(main): `quote' the offending argument.

[coreutils.git] / doc / textutils.texi

\input texinfo
@c %**start of header
@setfilename textutils.info
@settitle @sc{gnu} text utilities
@c %**end of header

@include version.texi
@include constants.texi

@c Define new indices.
@defcodeindex op

@c Put everything in one index (arbitrarily chosen to be the concept index).
@syncodeindex fn cp
@syncodeindex ky cp
@syncodeindex op cp
@syncodeindex pg cp
@syncodeindex vr cp

@ifinfo
@format
START-INFO-DIR-ENTRY
* Text utilities: (textutils).                  GNU text utilities.
* cat: (textutils)cat invocation.               Concatenate and write files.
* cksum: (textutils)cksum invocation.           Print @sc{posix} CRC checksum.
* comm: (textutils)comm invocation.             Compare sorted files by line.
* csplit: (textutils)csplit invocation.         Split by context.
* cut: (textutils)cut invocation.               Print selected parts of lines.
* expand: (textutils)expand invocation.         Convert tabs to spaces.
* fmt: (textutils)fmt invocation.               Reformat paragraph text.
* fold: (textutils)fold invocation.             Wrap long input lines.
* head: (textutils)head invocation.             Output the first part of files.
* join: (textutils)join invocation.             Join lines on a common field.
* md5sum: (textutils)md5sum invocation.         Print or check message-digests.
* nl: (textutils)nl invocation.                 Number lines and write files.
* od: (textutils)od invocation.                 Dump files in octal, etc.
* paste: (textutils)paste invocation.           Merge lines of files.
* pr: (textutils)pr invocation.                 Paginate or columnate files.
* ptx: (textutils)ptx invocation.               Produce permuted indexes.
* sort: (textutils)sort invocation.             Sort text files.
* split: (textutils)split invocation.           Split into fixed-size pieces.
* sum: (textutils)sum invocation.               Print traditional checksum.
* tac: (textutils)tac invocation.               Reverse files.
* tail: (textutils)tail invocation.             Output the last part of files.
* tsort: (textutils)tsort invocation.           Topological sort.
* tr: (textutils)tr invocation.                 Translate characters.
* unexpand: (textutils)unexpand invocation.     Convert spaces to tabs.
* uniq: (textutils)uniq invocation.             Uniquify files.
* wc: (textutils)wc invocation.                 Byte, word, and line counts.
END-INFO-DIR-ENTRY
@end format
@end ifinfo

@ifinfo
This file documents the GNU text utilities.

Copyright (C) 1994, 95, 96 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1
or any later version published by the Free Software Foundation;
with no Invariant Sections, with no
Front-Cover Texts, and with no Back-Cover Texts.
A copy of the license is included in the section entitled ``GNU
Free Documentation License''.

@end ifinfo

@titlepage
@title @sc{gnu} @code{textutils}
@subtitle A set of text utilities
@subtitle for version @value{VERSION}, @value{UPDATED}
@author David MacKenzie et al.

@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1994, 95, 96 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1
or any later version published by the Free Software Foundation;
with no Invariant Sections, with no
Front-Cover Texts, and with no Back-Cover Texts.
A copy of the license is included in the section entitled ``GNU
Free Documentation License''.
@end titlepage


@c If your makeinfo doesn't grok this @ifnottex directive, then either
@c get a newer version of makeinfo or do s/ifnottex/ifinfo/ here and on
@c the matching @end directive below.
@ifnottex
@node Top
@top GNU text utilities

@cindex text utilities
@cindex utilities for text handling

This manual documents version @value{VERSION} of the @sc{gnu} text utilities.

@menu
* Introduction::                       Caveats, overview, and authors.
* Common options::                     Common options.
* Output of entire files::             cat tac nl od
* Formatting file contents::           fmt pr fold
* Output of parts of files::           head tail split csplit
* Summarizing files::                  wc sum cksum md5sum
* Operating on sorted files::          sort uniq comm ptx tsort
* Operating on fields within a line::  cut paste join
* Operating on characters::            tr expand unexpand
* Opening the software toolbox::       The software tools philosophy.
* Index::                              General index.

@detailmenu
 --- The Detailed Node Listing ---

Output of entire files

* cat invocation::              Concatenate and write files.
* tac invocation::              Concatenate and write files in reverse.
* nl invocation::               Number lines and write files.
* od invocation::               Write files in octal or other formats.

Formatting file contents

* fmt invocation::              Reformat paragraph text.
* pr invocation::               Paginate or columnate files for printing.
* fold invocation::             Wrap input lines to fit in specified width.

Output of parts of files

* head invocation::             Output the first part of files.
* tail invocation::             Output the last part of files.
* split invocation::            Split a file into fixed-size pieces.
* csplit invocation::           Split a file into context-determined pieces.

Summarizing files

* wc invocation::               Print byte, word, and line counts.
* sum invocation::              Print checksum and block counts.
* cksum invocation::            Print CRC checksum and byte counts.
* md5sum invocation::           Print or check message-digests.

Operating on sorted files

* sort invocation::             Sort text files.
* uniq invocation::             Uniquify files.
* comm invocation::             Compare two sorted files line by line.
* ptx invocation::              Produce a permuted index of file contents.
* tsort invocation::            Topological sort.

@code{ptx}: Produce permuted indexes

* General options in ptx::      Options which affect general program behavior.
* Charset selection in ptx::    Underlying character set considerations.
* Input processing in ptx::     Input fields, contexts, and keyword selection.
* Output formatting in ptx::    Types of output format, and sizing the fields.
* Compatibility in ptx::        The GNU extensions to @code{ptx}

Operating on fields within a line

* cut invocation::              Print selected parts of lines.
* paste invocation::            Merge lines of files.
* join invocation::             Join lines on a common field.

Operating on characters

* tr invocation::               Translate, squeeze, and/or delete characters.
* expand invocation::           Convert tabs to spaces.
* unexpand invocation::         Convert spaces to tabs.

@code{tr}: Translate, squeeze, and/or delete characters

* Character sets::              Specifying sets of characters.
* Translating::                 Changing one characters to another.
* Squeezing::                   Squeezing repeats and deleting.
* Warnings in tr::              Warning messages.

Opening the software toolbox

* Toolbox introduction::        Toolbox introduction
* I/O redirection::             I/O redirection
* The who command::             The @code{who} command
* The cut command::             The @code{cut} command
* The sort command::            The @code{sort} command
* The uniq command::            The @code{uniq} command
* Putting the tools together::  Putting the tools together

@end detailmenu
@end menu

@end ifnottex


@node Introduction
@chapter Introduction

@cindex introduction

This manual is incomplete: No attempt is made to explain basic concepts
in a way suitable for novices.  Thus, if you are interested, please get
involved in improving this manual.  The entire @sc{gnu} community will
benefit.

@cindex POSIX.2
The @sc{gnu} text utilities are mostly compatible with the @sc{posix.2}
standard.

@c This paragraph appears in all of fileutils.texi, textutils.texi, and
@c sh-utils.texi too -- so be sure to keep them consistent.
@cindex bugs, reporting
Please report bugs to @email{bug-textutils@@gnu.org}.  Remember
to include the version number, machine architecture, input files, and
any other information needed to reproduce the bug: your input, what you
expected, what you got, and why it is wrong.  Diffs are welcome, but
please include a description of the problem as well, since this is
sometimes difficult to infer. @xref{Bugs, , , gcc, GNU CC}.

This manual was originally derived from the Unix man pages in the
distribution, which were written by David MacKenzie and updated by Jim
Meyering.  What you are reading now is the authoritative documentation
for these utilities;  the man pages are no longer being maintained.
The original @code{fmt} man page was written by Ross Paterson.
Fran@,{c}ois Pinard did the initial conversion to Texinfo format.
Karl Berry did the indexing, some reorganization, and editing of the results.
Richard Stallman contributed his usual invaluable insights to the
overall process.


@node Common options
@chapter Common options

@cindex common options

Certain options are available in all of these programs.  Rather than
writing identical descriptions for each of the programs, they are
described here.  (In fact, every @sc{gnu} program accepts (or should accept)
these options.)

Some of these programs recognize the @samp{--help} and @samp{--version}
options only when one of them is the sole command line argument.

@table @samp

@item --help
@opindex --help
@cindex help, online
Print a usage message listing all available options, then exit successfully.

@item --version
@opindex --version
@cindex version number, finding
Print the version number, then exit successfully.

@end table


@node Output of entire files
@chapter Output of entire files

@cindex output of entire files
@cindex entire files, output of

These commands read and write entire files, possibly transforming them
in some way.

