AWK FAQ (1998/03/26)

Archive-name: computer-lang/awk/faq
Author: awkfaq@locutus.ofB.ORG (Russell Schulz)
Comp-lang-awk-archive-name: faq
Posting-Frequency: biweekly
Last-modified: 1998-Mar-26

Frequently Asked Questions == FAQ
=====

:*:Contents:

1. Disclaimer
2. Spam
3. Can you answer my awk question?
4. How can I add a FAQ and its answer to the FAQ list?
5. What is awk?
6. What well-maintained awk-compatible languages are there?
6.1 nawk
6.2 gawk
6.3 mawk
6.4 tawk
6.5 mksawk
6.6 awkcc
6.7 awk2c
6.8 a2p
7. Where can I buy awk?
7.1 AT&T (awk, awkcc)
7.2 Thompson Automation (tawk)
7.3 MKS (awk, can generate standalone interpreted .exe)
8. Where can I get awk for free? For what platforms?
8.0 meta-answer
8.1 the one true awk
8.2 gawk
8.2.1 gawk precompiled for MS-DOS or OS/2
8.2.2 gawk precompiled for Macintosh
8.2.3 jgawk (Japanese gawk)
8.2.4 gawk.dll
8.3 mawk
8.4 awk2c
8.5 various old binary-only distributions for MSDOS
8.99 awkcc
9. Why would anyone still use awk instead of perl?
10. How can I learn awk?
11. What are some other awk resources?
12. How do I report a bug in gawk?
13. What's wrong with gawk on Digital's OSF/1?
14. How can I access shell or environment variables in an awk script?
14.1 Environment variables in general
14.2 Unix Shell Quoting
14.3 ENVIRON and "env"|
14.4 exporting environment variables back to the parent process
15. Is there an easy way to determine if you have oawk or nawk?
16. How does awk deal with multiple files?
16.0 Version warning
16.1 How can awk test for the existence of a file?
16.2 How can I get awk to read multiple files?
16.3 How can I tell from which file my input is coming?
16.4 How can I get awk to open multiple files (selected at runtime)?
16.5 How can I treat the first file specially?
16.6 How can I explicitly pass in a filename to treat specially?
17. How many elements were created by split()?
18. How can I split a string into characters?
19. How do I use tolower() in the SunOS nawk?
20. How do I have dynamic-width printf strings, like C?
21. Why doesn't "\\$" behave like /\\$/ ? Why don't parentheses match?
22. What is gawk's exit code?
23. How can I get awk to be case-insensitive?
24. How can I force a numeric/non-numeric comparison?
25. Why does { FS=":"; print $1 } not split the first record?
26. Did ^ and $ and . change in gawk?
27. Why doesn't awk 'begin {...}' work?
98. Miscellaneous
99. Credits

=====
:*:1. Disclaimer
Read at your own risk. The current, previous, or original authors make no claim as to fitness for any purpose or absence of any errors, and offer no warranty. Do not eat.

=====
:*:2. Spam
you wouldn't believe how much spam I get to this address.

=====
:*:3. Can you answer my awk question?

Probably not. Please don't mail it to me.

Read the FAQ, and the materials pointed to by it, and if you can't find an answer there, by all means post to the newsgroup.

A FAQ list is intended to reduce traffic on a newsgroup, not eliminate it.

=====
:*:4. How can I add a FAQ and its answer to the FAQ list?

Mail BOTH of them to me. Then I can add them to the FAQ and it should help people who have that same question later, as well as everyone who reads the group, because they won't see it asked and answered so often.

I do not work on this FAQ every day, but I will try to get updates incorporated in a timely manner.

=====
:*:5. What is awk?

awk is a programming language, named after its three original authors:

Alfred V. Aho
Brian W. Kernighan
Peter J. Weinberger

they write:

"Awk is a convenient and expressive programming language that can be applied to a wide variety of computing and data-manipulation tasks."

The title of the book uses `AWK', but the contents of the book use `awk' (except at the beginning of sentences, as above). I will attempt to do the same (except perhaps at the beginning of sentences, as above).

Most implementations of awk are interpreters which read your awk
source program and parse it and act on it directly.

Some vendors have developed awk compilers which will produce an `executable' that may be run stand-alone -- thus, the end user does not have access to the source code. there are also various awk->C converters which allow you to achieve the same functionality (by compiling the resulting C code later).

One of the most popular compilers, from Thompson Automation, continues to be the subject of many positive posts in the group.

[ I don't really want to start a reviews section, but it may be appropriate. I think it's of general interest, and a good thing for the FAQ, but I don't want to be given any grief by a negative review I didn't write just because I'm distributing it. If you have a review you'd like me to put a pointer to, please inform me -- I already have some pointers of this form listed. ]

=====
:*:6. What well-maintained awk-compatible languages are there?

