The @code{toupper} and @code{tolower} built-in string functions for case translation (@pxref{String Functions, ,Built-in Functions for String Manipulation}).@refill @item A cleaner specification for the @samp{%c} format-control letter in the @code{printf} function (@pxref{Printf, ,Using @code{printf} Statements for Fancier Printing}).@refill @item The ability to dynamically pass the field width and precision (@code{"%*.*d"}) in the argument list of the @code{printf} function (@pxref{Printf, ,Using @code{printf} Statements for Fancier Printing}).@refill @item The use of constant regexps such as @code{/foo/} as expressions, where they are equivalent to use of the matching operator, as in @code{$0 ~ /foo/} (@pxref{Constants, ,Constant Expressions}). @end itemize @node POSIX, POSIX/GNU, S5R4, Language History @section Changes between S5R4 and POSIX @code{awk} The @sc{posix} Command Language and Utilities standard for @code{awk} introduced the following changes into the language: @itemize @bullet{} @item The use of @samp{-W} for implementation-specific options. @item The use of @code{CONVFMT} for controlling the conversion of numbers to strings (@pxref{Conversion, ,Conversion of Strings and Numbers}). @item The concept of a numeric string, and tighter comparison rules to go with it (@pxref{Comparison Ops, ,Comparison Expressions}). @item More complete documentation of many of the previously undocumented features of the language. @end itemize @node POSIX/GNU, , POSIX, Language History @section Extensions in @code{gawk} not in POSIX @code{awk} The GNU implementation, @code{gawk}, adds these features: @itemize @bullet @item The @code{AWKPATH} environment variable for specifying a path search for the @samp{-f} command line option (@pxref{Command Line, ,Invoking @code{awk}}).@refill @item The various @code{gawk} specific features available via the @samp{-W} command line option (@pxref{Command Line, ,Invoking @code{awk}}). @item The @code{ARGIND} variable, that tracks the movement of @code{FILENAME} through @code{ARGV}. (@pxref{Built-in Variables}). @item The @code{ERRNO} variable, that contains the system error message when @code{getline} returns @minus{}1, or when @code{close} fails. (@pxref{Built-in Variables}). @item The @code{IGNORECASE} variable and its effects (@pxref{Case-sensitivity, ,Case-sensitivity in Matching}).@refill @item The @code{FIELDWIDTHS} variable and its effects (@pxref{Constant Size, ,Reading Fixed-width Data}).@refill @item The @code{next file} statement for skipping to the next data file (@pxref{Next File Statement, ,The @code{next file} Statement}).@refill @item The @code{systime} and @code{strftime} built-in functions for obtaining and printing time stamps (@pxref{Time Functions, ,Functions for Dealing with Time Stamps}).@refill @item The @file{/dev/stdin}, @file{/dev/stdout}, @file{/dev/stderr}, and @file{/dev/fd/@var{n}} file name interpretation (@pxref{Special Files, ,Standard I/O Streams}).@refill @item The @samp{-W compat} option to turn off these extensions (@pxref{Command Line, ,Invoking @code{awk}}).@refill @item The @samp{-W posix} option for full @sc{posix} compliance (@pxref{Command Line, ,Invoking @code{awk}}).@refill @end itemize @node Installation, Gawk Summary, Language History, Top @chapter Installing @code{gawk} This chapter provides instructions for installing @code{gawk} on the various platforms that are supported by the developers. The primary developers support Unix (and one day, GNU), while the other ports were contributed. The file @file{ACKNOWLEDGMENT} in the @code{gawk} distribution lists the electronic mail addresses of the people who did the respective ports.@refill @menu * Gawk Distribution:: What is in the @code{gawk} distribution. * Unix Installation:: Installing @code{gawk} under various versions of Unix. * VMS Installation:: Installing @code{gawk} on VMS. * MS-DOS Installation:: Installing @code{gawk} on MS-DOS. * Atari Installation:: Installing @code{gawk} on the Atari ST. @end menu @node Gawk Distribution, Unix Installation, Installation, Installation @section The @code{gawk} Distribution This section first describes how to get and extract the @code{gawk} distribution, and then discusses what is in the various files and subdirectories. @menu * Extracting:: How to get and extract the distribution. * Distribution contents:: What is in the distribution. @end menu @node Extracting, Distribution contents, Gawk Distribution, Gawk Distribution @subsection Getting the @code{gawk} Distribution @cindex getting gawk @cindex anonymous ftp @cindex anonymous uucp @cindex ftp, anonymous @cindex uucp, anonymous @code{gawk} is distributed as a @code{tar} file compressed with the GNU Zip program, @code{gzip}. You can get it via anonymous @code{ftp} to the Internet host @code{prep.ai.mit.edu}. Like all GNU software, it will be archived at other well known systems, from which it will be possible to use some sort of anonymous @code{uucp} to obtain the distribution as well. You can also order @code{gawk} on tape or CD-ROM directly from the Free Software Foundation. (The address is on the copyright page.) Doing so directly contributes to the support of the foundation and to the production of more free software. Once you have the distribution (for example, @file{gawk-2.15.0.tar.z}), first use @code{gzip} to expand the file, and then use @code{tar} to extract it. You can use the following pipeline to produce the @code{gawk} distribution: @example # Under System V, add 'o' to the tar flags gzip -d -c gawk-2.15.0.tar.z | tar -xvpf - @end example @noindent This will create a directory named @file{gawk-2.15} in the current directory. The distribution file name is of the form @file{gawk-2.15.@var{n}.tar.Z}. The @var{n} represents a @dfn{patchlevel}, meaning that minor bugs have been fixed in the major release. The current patchlevel is 0, but when retrieving distributions, you should get the version with the highest patchlevel.@refill If you are not on a Unix system, you will need to make other arrangements for getting and extracting the @code{gawk} distribution. You should consult a local expert. @node Distribution contents, , Extracting, Gawk Distribution @subsection Contents of the @code{gawk} Distribution @code{gawk} has a number of C source files, documentation files, subdirectories and files related to the configuration process (@pxref{Unix Installation, ,Compiling and Installing @code{gawk} on Unix}), and several subdirectories related to different, non-Unix, operating systems.@refill @table @asis @item various @samp{.c}, @samp{.y}, and @samp{.h} files The C and YACC source files are the actual @code{gawk} source code. @end table @table @file @item README @itemx README.VMS @itemx README.dos @itemx README.rs6000 @itemx README.ultrix Descriptive files: @file{README} for @code{gawk} under Unix, and the rest for the various hardware and software combinations. @item PORTS A list of systems to which @code{gawk} has been ported, and which have successfully run the test suite. @item ACKNOWLEDGMENT A list of the people who contributed major parts of the code or documentation. @item NEWS A list of changes to @code{gawk} since the last release or patch. @item COPYING The GNU General Public License. @item FUTURES A brief list of features and/or changes being contemplated for future releases, with some indication of the time frame for the feature, based on its difficulty. @item LIMITATIONS A list of those factors that limit @code{gawk}'s performance. Most of these depend on the hardware or operating system software, and are not limits in @code{gawk} itself.@refill @item PROBLEMS A file describing known problems with the current release. @item gawk.1 The @code{troff} source for a manual page describing @code{gawk}. @item gawk.texinfo @ifinfo The @code{texinfo} source file for this Info file. It should be processed with @TeX{} to produce a printed manual, and with @code{makeinfo} to produce the Info file.@refill @end ifinfo @iftex The @code{texinfo} source file for this manual. It should be processed with @TeX{} to produce a printed manual, and with @code{makeinfo} to produce the Info file.@refill @end iftex @item Makefile.in @itemx config @itemx config.in @itemx configure @itemx missing @itemx mungeconf These files and subdirectories are used when configuring @code{gawk} for various Unix systems. They are explained in detail in @ref{Unix Installation, ,Compiling and Installing @code{gawk} on Unix}.@refill @item atari Files needed for building @code{gawk} on an Atari ST. @xref{Atari Installation, ,Installing @code{gawk} on the Atari ST}, for details. @item pc Files needed for building @code{gawk} under MS-DOS. @xref{MS-DOS Installation, ,Installing @code{gawk} on MS-DOS}, for details. @item vms Files needed for building @code{gawk} under VMS. @xref{VMS Installation, ,Compiling Installing and Running @code{gawk} on VMS}, for details. @item test Many interesting @code{awk} programs, provided as a test suite for @code{gawk}. You can use @samp{make test} from the top level @code{gawk} directory to run your version of @code{gawk} against the test suite. @c There are many programs here that are useful in their own right. If @code{gawk} successfully passes @samp{make test} then you can be confident of a successful port.@refill @end table @node Unix Installation, VMS Installation, Gawk Distribution, Installation @section Compiling and Installing @code{gawk} on Unix Often, you can compile and install @code{gawk} by typing only two commands. However, if you do not use a supported system, you may need to configure @code{gawk} for your system yourself. @menu * Quick Installation:: Compiling @code{gawk} on a supported Unix version. * Configuration Philosophy:: How it's all supposed to work. * New Configurations:: What to do if there is no supplied configuration for your system. @end menu @node Quick Installation, Configuration Philosophy, Unix Installation, Unix Installation @subsection Compiling @code{gawk} for a Supported Unix Version @cindex installation, unix After you have extracted the @code{gawk} distribution, @code{cd} to @file{gawk-2.15}. Look in the @file{config} subdirectory for a file that matches your hardware/software combination. In general, only the software is relevant; for example @code{sunos41} is used for SunOS 4.1, on both Sun 3 and Sun 4 hardware.@refill If you find such a file, run the command: @example # assume you have SunOS 4.1 ./configure sunos41 @end example This produces a @file{Makefile} and @file{config.h} tailored to your system. You may wish to edit the @file{Makefile} to use a different C compiler, such as @code{gcc}, the GNU C compiler, if you have it. You may also wish to change the @code{CFLAGS} variable, which controls the command line options that are passed to the C compiler (such as optimization levels, or compiling for debugging).