@menu
* cat invocation::              Concatenate and write files.
* tac invocation::              Concatenate and write files in reverse.
* nl invocation::               Number lines and write files.
* od invocation::               Write files in octal or other formats.
@end menu

@node cat invocation
@section @code{cat}: Concatenate and write files

@pindex cat
@cindex concatenate and write files
@cindex copying files

@code{cat} copies each @var{file} (@samp{-} means standard input), or
standard input if none are given, to standard output.  Synopsis:

@example
cat [@var{option}] [@var{file}]@dots{}
@end example

The program accepts the following options.  Also see @ref{Common options}.

@table @samp

@item -A
@itemx --show-all
@opindex -A
@opindex --show-all
Equivalent to @samp{-vET}.

@item -B
@itemx --binary
@opindex -B
@opindex --binary
@cindex binary and text I/O in cat
On MS-DOS and MS-Windows only, read and write the files in binary mode.
By default, @code{cat} on MS-DOS/MS-Windows uses binary mode only when
standard output is redirected to a file or a pipe; this option overrides
that.  Binary file I/O is used so that the files retain their format
(Unix text as opposed to DOS text and binary), because @code{cat} is
frequently used as a file-copying program.  Some options (see below)
cause @code{cat} to read and write files in text mode because in those
cases the original file contents aren't important (e.g., when lines are
numbered by @code{cat}, or when line endings should be marked).  This is
so these options work as DOS/Windows users would expect; for example,
DOS-style text files have their lines end with the CR-LF pair of
characters, which won't be processed as an empty line by @samp{-b} unless
the file is read in text mode.

@item -b
@itemx --number-nonblank
@opindex -b
@opindex --number-nonblank
Number all nonblank output lines, starting with 1.  On MS-DOS and
MS-Windows, this option causes @code{cat} to read and write files in
text mode.

@item -e
@opindex -e
Equivalent to @samp{-vE}.

@item -E
@itemx --show-ends
@opindex -E
@opindex --show-ends
Display a @samp{$} after the end of each line.  On MS-DOS and
MS-Windows, this option causes @code{cat} to read and write files in
text mode.

@item -n
@itemx --number
@opindex -n
@opindex --number
Number all output lines, starting with 1.  On MS-DOS and MS-Windows,
this option causes @code{cat} to read and write files in text mode.

@item -s
@itemx --squeeze-blank
@opindex -s
@opindex --squeeze-blank
@cindex squeezing blank lines
Replace multiple adjacent blank lines with a single blank line.  On
MS-DOS and MS-Windows, this option causes @code{cat} to read and write
files in text mode.

@item -t
@opindex -t
Equivalent to @samp{-vT}.

@item -T
@itemx --show-tabs
@opindex -T
@opindex --show-tabs
Display TAB characters as @samp{^I}.

@item -u
@opindex -u
Ignored; for Unix compatibility.

@item -v
@itemx --show-nonprinting
@opindex -v
@opindex --show-nonprinting
Display control characters except for LFD and TAB using
@samp{^} notation and precede characters that have the high bit set with
@samp{M-}.  On MS-DOS and MS-Windows, this option causes @code{cat} to
read files and standard input in DOS binary mode, so the CR
characters at the end of each line are also visible.

@end table


@node tac invocation
@section @code{tac}: Concatenate and write files in reverse

@pindex tac
@cindex reversing files

@code{tac} copies each @var{file} (@samp{-} means standard input), or
standard input if none are given, to standard output, reversing the
records (lines by default) in each separately.  Synopsis:

@example
tac [@var{option}]@dots{} [@var{file}]@dots{}
@end example

@dfn{Records} are separated by instances of a string (newline by
default).  By default, this separator string is attached to the end of
the record that it follows in the file.

The program accepts the following options.  Also see @ref{Common options}.

@table @samp

@item -b
@itemx --before
@opindex -b
@opindex --before
The separator is attached to the beginning of the record that it
precedes in the file.

@item -r
@itemx --regex
@opindex -r
@opindex --regex
Treat the separator string as a regular expression.  Users of @code{tac}
on MS-DOS/MS-Windows should note that, since @code{tac} reads files in
binary mode, each line of a text file might end with a CR/LF pair
instead of the Unix-style LF.

@item -s @var{separator}
@itemx --separator=@var{separator}
@opindex -s
@opindex --separator
Use @var{separator} as the record separator, instead of newline.

@end table


@node nl invocation
@section @code{nl}: Number lines and write files

@pindex nl
@cindex numbering lines
@cindex line numbering

@code{nl} writes each @var{file} (@samp{-} means standard input), or
standard input if none are given, to standard output, with line numbers
added to some or all of the lines.  Synopsis:

@example
nl [@var{option}]@dots{} [@var{file}]@dots{}
@end example

@cindex logical pages, numbering on
@code{nl} decomposes its input into (logical) pages; by default, the
line number is reset to 1 at the top of each logical page.  @code{nl}
treats all of the input files as a single document; it does not reset
line numbers or logical pages between files.

@cindex headers, numbering
@cindex body, numbering
@cindex footers, numbering
A logical page consists of three sections: header, body, and footer.
Any of the sections can be empty.  Each can be numbered in a different
style from the others.

The beginnings of the sections of logical pages are indicated in the
input file by a line containing exactly one of these delimiter strings:

@table @samp
@item \:\:\:
start of header;
@item \:\:
start of body;
@item \:
start of footer.
@end table

The two characters from which these strings are made can be changed from
@samp{\} and @samp{:} via options (see below), but the pattern and
length of each string cannot be changed.

A section delimiter is replaced by an empty line on output.  Any text
that comes before the first section delimiter string in the input file
is considered to be part of a body section, so @code{nl} treats a
file that contains no section delimiters as a single body section.

The program accepts the following options.  Also see @ref{Common options}.

@table @samp

@item -b @var{style}
@itemx --body-numbering=@var{style}
@opindex -b
@opindex --body-numbering
Select the numbering style for lines in the body section of each
logical page.  When a line is not numbered, the current line number
is not incremented, but the line number separator character is still
prepended to the line.  The styles are:

@table @samp
@item a
number all lines,
@item t
number only nonempty lines (default for body),
@item n
do not number lines (default for header and footer),
@item p@var{regexp}
number only lines that contain a match for @var{regexp}.
@end table

@item -d @var{cd}
@itemx --section-delimiter=@var{cd}
@opindex -d
@opindex --section-delimiter
@cindex section delimiters of pages
Set the section delimiter characters to @var{cd}; default is
@samp{\:}. If only @var{c} is given, the second remains @samp{:}.
(Remember to protect @samp{\} or other metacharacters from shell
expansion with quotes or extra backslashes.)

@item -f @var{style}
@itemx --footer-numbering=@var{style}
@opindex -f
@opindex --footer-numbering
Analogous to @samp{--body-numbering}.

@item -h @var{style}
@itemx --header-numbering=@var{style}
@opindex -h
@opindex --header-numbering
Analogous to @samp{--body-numbering}.

@item -i @var{number}
@itemx --page-increment=@var{number}
@opindex -i
@opindex --page-increment
Increment line numbers by @var{number} (default 1).

@item -l @var{number}
@itemx --join-blank-lines=@var{number}
@opindex -l
@opindex --join-blank-lines
@cindex empty lines, numbering
@cindex blank lines, numbering
Consider @var{number} (default 1) consecutive empty lines to be one
logical line for numbering, and only number the last one.  Where fewer
than @var{number} consecutive empty lines occur, do not number them.
An empty line is one that contains no characters, not even spaces
or tabs.

@item -n @var{format}
@itemx --number-format=@var{format}
@opindex -n
@opindex --number-format
Select the line numbering format (default is @code{rn}):

@table @samp
@item ln
@opindex ln @r{format for @code{nl}}
left justified, no leading zeros;
@item rn
@opindex rn @r{format for @code{nl}}
right justified, no leading zeros;
@item rz
@opindex rz @r{format for @code{nl}}
right justified, leading zeros.
@end table

@item -p
@itemx --no-renumber
@opindex -p
@opindex --no-renumber
Do not reset the line number at the start of a logical page.

@item -s @var{string}
@itemx --number-separator=@var{string}
@opindex -s
@opindex --number-separator
Separate the line number from the text line in the output with
@var{string} (default is the TAB character).

@item -v @var{number}
@itemx --starting-line-number=@var{number}
@opindex -v
@opindex --starting-line-number
Set the initial line number on each logical page to @var{number} (default 1).

@item -w @var{number}
@itemx --number-width=@var{number}
@opindex -w
@opindex --number-width
Use @var{number} characters for line numbers (default 6).

@end table


@node od invocation
@section @code{od}: Write files in octal or other formats

@pindex od
@cindex octal dump of files
@cindex hex dump of files
@cindex ASCII dump of files
@cindex file contents, dumping unambiguously

@code{od} writes an unambiguous representation of each @var{file}
(@samp{-} means standard input), or standard input if none are given.
Synopsis:

@example
od [@var{option}]@dots{} [@var{file}]@dots{}
od -C [@var{file}] [[+]@var{offset} [[+]@var{label}]]
@end example

Each line of output consists of the offset in the input, followed by
groups of data from the file. By default, @code{od} prints the offset in
octal, and each group of file data is two bytes of input printed as a
single octal number.