6.1 nawk
AT&T's `new awk' -- probably nobody uses the `old awk' anymore.
interpreter
might NOT be well-maintained

6.2 gawk
from the GNU project
interpreter

6.3 mawk
from Michael Brennan <mailto:brennan@whidbey.com>
interpreter

6.4 tawk
from Thompson Automation
interpreter
compiler
MS-Windows DLL

6.5 mksawk
interpreter
compiler
from Mortice Kern Systems

an old version of mksawk is shipped as `nawk' on Ultrix and
OSF/1.

6.6 awkcc
translator to C
might NOT be well-maintained

6.7 awk2c
translator to C
uses GNU awk libraries extensively, and is subject to GPL
might NOT be well-maintained

6.8 a2p
translator to Perl
comes with Perl
doesn't handle multiple concatenations: e.g., var="x" "y" "z"
-> must be in pairs: e.g., var=( "x" "y" ) "z"
doesn't handle redirection: e.g., { print("data") > "filename" }
-> no known workaround

=====
:*:7. Where can I buy awk?

7.1 AT&T (awk, awkcc)

_The AWK Programming Language_ says:
phone
+1 201 522 6900 [is this number still valid?]
and login as `guest'.

<http://www.unipress.com/att/new/awk.html>
<http://www.unipress.com/att/new/awkcc.html>
<http://www.bell-labs.com/org/ssg/html/awk.htm>
<http://www.bell-labs.com/org/ssg/html/awkcc.htm>

these versions might NOT be well-maintained

7.2 Thompson Automation (tawk)

<http://www.tasoft.com/~thompson/tawk.html>

<http://www.teleport.com/~thompson/>

Thompson Automation Software
5616 SW Jefferson
Portland, OR 97221
USA

North America: 800-944-0139
Phone: +1 503 224 1639
Fax: +1 503 224 3230

7.3 MKS (awk, can generate standalone interpreted .exe)

<http://www.mks.ca/solution/tk/>

Mortice Kern Systems
185 Columbia Street W
Waterloo, ON
N2L 5Z5
Canada

North America: 800-265-2797
Phone: +1 519 884 2251
Fax: +1 519 884 8861

=====
:*:8. Where can I get awk for free? For what platforms?

8.0 meta-answer
Obtaining Awk and Perl
<http://uts.cc.utexas.edu/~churchh/awk-perl.html>

8.1 the one true awk
<http://cm.bell-labs.com/who/bwk/>
<http://plan9.bell-labs.com/who/bwk/>
<ftp://netlib.bell-labs.com/netlib/research/awk.bundle.Z>
[ appears to no longer be available via ftp 1997/Oct/23 ]

This is the version of awk described in "The Awk Programming Language",
by A. V. Aho, B. W. Kernighan, and P. J. Weinberger
(Addison-Wesley, 1988, ISBN 0-201-07981-X).
Changes, mostly bug fixes, are listed in FIXES.

8.1.1 the one true awk precompiled for MS-DOS, Win32, or OS/2

<http://www.simtel.net/simtel.net/>
<ftp://ftp.simtel.net/pub/simtelnet/gnu/djgpp/>

8.2 gawk
NOTE: gawk 3.0.2 has a per-record memory leak which is fixed
in (the wide beta and later) of 3.0.3 .

ftp.gnu.ai.mit.edu/pub/gnu/gawk*
<ftp://ftp.gnu.ai.mit.edu/pub/gnu/>
e.g.,
<ftp://ftp.gnu.ai.mit.edu/pub/gnu/gawk-3.0.3.tar.gz>

8.2.1 gawk precompiled for MS-DOS, Win32, or OS/2

The djgpp collection contains a 32-bit DOS gawk, along with
many GNU utilities which may be useful with gawk (djgpp ports
understand long filenames on Windows 95):

<http://www.simtel.net/simtel.net/>
<ftp://ftp.simtel.net/pub/simtelnet/gnu/djgpp/v2gnu/>
(look for gwk*.zip)

32-bit DOS (djgpp), Win32, and 16-bit OS/2 and DOS versions are
part of the GNUish project:

<http://www.simtel.net/simtel.net/>
<ftp://ftp.simtel.net/pub/simtelnet/gnu/gnuish/>
<http://wuarchive.wustl.edu/systems/msdos/gnuish/>
<http://ftp.coast.net/SimTel/gnu/gnuish.html>

32-bit OS/2, Win32, and DOS (emx) versions:

<http://www.leo.org/pub/comp/os/os2/gnu/script/gnuawk.zip> (Germany)
<ftp://ftp-os2.cdrom.com/pub/os2/lang/gnuawk.zip> (US)