@refill After you have configured @file{Makefile} and @file{config.h}, type: @example make @end example @noindent and shortly thereafter, you should have an executable version of @code{gawk}. That's all there is to it! @node Configuration Philosophy, New Configurations, Quick Installation, Unix Installation @subsection The Configuration Process (This section is of interest only if you know something about using the C language and the Unix operating system.) The source code for @code{gawk} generally attempts to adhere to industry standards wherever possible. This means that @code{gawk} uses library routines that are specified by the @sc{ansi} C standard and by the @sc{posix} operating system interface standard. When using an @sc{ansi} C compiler, function prototypes are provided to help improve the compile-time checking. Many older Unix systems do not support all of either the @sc{ansi} or the @sc{posix} standards. The @file{missing} subdirectory in the @code{gawk} distribution contains replacement versions of those subroutines that are most likely to be missing. The @file{config.h} file that is created by the @code{configure} program contains definitions that describe features of the particular operating system where you are attempting to compile @code{gawk}. For the most part, it lists which standard subroutines are @emph{not} available. For example, if your system lacks the @samp{getopt} routine, then @samp{GETOPT_MISSING} would be defined. @file{config.h} also defines constants that describe facts about your variant of Unix. For example, there may not be an @samp{st_blksize} element in the @code{stat} structure. In this case @samp{BLKSIZE_MISSING} would be defined. Based on the list in @file{config.h} of standard subroutines that are missing, @file{missing.c} will do a @samp{#include} of the appropriate file(s) from the @file{missing} subdirectory.@refill Conditionally compiled code in the other source files relies on the other definitions in the @file{config.h} file. Besides creating @file{config.h}, @code{configure} produces a @file{Makefile} from @file{Makefile.in}. There are a number of lines in @file{Makefile.in} that are system or feature specific. For example, there is line that begins with @samp{##MAKE_ALLOCA_C##}. This is normally a comment line, since it starts with @samp{#}. If a configuration file has @samp{MAKE_ALLOCA_C} in it, then @code{configure} will delete the @samp{##MAKE_ALLOCA_C##} from the beginning of the line. This will enable the rules in the @file{Makefile} that use a C version of @samp{alloca}. There are several similar features that work in this fashion.@refill @node New Configurations, , Configuration Philosophy, Unix Installation @subsection Configuring @code{gawk} for a New System (This section is of interest only if you know something about using the C language and the Unix operating system, and if you have to install @code{gawk} on a system that is not supported by the @code{gawk} distribution. If you are a C or Unix novice, get help from a local expert.) If you need to configure @code{gawk} for a Unix system that is not supported in the distribution, first see @ref{Configuration Philosophy, ,The Configuration Process}. Then, copy @file{config.in} to @file{config.h}, and copy @file{Makefile.in} to @file{Makefile}.@refill Next, edit both files. Both files are liberally commented, and the necessary changes should be straightforward. While editing @file{config.h}, you need to determine what library routines you do or do not have by consulting your system documentation, or by perusing your actual libraries using the @code{ar} or @code{nm} utilities. In the worst case, simply do not define @emph{any} of the macros for missing subroutines. When you compile @code{gawk}, the final link-editing step will fail. The link editor will provide you with a list of unresolved external references---these are the missing subroutines. Edit @file{config.h} again and recompile, and you should be set.@refill Editing the @file{Makefile} should also be straightforward. Enable or disable the lines that begin with @samp{##MAKE_@var{whatever}##}, as appropriate. Select the correct C compiler and @code{CFLAGS} for it. Then run @code{make}. Getting a correct configuration is likely to be an iterative process. Do not be discouraged if it takes you several tries. If you have no luck whatsoever, please report your system type, and the steps you took. Once you do have a working configuration, please send it to the maintainers so that support for your system can be added to the official release. @xref{Bugs, ,Reporting Problems and Bugs}, for information on how to report problems in configuring @code{gawk}. You may also use the same mechanisms for sending in new configurations.@refill @node VMS Installation, MS-DOS Installation, Unix Installation, Installation @section Compiling, Installing, and Running @code{gawk} on VMS @c based on material from @c Pat Rankin @cindex installation, vms This section describes how to compile and install @code{gawk} under VMS. @menu * VMS Compilation:: How to compile @code{gawk} under VMS. * VMS Installation Details:: How to install @code{gawk} under VMS. * VMS Running:: How to run @code{gawk} under VMS. * VMS POSIX:: Alternate instructions for VMS POSIX. @end menu @node VMS Compilation, VMS Installation Details, VMS Installation, VMS Installation @subsection Compiling @code{gawk} under VMS To compile @code{gawk} under VMS, there is a @code{DCL} command procedure that will issue all the necessary @code{CC} and @code{LINK} commands, and there is also a @file{Makefile} for use with the @code{MMS} utility. From the source directory, use either @smallexample $ @@[.VMS]VMSBUILD.COM @end smallexample @noindent or @smallexample $ MMS/DESCRIPTION=[.VMS]DECSRIP.MMS GAWK @end smallexample 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.@refill @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} should be needed. @item DEC C Edit @file{vmsbuild.com} or @file{descrip.mms} according to their comments. No changes to @file{config.h} should be needed. @end table @code{gawk} 2.15 has been tested under VAX/VMS 5.5-1 using VAX C V3.2, GNU C 1.40 and 2.3. It should work without modifications for VMS V4.6 and up. @node VMS Installation Details, VMS Running, VMS Compilation, VMS Installation @subsection Installing @code{gawk} on VMS To install @code{gawk}, all you need is a ``foreign'' command, which is a @code{DCL} symbol whose value begins with a dollar sign. @smallexample $ GAWK :== $device:[directory]GAWK @end smallexample @noindent (Substitute the actual location of @code{gawk.exe} for @samp{device:[directory]}.) The symbol should be placed in the @file{login.com} of any user who wishes to run @code{gawk}, so that it will be defined every time the user logs on. Alternatively, the symbol may be placed in the system-wide @file{sylogin.com} procedure, which will allow all users to run @code{gawk}.@refill Optionally, the help entry can be loaded into a VMS help library: @smallexample $ LIBRARY/HELP SYS$HELP:HELPLIB [.VMS]GAWK.HLP @end smallexample @noindent (You may want to substitute a site-specific help library rather than the standard VMS library @samp{HELPLIB}.) After loading the help text, @c this is so tiny, but `should' be smallexample for consistency sake... @c I didn't because it was so short. --mew 29jan1992 @example $ HELP GAWK @end example @noindent will provide information about both the @code{gawk} implementation and the @code{awk} programming language. The logical name @samp{AWK_LIBRARY} can designate a default location for @code{awk} program files. For the @samp{-f} option, if the specified filename has no device or directory path information in it, @code{gawk} will look in the current directory first, then in the directory specified by the translation of @samp{AWK_LIBRARY} if the file was not found. If after searching in both directories, the file still is not found, then @code{gawk} appends the suffix @samp{.awk} to the filename and the file search will be re-tried. If @samp{AWK_LIBRARY} is not defined, that portion of the file search will fail benignly.@refill @node VMS Running, VMS POSIX, VMS Installation Details, VMS Installation @subsection Running @code{gawk} on VMS Command line parsing and quoting conventions are significantly different on VMS, so examples in this manual or from other sources often need minor changes. They @emph{are} minor though, and all @code{awk} programs should run correctly. Here are a couple of trivial tests: @smallexample $ gawk -- "BEGIN @{print ""Hello, World!""@}" $ gawk -"W" version ! could also be -"W version" or "-W version" @end smallexample @noindent Note that upper-case and mixed-case text must be quoted. The VMS port of @code{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 @samp{--} flag is required to force Unix style rather than @code{DCL} parsing. If any other dash-type options (or multiple parameters such as data files to be processed) are present, there is no ambiguity and @samp{--} can be omitted. The default search path when looking for @code{awk} program files specified by the @samp{-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 multi-translation @code{RMS} searchlist. @node VMS POSIX, , VMS Running, VMS Installation @subsection Building and using @code{gawk} under VMS POSIX Ignore the instructions above, although @file{vms/gawk.hlp} should still be made available in a help library. Make sure that the two scripts, @file{configure} and @file{mungeconf}, are executable; use @samp{chmod +x} on them if necessary. Then execute the following commands: @smallexample $ POSIX psx> configure vms-posix psx> make awktab.c gawk @end smallexample @noindent The first command will construct files @file{config.h} and @file{Makefile} out of templates. The second command will compile and link @code{gawk}. Due to a @code{make} bug in VMS POSIX V1.0 and V1.1, the file @file{awktab.c} must be given as an explicit target or it will not be built and the final link step will fail. Ignore the warning @samp{"Could not find lib m in lib list"}; it is harmless, caused by the explicit use of @samp{-lm} as a linker option which is not needed under VMS POSIX. Under V1.1 (but not V1.0) a problem with the @code{yacc} skeleton @file{/etc/yyparse.c} will cause a compiler warning for @file{awktab.c}, followed by a linker warning about compilation warnings in the resulting object module. These warnings can be ignored.