The program accepts the following options.  Also see @ref{Common options}.

@table @samp

@item -A @var{radix}
@itemx --address-radix=@var{radix}
@opindex -A
@opindex --address-radix
@cindex radix for file offsets
@cindex file offset radix
Select the base in which file offsets are printed.  @var{radix} can
be one of the following:

@table @samp
@item d
decimal;
@item o
octal;
@item x
hexadecimal;
@item n
none (do not print offsets).
@end table

The default is octal.

@item -j @var{bytes}
@itemx --skip-bytes=@var{bytes}
@opindex -j
@opindex --skip-bytes
Skip @var{bytes} input bytes before formatting and writing.  If
@var{bytes} begins with @samp{0x} or @samp{0X}, it is interpreted in
hexadecimal; otherwise, if it begins with @samp{0}, in octal; otherwise,
in decimal.  Appending @samp{b} multiplies @var{bytes} by 512, @samp{k}
by 1024, and @samp{m} by 1048576.

@item -N @var{bytes}
@itemx --read-bytes=@var{bytes}
@opindex -N
@opindex --read-bytes
Output at most @var{bytes} bytes of the input.  Prefixes and suffixes on
@code{bytes} are interpreted as for the @samp{-j} option.

@item -s [@var{n}]
@itemx --strings[=@var{n}]
@opindex -s
@opindex --strings
@cindex string constants, outputting
Instead of the normal output, output only @dfn{string constants}: at
least @var{n} (3 by default) consecutive @sc{ascii} graphic characters,
followed by a null (zero) byte.

@item -t @var{type}
@itemx --format=@var{type}
@opindex -t
@opindex --format
Select the format in which to output the file data.  @var{type} is a
string of one or more of the below type indicator characters.  If you
include more than one type indicator character in a single @var{type}
string, or use this option more than once, @code{od} writes one copy
of each output line using each of the data types that you specified,
in the order that you specified.

Adding a trailing ``z'' to any type specification appends a display
of the @sc{ascii} character representation of the printable characters
to the output line generated by the type specification.

@table @samp
@item a
named character
@item c
@sc{ascii} character or backslash escape,
@item d
signed decimal
@item f
floating point
@item o
octal
@item u
unsigned decimal
@item x
hexadecimal
@end table

The type @code{a} outputs things like @samp{sp} for space, @samp{nl} for
newline, and @samp{nul} for a null (zero) byte.  Type @code{c} outputs
@samp{ }, @samp{\n}, and @code{\0}, respectively.

@cindex type size
Except for types @samp{a} and @samp{c}, you can specify the number
of bytes to use in interpreting each number in the given data type
by following the type indicator character with a decimal integer.
Alternately, you can specify the size of one of the C compiler's
built-in data types by following the type indicator character with
one of the following characters.  For integers (@samp{d}, @samp{o},
@samp{u}, @samp{x}):

@table @samp
@item C
char
@item S
short
@item I
int
@item L
long
@end table

For floating point (@code{f}):

@table @asis
@item F
float
@item D
double
@item L
long double
@end table

@item -v
@itemx --output-duplicates
@opindex -v
@opindex --output-duplicates
Output consecutive lines that are identical.  By default, when two or
more consecutive output lines would be identical, @code{od} outputs only
the first line, and puts just an asterisk on the following line to
indicate the elision.

@item -w[@var{n}]
@itemx --width[=@var{n}]
@opindex -w
@opindex --width
Dump @code{n} input bytes per output line.  This must be a multiple of
the least common multiple of the sizes associated with the specified
output types.  If @var{n} is omitted, the default is 32.  If this option
is not given at all, the default is 16.

@end table

The next several options map the old, pre-@sc{posix} format specification
options to the corresponding @sc{posix} format specs.
@sc{gnu} @code{od} accepts
any combination of old- and new-style options.  Format specification
options accumulate.

@table @samp

@item -a
@opindex -a
Output as named characters.  Equivalent to @samp{-ta}.

@item -b
@opindex -b
Output as octal bytes.  Equivalent to @samp{-toC}.

@item -c
@opindex -c
Output as @sc{ascii} characters or backslash escapes.  Equivalent to
@samp{-tc}.

@item -d
@opindex -d
Output as unsigned decimal shorts.  Equivalent to @samp{-tu2}.

@item -f
@opindex -f
Output as floats.  Equivalent to @samp{-tfF}.

@item -h
@opindex -h
Output as hexadecimal shorts.  Equivalent to @samp{-tx2}.

@item -i
@opindex -i
Output as decimal shorts.  Equivalent to @samp{-td2}.

@item -l
@opindex -l
Output as decimal longs.  Equivalent to @samp{-td4}.

@item -o
@opindex -o
Output as octal shorts.  Equivalent to @samp{-to2}.

@item -x
@opindex -x
Output as hexadecimal shorts.  Equivalent to @samp{-tx2}.

@item -C
@itemx --traditional
@opindex --traditional
Recognize the pre-@sc{posix} non-option arguments that traditional @code{od}
accepted.  The following syntax:

@smallexample
od --traditional [@var{file}] [[+]@var{offset}[.][b] [[+]@var{label}[.][b]]]
@end smallexample

@noindent
can be used to specify at most one file and optional arguments
specifying an offset and a pseudo-start address, @var{label}.  By
default, @var{offset} is interpreted as an octal number specifying how
many input bytes to skip before formatting and writing.  The optional
trailing decimal point forces the interpretation of @var{offset} as a
decimal number.  If no decimal is specified and the offset begins with
@samp{0x} or @samp{0X} it is interpreted as a hexadecimal number.  If
there is a trailing @samp{b}, the number of bytes skipped will be
@var{offset} multiplied by 512.  The @var{label} argument is interpreted
just like @var{offset}, but it specifies an initial pseudo-address.  The
pseudo-addresses are displayed in parentheses following any normal
address.

@end table


@node Formatting file contents
@chapter Formatting file contents

@cindex formatting file contents

These commands reformat the contents of files.

@menu
* fmt invocation::              Reformat paragraph text.
* pr invocation::               Paginate or columnate files for printing.
* fold invocation::             Wrap input lines to fit in specified width.
@end menu


@node fmt invocation
@section @code{fmt}: Reformat paragraph text

@pindex fmt
@cindex reformatting paragraph text
@cindex paragraphs, reformatting
@cindex text, reformatting

@code{fmt} fills and joins lines to produce output lines of (at most)
a given number of characters (75 by default).  Synopsis:

@example
fmt [@var{option}]@dots{} [@var{file}]@dots{}
@end example

@code{fmt} reads from the specified @var{file} arguments (or standard
input if none are given), and writes to standard output.

By default, blank lines, spaces between words, and indentation are
preserved in the output; successive input lines with different
indentation are not joined; tabs are expanded on input and introduced on
output.

@cindex line-breaking
@cindex sentences and line-breaking
@cindex Knuth, Donald E.
@cindex Plass, Michael F.
@code{fmt} prefers breaking lines at the end of a sentence, and tries to
avoid line breaks after the first word of a sentence or before the last
word of a sentence.  A @dfn{sentence break} is defined as either the end
of a paragraph or a word ending in any of @samp{.?!}, followed by two
spaces or end of line, ignoring any intervening parentheses or quotes.
Like @TeX{}, @code{fmt} reads entire ``paragraphs'' before choosing line
breaks; the algorithm is a variant of that in ``Breaking Paragraphs Into
Lines'' (Donald E. Knuth and Michael F. Plass, @cite{Software---Practice
and Experience}, 11 (1981), 1119--1184).

The program accepts the following options.  Also see @ref{Common options}.

@table @samp

@item -c
@itemx --crown-margin
@opindex -c
@opindex --crown-margin
@cindex crown margin
@dfn{Crown margin} mode: preserve the indentation of the first two
lines within a paragraph, and align the left margin of each subsequent
line with that of the second line.

@item -t
@itemx --tagged-paragraph
@opindex -t
@opindex --tagged-paragraph
@cindex tagged paragraphs
@dfn{Tagged paragraph} mode: like crown margin mode, except that if
indentation of the first line of a paragraph is the same as the
indentation of the second, the first line is treated as a one-line
paragraph.

@item -s
@itemx --split-only
@opindex -s
@opindex --split-only
Split lines only.  Do not join short lines to form longer ones.  This
prevents sample lines of code, and other such ``formatted'' text from
being unduly combined.

@item -u
@itemx --uniform-spacing
@opindex -u
@opindex --uniform-spacing
Uniform spacing.  Reduce spacing between words to one space, and spacing
between sentences to two spaces.

@item -@var{width}
@itemx -w @var{width}
@itemx --width=@var{width}
@opindex -@var{width}
@opindex -w
@opindex --width
Fill output lines up to @var{width} characters (default 75).  @code{fmt}
initially tries to make lines about 7% shorter than this, to give it
room to balance line lengths.