8.2.2 gawk precompiled for Macintosh

<ftp://ftp.funet.fi/pub/mac/programming/>
<ftp://ezinfo.ethz.ch/mac/programming/>
<ftp://ftp.uwtc.washington.edu/pub/Mac/Programming/>
<ftp://ftp.eos.hokudai.ac.jp/pub/mac/util/Gawk/>
<ftp://ftp.fu-berlin.de/mac/lang/MPW/>
<ftp://pascal.zedat.fu-berlin.de/mac/lang/MPW/>
<ftp://ftp.cs.tu-berlin.de/pub/mac/lang/MPW/>
<ftp://nic.switch.ch/software/mac/src/mpw_c/>

8.2.3 jgawk (Japanese gawk)

<ftp://ftp.eos.hokudai.ac.jp/pub/mac/util/jgawk/>

8.2.4 gawk.dll

<http://www.muc.de/~walkerj/>
Gawk 2.15.2 plus extensions
+ Read/Write functions for INI files
+ Read-only functions for DBF files

8.3 mawk
NOTE: do not use mawk 1.3.2 (a one-character change yields 1.3.3)
due to an obscure (and rarely-appearing) regex problem.

<ftp://ftp.whidbey.net/pub/brennan/>
e.g.,
<ftp://ftp.whidbey.net/pub/brennan/mawk1.3.3.tar.gz>

<ftp://oxy.edu/public/> [not responding 1997/Dec/30]
e.g.,
<ftp://oxy.edu/public/mawk1.2.2.tar.gz>
<ftp://oxy.edu/public/mawk1.3beta.tar.gz>

8.4 awk2c
<ftp://sunsite.unc.edu/pub/Linux/utils/text/awk2c050.tgz>

8.5 various old binary-only distributions for MSDOS
<ftp://ftp.coast.net/pub/SimTel/msdos/awk/>
<http://www.coast.net/SimTel/msdos/awk.html>

8.99 awkcc [unknown]
Previous versions of this FAQ mentioned a file on MKS' web site.
Neil Mahoney spent some time examining it and discovered it is
just a package BUILT with awkcc, not awkcc itself. eventually,
this notice will be removed.

Some of Neil's explorations are interesting for those looking
for the real awkcc:

Get file copied to my Sun 4 file system (where I do most of my work)

$ ls -l awkcc
-rw-r--r-- 1 neilm 1827 May 20 11:20 README
-rw-r--r-- 1 neilm 1378 May 20 11:20 awk.h
-rwxr-xr-x 1 neilm 118 May 20 11:20 awkcc.sh
-rw-r--r-- 1 neilm 824 May 20 11:20 dollars.h
-rw-r--r-- 1 neilm 3858 May 20 11:20 ear.h
-rw-r--r-- 1 neilm 993 May 20 11:20 hash.h
-rw-r--r-- 1 neilm 1707 May 20 11:20 header.h
-rw-r--r-- 1 neilm 103468 May 20 11:21 libAWK.a
-rw-r--r-- 1 neilm 4136 May 20 11:20 specassign.h
-rw-r--r-- 1 neilm 14467 May 20 11:20 unipen.c
-rw-r--r-- 1 neilm 275 May 20 11:20 y.tab.h

Looking good!

$ awkcc.sh
fix errors found by run
$ awkcc.sh

no awkcc executable... what is this ?
-rwxr-xr-x 1 neilm 106496 May 20 11:21 uniparse

$ head -20 README

##############################################
To compile the UNIPEN 1.0 parser, run awkcc.sh
##############################################

The following files are from the awkcc package,
provided by Chris Ramming (jcr@research.att.com),
Copyright (c) 1991 AT&T.
All Rights Reserved

awk.h awkcc.sh
copyright dollars.h
ear.h hash.h
header.h libAWK.a
specassign.h y.tab.h

The file unipen.c is machine generated c code from the unipen.awk
parser, using the awkcc package.
Copyright (c) 1994 - I. Guyon, AT&T Bell Labs.

# DISCLAIMER: #

=====
:*:9. Why would anyone still use awk instead of perl?

A valid question, since awk is a subset of perl; also, the authors of perl have historically known awk (and sed, and C, and a host of other Unix tools) very well, and still decided to move on.

There are some things that perl has built-in support for that no version of awk can do without great difficulty (if at all); if you need to do these things, there is no choice to make. for instance, no reasonable person would try to write a web server in awk (over using perl or even C).

However, there are some things in awk's favor compared to perl:

- awk is simpler (especially important if deciding which to learn first)
- you may already know awk well enough for the task at hand
- you may already have only awk installed
- awk can be smaller, thus much quicker to execute for small programs
- awk variables don't have `$' in front of them :-)
- clear perl code is better than unclear awk code; but NOTHING comes close to unclear perl code

=====
:*:10. How can I learn awk?