@refill Once built, @code{gawk} will work like any other shell utility. Unlike the normal VMS port of @code{gawk}, no special command line manipulation is needed in the VMS POSIX environment. @node MS-DOS Installation, Atari Installation, VMS Installation, Installation @section Installing @code{gawk} on MS-DOS @cindex installation, ms-dos The first step is to get all the files in the @code{gawk} distribution onto your PC. Move all the files from the @file{pc} directory into the main directory where the other files are. Edit the file @file{make.bat} so that it will be an acceptable MS-DOS batch file. This means making sure that all lines are terminated with the ASCII carriage return and line feed characters. restrictions. @code{gawk} has only been compiled with version 5.1 of the Microsoft C compiler. The file @file{make.bat} from the @file{pc} directory assumes that you have this compiler. Copy the file @file{setargv.obj} from the library directory where it resides to the @code{gawk} source code directory. Run @file{make.bat}. This will compile @code{gawk} for you, and link it. That's all there is to it! @node Atari Installation, , MS-DOS Installation, Installation @section Installing @code{gawk} on the Atari ST @c based on material from @c Michal Jaegermann @cindex installation, atari This section assumes that you are running TOS. It applies to other Atari models (STe, TT) as well. In order to use @code{gawk}, you need to have a shell, either text or graphics, that does not map all the characters of a command line to upper case. Maintaining case distinction in option flags is very important (@pxref{Command Line, ,Invoking @code{awk}}). Popular shells like @code{gulam} or @code{gemini} will work, as will newer versions of @code{desktop}. Support for I/O redirection is necessary to make it easy to import @code{awk} programs from other environments. Pipes are nice to have, but not vital. If you have received an executable version of @code{gawk}, place it, as usual, anywhere in your @code{PATH} where your shell will find it. While executing, @code{gawk} creates a number of temporary files. @code{gawk} looks for either of the environment variables @code{TEMP} or @code{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 @sc{ram} drive. If neither @code{TEMP} nor @code{TMPDIR} are found, then @code{gawk} uses the current directory for its temporary files. The ST version of @code{gawk} searches for its program files as described in @ref{AWKPATH Variable, ,The @code{AWKPATH} Environment Variable}. On the ST, the default value for the @code{AWKPATH} variable is @code{@w{".,c:\lib\awk,c:\gnu\lib\awk"}}. The search path can be modified by explicitly setting @code{AWKPATH} to whatever you wish. Note that colons cannot be used on the ST to separate elements in the @code{AWKPATH} variable, since they have another, reserved, meaning. Instead, you must use a comma to separate elements in the path. If you are recompiling @code{gawk} on the ST, then you can choose a new default search path, by setting the value of @samp{DEFPATH} in the file @file{...\config\atari}. You may choose a different separator character by setting the value of @samp{ENVSEP} in the same file. The new values will be used when creating the header file @file{config.h}.@refill @ignore As a last resort, small adjustments can be made directly on the executable version of @code{gawk} using a binary editor.@refill @end ignore Although @code{awk} allows great flexibility in doing I/O redirections from within a program, this facility should be used with care on the ST. 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 @code{awk} program using @code{print} statements explicitly redirected to @code{"/dev/stdout"}, while other @code{print} statements use the default standard output, and a calling shell has redirected standard output to a file.@refill @c whew! When @code{gawk} is compiled with the ST version of @code{gcc} and its usual libraries, it will accept both @samp{/} and @samp{\} as path separators. While this is convenient, it should be remembered that this removes one, technically legal, character (@samp{/}) from your file names, and that it may 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 @code{gawk} will be used by some other program, use only backslashes. Also remember that in @code{awk}, backslashes in strings have to be doubled in order to get literal backslashes. The initial port of @code{gawk} to the ST was done with @code{gcc}. If you wish to recompile @code{gawk} from scratch, you will need to use a compiler that accepts @sc{ansi} standard C (such as @code{gcc}, Turbo C, or Prospero C). If @code{sizeof(int) != @w{sizeof(int *)}}, the correctness of the generated code depends heavily on the fact that all function calls have function prototypes in the current scope. If your compiler does not accept function prototypes, you will probably have to add a number of casts to the code.@refill If you are using @code{gcc}, make sure that you have up-to-date libraries. Older versions have problems with some library functions (@code{atan2()}, @code{strftime()}, the @samp{%g} conversion in @code{sprintf()}) which may affect the operation of @code{gawk}. In the @file{atari} subdirectory of the @code{gawk} distribution is a version of the @code{system()} function that has been tested with @code{gulam} and @code{msh}; it should work with other shells as well. With @code{gulam}, it passes the string to be executed without spawning an extra copy of a shell. It is possible to replace this version of @code{system()} with a similar function from a library or from some other source if that version would be a better choice for the shell you prefer. The files needed to recompile @code{gawk} on the ST can be found in the @file{atari} directory. The provided files and instructions below assume that you have the GNU C compiler (@code{gcc}), the @code{gulam} shell, and an ST version of @code{sed}. The @file{Makefile} is set up to use @file{byacc} as a @file{yacc} replacement. With a different set of tools some adjustments and/or editing will be needed.@refill @code{cd} to the @file{atari} directory. Copy @file{Makefile.st} to @file{makefile} in the source (parent) directory. Possibly adjust @file{../config/atari} to suit your system. Execute the script @file{mkconf.g} which will create the header file @file{../config.h}. Go back to the source directory. If you are not using @code{gcc}, check the file @file{missing.c}. It may be necessary to change forward slashes in the references to files from the @file{atari} subdirectory into backslashes. Type @code{make} and enjoy.@refill Compilation with @code{gcc} of some of the bigger modules, like @file{awk_tab.c}, may require a full four megabytes of memory. On smaller machines you would need to cut down on optimizations, or you would have to switch to another, less memory hungry, compiler.@refill @node Gawk Summary, Sample Program, Installation, Top @appendix @code{gawk} Summary This appendix provides a brief summary of the @code{gawk} command line and the @code{awk} language. It is designed to serve as ``quick reference.'' It is therefore terse, but complete. @menu * Command Line Summary:: Recapitulation of the command line. * Language Summary:: A terse review of the language. * Variables/Fields:: Variables, fields, and arrays. * Rules Summary:: Patterns and Actions, and their component parts. * Functions Summary:: Defining and calling functions. * Historical Features:: Some undocumented but supported ``features''. @end menu @node Command Line Summary, Language Summary, Gawk Summary, Gawk Summary @appendixsec Command Line Options Summary The command line consists of options to @code{gawk} itself, the @code{awk} program text (if not supplied via the @samp{-f} option), and values to be made available in the @code{ARGC} and @code{ARGV} predefined @code{awk} variables: @example awk @r{[@var{POSIX or GNU style options}]} -f source-file @r{[@code{--}]} @var{file} @dots{} awk @r{[@var{POSIX or GNU style options}]} @r{[@code{--}]} '@var{program}' @var{file} @dots{} @end example The options that @code{gawk} accepts are: @table @code @item -F @var{fs} @itemx --field-separator=@var{fs} Use @var{fs} for the input field separator (the value of the @code{FS} predefined variable). @item -f @var{program-file} @itemx --file=@var{program-file} Read the @code{awk} program source from the file @var{program-file}, instead of from the first command line argument. @item -v @var{var}=@var{val} @itemx --assign=@var{var}=@var{val} Assign the variable @var{var} the value @var{val} before program execution begins. @item -W compat @itemx --compat Specifies compatibility mode, in which @code{gawk} extensions are turned off. @item -W copyleft @itemx -W copyright @itemx --copyleft @itemx --copyright Print the short version of the General Public License on the error output. This option may disappear in a future version of @code{gawk}. @item -W help @itemx -W usage @itemx --help @itemx --usage Print a relatively short summary of the available options on the error output. @item -W lint @itemx --lint Give warnings about dubious or non-portable @code{awk} constructs. @item -W posix @itemx --posix Specifies @sc{posix} compatibility mode, in which @code{gawk} extensions are turned off and additional restrictions apply. @item -W source=@var{program-text} @itemx --source=@var{program-text} Use @var{program-text} as @code{awk} program source code. This option allows mixing command line source code with source code from files, and is particularly useful for mixing command line programs with library functions. @item -W version @itemx --version Print version information for this particular copy of @code{gawk} on the error output. This option may disappear in a future version of @code{gawk}. @item -- Signal the end of options. This is useful to allow further arguments to the @code{awk} program itself to start with a @samp{-}. This is mainly for consistency with the argument parsing conventions of @sc{posix}. @end table Any other options are flagged as invalid, but are otherwise ignored. @xref{Command Line, ,Invoking @code{awk}}, for more details. @node Language Summary, Variables/Fields, Command Line Summary, Gawk Summary @appendixsec Language Summary An @code{awk} program consists of a sequence of pattern-action statements and optional function definitions. @example @var{pattern} @{ @var{action statements} @} function @var{name}(@var{parameter list}) @{ @var{action statements} @} @end example @code{gawk} first reads the program source from the @var{program-file}(s) if specified, or from the first non-option argument on the command line. The @samp{-f} option may be used multiple times on the command line. @code{gawk} reads the program text from all the @var{program-file} files, effectively concatenating them in the order they are specified. This is useful for building libraries of @code{awk} functions, without having to include them in each new @code{awk} program that uses them. To use a library function in a file from a program typed in on the command line, specify @samp{-f /dev/tty}; then type your program, and end it with a @kbd{Control-d}. @xref{Command Line, ,Invoking @code{awk}}.