@item -p @var{prefix}
@itemx --prefix=@var{prefix}
Only lines beginning with @var{prefix} (possibly preceded by whitespace)
are subject to formatting. The prefix and any preceding whitespace are
stripped for the formatting and then re-attached to each formatted output
line.  One use is to format certain kinds of program comments, while
leaving the code unchanged.

@end table


@node pr invocation
@section @code{pr}: Paginate or columnate files for printing

@pindex pr
@cindex printing, preparing files for
@cindex multicolumn output, generating
@cindex merging files in parallel

@code{pr} writes each @var{file} (@samp{-} means standard input), or
standard input if none are given, to standard output, paginating and
optionally outputting in multicolumn format; optionally merges all
@var{file}s, printing all in parallel, one per column.  Synopsis:

@example
pr [@var{option}]@dots{} [@var{file}]@dots{}
@end example

By default, a 5-line header is printed at each page: two blank lines;
a line with the date, the filename, and the page count; and two more
blank lines.  A footer of five blank lines is also printed.  With the @samp{-F}
option, a 3-line header is printed: the leading two blank lines are
omitted; no footer is used.  The default @var{page_length} in both cases is 66
lines.  The default number of text lines changes from 56 (without @samp{-F})
to 63 (with @samp{-F}).  The text line of the header takes up the full
@var{page_width} in the form @samp{yyyy-mm-dd HH:MM string Page nnnn}.
String is a centered header string.

Form feeds in the input cause page breaks in the output.  Multiple form
feeds produce empty pages.

Columns are of equal width, separated by an optional string (default
is @samp{space}).  For multicolumn output, lines will always be truncated to
@var{page_width} (default 72), unless you use the @samp{-J} option.  For single
column output no line truncation occurs by default.  Use @samp{-W} option to
truncate lines in that case.

The following changes were made in version 1.22i and apply to later
versions of @command{pr}:
@c FIXME: this whole section here sounds very awkward to me. I
@c made a few small changes, but really it all needs to be redone. - Brian
@c OK, I fixed another sentence or two, but some of it I just don't understand.
@ - Brian
@itemize @bullet

@item
Some small @var{letter options} (@samp{-s}, @samp{-w}) have been
redefined for better @sc{posix} compliance.  The output of some further
cases has been adapted to other Unix systems.  These changes are not
compatible with earlier versions of the program.

@item
Some @var{new capital letter} options (@samp{-J}, @samp{-S}, @samp{-W})
have been introduced to turn off unexpected interferences of small letter
options.  The @samp{-N} option and the second argument @var{last_page}
of @samp{+FIRST_PAGE} offer more flexibility.  The detailed handling of
form feeds set in the input files requires the @samp{-T} option.

@item
Capital letter options override small letter ones.

@item
Some of the option-arguments (compare @samp{-s}, @samp{-S}, @samp{-e},
@samp{-i}, @samp{-n}) cannot be specified as separate arguments from the
preceding option letter (already stated in the @sc{posix} specification).
@end itemize

The program accepts the following options.  Also see @ref{Common options}.

@table @samp

@item +@var{first_page}[:@var{last_page}]
@itemx --pages=@var{first_page}[:@var{last_page}]
@opindex +@var{first_page}[:@var{last_page}]
@opindex --pages
Begin printing with page @var{first_page} and stop with @var{last_page}.
Missing @samp{:@var{last_page}} implies end of file.  While estimating
the number of skipped pages each form feed in the input file results
in a new page.  Page counting with and without @samp{+@var{first_page}}
is identical.  By default, counting starts with the first page of input
file (not first page printed).  Line numbering may be altered by @samp{-N}
option.

@item -@var{column}
@itemx --columns=@var{column}
@opindex -@var{column}
@opindex --columns
@cindex down columns
With each single @var{file}, produce @var{column} columns of output
(default is 1) and print columns down, unless @samp{-a} is used.  The
column width is automatically decreased as @var{column} increases; unless
you use the @samp{-W/-w} option to increase @var{page_width} as well.
This option might well cause some lines to be truncated.  The number of
lines in the columns on each page are balanced.  The options @samp{-e}
and @samp{-i} are on for multiple text-column output.  Together with
@samp{-J} option column alignment and line truncation is turned off.
Lines of full length are joined in a free field format and @samp{-S}
option may set field separators.  @samp{-@var{column}} may not be used
with @samp{-m} option.

@item -a
@itemx --across
@opindex -a
@opindex --across
@cindex across columns
With each single @var{file}, print columns across rather than down.  The
@samp{-@var{column}} option must be given with @var{column} greater than one.
If a line is too long to fit in a column, it is truncated.

@item -c
@itemx --show-control-chars
@opindex -c
@opindex --show-control-chars
Print control characters using hat notation (e.g., @samp{^G}); print
other unprintable characters in octal backslash notation.  By default,
unprintable characters are not changed.

@item -d
@itemx --double-space
@opindex -d
@opindex --double-space
@cindex double spacing
Double space the output.

@item -e[@var{in-tabchar}[@var{in-tabwidth}]]
@itemx --expand-tabs[=@var{in-tabchar}[@var{in-tabwidth}]]
@opindex -e
@opindex --expand-tabs
@cindex input tabs
Expand @var{tab}s to spaces on input.  Optional argument @var{in-tabchar} is
the input tab character (default is the TAB character).  Second optional
argument @var{in-tabwidth} is the input tab character's width (default
is 8).

@item -f
@itemx -F
@itemx --form-feed
@opindex -F
@opindex -f
@opindex --form-feed
Use a form feed instead of newlines to separate output pages.  The default
page length of 66 lines is not altered.  But the number of lines of text
per page changes from default 56 to 63 lines.

@item -h @var{HEADER}
@itemx --header=@var{HEADER}
@opindex -h
@opindex --header
Replace the filename in the header with the centered string @var{header}.
Left-hand-side truncation (marked by a @samp{*}) may occur if the total
header line @samp{yyyy-mm-dd HH:MM HEADER Page nnnn} becomes larger than
@var{page_width}.  @samp{-h ""} prints a blank line header.  Don't use
@samp{-h""}.
A space between the @samp{-h} option and the argument is always
indispensable.

@item -i[@var{out-tabchar}[@var{out-tabwidth}]]
@itemx --output-tabs[=@var{out-tabchar}[@var{out-tabwidth}]]
@opindex -i
@opindex --output-tabs
@cindex output tabs
Replace spaces with @var{tab}s on output.  Optional argument @var{out-tabchar}
is the output tab character (default is the TAB character).  Second optional
argument @var{out-tabwidth} is the output tab character's width (default
is 8).

@item -J
@itemx --join-lines
@opindex -J
@opindex --join-lines
Merge lines of full length.  Used together with the column options
@samp{-@var{column}}, @samp{-a -@var{column}} or @samp{-m}.  Turns off
@samp{-W/-w} line truncation;
no column alignment used; may be used with @samp{-S[@var{string}]}.
@samp{-J} has been introduced (together with @samp{-W} and @samp{-S})
to disentangle the old (@sc{posix}-compliant) options @samp{-w} and
@samp{-s} along with the three column options.


@item -l @var{page_length}
@itemx --length=@var{page_length}
@opindex -l
@opindex --length
Set the page length to @var{page_length} (default 66) lines, including
the lines of the header [and the footer].  If @var{page_length} is less
than or equal to 10 (or <= 3 with @samp{-F}), the header and footer are
omitted, and all form feeds set in input files are eliminated, as if
the @samp{-T} option had been given.

@item -m
@itemx --merge
@opindex -m
@opindex --merge
Merge and print all @var{file}s in parallel, one in each column.  If a
line is too long to fit in a column, it is truncated, unless the @samp{-J}
option is used.  @samp{-S[@var{string}]} may be used.  Empty pages in
some @var{file}s (form feeds set) produce empty columns, still marked
by @var{string}.  The result is a continuous line numbering and column
marking throughout the whole merged file.  Completely empty merged pages
show no separators or line numbers.  The default header becomes
@samp{yyyy-mm-dd HH:MM <blanks> Page nnnn}; may be used with
@samp{-h @var{header}} to fill up the middle blank part.

@item -n[@var{number-separator}[@var{digits}]]
@itemx --number-lines[=@var{number-separator}[@var{digits}]]
@opindex -n
@opindex --number-lines
Provide @var{digits} digit line numbering (default for @var{digits} is
5).  With multicolumn output the number occupies the first @var{digits}
column positions of each text column or only each line of @samp{-m}
output.  With single column output the number precedes each line just as
@samp{-m} does.  Default counting of the line numbers starts with the
first line of the input file (not the first line printed, compare the
@samp{--page} option and @samp{-N} option).
Optional argument @var{number-separator} is the character appended to
the line number to separate it from the text followed.  The default
separator is the TAB character.  In a strict sense a TAB is always
printed with single column output only.  The @var{TAB}-width varies
with the @var{TAB}-position, e.g. with the left @var{margin} specified
by @samp{-o} option.  With multicolumn output priority is given to
@samp{equal width of output columns} (a @sc{posix} specification).
The @var{TAB}-width is fixed to the value of the first column and does
not change with different values of left @var{margin}.  That means a
fixed number of spaces is always printed in the place of the
@var{number-separator tab}.  The tabification depends upon the output
position.