The commercial vendors of DOS versions (MKS and Thompson) each have their own well written books with examples. [available separately?]

English Book:

_The AWK Programming Language_, by Aho, Kernighan and Weinberger, who invented the language. Published by Addison-Wesley. Lots of good material in not a lot of space. A little out of date with regard to POSIX awk. ISBN 0-201-07981-X

<http://heg-school.aw.com/cseng/authors/aho/awk/awk.html>
[ just leads to <http://cseng.aw.com> 1997/Dec/30 ]
<http://cseng.aw.com/bookdetail.qry?ISBN=0%2D201%2D07981%2DX&ptype=0>

English Book:

_Effective AWK Programming_ by Arnold Robbins. Published by SSC (+1 206-FOR-UNIX, <http://www.ssc.com>, <mailto:sales@ssc.com>). Also published by the FSF as "The GNU AWK User's Guide"; Texinfo source is included with the gawk distribution, so you can also print this yourself. ISBN 1-57831-000-8

Russell recommends buying the book instead of trying to print it all out, for three reasons:

1. it's probably cheaper than using your own toner and paper.

2. some money goes back to help further development, both to Arnold Robbins (only if you buy from SSC) and the Free Software Foundation (if you buy from either SSC or the FSF).

3. it helps convince publishers that we _like_ having full documentation available on-line (e.g., for searching), but will still pay for a compact, bound copy.

information, including an errata list, is on the web site.
<http://www.ssc.com/ssc/eap/>
ISBN 0-916151-88-3 (first edition)

English Book:

second edition:

_Sed & Awk_, by Dale Dougherty & Arnold Robbins, published by O'Reilly and Associates.
_sed & awk_ describes two text manipulation programs that are mainstays of the UNIX programmer's toolbox. This new edition covers the sed and awk programs as they are now mandated by the POSIX standard and includes discussion of the GNU versions of these programs.
<http://www.ora.com/catalog/sed2/noframes.html>
<http://www.ora.com/catalog/covers/sedawk-t.gif>
ISBN 1-56592-225-5

An errata for the second edition of Sed & Awk is at
<http://www.cs.colostate.edu/~dzubera/sedawk2.txt>

historical notes on the first edition:

_Sed & Awk_, by Dale Dougherty, published by O'Reilly and Associates. A nice introduction to sed and awk, showing how they relate to each other. However, the first edition is `full of typos and out-and-out mistakes'.

<http://www.ora.com/catalog/sed/noframes.html>
ISBN 0-937175-59-5

a `by no means complete' errata list is available. the author mentions `later printings of the book have many of the errors fixed.'
<http://www.cs.colostate.edu/~dzubera/sedawk.txt>

Deutsch Book:

Awk und Sed, by Helmut Herold.
<http://www.addison-wesley.de/KD/BS/685.html>
[URL fails 1997/Sep/15 -- search in Katalog does not find `awk']
awk und sed / Helmut Herold. - 2., ueberarb. und erw. Auflage - Bonn; Paris; Reading, Mass. [u.a.]: Addison-Wesley, 1994 (UNIX und seine Werkzeuge). ISBN 3-89319-685-4. VVA-Nr. 563-00685-8

Web Site:
<http://www.cs.hmc.edu/FAQ/qref/awk.html>
Getting started with Awk

Web Site:
<http://www.uga.edu/~ucns/sss/unix/awk.html>
Awk introduction

Web Site:
<http://www.mbnet.mb.ca/~natewild/awk/awk.html>
[ no longer available 1997/Oct/28 ]
Information about Tawk; Awk sample source code

Ian Gordon's Introduction to Gawk from Linux Journal
<http://www.ssc.com/lj/issue25/1156.html>

Awk Introduction
<http://www.ntua.gr/unix/>
<ftp://www.brooks.af.mil/pub/unix/white_papers/awk.ps>
Awk introduction (PostScript and text) by awk authors (somewhat old, doesn't cover the many recent extensions, but still a valid introduction to the language)

=====
:*:11. What are some other awk resources?