@refill The environment variable @code{AWKPATH} specifies a search path to use when finding source files named with the @samp{-f} option. The default path, which is @samp{.:/usr/lib/awk:/usr/local/lib/awk} is used if @code{AWKPATH} is not set. If a file name given to the @samp{-f} option contains a @samp{/} character, no path search is performed. @xref{AWKPATH Variable, ,The @code{AWKPATH} Environment Variable}, for a full description of the @code{AWKPATH} environment variable.@refill @code{gawk} compiles the program into an internal form, and then proceeds to read each file named in the @code{ARGV} array. If there are no files named on the command line, @code{gawk} reads the standard input. If a ``file'' named on the command line has the form @samp{@var{var}=@var{val}}, it is treated as a variable assignment: the variable @var{var} is assigned the value @var{val}. If any of the files have a value that is the null string, that element in the list is skipped.@refill For each line in the input, @code{gawk} tests to see if it matches any @var{pattern} in the @code{awk} program. For each pattern that the line matches, the associated @var{action} is executed. @node Variables/Fields, Rules Summary, Language Summary, Gawk Summary @appendixsec Variables and Fields @code{awk} variables are dynamic; they come into existence when they are first used. Their values are either floating-point numbers or strings. @code{awk} also has one-dimension arrays; multiple-dimensional arrays may be simulated. There are several predefined variables that @code{awk} sets as a program runs; these are summarized below. @menu * Fields Summary:: Input field splitting. * Built-in Summary:: @code{awk}'s built-in variables. * Arrays Summary:: Using arrays. * Data Type Summary:: Values in @code{awk} are numbers or strings. @end menu @node Fields Summary, Built-in Summary, Variables/Fields, Variables/Fields @appendixsubsec Fields As each input line is read, @code{gawk} splits the line into @var{fields}, using the value of the @code{FS} variable as the field separator. If @code{FS} is a single character, fields are separated by that character. Otherwise, @code{FS} is expected to be a full regular expression. In the special case that @code{FS} is a single blank, fields are separated by runs of blanks and/or tabs. Note that the value of @code{IGNORECASE} (@pxref{Case-sensitivity, ,Case-sensitivity in Matching}) also affects how fields are split when @code{FS} is a regular expression.@refill Each field in the input line may be referenced by its position, @code{$1}, @code{$2}, and so on. @code{$0} is the whole line. The value of a field may be assigned to as well. Field numbers need not be constants: @example n = 5 print $n @end example @noindent prints the fifth field in the input line. The variable @code{NF} is set to the total number of fields in the input line. References to nonexistent fields (i.e., fields after @code{$NF}) return the null-string. However, assigning to a nonexistent field (e.g., @code{$(NF+2) = 5}) increases the value of @code{NF}, creates any intervening fields with the null string as their value, and causes the value of @code{$0} to be recomputed, with the fields being separated by the value of @code{OFS}.@refill @xref{Reading Files, ,Reading Input Files}, for a full description of the way @code{awk} defines and uses fields. @node Built-in Summary, Arrays Summary, Fields Summary, Variables/Fields @appendixsubsec Built-in Variables @code{awk}'s built-in variables are: @table @code @item ARGC The number of command line arguments (not including options or the @code{awk} program itself). @item ARGIND The index in @code{ARGV} of the current file being processed. It is always true that @samp{FILENAME == ARGV[ARGIND]}. @item ARGV The array of command line arguments. The array is indexed from 0 to @code{ARGC} @minus{} 1. Dynamically changing the contents of @code{ARGV} can control the files used for data.@refill @item CONVFMT The conversion format to use when converting numbers to strings. @item FIELDWIDTHS A space separated list of numbers describing the fixed-width input data. @item ENVIRON An array containing the values of the environment variables. The array is indexed by variable name, each element being the value of that variable. Thus, the environment variable @code{HOME} would be in @code{ENVIRON["HOME"]}. Its value might be @file{/u/close}. Changing this array does not affect the environment seen by programs which @code{gawk} spawns via redirection or the @code{system} function. (This may change in a future version of @code{gawk}.) Some operating systems do not have environment variables. The array @code{ENVIRON} is empty when running on these systems. @item ERRNO The system error message when an error occurs using @code{getline} or @code{close}. @item FILENAME The name of the current input file. If no files are specified on the command line, the value of @code{FILENAME} is @samp{-}. @item FNR The input record number in the current input file. @item FS The input field separator, a blank by default. @item IGNORECASE The case-sensitivity flag for regular expression operations. If @code{IGNORECASE} has a nonzero value, then pattern matching in rules, field splitting with @code{FS}, regular expression matching with @samp{~} and @samp{!~}, and the @code{gsub}, @code{index}, @code{match}, @code{split} and @code{sub} predefined functions all ignore case when doing regular expression operations.@refill @item NF The number of fields in the current input record. @item NR The total number of input records seen so far. @item OFMT The output format for numbers for the @code{print} statement, @code{"%.6g"} by default. @item OFS The output field separator, a blank by default. @item ORS The output record separator, by default a newline. @item RS The input record separator, by default a newline. @code{RS} is exceptional in that only the first character of its string value is used for separating records. If @code{RS} is set to the null string, then records are separated by blank lines. When @code{RS} is set to the null string, then the newline character always acts as a field separator, in addition to whatever value @code{FS} may have.@refill @item RSTART The index of the first character matched by @code{match}; 0 if no match. @item RLENGTH The length of the string matched by @code{match}; @minus{}1 if no match. @item SUBSEP The string used to separate multiple subscripts in array elements, by default @code{"\034"}. @end table @xref{Built-in Variables}, for more information. @node Arrays Summary, Data Type Summary, Built-in Summary, Variables/Fields @appendixsubsec Arrays Arrays are subscripted with an expression between square brackets (@samp{[} and @samp{]}). Array subscripts are @emph{always} strings; numbers are converted to strings as necessary, following the standard conversion rules (@pxref{Conversion, ,Conversion of Strings and Numbers}).@refill If you use multiple expressions separated by commas inside the square brackets, then the array subscript is a string consisting of the concatenation of the individual subscript values, converted to strings, separated by the subscript separator (the value of @code{SUBSEP}). The special operator @code{in} may be used in an @code{if} or @code{while} statement to see if an array has an index consisting of a particular value. @example if (val in array) print array[val] @end example If the array has multiple subscripts, use @code{(i, j, @dots{}) in array} to test for existence of an element. The @code{in} construct may also be used in a @code{for} loop to iterate over all the elements of an array. @xref{Scanning an Array, ,Scanning all Elements of an Array}.@refill An element may be deleted from an array using the @code{delete} statement. @xref{Arrays, ,Arrays in @code{awk}}, for more detailed information. @node Data Type Summary, , Arrays Summary, Variables/Fields @appendixsubsec Data Types The value of an @code{awk} expression is always either a number or a string. Certain contexts (such as arithmetic operators) require numeric values. They convert strings to numbers by interpreting the text of the string as a numeral. If the string does not look like a numeral, it converts to 0. Certain contexts (such as concatenation) require string values. They convert numbers to strings by effectively printing them with @code{sprintf}. @xref{Conversion, ,Conversion of Strings and Numbers}, for the details.@refill To force conversion of a string value to a number, simply add 0 to it. If the value you start with is already a number, this does not change it. To force conversion of a numeric value to a string, concatenate it with the null string. The @code{awk} language defines comparisons as being done numerically if both operands are numeric, or if one is numeric and the other is a numeric string. Otherwise one or both operands are converted to strings and a string comparison is performed. Uninitialized variables have the string value @code{""} (the null, or empty, string). In contexts where a number is required, this is equivalent to 0. @xref{Variables}, for more information on variable naming and initialization; @pxref{Conversion, ,Conversion of Strings and Numbers}, for more information on how variable values are interpreted.@refill @node Rules Summary, Functions Summary, Variables/Fields, Gawk Summary @appendixsec Patterns and Actions @menu * Pattern Summary:: Quick overview of patterns. * Regexp Summary:: Quick overview of regular expressions. * Actions Summary:: Quick overview of actions. @end menu An @code{awk} program is mostly composed of rules, each consisting of a pattern followed by an action. The action is enclosed in @samp{@{} and @samp{@}}. Either the pattern may be missing, or the action may be missing, but, of course, not both. If the pattern is missing, the action is executed for every single line of input. A missing action is equivalent to this action, @example @{ print @} @end example @noindent which prints the entire line. Comments begin with the @samp{#} character, and continue until the end of the line. Blank lines may be used to separate statements. Normally, a statement ends with a newline, however, this is not the case for lines ending in a @samp{,}, @samp{@{}, @samp{?}, @samp{:}, @samp{&&}, or @samp{||}. Lines ending in @code{do} or @code{else} also have their statements automatically continued on the following line. In other cases, a line can be continued by ending it with a @samp{\}, in which case the newline is ignored.