@item -N @var{line_number}
@itemx --first-line-number=@var{line_number}
@opindex -N
@opindex --first-line-number
Start line counting with the number @var{line_number} at first line of
first page printed (in most cases not the first line of the input file).

@item -o @var{margin}
@itemx --indent=@var{margin}
@opindex -o
@opindex --indent
@cindex indenting lines
@cindex left margin
Indent each line with a margin @var{margin} spaces wide (default is zero).
The total page width is the size of the margin plus the @var{page_width}
set with the @samp{-W/-w} option.  A limited overflow may occur with
numbered single column output (compare @samp{-n} option).

@item -r
@itemx --no-file-warnings
@opindex -r
@opindex --no-file-warnings
Do not print a warning message when an argument @var{file} cannot be
opened.  (The exit status will still be nonzero, however.)

@item -s[@var{char}]
@itemx --separator[=@var{char}]
@opindex -s
@opindex --separator
Separate columns by a single character @var{char}.  The default for
@var{char} is the TAB character without @samp{-w} and @samp{no
character} with @samp{-w}.  Without @samp{-s} the default separator
@samp{space} is set.  @samp{-s[char]} turns off line truncation of all
three column options (@samp{-COLUMN}|@samp{-a -COLUMN}|@samp{-m}) unless
@samp{-w} is set.  This is a @sc{posix}-compliant formulation.


@item -S[@var{string}]
@itemx --sep-string[=@var{string}]
@opindex -S
@opindex --sep-string
Use @var{string} to separate output columns.  The @samp{-S} option doesn't
affect the @samp{-W/-w} option, unlike the @samp{-s} option which does.  It
does not affect line truncation or column alignment.
Without @samp{-S}, and with @samp{-J}, @code{pr} uses the default output
separator, TAB.
Without @samp{-S} or @samp{-J}, @code{pr} uses a @samp{space}
(same as @samp{-S" "}).
Using @samp{-S} with no @var{string} is equivalent to @samp{-S""}.
Note that for some of @code{pr}'s options the single-letter option
character must be followed immediately by any corresponding argument;
there may not be any intervening white space.
@samp{-S/-s} is one of them.  Don't use @samp{-S "STRING"}.
@sc{posix} requires this.

@item -t
@itemx --omit-header
@opindex -t
@opindex --omit-header
Do not print the usual header [and footer] on each page, and do not fill
out the bottom of pages (with blank lines or a form feed).  No page
structure is produced, but form feeds set in the input files are retained.
The predefined pagination is not changed.  @samp{-t} or @samp{-T} may be
useful together with other options; e.g.: @samp{-t -e4}, expand TAB characters
in the input file to 4 spaces but don't make any other changes.  Use of
@samp{-t} overrides @samp{-h}.

@item -T
@itemx --omit-pagination
@opindex -T
@opindex --omit-pagination
Do not print header [and footer].  In addition eliminate all form feeds
set in the input files.

@item -v
@itemx --show-nonprinting
@opindex -v
@opindex --show-nonprinting
Print unprintable characters in octal backslash notation.

@item -w @var{page_width}
@itemx --width=@var{page_width}
@opindex -w
@opindex --width
Set page width to @var{page_width} characters for multiple text-column
output only (default for @var{page_width} is 72).  @samp{-s[CHAR]} turns
off the default page width and any line truncation and column alignment.
Lines of full length are merged, regardless of the column options
set.  No @var{page_width} setting is possible with single column output.
A @sc{posix}-compliant formulation.

@item -W @var{page_width}
@itemx --page_width=@var{page_width}
@opindex -W
@opindex --page_width
Set the page width to @var{page_width} characters.  That's valid with and
without a column option.  Text lines are truncated, unless @samp{-J}
is used.  Together with one of the three column options
(@samp{-@var{column}}, @samp{-a -@var{column}} or @samp{-m}) column
alignment is always used.  The separator options @samp{-S} or @samp{-s}
don't affect the @samp{-W} option.  Default is 72 characters.  Without
@samp{-W @var{page_width}} and without any of the column options NO line
truncation is used (defined to keep downward compatibility and to meet
most frequent tasks).  That's equivalent to @samp{-W 72 -J}.  With and
without @samp{-W @var{page_width}} the header line is always truncated
to avoid line overflow.

@end table


@node fold invocation
@section @code{fold}: Wrap input lines to fit in specified width

@pindex fold
@cindex wrapping long input lines
@cindex folding long input lines

@code{fold} writes each @var{file} (@samp{-} means standard input), or
standard input if none are given, to standard output, breaking long
lines.  Synopsis:

@example
fold [@var{option}]@dots{} [@var{file}]@dots{}
@end example

By default, @code{fold} breaks lines wider than 80 columns.  The output
is split into as many lines as necessary.

@cindex screen columns
@code{fold} counts screen columns by default; thus, a tab may count more
than one column, backspace decreases the column count, and carriage
return sets the column to zero.

The program accepts the following options.  Also see @ref{Common options}.

@table @samp

@item -b
@itemx --bytes
@opindex -b
@opindex --bytes
Count bytes rather than columns, so that tabs, backspaces, and carriage
returns are each counted as taking up one column, just like other
characters.

@item -s
@itemx --spaces
@opindex -s
@opindex --spaces
Break at word boundaries: the line is broken after the last blank before
the maximum line length.  If the line contains no such blanks, the line
is broken at the maximum line length as usual.

@item -w @var{width}
@itemx --width=@var{width}
@opindex -w
@opindex --width
Use a maximum line length of @var{width} columns instead of 80.

@end table


@node Output of parts of files
@chapter Output of parts of files

@cindex output of parts of files
@cindex parts of files, output of

These commands output pieces of the input.

@menu
* head invocation::             Output the first part of files.
* tail invocation::             Output the last part of files.
* split invocation::            Split a file into fixed-size pieces.
* csplit invocation::           Split a file into context-determined pieces.
@end menu

@node head invocation
@section @code{head}: Output the first part of files

@pindex head
@cindex initial part of files, outputting
@cindex first part of files, outputting

@code{head} prints the first part (10 lines by default) of each
@var{file}; it reads from standard input if no files are given or
when given a @var{file} of @samp{-}.  Synopses:

@example
head [@var{option}]@dots{} [@var{file}]@dots{}
head -@var{number} [@var{option}]@dots{} [@var{file}]@dots{}
@end example

If more than one @var{file} is specified, @code{head} prints a
one-line header consisting of
@example
==> @var{file name} <==
@end example
@noindent
before the output for each @var{file}.

@code{head} accepts two option formats: the new one, in which numbers
are arguments to the options (@samp{-q -n 1}), and the old one, in which
the number precedes any option letters (@samp{-1q}).

The program accepts the following options.  Also see @ref{Common options}.

@table @samp

@item -@var{count}@var{options}
@opindex -@var{count}
This option is only recognized if it is specified first.  @var{count} is
a decimal number optionally followed by a size letter (@samp{b},
@samp{k}, @samp{m}) as in @code{-c}, or @samp{l} to mean count by lines,
or other option letters (@samp{cqv}).

@item -c @var{bytes}
@itemx --bytes=@var{bytes}
@opindex -c
@opindex --bytes
Print the first @var{bytes} bytes, instead of initial lines.  Appending
@samp{b} multiplies @var{bytes} by 512, @samp{k} by 1024, and @samp{m}
by 1048576.

@itemx -n @var{n}
@itemx --lines=@var{n}
@opindex -n
@opindex --lines
Output the first @var{n} lines.

@item -q
@itemx --quiet
@itemx --silent
@opindex -q
@opindex --quiet
@opindex --silent
Never print file name headers.

@item -v
@itemx --verbose
@opindex -v
@opindex --verbose
Always print file name headers.

@end table


@node tail invocation
@section @code{tail}: Output the last part of files

@pindex tail
@cindex last part of files, outputting

@code{tail} prints the last part (10 lines by default) of each
@var{file}; it reads from standard input if no files are given or
when given a @var{file} of @samp{-}.  Synopses:

@example
tail [@var{option}]@dots{} [@var{file}]@dots{}
tail -@var{number} [@var{option}]@dots{} [@var{file}]@dots{}
tail +@var{number} [@var{option}]@dots{} [@var{file}]@dots{}
@end example

If more than one @var{file} is specified, @code{tail} prints a
one-line header consisting of
@example
==> @var{file name} <==
@end example
@noindent
before the output for each @var{file}.

