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"To boldly go where no man has gone before" is a Registered Trademark of Paramount Pictures Corporation.

Copyright (C) 1989, 1991, 92, 93, 96, 97 Free Software Foundation, Inc.

This is Edition 1.0.3 of Effective AWK Programming,
for the 3.0.3 (or later) version of the GNU implementation of AWK.

Published jointly by:

@multitable {Specialized Systems Consultants, Inc. (SSC)} {Boston, MA 02111-1307 USA}

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    ISBN 1-57831-000-8
    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.

    Cover art by Amy Wells Wood.

    To Miriam, for making me complete.

    To Chana, for the joy you bring us.

    To Rivka, for the exponential increase.

    To Nachum, for the added dimension.


    This book teaches you about the awk language and how you can use it effectively. You should already be familiar with basic system commands, such as cat and ls,(1) and basic shell facilities, such as Input/Output (I/O) redirection and pipes.

    Implementations of the awk language are available for many different computing environments. This book, while describing the awk language in general, also describes a particular implementation of awk called gawk (which stands for "GNU Awk"). gawk runs on a broad range of Unix systems, ranging from 80386 PC-based computers, up through large scale systems, such as Crays. gawk has also been ported to MS-DOS and OS/2 PC's, Atari and Amiga micro-computers, and VMS.

    History of awk and gawk

    The name awk comes from the initials of its designers: Alfred V. Aho, Peter J. Weinberger, and Brian W. Kernighan. The original version of awk was written in 1977 at AT&T Bell Laboratories. In 1985 a new version made the programming language more powerful, introducing user-defined functions, multiple input streams, and computed regular expressions. This new version became generally available with Unix 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 awk in the POSIX Command Language and Utilities standard further clarified the language based on feedback from both the gawk designers, and the original Bell Labs awk designers.

    The GNU implementation, 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 gawk for compatibility with the newer awk. Current development focuses on bug fixes, performance improvements, standards compliance, and occasionally, new features.

    The GNU Project and This Book

    The Free Software Foundation (FSF) is a non-profit organization dedicated to the production and distribution of freely distributable software. It was founded by Richard M. Stallman, the author of the original Emacs editor. GNU Emacs is the most widely used version of Emacs today.

    The GNU project is an on-going effort on the part of the Free Software Foundation to create a complete, freely distributable, POSIX compliant computing environment. (GNU stands for "GNU's not Unix".) The FSF uses the "GNU General Public License" (or GPL) to ensure that source code for their software is always available to the end user. A copy of the GPL is included for your reference (see section GNU GENERAL PUBLIC LICENSE). The GPL applies to the C language source code for gawk.

    A shell, an editor (Emacs), highly portable optimizing C, C++, and Objective-C compilers, a symbolic debugger, and dozens of large and small utilities (such as gawk), have all been completed and are freely available. As of this writing (early 1997), the GNU operating system kernel (the HURD), has been released, but is still in an early stage of development.

    Until the GNU operating system is more fully developed, you should consider using Linux, a freely distributable, Unix-like operating system for 80386, DEC Alpha, Sun SPARC and other systems. There are many books on Linux. One freely available one is Linux Installation and Getting Started, by Matt Welsh. Many Linux distributions are available, often in computer stores or bundled on CD-ROM with books about Linux. (There are three other freely available, Unix-like operating systems for 80386 and other systems, NetBSD, FreeBSD,and OpenBSD. All are based on the 4.4-Lite Berkeley Software Distribution, and they use recent versions of gawk for their versions of awk.)

    This book you are reading now is actually free. The information in it is freely available to anyone, the machine readable source code for the book comes with gawk, and anyone may take this book to a copying machine and make as many copies of it as they like. (Take a moment to check the copying permissions on the Copyright page.)

    If you paid money for this book, what you actually paid for was the book's nice printing and binding, and the publisher's associated costs to produce it. We have made an effort to keep these costs reasonable; most people would prefer a bound book to over 330 pages of photo-copied text that would then have to be held in a loose-leaf binder (not to mention the time and labor involved in doing the copying). The same is true of producing this book from the machine readable source; the retail price is only slightly more than the cost per page of printing it on a laser printer.