@refill Multiple statements may be put on one line by separating them with a @samp{;}. This applies to both the statements within the action part of a rule (the usual case), and to the rule statements. @xref{Comments, ,Comments in @code{awk} Programs}, for information on @code{awk}'s commenting convention; @pxref{Statements/Lines, ,@code{awk} Statements versus Lines}, for a description of the line continuation mechanism in @code{awk}.@refill @node Pattern Summary, Regexp Summary, Rules Summary, Rules Summary @appendixsubsec Patterns @code{awk} patterns may be one of the following: @example /@var{regular expression}/ @var{relational expression} @var{pattern} && @var{pattern} @var{pattern} || @var{pattern} @var{pattern} ? @var{pattern} : @var{pattern} (@var{pattern}) ! @var{pattern} @var{pattern1}, @var{pattern2} BEGIN END @end example @code{BEGIN} and @code{END} are two special kinds of patterns that are not tested against the input. The action parts of all @code{BEGIN} rules are merged as if all the statements had been written in a single @code{BEGIN} rule. They are executed before any of the input is read. Similarly, all the @code{END} rules are merged, and executed when all the input is exhausted (or when an @code{exit} statement is executed). @code{BEGIN} and @code{END} patterns cannot be combined with other patterns in pattern expressions. @code{BEGIN} and @code{END} rules cannot have missing action parts.@refill For @samp{/@var{regular-expression}/} patterns, the associated statement is executed for each input line that matches the regular expression. Regular expressions are extensions of those in @code{egrep}, and are summarized below. A @var{relational expression} may use any of the operators defined below in the section on actions. These generally test whether certain fields match certain regular expressions. The @samp{&&}, @samp{||}, and @samp{!} operators are logical ``and,'' logical ``or,'' and logical ``not,'' respectively, as in C. They do short-circuit evaluation, also as in C, and are used for combining more primitive pattern expressions. As in most languages, parentheses may be used to change the order of evaluation. The @samp{?:} operator is like the same operator in C. If the first pattern matches, then the second pattern is matched against the input record; otherwise, the third is matched. Only one of the second and third patterns is matched. The @samp{@var{pattern1}, @var{pattern2}} form of a pattern is called a range pattern. It matches all input lines starting with a line that matches @var{pattern1}, and continuing until a line that matches @var{pattern2}, inclusive. A range pattern cannot be used as an operand to any of the pattern operators. @xref{Patterns}, for a full description of the pattern part of @code{awk} rules. @node Regexp Summary, Actions Summary, Pattern Summary, Rules Summary @appendixsubsec Regular Expressions Regular expressions are the extended kind found in @code{egrep}. They are composed of characters as follows: @table @code @item @var{c} matches the character @var{c} (assuming @var{c} is a character with no special meaning in regexps). @item \@var{c} matches the literal character @var{c}. @item . matches any character except newline. @item ^ matches the beginning of a line or a string. @item $ matches the end of a line or a string. @item [@var{abc}@dots{}] matches any of the characters @var{abc}@dots{} (character class). @item [^@var{abc}@dots{}] matches any character except @var{abc}@dots{} and newline (negated character class). @item @var{r1}|@var{r2} matches either @var{r1} or @var{r2} (alternation). @item @var{r1r2} matches @var{r1}, and then @var{r2} (concatenation). @item @var{r}+ matches one or more @var{r}'s. @item @var{r}* matches zero or more @var{r}'s. @item @var{r}? matches zero or one @var{r}'s. @item (@var{r}) matches @var{r} (grouping). @end table @xref{Regexp, ,Regular Expressions as Patterns}, for a more detailed explanation of regular expressions. The escape sequences allowed in string constants are also valid in regular expressions (@pxref{Constants, ,Constant Expressions}). @node Actions Summary, , Regexp Summary, Rules Summary @appendixsubsec Actions Action statements are enclosed in braces, @samp{@{} and @samp{@}}. Action statements consist of the usual assignment, conditional, and looping statements found in most languages. The operators, control statements, and input/output statements available are patterned after those in C. @menu * Operator Summary:: @code{awk} operators. * Control Flow Summary:: The control statements. * I/O Summary:: The I/O statements. * Printf Summary:: A summary of @code{printf}. * Special File Summary:: Special file names interpreted internally. * Numeric Functions Summary:: Built-in numeric functions. * String Functions Summary:: Built-in string functions. * Time Functions Summary:: Built-in time functions. * String Constants Summary:: Escape sequences in strings. @end menu @node Operator Summary, Control Flow Summary, Actions Summary, Actions Summary @appendixsubsubsec Operators The operators in @code{awk}, in order of increasing precedence, are: @table @code @item = += -= *= /= %= ^= Assignment. Both absolute assignment (@code{@var{var}=@var{value}}) and operator assignment (the other forms) are supported. @item ?: A conditional expression, as in C. This has the form @code{@var{expr1} ? @var{expr2} : @var{expr3}}. If @var{expr1} is true, the value of the expression is @var{expr2}; otherwise it is @var{expr3}. Only one of @var{expr2} and @var{expr3} is evaluated.@refill @item || Logical ``or''. @item && Logical ``and''. @item ~ !~ Regular expression match, negated match. @item < <= > >= != == The usual relational operators. @item @var{blank} String concatenation. @item + - Addition and subtraction. @item * / % Multiplication, division, and modulus. @item + - ! Unary plus, unary minus, and logical negation. @item ^ Exponentiation (@samp{**} may also be used, and @samp{**=} for the assignment operator, but they are not specified in the @sc{posix} standard). @item ++ -- Increment and decrement, both prefix and postfix. @item $ Field reference. @end table @xref{Expressions, ,Expressions as Action Statements}, for a full description of all the operators listed above. @xref{Fields, ,Examining Fields}, for a description of the field reference operator.@refill @node Control Flow Summary, I/O Summary, Operator Summary, Actions Summary @appendixsubsubsec Control Statements The control statements are as follows: @example if (@var{condition}) @var{statement} @r{[} else @var{statement} @r{]} while (@var{condition}) @var{statement} do @var{statement} while (@var{condition}) for (@var{expr1}; @var{expr2}; @var{expr3}) @var{statement} for (@var{var} in @var{array}) @var{statement} break continue delete @var{array}[@var{index}] exit @r{[} @var{expression} @r{]} @{ @var{statements} @} @end example @xref{Statements, ,Control Statements in Actions}, for a full description of all the control statements listed above. @node I/O Summary, Printf Summary, Control Flow Summary, Actions Summary @appendixsubsubsec I/O Statements The input/output statements are as follows: @table @code @item getline Set @code{$0} from next input record; set @code{NF}, @code{NR}, @code{FNR}. @item getline <@var{file} Set @code{$0} from next record of @var{file}; set @code{NF}. @item getline @var{var} Set @var{var} from next input record; set @code{NF}, @code{FNR}. @item getline @var{var} <@var{file} Set @var{var} from next record of @var{file}. @item next Stop processing the current input record. The next input record is read and processing starts over with the first pattern in the @code{awk} program. If the end of the input data is reached, the @code{END} rule(s), if any, are executed. @item next file Stop processing the current input file. The next input record read comes from the next input file. @code{FILENAME} is updated, @code{FNR} is set to 1, and processing starts over with the first pattern in the @code{awk} program. If the end of the input data is reached, the @code{END} rule(s), if any, are executed. @item print Prints the current record. @item print @var{expr-list} Prints expressions. @item print @var{expr-list} > @var{file} Prints expressions on @var{file}. @item printf @var{fmt, expr-list} Format and print. @item printf @var{fmt, expr-list} > file Format and print on @var{file}. @end table Other input/output redirections are also allowed. For @code{print} and @code{printf}, @samp{>> @var{file}} appends output to the @var{file}, and @samp{| @var{command}} writes on a pipe. In a similar fashion, @samp{@var{command} | getline} pipes input into @code{getline}. @code{getline} returns 0 on end of file, and @minus{}1 on an error.@refill @xref{Getline, ,Explicit Input with @code{getline}}, for a full description of the @code{getline} statement. @xref{Printing, ,Printing Output}, for a full description of @code{print} and @code{printf}. Finally, @pxref{Next Statement, ,The @code{next} Statement}, for a description of how the @code{next} statement works.@refill @node Printf Summary, Special File Summary, I/O Summary, Actions Summary @appendixsubsubsec @code{printf} Summary The @code{awk} @code{printf} statement and @code{sprintf} function accept the following conversion specification formats: @table @code @item %c An ASCII character. If the argument used for @samp{%c} is numeric, it is treated as a character and printed. Otherwise, the argument is assumed to be a string, and the only first character of that string is printed. @item %d @itemx %i A decimal number (the integer part). @item %e A floating point number of the form @samp{@r{[}-@r{]}d.ddddddE@r{[}+-@r{]}dd}.@refill @item %f A floating point number of the form @r{[}@code{-}@r{]}@code{ddd.dddddd}. @item %g Use @samp{%e} or @samp{%f} conversion, whichever produces a shorter string, with nonsignificant zeros suppressed. @item %o An unsigned octal number (again, an integer). @item %s A character string. @item %x An unsigned hexadecimal number (an integer). @item %X Like @samp{%x}, except use @samp{A} through @samp{F} instead of @samp{a} through @samp{f} for decimal 10 through 15.@refill @item %% A single @samp{%} character; no argument is converted. @end table There are optional, additional parameters that may lie between the @samp{%} and the control letter: @table @code @item - The expression should be left-justified within its field. @item @var{width} The field should be padded to this width. If @var{width} has a leading zero, then the field is padded with zeros. Otherwise it is padded with blanks. @item .