@cindex BSD @code{tail}
@sc{gnu} @code{tail} can output any amount of data (some other versions of
@code{tail} cannot).  It also has no @samp{-r} option (print in
reverse), since reversing a file is really a different job from printing
the end of a file; BSD @code{tail} (which is the one with @code{-r}) can
only reverse files that are at most as large as its buffer, which is
typically 32k.  A more reliable and versatile way to reverse files is
the @sc{gnu} @code{tac} command.

@code{tail} accepts two option formats: the new one, in which numbers
are arguments to the options (@samp{-n 1}), and the old one, in which
the number precedes any option letters (@samp{-1} or @samp{+1}).

If any option-argument is a number @var{n} starting with a @samp{+},
@code{tail} begins printing with the @var{n}th item from the start of
each file, instead of from the end.

The program accepts the following options.  Also see @ref{Common options}.

@table @samp

@item -@var{count}
@itemx +@var{count}
@opindex -@var{count}
@opindex +@var{count}
This option is only recognized if it is specified first.  @var{count} is
a decimal number optionally followed by a size letter (@samp{b},
@samp{k}, @samp{m}) as in @code{-c}, or @samp{l} to mean count by lines,
or other option letters (@samp{cfqv}).

@item -c @var{bytes}
@itemx --bytes=@var{bytes}
@opindex -c
@opindex --bytes
Output the last @var{bytes} bytes, instead of final lines.  Appending
@samp{b} multiplies @var{bytes} by 512, @samp{k} by 1024, and @samp{m}
by 1048576.

@item -f
@itemx --follow[=@var{how}]
@opindex -f
@opindex --follow
@cindex growing files
@vindex name @r{follow option}
@vindex descriptor @r{follow option}
Loop forever trying to read more characters at the end of the file,
presumably because the file is growing.  This option is ignored when
reading from a pipe.
If more than one file is given, @code{tail} prints a header whenever it
gets output from a different file, to indicate which file that output is
from.

There are two ways to specify how you'd like to track files with this option,
but that difference is noticeable only when a followed file is removed or
renamed.
If you'd like to continue to track the end of a growing file even after
it has been unlinked, use @samp{--follow=descriptor}.  This is the default
behavior, but it is not useful if you're tracking a log file that may be
rotated (removed or renamed, then reopened).  In that case, use
@samp{--follow=name} to track the named file by reopening it periodically
to see if it has been removed and recreated by some other program.

No matter which method you use, if the tracked file is determined to have
shrunk, @code{tail} prints a message saying the file has been truncated
and resumes tracking the end of the file from the newly-determined endpoint.

When a file is removed, @code{tail}'s behavior depends on whether it is
following the name or the descriptor.  When following by name, tail can
detect that a file has been removed and gives a message to that effect,
and if @samp{--retry} has been specified it will continue checking
periodically to see if the file reappears.
When following a descriptor, tail does not detect that the file has
been unlinked or renamed and issues no message;  even though the file
may no longer be accessible via its original name, it may still be
growing.

The option values @samp{descriptor} and @samp{name} may be specified only
with the long form of the option, not with @samp{-f}.

@itemx --retry
@opindex --retry
This option is meaningful only when following by name.
Without this option, when tail encounters a file that doesn't
exist or is otherwise inaccessible, it reports that fact and
never checks it again.

@itemx --sleep-interval=@var{n}
@opindex --sleep-interval
Change the number of seconds to wait between iterations (the default is 1).
During one iteration, every specified file is checked to see if it has
changed size.

@itemx --pid=@var{pid}
@opindex --pid
When following by name or by descriptor, you may specify the process ID,
@var{pid}, of the sole writer of all @var{file} arguments.  Then, shortly
after that process terminates, tail will also terminate.  This will
work properly only if the writer and the tailing process are running on
the same machine.  For example, to save the output of a build in a file
and to watch the file grow, if you invoke @code{make} and @code{tail}
like this then the tail process will stop when your build completes.
Without this option, you would have had to kill the @code{tail -f}
process yourself.
@example
$ make >& makerr & tail --pid=$! -f makerr
@end example
If you specify a @var{pid} that is not in use or that does not correspond
to the process that is writing to the tailed files, then @code{tail}
may terminate long before any @var{file}s stop growing or it may not
terminate until long after the real writer has terminated.
Note that @samp{--pid} cannot be supported on some systems; @code{tail}
will print a warning if this is the case.

@itemx --max-unchanged-stats=@var{n}
@opindex --max-unchanged-stats
When tailing a file by name, if there have been @var{n} (default
N=@value{DEFAULT_MAX_N_UNCHANGED_STATS_BETWEEN_OPENS}) consecutive
iterations for which the size has remained the same, then
@code{open}/@code{fstat} the file to determine if that file name is
still associated with the same device/inode-number pair as before.
When following a log file that is rotated, this is approximately the
number of seconds between when tail prints the last pre-rotation lines
and when it prints the lines that have accumulated in the new log file.
This option is meaningful only when following by name.

@itemx -n @var{n}
@itemx --lines=@var{n}
@opindex -n
@opindex --lines
Output the last @var{n} lines.

@item -q
@itemx -quiet
@itemx --silent
@opindex -q
@opindex --quiet
@opindex --silent
Never print file name headers.

@item -v
@itemx --verbose
@opindex -v
@opindex --verbose
Always print file name headers.

@end table


@node split invocation
@section @code{split}: Split a file into fixed-size pieces

@pindex split
@cindex splitting a file into pieces
@cindex pieces, splitting a file into

@code{split} creates output files containing consecutive sections of
@var{input} (standard input if none is given or @var{input} is
@samp{-}).  Synopsis:

@example
split [@var{option}] [@var{input} [@var{prefix}]]
@end example

By default, @code{split} puts 1000 lines of @var{input} (or whatever is
left over for the last section), into each output file.

@cindex output file name prefix
The output files' names consist of @var{prefix} (@samp{x} by default)
followed by a group of letters @samp{aa}, @samp{ab}, and so on, such
that concatenating the output files in sorted order by file name produces
the original input file.  (If more than 676 output files are required,
@code{split} uses @samp{zaa}, @samp{zab}, etc.)

The program accepts the following options.  Also see @ref{Common options}.

@table @samp

@item -@var{lines}
@itemx -l @var{lines}
@itemx --lines=@var{lines}
@opindex -l
@opindex --lines
Put @var{lines} lines of @var{input} into each output file.

@item -b @var{bytes}
@itemx --bytes=@var{bytes}
@opindex -b
@opindex --bytes
Put the first @var{bytes} bytes of @var{input} into each output file.
Appending @samp{b} multiplies @var{bytes} by 512, @samp{k} by 1024, and
@samp{m} by 1048576.

@item -C @var{bytes}
@itemx --line-bytes=@var{bytes}
@opindex -C
@opindex --line-bytes
Put into each output file as many complete lines of @var{input} as
possible without exceeding @var{bytes} bytes.  For lines longer than
@var{bytes} bytes, put @var{bytes} bytes into each output file until
less than @var{bytes} bytes of the line are left, then continue
normally.  @var{bytes} has the same format as for the @samp{--bytes}
option.

@itemx --verbose
@opindex --verbose
Write a diagnostic to standard error just before each output file is opened.

@end table


@node csplit invocation
@section @code{csplit}: Split a file into context-determined pieces

@pindex csplit
@cindex context splitting
@cindex splitting a file into pieces by context

@code{csplit} creates zero or more output files containing sections of
@var{input} (standard input if @var{input} is @samp{-}).  Synopsis:

@example
csplit [@var{option}]@dots{} @var{input} @var{pattern}@dots{}
@end example

The contents of the output files are determined by the @var{pattern}
arguments, as detailed below.  An error occurs if a @var{pattern}
argument refers to a nonexistent line of the input file (e.g., if no
remaining line matches a given regular expression).  After every
@var{pattern} has been matched, any remaining input is copied into one
last output file.

By default, @code{csplit} prints the number of bytes written to each
output file after it has been created.

The types of pattern arguments are:

@table @samp

@item @var{n}
Create an output file containing the input up to but not including line
@var{n} (a positive integer).  If followed by a repeat count, also
create an output file containing the next @var{line} lines of the input
file once for each repeat.

@item /@var{regexp}/[@var{offset}]
Create an output file containing the current line up to (but not
including) the next line of the input file that contains a match for
@var{regexp}.  The optional @var{offset} is a @samp{+} or @samp{-}
followed by a positive integer.  If it is given, the input up to the
matching line plus or minus @var{offset} is put into the output file,
and the line after that begins the next section of input.

@item %@var{regexp}%[@var{offset}]
Like the previous type, except that it does not create an output
file, so that section of the input file is effectively ignored.