    This book itself has gone through several previous, preliminary editions. I started working on a preliminary draft of The GAWK Manual, by Diane Close, Paul Rubin, and Richard Stallman in the fall of 1988. It was around 90 pages long, and barely described the original, "old" version of awk. After substantial revision, the first version of the The GAWK Manual to be released was Edition 0.11 Beta in October of 1989. The manual then underwent more substantial revision for Edition 0.13 of December 1991. David Trueman, Pat Rankin, and Michal Jaegermann contributed sections of the manual for Edition 0.13. That edition was published by the FSF as a bound book early in 1992. Since then there have been several minor revisions, notably Edition 0.14 of November 1992 that was published by the FSF in January of 1993, and Edition 0.16 of August 1993.

    Edition 1.0 of Effective AWK Programming represents a significant re-working of The GAWK Manual, with much additional material. The FSF and I agree that I am now the primary author. I also felt that it needed a more descriptive title.

    Effective AWK Programming will undoubtedly continue to evolve. An electronic version comes with the gawk distribution from the FSF. If you find an error in this book, please report it! See section Reporting Problems and Bugs, for information on submitting problem reports electronically, or write to me in care of the FSF.


    I would like to acknowledge Richard M. Stallman, for his vision of a better world, and for his courage in founding the FSF and starting the GNU project.

    The initial draft of The GAWK Manual had the following acknowledgements:

    Many people need to be thanked for their assistance in producing this manual. Jay Fenlason contributed many ideas and sample programs. Richard Mlynarik and Robert Chassell gave helpful comments on drafts of this manual. The paper A Supplemental Document for awk by John W. Pierce of the Chemistry Department at UC San Diego, pinpointed several issues relevant both to awk implementation and to this manual, that would otherwise have escaped us.

    The following people provided many helpful comments on Edition 0.13 of The GAWK Manual: Rick Adams, Michael Brennan, Rich Burridge, Diane Close, Christopher ("Topher") Eliot, Michael Lijewski, Pat Rankin, Miriam Robbins, and Michal Jaegermann.

    The following people provided many helpful comments for Edition 1.0 of Effective AWK Programming: Karl Berry, Michael Brennan, Darrel Hankerson, Michal Jaegermann, Michael Lijewski, and Miriam Robbins. Pat Rankin, Michal Jaegermann, Darrel Hankerson and Scott Deifik updated their respective sections for Edition 1.0.

    Robert J. Chassell provided much valuable advice on the use of Texinfo. He also deserves special thanks for convincing me not to title this book How To Gawk Politely. Karl Berry helped significantly with the TeX part of Texinfo.

    David Trueman deserves special credit; he has done a yeoman job of evolving gawk so that it performs well, and without bugs. Although he is no longer involved with gawk, working with him on this project was a significant pleasure.

    Scott Deifik, Darrel Hankerson, Kai Uwe Rommel, Pat Rankin, and Michal Jaegermann (in no particular order) are long time members of the gawk "crack portability team." Without their hard work and help, gawk would not be nearly the fine program it is today. It has been and continues to be a pleasure working with this team of fine people.

    Jeffrey Friedl provided invaluable help in tracking down a number of last minute problems with regular expressions in gawk 3.0.

    David and I would like to thank Brian Kernighan of Bell Labs for invaluable assistance during the testing and debugging of gawk, and for help in clarifying numerous points about the language. We could not have done nearly as good a job on either gawk or its documentation without his help.

    I would like to thank Marshall and Elaine Hartholz of Seattle, and Dr. Bert and Rita Schreiber of Detroit for large amounts of quiet vacation time in their homes, which allowed me to make significant progress on this book and on gawk itself. Phil Hughes of SSC contributed in a very important way by loaning me his laptop Linux system, not once, but twice, allowing me to do a lot of work while away from home.

    Finally, I must thank my wonderful wife, Miriam, for her patience through the many versions of this project, for her proof-reading, and for sharing me with the computer. I would like to thank my parents for their love, and for the grace with which they raised and educated me. I also must acknowledge my gratitude to G-d, for the many opportunities He has sent my way, as well as for the gifts He has given me with which to take advantage of those opportunities.

    Arnold Robbins
    Atlanta, Georgia
    February, 1997

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