@var{prec} A number indicating the maximum width of strings or digits to the right of the decimal point. @end table Either or both of the @var{width} and @var{prec} values may be specified as @samp{*}. In that case, the particular value is taken from the argument list. @xref{Printf, ,Using @code{printf} Statements for Fancier Printing}, for examples and for a more detailed description. @node Special File Summary, Numeric Functions Summary, Printf Summary, Actions Summary @appendixsubsubsec Special File Names When doing I/O redirection from either @code{print} or @code{printf} into a file, or via @code{getline} from a file, @code{gawk} recognizes certain special file names internally. These file names allow access to open file descriptors inherited from @code{gawk}'s parent process (usually the shell). The file names are: @table @file @item /dev/stdin The standard input. @item /dev/stdout The standard output. @item /dev/stderr The standard error output. @item /dev/fd/@var{n} The file denoted by the open file descriptor @var{n}. @end table In addition the following files provide process related information about the running @code{gawk} program. @table @file @item /dev/pid Reading this file returns the process ID of the current process, in decimal, terminated with a newline. @item /dev/ppid Reading this file returns the parent process ID of the current process, in decimal, terminated with a newline. @item /dev/pgrpid Reading this file returns the process group ID of the current process, in decimal, terminated with a newline. @item /dev/user Reading this file returns a single record terminated with a newline. The fields are separated with blanks. The fields represent the following information: @table @code @item $1 The value of the @code{getuid} system call. @item $2 The value of the @code{geteuid} system call. @item $3 The value of the @code{getgid} system call. @item $4 The value of the @code{getegid} system call. @end table If there are any additional fields, they are the group IDs returned by @code{getgroups} system call. (Multiple groups may not be supported on all systems.)@refill @end table @noindent These file names may also be used on the command line to name data files. These file names are only recognized internally if you do not actually have files by these names on your system. @xref{Special Files, ,Standard I/O Streams}, for a longer description that provides the motivation for this feature. @node Numeric Functions Summary, String Functions Summary, Special File Summary, Actions Summary @appendixsubsubsec Numeric Functions @code{awk} has the following predefined arithmetic functions: @table @code @item atan2(@var{y}, @var{x}) returns the arctangent of @var{y/x} in radians. @item cos(@var{expr}) returns the cosine in radians. @item exp(@var{expr}) the exponential function. @item int(@var{expr}) truncates to integer. @item log(@var{expr}) the natural logarithm function. @item rand() returns a random number between 0 and 1. @item sin(@var{expr}) returns the sine in radians. @item sqrt(@var{expr}) the square root function. @item srand(@var{expr}) use @var{expr} as a new seed for the random number generator. If no @var{expr} is provided, the time of day is used. The return value is the previous seed for the random number generator. @end table @node String Functions Summary, Time Functions Summary, Numeric Functions Summary, Actions Summary @appendixsubsubsec String Functions @code{awk} has the following predefined string functions: @table @code @item gsub(@var{r}, @var{s}, @var{t}) for each substring matching the regular expression @var{r} in the string @var{t}, substitute the string @var{s}, and return the number of substitutions. If @var{t} is not supplied, use @code{$0}. @item index(@var{s}, @var{t}) returns the index of the string @var{t} in the string @var{s}, or 0 if @var{t} is not present. @item length(@var{s}) returns the length of the string @var{s}. The length of @code{$0} is returned if no argument is supplied. @item match(@var{s}, @var{r}) returns the position in @var{s} where the regular expression @var{r} occurs, or 0 if @var{r} is not present, and sets the values of @code{RSTART} and @code{RLENGTH}. @item split(@var{s}, @var{a}, @var{r}) splits the string @var{s} into the array @var{a} on the regular expression @var{r}, and returns the number of fields. If @var{r} is omitted, @code{FS} is used instead. @item sprintf(@var{fmt}, @var{expr-list}) prints @var{expr-list} according to @var{fmt}, and returns the resulting string. @item sub(@var{r}, @var{s}, @var{t}) this is just like @code{gsub}, but only the first matching substring is replaced. @item substr(@var{s}, @var{i}, @var{n}) returns the @var{n}-character substring of @var{s} starting at @var{i}. If @var{n} is omitted, the rest of @var{s} is used. @item tolower(@var{str}) returns a copy of the string @var{str}, with all the upper-case characters in @var{str} translated to their corresponding lower-case counterparts. Nonalphabetic characters are left unchanged. @item toupper(@var{str}) returns a copy of the string @var{str}, with all the lower-case characters in @var{str} translated to their corresponding upper-case counterparts. Nonalphabetic characters are left unchanged. @item system(@var{cmd-line}) Execute the command @var{cmd-line}, and return the exit status. @end table @node Time Functions Summary, String Constants Summary, String Functions Summary, Actions Summary @appendixsubsubsec Built-in time functions The following two functions are available for getting the current time of day, and for formatting time stamps. @table @code @item systime() returns the current time of day as the number of seconds since a particular epoch (Midnight, January 1, 1970 @sc{utc}, on @sc{posix} systems). @item strftime(@var{format}, @var{timestamp}) formats @var{timestamp} according to the specification in @var{format}. The current time of day is used if no @var{timestamp} is supplied. @xref{Time Functions, ,Functions for Dealing with Time Stamps}, for the details on the conversion specifiers that @code{strftime} accepts.@refill @end table @iftex @xref{Built-in, ,Built-in Functions}, for a description of all of @code{awk}'s built-in functions. @end iftex @node String Constants Summary, , Time Functions Summary, Actions Summary @appendixsubsubsec String Constants String constants in @code{awk} are sequences of characters enclosed between double quotes (@code{"}). Within strings, certain @dfn{escape sequences} are recognized, as in C. These are: @table @code @item \\ A literal backslash. @item \a The ``alert'' character; usually the ASCII BEL character. @item \b Backspace. @item \f Formfeed. @item \n Newline. @item \r Carriage return. @item \t Horizontal tab. @item \v Vertical tab. @item \x@var{hex digits} The character represented by the string of hexadecimal digits following the @samp{\x}. As in @sc{ansi} C, all following hexadecimal digits are considered part of the escape sequence. (This feature should tell us something about language design by committee.) E.g., @code{"\x1B"} is a string containing the ASCII ESC (escape) character. (The @samp{\x} escape sequence is not in @sc{posix} @code{awk}.) @item \@var{ddd} The character represented by the 1-, 2-, or 3-digit sequence of octal digits. Thus, @code{"\033"} is also a string containing the ASCII ESC (escape) character. @item \@var{c} The literal character @var{c}. @end table The escape sequences may also be used inside constant regular expressions (e.g., the regexp @code{@w{/[@ \t\f\n\r\v]/}} matches whitespace characters).@refill @xref{Constants, ,Constant Expressions}. @node Functions Summary, Historical Features, Rules Summary, Gawk Summary @appendixsec Functions Functions in @code{awk} are defined as follows: @example function @var{name}(@var{parameter list}) @{ @var{statements} @} @end example Actual parameters supplied in the function call are used to instantiate the formal parameters declared in the function. Arrays are passed by reference, other variables are passed by value. If there are fewer arguments passed than there are names in @var{parameter-list}, the extra names are given the null string as value. Extra names have the effect of local variables. The open-parenthesis in a function call of a user-defined function must immediately follow the function name, without any intervening white space. This is to avoid a syntactic ambiguity with the concatenation operator. The word @code{func} may be used in place of @code{function} (but not in @sc{posix} @code{awk}). Use the @code{return} statement to return a value from a function. @xref{User-defined, ,User-defined Functions}, for a more complete description. @node Historical Features, , Functions Summary, Gawk Summary @appendixsec Historical Features There are two features of historical @code{awk} implementations that @code{gawk} supports. First, it is possible to call the @code{length} built-in function not only with no arguments, but even without parentheses! @example a = length @end example @noindent is the same as either of @example a = length() a = length($0) @end example @noindent This feature is marked as ``deprecated'' in the @sc{posix} standard, and @code{gawk} will issue a warning about its use if @samp{-W lint} is specified on the command line. The other feature is the use of the @code{continue} statement outside the body of a @code{while}, @code{for}, or @code{do} loop. Traditional @code{awk} implementations have treated such usage as equivalent to the @code{next} statement. @code{gawk} will support this usage if @samp{-W posix} has not been specified. @node Sample Program, Bugs, Gawk Summary, Top @appendix Sample Program The following example is a complete @code{awk} program, which prints the number of occurrences of each word in its input. It illustrates the associative nature of @code{awk} arrays by using strings as subscripts. It also demonstrates the @samp{for @var{x} in @var{array}} construction. Finally, it shows how @code{awk} can be used in conjunction with other utility programs to do a useful task of some complexity with a minimum of effort. Some explanations follow the program listing.@refill @example awk ' # Print list of word frequencies @{ for (i = 1; i <= NF; i++) freq[$i]++ @} END @{ for (word in freq) printf "%s\t%d\n", word, freq[word] @}' @end example The first thing to notice about this program is that it has two rules. The first rule, because it has an empty pattern, is executed on every line of the input. It uses @code{awk}'s field-accessing mechanism (@pxref{Fields, ,Examining 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.@refill For each input word, an element of the array @code{freq} is incremented to reflect that the word has been seen an additional time.@refill 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.