Unix and awk courseware:
<http://www.cit.ac.nz/smac/os202/default.htm>

Awk course
<http://www.ee.ic.ac.uk/course/advanced/awk/awk.html>

Developer information on awk
<http://www.devinfo.com/languages/awk/>

Spatial Analysis with Awk (course)
<http://www.udel.edu/johnmack/frec682/682awk.html>

Debugger and Assertion Checker for Awk
<http://www.irisa.fr/EXTERNE/manifestations/AADEBUG95/Abstracts/auguston2.html>

Free Compilers and Interpreters List
<http://www.idiom.com/free-compilers/LANG/awk-1.html>

Voicenet.com awk page
<http://www.voicenet.com/tech/comp/prog/awk/>

Four awk implementations for MS-DOS: How do they compare?
<http://www.voicenet.com/tech/comp/prog/awk/awk2.rev>

Gawk 3 manual
<http://www.tec.ualberta.ca/Documentation/Info/by-chapter/gawk-3.0.3/>
<ftp://ftp.tec.ualberta.ca/pub/Documentation/Info/by-chapter/gawk-3.0.3-html.tar.gz>

Unix Vault
<http://www.nda.com/~jblaine/vault/>
<http://www2.shore.net/~jblaine/vault/>

Yahoo's awk links
<http://www.yahoo.com/Computers_and_Internet/Programming_Languages/Awk/>

New Mexico Tech awk information
<http://www.nmt.edu/tcc/help/lang/awk.html>
<http://www.nmt.edu/bin/man?awk>
<http://www.nmt.edu/bin/man?gawk>

A Supplemental Document For AWK - or - Things Al, Pete, And Brian Didn't Mention Much
<http://oak.oakland.edu/pub/unix-c/info/awk-supplement.txt>
[ interesting historically -- I always wondered exactly why I mistrusted setting `$n' and expecting `$0' to change -- and this document explains why. (it works in almost all versions now.) ]

A* - an awk extension (paper by D.A.Ladd and J.C.Ramming)
<http://sunsite.ust.hk/dblp/db/journals/tse/tse21.html>
<http://www.usenix.org/~jcr/>
[ does anyone know what issue of _Unix Review_ mentioned it? ]

ftwalk - not strictly a version of awk
`it is very much an awk-like language'
<http://www.contex.com/ftwalk/>

Konrad Hambrick's `rawketry' and AltAcc data reduction scripts
ftp://ftp.netcom.com/pub/ko/konrad/rawketry/
ftp://ftp.netcom.com/pub/ko/konrad/altacc/software/

Ralph Becket's CGI and HTML Awk libraries
http://www.cam.sri.com/people/becket/awk/awk.html

English Book
Language and Computers
ISBN 0-7486-0785-4 (paperback)
ISBN 0-7486-0848-6 (hardcover)
New Scientist #2071, 1997/Mar/01, p45; paragraph titled `Dream in awk'
<http://www.ling.lancs.ac.uk/staff/andrew/etel.htm>
<http://www.ling.lancs.ac.uk/staff/andrew/pubs.htm>
<http://www.ed.ac.uk/~eup/subject.html>
<http://www.ed.ac.uk/~eup/highlights.html#barnbrook>
[ Blackwell's in Oxford has 3 copies under General Linguistics ]
a book about computer-aided linguistics which uses awk as its implementation language

=====
:*:12. How do I report a bug in gawk?

This is described in great detail in the gawk documentation. In brief,

1. Make sure what you've discovered is really a bug by checking the documentation and, if possible, comparing with nawk and mawk.

2. Cut down the program and data to as small as possible a test case that will illustrate the bug.

3. Send mail to <mailto:bug-gnu-utils@prep.ai.mit.edu>, with a carbon copy to <mailto:arnold@gnu.ai.mit.edu>.

4. Do not just post in comp.lang.awk; Arnold's readership there is sporadic.

=====
:*:13. What's wrong with gawk on Digital's OSF/1?

The version of gawk shipped with OSF/1 is very old, based on gawk 2.14. Get the current version from a GNU mirror near you, and if you still have a problem, report it as per the directions in the gawk documentation.

=====
:*:14. How can I access shell or environment variables in an awk script?

14.0 shells

the examples using quoting are intended for use with any standard (sh-compatible-quoting) Unix shell.

non-sh-compatible shells will require different quoting. if you're not even using Unix (or a ported Unix shell), just ignore the whole section on quoting.

14.1 Environment variables in general

Answer 1: on Unix, use "alternate quoting", e.g.

awk -F: '$1 ~ /'"$USER"'/ {print $5}' /etc/passwd
^^^^^^^^*******^^^^^^^^^^^^^^

any standard Unix shell will send the underlined part as one long argument (with embedded spaces) to awk, for instance:

$1 ~ /bwk/ {print $5}

Note that there may not be any spaces between the quoted
parts. Otherwise, you wouldn't end up a single, long script
argument, because Unix shells break arguments on spaces
(unless they are `escaped' with `\', or in '' or "", as the
above example shows).