@item @{@var{repeat-count}@}
Repeat the previous pattern @var{repeat-count} additional
times. @var{repeat-count} can either be a positive integer or an
asterisk, meaning repeat as many times as necessary until the input is
exhausted.

@end table

The output files' names consist of a prefix (@samp{xx} by default)
followed by a suffix.  By default, the suffix is an ascending sequence
of two-digit decimal numbers from @samp{00} to @samp{99}.  In any case,
concatenating the output files in sorted order by filename produces the
original input file.

By default, if @code{csplit} encounters an error or receives a hangup,
interrupt, quit, or terminate signal, it removes any output files
that it has created so far before it exits.

The program accepts the following options.  Also see @ref{Common options}.

@table @samp

@item -f @var{prefix}
@itemx --prefix=@var{prefix}
@opindex -f
@opindex --prefix
@cindex output file name prefix
Use @var{prefix} as the output file name prefix.

@item -b @var{suffix}
@itemx --suffix=@var{suffix}
@opindex -b
@opindex --suffix
@cindex output file name suffix
Use @var{suffix} as the output file name suffix.  When this option is
specified, the suffix string must include exactly one
@code{printf(3)}-style conversion specification, possibly including
format specification flags, a field width, a precision specifications,
or all of these kinds of modifiers.  The format letter must convert a
binary integer argument to readable form; thus, only @samp{d}, @samp{i},
@samp{u}, @samp{o}, @samp{x}, and @samp{X} conversions are allowed.  The
entire @var{suffix} is given (with the current output file number) to
@code{sprintf(3)} to form the file name suffixes for each of the
individual output files in turn.  If this option is used, the
@samp{--digits} option is ignored.

@item -n @var{digits}
@itemx --digits=@var{digits}
@opindex -n
@opindex --digits
Use output file names containing numbers that are @var{digits} digits
long instead of the default 2.

@item -k
@itemx --keep-files
@opindex -k
@opindex --keep-files
Do not remove output files when errors are encountered.

@item -z
@itemx --elide-empty-files
@opindex -z
@opindex --elide-empty-files
Suppress the generation of zero-length output files.  (In cases where
the section delimiters of the input file are supposed to mark the first
lines of each of the sections, the first output file will generally be a
zero-length file unless you use this option.)  The output file sequence
numbers always run consecutively starting from 0, even when this option
is specified.

@item -s
@itemx -q
@itemx --silent
@itemx --quiet
@opindex -s
@opindex -q
@opindex --silent
@opindex --quiet
Do not print counts of output file sizes.

@end table


@node Summarizing files
@chapter Summarizing files

@cindex summarizing files

These commands generate just a few numbers representing entire
contents of files.

@menu
* wc invocation::               Print byte, word, and line counts.
* sum invocation::              Print checksum and block counts.
* cksum invocation::            Print CRC checksum and byte counts.
* md5sum invocation::           Print or check message-digests.
@end menu


@node wc invocation
@section @code{wc}: Print byte, word, and line counts

@pindex wc
@cindex byte count
@cindex character count
@cindex word count
@cindex line count

@code{wc} counts the number of bytes, characters, whitespace-separated
words, and newlines in each given @var{file}, or standard input if none
are given or for a @var{file} of @samp{-}.  Synopsis:

@example
wc [@var{option}]@dots{} [@var{file}]@dots{}
@end example

@cindex total counts
@vindex POSIXLY_CORRECT
@code{wc} prints one line of counts for each file, and if the file was
given as an argument, it prints the file name following the counts.  If
more than one @var{file} is given, @code{wc} prints a final line
containing the cumulative counts, with the file name @file{total}.  The
counts are printed in this order: newlines, words, characters, bytes.
By default, each count is output right-justified in a 7-byte field with
one space between fields so that the numbers and file names line up nicely
in columns.  However, @sc{posix} requires that there be exactly one space
separating columns.  You can make @code{wc} use the @sc{posix}-mandated
output format by setting the @env{POSIXLY_CORRECT} environment variable.

By default, @code{wc} prints three counts: the newline, words, and byte
counts.  Options can specify that only certain counts be printed.
Options do not undo others previously given, so

@example
wc --bytes --words
@end example

@noindent
prints both the byte counts and the word counts.

With the @code{--max-line-length} option, @code{wc} prints the length
of the longest line per file, and if there is more than one file it
prints the maximum (not the sum) of those lengths.

The program accepts the following options.  Also see @ref{Common options}.

@table @samp

@item -c
@itemx --bytes
@opindex -c
@opindex --bytes
Print only the byte counts.

@item -m
@itemx --chars
@opindex -m
@opindex --chars
Print only the character counts.

@item -w
@itemx --words
@opindex -w
@opindex --words
Print only the word counts.

@item -l
@itemx --lines
@opindex -l
@opindex --lines
Print only the newline counts.

@item -L
@itemx --max-line-length
@opindex -L
@opindex --max-line-length
Print only the maximum line lengths.

@end table


@node sum invocation
@section @code{sum}: Print checksum and block counts

@pindex sum
@cindex 16-bit checksum
@cindex checksum, 16-bit

@code{sum} computes a 16-bit checksum for each given @var{file}, or
standard input if none are given or for a @var{file} of @samp{-}.  Synopsis:

@example
sum [@var{option}]@dots{} [@var{file}]@dots{}
@end example

@code{sum} prints the checksum for each @var{file} followed by the
number of blocks in the file (rounded up).  If more than one @var{file}
is given, file names are also printed (by default).  (With the
@samp{--sysv} option, corresponding file names are printed when there is
at least one file argument.)

By default, @sc{gnu} @code{sum} computes checksums using an algorithm
compatible with BSD @code{sum} and prints file sizes in units of
1024-byte blocks.

The program accepts the following options.  Also see @ref{Common options}.

@table @samp

@item -r
@opindex -r
@cindex BSD @code{sum}
Use the default (BSD compatible) algorithm.  This option is included for
compatibility with the System V @code{sum}.  Unless @samp{-s} was also
given, it has no effect.

@item -s
@itemx --sysv
@opindex -s
@opindex --sysv
@cindex System V @code{sum}
Compute checksums using an algorithm compatible with System V
@code{sum}'s default, and print file sizes in units of 512-byte blocks.

@end table

@code{sum} is provided for compatibility; the @code{cksum} program (see
next section) is preferable in new applications.


@node cksum invocation
@section @code{cksum}: Print CRC checksum and byte counts

@pindex cksum
@cindex cyclic redundancy check
@cindex CRC checksum

@code{cksum} computes a cyclic redundancy check (CRC) checksum for each
given @var{file}, or standard input if none are given or for a
@var{file} of @samp{-}.  Synopsis:

@example
cksum [@var{option}]@dots{} [@var{file}]@dots{}
@end example

@code{cksum} prints the CRC checksum for each file along with the number
of bytes in the file, and the filename unless no arguments were given.

@code{cksum} is typically used to ensure that files
transferred by unreliable means (e.g., netnews) have not been corrupted,
by comparing the @code{cksum} output for the received files with the
@code{cksum} output for the original files (typically given in the
distribution).

The CRC algorithm is specified by the @sc{posix.2} standard.  It is not
compatible with the BSD or System V @code{sum} algorithms (see the
previous section); it is more robust.

The only options are @samp{--help} and @samp{--version}.  @xref{Common
options}.


@node md5sum invocation
@section @code{md5sum}: Print or check message-digests

@pindex md5sum
@cindex 128-bit checksum
@cindex checksum, 128-bit
@cindex fingerprint, 128-bit
@cindex message-digest, 128-bit

@code{md5sum} computes a 128-bit checksum (or @dfn{fingerprint} or
@dfn{message-digest}) for each specified @var{file}.
If a @var{file} is specified as @samp{-} or if no files are given
@code{md5sum} computes the checksum for the standard input.
@code{md5sum} can also determine whether a file and checksum are
consistent. Synopses:

@example
md5sum [@var{option}]@dots{} [@var{file}]@dots{}
md5sum [@var{option}]@dots{} --check [@var{file}]
@end example

For each @var{file}, @samp{md5sum} outputs the MD5 checksum, a flag
indicating a binary or text input file, and the filename.
If @var{file} is omitted or specified as @samp{-}, standard input is read.

The program accepts the following options.  Also see @ref{Common options}.

@table @samp

@item -b
@itemx --binary
@opindex -b
@opindex --binary
@cindex binary input files
Treat all input files as binary.  This option has no effect on Unix
systems, since they don't distinguish between binary and text files.
This option is useful on systems that have different internal and
external character representations.  On MS-DOS and MS-Windows, this is
the default.