@refill Note that this program has several problems that would prevent it from being useful by itself on real text files:@refill @itemize @bullet @item Words are detected using the @code{awk} convention that fields are separated by whitespace and that other characters in the input (except newlines) don't have any special meaning to @code{awk}. This means that punctuation characters count as part of words.@refill @item The @code{awk} language considers upper and lower case characters to be distinct. Therefore, @samp{foo} and @samp{Foo} are not treated by this program 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 that.@refill @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 having an alphabetized table of how frequently each word occurs.@refill @end itemize The way to solve these problems is to use some of the more advanced features of the @code{awk} language. First, we use @code{tolower} to remove case distinctions. Next, we use @code{gsub} to remove punctuation characters. Finally, we use the system @code{sort} utility to process the output of the @code{awk} script. First, here is the new version of the program:@refill @example awk ' # Print list of word frequencies @{ $0 = tolower($0) # remove case distinctions gsub(/[^a-z0-9_ \t]/, "", $0) # remove punctuation for (i = 1; i <= NF; i++) freq[$i]++ @} END @{ for (word in freq) printf "%s\t%d\n", word, freq[word] @}' @end example Assuming we have saved this program in a file named @file{frequency.awk}, and that the data is in @file{file1}, the following pipeline @example awk -f frequency.awk file1 | sort +1 -nr @end example @noindent produces a table of the words appearing in @file{file1} in order of decreasing frequency. The @code{awk} program suitably massages the data and produces a word frequency table, which is not ordered. The @code{awk} script's output is then sorted by the @code{sort} command and printed on the terminal. The options given to @code{sort} in this example specify to sort using 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.@refill We could have even done the @code{sort} from within the program, by changing the @code{END} action to: @example END @{ sort = "sort +1 -nr" for (word in freq) printf "%s\t%d\n", word, freq[word] | sort close(sort) @}' @end example See the general operating system documentation for more information on how to use the @code{sort} command.@refill @ignore @strong{ADR: I have some more substantial programs courtesy of Rick Adams at UUNET. I am planning on incorporating those either in addition to or instead of this program.} @strong{I would also like to incorporate the general @code{translate} function that I have written.} @strong{I have a ton of other sample programs to include too.} @end ignore @node Bugs, Notes, Sample Program, Top @appendix Reporting Problems and Bugs @c This chapter stolen shamelessly from the GNU m4 manual. @c This chapter has been unshamelessly altered to emulate changes made to @c make.texi from whence it was originally shamelessly stolen! :-} --mew If you have problems with @code{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 @code{awk} program and input data file that reproduces the problem. Then send us the program and data file, some idea of what kind of Unix system you're using, and the exact results @code{gawk} gave you. Also say what you expected to occur; this will help us decide whether the problem was really in the documentation. Once you have a precise problem, send e-mail to (Internet) @samp{bug-gnu-utils@@prep.ai.mit.edu} or (UUCP) @samp{mit-eddie!prep.ai.mit.edu!bug-gnu-utils}. Please include the version number of @code{gawk} you are using. You can get this information with the command @samp{gawk -W version '@{@}' /dev/null}. You should send carbon copies of your mail to David Trueman at @samp{david@@cs.dal.ca}, and to Arnold Robbins, who can be reached at @samp{arnold@@skeeve.atl.ga.us}. David is most likely to fix code problems, while Arnold is most likely to fix documentation problems.@refill 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 Arnold Robbins; he will try to help you out, although he may not have the time to fix the problem. You can send him electronic mail at the Internet address above. If you find bugs in one of the non-Unix ports of @code{gawk}, please send an electronic mail message to the person who maintains that port. They are listed below, and also in the @file{README} file in the @code{gawk} distribution. Information in the @code{README} file should be considered authoritative if it conflicts with this manual. The people maintaining the non-Unix ports of @code{gawk} are: @table @asis @item MS-DOS The port to MS-DOS is maintained by Scott Deifik. His electronic mail address is @samp{scottd@@amgen.com}. @item VMS The port to VAX VMS is maintained by Pat Rankin. His electronic mail address is @samp{rankin@@eql.caltech.edu}. @item Atari ST The port to the Atari ST is maintained by Michal Jaegermann. His electronic mail address is @samp{ntomczak@@vm.ucs.ualberta.ca}. @end table If your bug is also reproducible under Unix, please send copies of your report to the general GNU bug list, as well as to Arnold Robbins and David Trueman, at the addresses listed above. @node Notes, Glossary, Bugs, Top @appendix Implementation Notes This appendix contains information mainly of interest to implementors and maintainers of @code{gawk}. Everything in it applies specifically to @code{gawk}, and not to other implementations. @menu * Compatibility Mode:: How to disable certain @code{gawk} extensions. * Future Extensions:: New features we may implement soon. * Improvements:: Suggestions for improvements by volunteers. @end menu @node Compatibility Mode, Future Extensions, Notes, Notes @appendixsec Downward Compatibility and Debugging @xref{POSIX/GNU, ,Extensions in @code{gawk} not in POSIX @code{awk}}, for a summary of the GNU extensions to the @code{awk} language and program. All of these features can be turned off by invoking @code{gawk} with the @samp{-W compat} option, or with the @samp{-W posix} option.@refill If @code{gawk} is compiled for debugging with @samp{-DDEBUG}, then there is one more option available on the command line: @table @samp @item -W parsedebug Print out the parse stack information as the program is being parsed. @end table This option is intended only for serious @code{gawk} developers, and not for the casual user. It probably has not even been compiled into your version of @code{gawk}, since it slows down execution. @node Future Extensions, Improvements, Compatibility Mode, Notes @appendixsec Probable Future Extensions This section briefly lists extensions that indicate the directions we are currently considering for @code{gawk}. The file @file{FUTURES} in the @code{gawk} distributions lists these extensions, as well as several others. @table @asis @item @code{RS} as a regexp The meaning of @code{RS} may be generalized along the lines of @code{FS}. @item Control of subprocess environment Changes made in @code{gawk} to the array @code{ENVIRON} may be propagated to subprocesses run by @code{gawk}. @item Databases It may be possible to map a GDBM/NDBM/SDBM file into an @code{awk} array. @item Single-character fields The null string, @code{""}, as a field separator, will cause field splitting and the @code{split} function to separate individual characters. Thus, @code{split(a, "abcd", "")} would yield @code{a[1] == "a"}, @code{a[2] == "b"}, and so on. @item More @code{lint} warnings There are more things that could be checked for portability. @item @code{RECLEN} variable for fixed length records Along with @code{FIELDWIDTHS}, this would speed up the processing of fixed-length records. @item @code{RT} variable to hold the record terminator It is occasionally useful to have access to the actual string of characters that matched the @code{RS} variable. The @code{RT} variable would hold these characters. @item A @code{restart} keyword After modifying @code{$0}, @code{restart} would restart the pattern matching loop, without reading a new record from the input. @item A @samp{|&} redirection The @samp{|&} redirection, in place of @samp{|}, would open a two-way pipeline for communication with a sub-process (via @code{getline} and @code{print} and @code{printf}). @item @code{IGNORECASE} affecting all comparisons The effects of the @code{IGNORECASE} variable may be generalized to all string comparisons, and not just regular expression operations. @item A way to mix command line source code and library files There may be a new option that would make it possible to easily use library functions from a program entered on the command line. @c probably a @samp{-s} option... @item GNU-style long options We will add GNU-style long options to @code{gawk} for compatibility with other GNU programs. (For example, @samp{--field-separator=:} would be equivalent to @samp{-F:}.)@refill @c this is @emph{very} long term --- not worth including right now. @ignore @item The C Comma Operator We may add the C comma operator, which takes the form @code{@var{expr1},@var{expr2}}. The first expression is evaluated, and the result is thrown away. The value of the full expression is the value of @var{expr2}.@refill @end ignore @end table @node Improvements, , Future Extensions, Notes @appendixsec Suggestions for Improvements Here are some projects that would-be @code{gawk} hackers might like to take on. They vary in size from a few days to a few weeks of programming, depending on which one you choose and how fast a programmer you are. Please send any improvements you write to the maintainers at the GNU project.@refill @enumerate @item Compilation of @code{awk} programs: @code{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.@refill It should be possible for @code{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 @code{gawk} library to provide all the needed functions (regexps, fields, associative arrays, type coercion, and so on).@refill An easier possibility might be for an intermediate phase of @code{awk} 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 @code{gawk} does now.@refill This may actually happen for the 3.0 version of @code{gawk}. @item An error message section has not been included in this version of the manual. Perhaps some nice beta testers will document some of the messages for the future. @item The programs in the test suite could use documenting in this manual. @item The programs and data files in the manual should be available in separate files to facilitate experimentation. @item See the @file{FUTURES} file for more ideas. Contact us if you would seriously like to tackle any of the items listed there. @end enumerate @node Glossary, Index, Notes, Top @appendix Glossary @table @asis @item Action A series of @code{awk} statements attached to a rule. If the rule's pattern matches an input record, the @code{awk} language executes the rule's action. Actions are always enclosed in curly braces. @xref{Actions, ,Overview of Actions}.