Answer 2: RTFM to see if and how your awk supports variable definitions on the command line, e.g.,

awk -F: -v name=$USER '$1 ~ name {print $5}' /etc/passwd

Answer 3: RTFM if your awk can access enviroment vars. Then perhaps

awk -F: '$1 ~ ENVIRON["USER"] {print $5}' /etc/passwd

Always remember for your /bin/sh scripts that it's easy to put
things into the environment for a single command run:

name=felix age=56 awk '... ENVIRON["name"] .....'

this also works with ksh and some other shells.

The first approach is extremely portable, but doesn't work with awk "-f" script files. In that case, it's better to use a shell script and stretch a long awk command argument in '...' across multiple lines if need be.

Also note: /bin/csh requires a \ before an embedded newline, /bin/sh not.

[NOTE: The examples in the following section will show misalligned because of
the need of a wider line display, and the use of ASCII diagrams - JMC.]

14.2 Unix Shell Quoting

Quoting can be such a headache for the novice, in shell programming, and especially in awk.

(see below for a verbose explanation of the first one, with 7 quotes)

awk 'BEGIN { q="'"'"'";print "Never say can"q"t."; exit }'

nawk -v q="'" 'BEGIN { print "Never say can"q"t."; exit }'

awk 'BEGIN { q=sprintf("%c",39); print "Never say can"q"t."; exit }'

awk 'BEGIN { q=sprintf("%c",39); print "Never say \"can"q"t.\""; exit }'

and

cat <<@@ > foo.awk
{ print "Never say can't." }
@@
awk -f foo.awk; rm foo.awk

But not:

awk 'BEGIN { q="\'"; print "Never say \"can"q"t.\""; exit }'

explanation of the 7-quote example:
note that it is quoted three different ways:

awk 'BEGIN { q="'
"'"
'";print "Never say can"q"t."; exit }'

and that argument comes out as the single string (with embedded spaces)

BEGIN { q="'";print "Never say can"q"t."; exit }

which is the same as

BEGIN { q="'"; print "Never say can" q "t."; exit }
^^^^^^^^^^^^^ ^ ^^
| | | ||
| | | ||
vvvvvvvvvvvvv | ||
Never say can v ||
' vv
t.

which gets you

Never say can't.

14.3 ENVIRON[] and "env"|getline

Modern versions of new awk (gawk, mawk, Bell Labs awk, any POSIX awk) all provide an array named ENVIRON. The array is indexed by environment variable name, the value is that variables value. For instance, ENVIRON["HOME"] might be "/home/chris". To print out all the names and values, use a simple loop:

for (i in ENVIRON)
printf("ENVIRON['%s'] = '%s'\n", i, ENVIRON[i])

What if my awk doesn't have ENVIRON[]?

Short answer, get a better awk. There are many freely available versions.

Longer answer, on Unix you can use a pipe from the `env' or `printenv' commands, but this is less pretty, and may be a problem if the values contain newlines:

# test this on your system before you depend on it!
while (("env" | getline line) > 0) {
n = split(line, v, "=")
# v[1] is variable name
# v[2] is variable value
if (n > 2)
# value contained "=", handle it ...
}

14.4 exporting environment variables back to the parent process

How can I put values into the environment of the program that called my awk program?

Short answer, you can't. Unix ain't Plan 9, and you can't tweak the parent's address space.

(DOS isn't even Unix, so it lets any program overwrite any memory location, including the parent's environment space. But the details are (obviously) going to be fairly icky. Avoid.)

Longer answer, write the results in a form the shell can parse to a temporary file, and have the shell "source" the file after running the awk program:

awk 'BEGIN { printf("NEWVAR='%s'\n", somevalue) }' > /tmp/awk.$$

. /tmp/awk.$$ # sh/ksh/bash/pdksh/zsh etc

rm /tmp/awk.$$

With many shells, you can use `eval', but this is also cumbersome:

eval `awk 'BEGIN { print "NEWVAR=" somevalue }'`

Csh syntax and more robust use of quotation marks are left as exercises for the reader.

=====
:*:15. Is there an easy way to determine if you have oawk or nawk?

The following in a BEGIN rule will do the trick.

if (ARGC == 0)
# old awk
else
# new awk

=====
:*:16. How does awk deal with multiple files?

16.0 Version warning

some of these techniques will require non-ancient versions of awk.