@refill @item Amazing @code{awk} Assembler Henry Spencer at the University of Toronto wrote a retargetable assembler completely as @code{awk} scripts. It is thousands of lines long, including machine descriptions for several 8-bit microcomputers. @c It is distributed with @code{gawk} (as part of the test suite) and It is a good example of a program that would have been better written in another language.@refill @item @sc{ansi} The American National Standards Institute. This organization produces many standards, among them the standard for the C programming language. @item Assignment An @code{awk} expression that changes the value of some @code{awk} variable or data object. An object that you can assign to is called an @dfn{lvalue}. @xref{Assignment Ops, ,Assignment Expressions}.@refill @item @code{awk} Language The language in which @code{awk} programs are written. @item @code{awk} Program An @code{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. @code{awk} programs may also contain function definitions.@refill @item @code{awk} Script Another name for an @code{awk} program. @item Built-in Function The @code{awk} language provides built-in functions that perform various numerical, time stamp related, and string computations. Examples are @code{sqrt} (for the square root of a number) and @code{substr} (for a substring of a string). @xref{Built-in, ,Built-in Functions}.@refill @item Built-in Variable @code{ARGC}, @code{ARGIND}, @code{ARGV}, @code{CONVFMT}, @code{ENVIRON}, @code{ERRNO}, @code{FIELDWIDTHS}, @code{FILENAME}, @code{FNR}, @code{FS}, @code{IGNORECASE}, @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 @code{awk}. Changing some of them affects @code{awk}'s running environment. @xref{Built-in Variables}.@refill @item Braces See ``Curly Braces.'' @item C The system programming language that most GNU software is written in. The @code{awk} programming language has C-like syntax, and this manual points out similarities between @code{awk} and C when appropriate.@refill @item CHEM A preprocessor for @code{pic} that reads descriptions of molecules and produces @code{pic} input for drawing them. It was written by Brian Kernighan, and is available from @code{netlib@@research.att.com}.@refill @item Compound Statement A series of @code{awk} statements, enclosed in curly braces. Compound statements may be nested. @xref{Statements, ,Control Statements in Actions}.@refill @item Concatenation Concatenating two strings means sticking them together, one after another, giving a new string. For example, the string @samp{foo} concatenated with the string @samp{bar} gives the string @samp{foobar}. @xref{Concatenation, ,String Concatenation}.@refill @item Conditional Expression An expression using the @samp{?:} ternary operator, such as @code{@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, ,Conditional Expressions}.@refill @item Constant Regular Expression A constant regular expression is a regular expression written within slashes, such as @samp{/foo/}. This regular expression is chosen when you write the @code{awk} program, and cannot be changed doing its execution. @xref{Regexp Usage, ,How to Use Regular Expressions}. @item Comparison Expression A relation that is either true or false, such as @code{(a < b)}. Comparison expressions are used in @code{if}, @code{while}, and @code{for} statements, and in patterns to select which input records to process. @xref{Comparison Ops, ,Comparison Expressions}.@refill @item Curly Braces The characters @samp{@{} and @samp{@}}. Curly braces are used in @code{awk} for delimiting actions, compound statements, and function bodies.@refill @item Data Objects These are numbers and strings of characters. Numbers are converted into strings and vice versa, as needed. @xref{Conversion, ,Conversion of Strings and Numbers}.@refill @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 may vary. @xref{Regexp Usage, ,How to Use Regular Expressions}. @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{Constants, ,Constant Expressions}. @item Field When @code{awk} reads an input record, it splits the record into pieces separated by whitespace (or by a separator regexp which 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{Records, ,How Input is Split into Records}.@refill @item Format Format strings are used to control the appearance of output in the @code{printf} statement. Also, data conversions from numbers to strings are controlled by the format string contained in the built-in variable @code{CONVFMT}. @xref{Control Letters, ,Format-Control Letters}.@refill @item Function A specialized group of statements often used to encapsulate general or program-specific tasks. @code{awk} has a number of built-in functions, and also allows you to define your own. @xref{Built-in, ,Built-in Functions}. Also, see @ref{User-defined, ,User-defined Functions}.@refill @item @code{gawk} The GNU implementation of @code{awk}. @item GNU ``GNU's not Unix''. An on-going project of the Free Software Foundation to create a complete, freely distributable, @sc{posix}-compliant computing environment. @item Input Record A single chunk of data read in by @code{awk}. Usually, an @code{awk} input record consists of one line of text. @xref{Records, ,How Input is Split into Records}.@refill @item Keyword In the @code{awk} language, a keyword is a word that has special meaning. Keywords are reserved and may not be used as variable names. @code{awk}'s keywords are: @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{function}, @code{func}, and @code{exit}.@refill @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 @code{awk}, a field designator can also be used as an lvalue.@refill @item Number A numeric valued data object. The @code{gawk} implementation uses double precision floating point to represent numbers.@refill @item Pattern Patterns tell @code{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{Patterns}.@refill @item @sc{posix} The name for a series of standards being developed by the @sc{ieee} that specify a Portable Operating System interface. The ``IX'' denotes the Unix heritage of these standards. The main standard of interest for @code{awk} users is P1003.2, the Command Language and Utilities standard. @item Range (of input lines) A sequence of consecutive lines from the input file. A pattern can specify ranges of input lines for @code{awk} to process, or it can specify single lines. @xref{Patterns}.@refill @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 other than the standard input stream, or output to 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{>>}, and @samp{|} operators. You can redirect input to the @code{getline} statement using the @samp{<} and @samp{|} operators. @xref{Redirection, ,Redirecting Output of @code{print} and @code{printf}}.@refill @item Regular Expression See ``regexp.'' @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 @code{awk}, regexps are used in patterns and in conditional expressions. Regexps may contain escape sequences. @xref{Regexp, ,Regular Expressions as Patterns}.@refill @item Rule A segment of an @code{awk} program, that specifies how to process single input records. A rule consists of a @dfn{pattern} and an @dfn{action}. @code{awk} reads an input record; then, for each rule, if the input record satisfies the rule's pattern, @code{awk} executes the rule's action. Otherwise, the rule does nothing for that input record.@refill @item Side Effect A side effect occurs when an expression has an effect aside from merely producing a value. Assignment expressions, increment expressions and function calls have side effects. @xref{Assignment Ops, ,Assignment Expressions}. @item Special File A file name interpreted internally by @code{gawk}, instead of being handed directly to the underlying operating system. For example, @file{/dev/stdin}. @xref{Special Files, ,Standard I/O Streams}. @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, and with interactive programs, which require input from the user.@refill @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 @code{awk} language, and may contain escape sequences. @xref{Constants, ,Constant Expressions}. @item Whitespace A sequence of blank or tab characters occurring inside an input record or a string.@refill @end table @node Index, , Glossary, Top @unnumbered Index @printindex cp @summarycontents @contents @bye Unresolved Issues: ------------------ 1. From: ntomczak@vm.ucs.ualberta.ca (Michal Jaegermann) Examples of usage tend to suggest that /../ and ".." delimiters can be used for regular expressions, even if definition is consistently using /../. I am not sure what the real rules are and in particular what of the following is a bug and what is a feature: # This program matches everything '"\(" { print }' # This one complains about mismatched parenthesis '$0 ~ "\(" { print }' # This one behaves in an expected manner '/\(/ { print }' You may also try to use "\(" as an argument to match() to see what will happen. 2. From ADR. The posix (and original Unix!) notion of awk values as both number and string values needs to be put into the manual. This involves major and minor rewrites of most of the manual, but should help in clarifying many of the weirder points of the language. 3. From ADR. The manual should be reorganized. Expressions should be introduced early, building up to regexps as expressions, and from there to their use as patterns and then in actions. Built-in vars should come earlier in the manual too. The 'expert info' sections marked with comments should get their own sections or subsections with nodes and titles. The manual should be gone over thoroughly for indexing. 4. 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. 5. From ADR in response to moraes@uunet.ca (This would make the beginnings of a good "puzzles" section...) Date: Mon, 2 Dec 91 10:08:05 EST From: gatech!cc!arnold (Arnold Robbins) To: cs.dal.ca!david, uunet.ca!moraes Subject: redirecting to /dev/stderr Cc: skeeve!arnold, boeing.com!brennan, research.att.com!bwk In 2.13.3 the following program no longer dumps core: BEGIN { print "hello" > /dev/stderr ; exit(1) } Instead, it creates a file named `0' with the word `hello' in it. AWK semantics strikes again. The meaning of the statement is print "hello" > (($0 ~ /dev/) stderr) /dev/ tests $0 for the pattern `dev'. This yields a 0. The variable stderr, having never been used, has a null string in it. The concatenation yields a string value of "0" which is used as the file name. Sigh. I think with some more time I can come up with a decent fix, but it will probably only print a diagnostic with -Wlint. Arnold