16.1 How can awk test for the existence of a file?

the best way is to simply try and read from the file.

function exists(file, dummy, ret)
{
if ((getline dummy < file) > 0)
# file exists and can be read
ret = 1
else
ret = 0
close(file)
return ret
}

[ I've read reports that earlier versions of mawk would write to stderr as well as getline returning <0 -- is this still true? ]

16.2 How can I get awk to read multiple files?

it's automatic (under Unix at least) -- use something like:

awk '/^#include/ {print $2}' *.c *.h

16.3 How can I tell from which file my input is coming?

use the built-in variable FILENAME:

awk '/^#include/ {print FILENAME,$2}' *.c *.h

16.4 How can I get awk to open multiple files (selected at runtime)?

use `getline', `close', and `print EXPR > FILENAME', like:

# assumes input file has at least 1 line, output file writeable
function double(infilename,outfilename, aline)
{
while ( (getline aline < infilename) >0 )
print(aline aline) > outfilename;
close(infilename);
close(outilename);
}

16.5 How can I treat the first file specially?

use FILENAME, thusly:

BEGIN { rulesfile="" }
rulesfile == "" { rulesfile = FILENAME; }
FILENAME == rulesfile { build_rule($0); }
FILENAME != rulesfile { apply_rule($0); }

Example:

Suppose you have a text-line "database" and you want to make some
batch changes to it, by replacing some old lines with new lines.

BEGIN { rulesfile="" }
rulesfile == "" { rulesfile = FILENAME; }
rulesfile == FILENAME { replace[$1] = $0; }
rulesfile != FILENAME \
{
if ($1 in replace)
print replace[$1];
else
print;
}

16.6 How can I explicitly pass in a filename to treat specially?

use `-v rulesfile=filename' like you would any other variable, and then use a `getline' loop (and `close') in your BEGIN statement.

BEGIN \
{
if (rulesfile=="")
{
print "must use -v rulesfile=filename";
exit(1);
}
while ( (getline < rulesfile) >0 )
replace[$1]=$0;
close(rulesfile);
}

{
if ($1 in replace)
print replace[$1];
else
print;
}

=====
:*:17. How many elements were created by split()?

when I do a split on a field, e.g.,

split($1,x,"string")

how can i find out how many elements x has (i mean other than
testing for null string or doing a for (n in x) test)?

use the return value of split()

n = split($1, x, "string")

=====
:*:18. How can I split a string into characters?

in portable POSIX awk, the only way to do this is to use substr to pull out each character, one by one. this is painful. however, gawk, mawk, and the newest version of the Bell Labs awk all allow you to set FS = "" and use "" as the third argument of split.

so, split("chars",anarray,"") results in the array anarray containing 5 elements -- "c", "h", "a", "r", "s".

if you don't have any ^As in your string, you could try:

{
string=$0;
gsub(".", "&\001", string)
n=split(string, anarray, "\001")
for (i=1;i<=n;i++) print i "=>" anarray[i];
}

=====
:*:19. How do I use tolower() in the SunOS nawk?

I want to use the tolower() function with SunOS nawk, but all I get is

nawk: calling undefined function tolower

The SunOS nawk is from a time before awk acquired the tolower() and
toupper() functions. Either use one of the freely available awks, or
write your own function to do it using index, substr, and gsub.

An example of such a function is in O'Reilly's _Sed & Awk_.

=====
:*:20. How do I have dynamic-width printf strings, like C?

with modern awks, just do it like you would in C:

maxlen=0

for (i in arr)
if (maxlen<length(arr[i]))
maxlen=length(arr[i])

for (i in arr)
printf("%-*s %s\n",maxlen,arr[i],i)

with old awks, just do it like you would do if you didn't know about %*:

maxlen=0

for (i in arr)
if (maxlen<length(arr[i]))
maxlen=length(arr[i])

printfstring="%-" maxlen "s %s\n";
for (i in arr)
printf(printfstring,arr[i],i)

=====
:*:21. Why doesn't "\\$" behave like /\\$/ ? Why don't parentheses match?

because the first is a string, so some of the escape characters get eaten up (like \" to escape a double-quote within the string).

/\\$/ => regular expression: literal backslash at end-of-expression

"\\$" => string: \$ => regular expression: literal dollar sign

to get behavior like the first case in a string, use "\\\\$" .

there are other, less obvious characters which need the same attention; under-quoting or over-quoting should be avoided:

parentheses are special for alternation:

/\(test\)/ => 6 characters `(test)'

"\(test\)" => /(test)/ => 4 characters `test'

an example of trying to match some compass directions:

/(N|S)(E|W)/ => `NE' or `NW' or `SE' or `SW' (correct)

"(N|S)(E|W)" => /(N|S)(E|W)/ (correct)

"\(N|S\)\(E|W\)" => /(N|S)(E|W)/ (correct) (all \ had no effect)

"\(N\|S\)\(E\|W\)" => /(N|S)(E|W)/ (correct) (all \ had no effect)

expressions that look similar but behave totally differently:

/\(N|S\)\(E|W\)/ => `(N' or `S)(E' or `W)'

/\(N\|S\)\(E\|W\)/ => `(N|S)(E|W)'

=====
:*:22. What is gawk's exit code?

normally, the `exit' command exits with a value of zero.

you can supply an optional numeric value to the `exit' command to make it exit with a value:

if (whatever)
exit 12

if you have an END block, control first transfers there. within the END block, an `exit' command exits immediately; if you had previously supplied a value, that value is used. but, if you give a new value to `exit' within the END block, the new value is used. this is documented in the GNU Awk User's Guide (gawk.texi).

if you have an END block you want to be able to skip sometimes, you may have to do something like this:

BEGIN {
...
exitcode=0;
...
}

{
...
if (fatal)
{
exitcode=12;
exit(exitcode);
}
...
}

END {
if (exitcode)
exit(exitcode);
...
}

=====
:*:23. How can I get awk to be case-insensitive?

23.1. use tolower()
- portable
- must be explicitly used for each comparison

instead of:
if (avar=="a" || avar=="A") { ... }
use:
if (tolower(avar)=="a") { ... }

23.2. use IGNORECASE=1;
- probably gawk only
- used for all comparisons

=====
:*:24. How can I force a numeric/non-numeric comparison?

these are the canonical, work-in-all-versions snippets. there are many others, most longer, some shorter (but possibly less portable).

to compare two variables as numbers ONLY, use
if (0+var1 == 0+var2)

to compare two variables as non-numeric strings ONLY, use
if ("" var1 == "" var2)

=====
:*:25. Why does { FS=":"; print $1 } not split the first record?

basically, you should set FS before it's needed to split $0 into fields. once awk encounters a `{', it may already be too late.

some awk implementations set the fields at the beginning of the block, and don't re-parse just because you changed FS. to get the desired behavior, you must set FS _before_ reading in a line.

e.g.,
BEGIN { FS=":" }
{ print $1 }

e.g.,
awk -F: '{ print $1 }'

if you run code like this

{ FS=":"; print $1 }

on this data:

first:second:third but not last:fourth
First:Second:Third But Not Last:Fourth
FIRST:SECOND:THIRD BUT NOT LAST:FOURTH

you may get either
this: or this:
---- -------
first first:second:third
First First
FIRST FIRST

=====
:*:26. Did ^ and $ and . change in gawk?

yes. early versions cared about \n (newlines) and treated them specially. version 3.* and later are more POSIX-compliant here.

=====
:*:27. Why doesn't awk 'begin {...}' work?

it needs to be `BEGIN' (i.e., it's case-sensitive).

=====
:*:98. Miscellaneous

=====
:*:99. Credits

I expect most of the information in this FAQ to be supplied by people other than myself -- it's just going to work better that way. The newsgroup readers have a LOT more awk experience than I ever will (unless I multiply myself by a few thousand, which is not legal with today's tax laws).

These people have contributed to the well-being of the FAQ:

arnold [at] gnu.ai.mit.edu (Arnold D. Robbins)
100335.120 [at] CompuServe.COM (James G. Walker)
jland [at] worldnet.att.net (Jim Land)
yuli.barcohen [at] telrad.co.il (Yuli Barcohen)
johnd [at] mozart.inet.co.th (John DeHaven)
amnonc [at] mercury.co.il (Amnon Cohen)
saguyami [at] post.tau.ac.il (Shay)
hankedr [at] mail.auburn.edu (Darrel Hankerson)
mark [at] ispc001.demon.co.uk (Mark Katz)
brennan [at] whidbey.com (Michael D. Brennan)
neitzel [at] gaertner.de (Martin Neitzel)
pjf [at] osiris.cs.uoguelph.ca (Peter Jaspers-Fayer)
dmckeon [at] swcp.com (Denis McKeon)
neil_mahoney [at] il.us.swissbank.com (Neil Mahoney)
dzubera [at] CS.ColoState.EDU (Zube)
allen [at] gateway.grumman.com (John L. Allen)
jerabek [at] rm6208.gud.siemens.co.at (Martin Jerabek)
tom [at] contex.com (Tom Hull)
bmarcum [at] iglou.com (Bill Marcum)
thobe [at] lafn.org (Glenn Thobe)
boffi [at] rachele.stru.polimi.it (giacomo boffi)
hastinga [at] tarim.dialogic.com (Austin Hastings)
konrad [at] netcom.com (Konrad Hambrick)
jmccann [at] WOLFENET.com (James McCann)
eia018 [at] comp.lancs.ac.uk (Dr Andrew Wilson)
Alex.Schoenmakers [at] lhs.be
rwab1 [at] cl.cam.ac.uk (Ralph Becket)

Thanks.

=====

thus endeth the awk FAQ.

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:*:Conversion to DOC format by:
Jesus M. Castagnetto
(jmcastagnetto@hotmail.com)
http://www.geocities.com/ResearchTriangle/Lab/1059/
April 18, 1998.
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