@item -c
@itemx --check
Read filenames and checksum information from the single @var{file}
(or from stdin if no @var{file} was specified) and report whether
each named file and the corresponding checksum data are consistent.
The input to this mode of @code{md5sum} is usually the output of
a prior, checksum-generating run of @samp{md5sum}.
Each valid line of input consists of an MD5 checksum, a binary/text
flag, and then a filename.
Binary files are marked with @samp{*}, text with @samp{ }.
For each such line, @code{md5sum} reads the named file and computes its
MD5 checksum.  Then, if the computed message digest does not match the
one on the line with the filename, the file is noted as having
failed the test.  Otherwise, the file passes the test.
By default, for each valid line, one line is written to standard
output indicating whether the named file passed the test.
After all checks have been performed, if there were any failures,
a warning is issued to standard error.
Use the @samp{--status} option to inhibit that output.
If any listed file cannot be opened or read, if any valid line has
an MD5 checksum inconsistent with the associated file, or if no valid
line is found, @code{md5sum} exits with nonzero status.  Otherwise,
it exits successfully.

@itemx --status
@opindex --status
@cindex verifying MD5 checksums
This option is useful only when verifying checksums.
When verifying checksums, don't generate the default one-line-per-file
diagnostic and don't output the warning summarizing any failures.
Failures to open or read a file still evoke individual diagnostics to
standard error.
If all listed files are readable and are consistent with the associated
MD5 checksums, exit successfully.  Otherwise exit with a status code
indicating there was a failure.

@item -t
@itemx --text
@opindex -t
@opindex --text
@cindex text input files
Treat all input files as text files.  This is the reverse of
@samp{--binary}.

@item -w
@itemx --warn
@opindex -w
@opindex --warn
@cindex verifying MD5 checksums
When verifying checksums, warn about improperly formatted MD5 checksum lines.
This option is useful only if all but a few lines in the checked input
are valid.

@end table


@node Operating on sorted files
@chapter Operating on sorted files

@cindex operating on sorted files
@cindex sorted files, operations on

These commands work with (or produce) sorted files.

@menu
* sort invocation::             Sort text files.
* uniq invocation::             Uniquify files.
* comm invocation::             Compare two sorted files line by line.
* ptx invocation::              Produce a permuted index of file contents.
* tsort invocation::            Topological sort.
@end menu


@node sort invocation
@section @code{sort}: Sort text files

@pindex sort
@cindex sorting files

@code{sort} sorts, merges, or compares all the lines from the given
files, or standard input if none are given or for a @var{file} of
@samp{-}.  By default, @code{sort} writes the results to standard
output.  Synopsis:

@example
sort [@var{option}]@dots{} [@var{file}]@dots{}
@end example

@code{sort} has three modes of operation: sort (the default), merge,
and check for sortedness.  The following options change the operation
mode:

@table @samp

@item -c
@opindex -c
@cindex checking for sortedness
Check whether the given files are already sorted: if they are not all
sorted, print an error message and exit with a status of 1.
Otherwise, exit successfully.

@item -m
@opindex -m
@cindex merging sorted files
Merge the given files by sorting them as a group.  Each input file must
always be individually sorted.  It always works to sort instead of
merge; merging is provided because it is faster, in the case where it
works.

@end table

@vindex LC_COLLATE
A pair of lines is compared as follows: if any key fields have been
specified, @code{sort} compares each pair of fields, in the order
specified on the command line, according to the associated ordering
options, until a difference is found or no fields are left.
Unless otherwise specified, all comparisons use the character
collating sequence specified by the @env{LC_COLLATE} locale.

If any of the global options @samp{Mbdfinr} are given but no key fields
are specified, @code{sort} compares the entire lines according to the
global options.

Finally, as a last resort when all keys compare equal (or if no
ordering options were specified at all), @code{sort} compares the entire
lines.  The last resort comparison
honors the @samp{-r} global option.  The @samp{-s} (stable) option
disables this last-resort comparison so that lines in which all fields
compare equal are left in their original relative order.  If no fields
or global options are specified, @samp{-s} has no effect.

@sc{gnu} @code{sort} (as specified for all @sc{gnu} utilities) has no limits on
input line length or restrictions on bytes allowed within lines.  In
addition, if the final byte of an input file is not a newline, @sc{gnu}
@code{sort} silently supplies one.  A line's trailing newline is not
part of the line for comparison purposes.@footnote{@sc{posix}.2-1992
requires that the trailing newline be part of the comparison, and some
@code{sort} implementations obey this requirement, but it is widely
considered to be a bug in the standard and the next version of
@sc{posix}.2 will likely remove this requirement.}

Upon any error, @code{sort} exits with a status of @samp{2}.

@vindex TMPDIR
If the environment variable @env{TMPDIR} is set, @code{sort} uses its
value as the directory for temporary files instead of @file{/tmp}.  The
@samp{-T @var{tempdir}} option in turn overrides the environment
variable.

The following options affect the ordering of output lines.  They may be
specified globally or as part of a specific key field.  If no key
fields are specified, global options apply to comparison of entire
lines; otherwise the global options are inherited by key fields that do
not specify any special options of their own.  In pre-@sc{posix}
versions of @command{sort}, global options affect only later key fields,
so portable shell scripts should specify global options first.

@table @samp

@item -b
@opindex -b
@cindex blanks, ignoring leading
@vindex LC_CTYPE
Ignore leading blanks when finding sort keys in each line.
The @env{LC_CTYPE} locale determines character types.

@item -d
@opindex -d
@cindex phone directory order
@cindex telephone directory order
@vindex LC_CTYPE
Sort in @dfn{phone directory} order: ignore all characters except
letters, digits and blanks when sorting.
The @env{LC_CTYPE} locale determines character types.

@item -f
@opindex -f
@cindex case folding
@vindex LC_CTYPE
Fold lowercase characters into the equivalent uppercase characters when
sorting so that, for example, @samp{b} and @samp{B} sort as equal.
The @env{LC_CTYPE} locale determines character types.

@item -g
@opindex -g
@cindex general numeric sort
@vindex LC_NUMERIC
Sort numerically, using the standard C function @code{strtod} to convert
a prefix of each line to a double-precision floating point number.
This allows floating point numbers to be specified in scientific notation,
like @code{1.0e-34} and @code{10e100}.
The @env{LC_NUMERIC} locale determines the decimal-point character.
Do not report overflow, underflow, or conversion errors.
Use the following collating sequence:

@itemize @bullet
@item
Lines that do not start with numbers (all considered to be equal).
@item
NaNs (``Not a Number'' values, in IEEE floating point arithmetic)
in a consistent but machine-dependent order.
@item
Minus infinity.
@item
Finite numbers in ascending numeric order (with @math{-0} and @math{+0} equal).
@item
Plus infinity.
@end itemize

Use this option only if there is no alternative; it is much slower than
@samp{-n} and it can lose information when converting to floating point.

@item -i
@opindex -i
@cindex unprintable characters, ignoring
@vindex LC_CTYPE
Ignore unprintable characters.
The @env{LC_CTYPE} locale determines character types.

@item -M
@opindex -M
@cindex months, sorting by
@vindex LC_TIME
An initial string, consisting of any amount of whitespace, followed
by a month name abbreviation, is folded to UPPER case and
compared in the order @samp{JAN} < @samp{FEB} < @dots{} < @samp{DEC}.
Invalid names compare low to valid names.  The @env{LC_TIME} locale
determines the month spellings.

@item -n
@opindex -n
@cindex numeric sort
@vindex LC_NUMERIC
Sort numerically: the number begins each line; specifically, it consists
of optional whitespace, an optional @samp{-} sign, and zero or more
digits possibly separated by thousands separators, optionally followed
by a decimal-point character and zero or more digits.  The @env{LC_NUMERIC}
locale specifies the decimal-point character and thousands separator.

@code{sort -n} uses what might be considered an unconventional method
to compare strings representing floating point numbers.  Rather than
first converting each string to the C @code{double} type and then
comparing those values, sort aligns the decimal-point characters in the two
strings and compares the strings a character at a time.  One benefit
of using this approach is its speed.  In practice this is much more
efficient than performing the two corresponding string-to-double (or even
string-to-integer) conversions and then comparing doubles.  In addition,
there is no corresponding loss of precision.  Converting each string to
@code{double} before comparison would limit precision to about 16 digits
on most systems.

Neither a leading @samp{+} nor exponential notation is recognized.
To compare such strings numerically, use the @samp{-g} option.

@item -r
@opindex -r
@cindex reverse sorting
Reverse the result of comparison, so that lines with greater key values
appear earlier in the output instead of later.

@end table

Other options are:

@table @samp

@item -o @var{output-file}
@opindex -o
@cindex overwriting of input, allowed
Write output to @var{output-file} instead of standard output.
If @var{output-file} is one of the input files, @code{sort} copies
it to a temporary file before sorting and writing the output to
@var{output-file}.

@item -S @var{size}
@opindex -S
@cindex size for main memory sorting
Use a main-memory sort buffer of the given @var{size}.  By default,
@var{size} is in units of 1,024 bytes.  Appending @samp{%} causes
@var{size} to be interpreted as a percentage of physical memory.
Appending @samp{k} multiplies @var{size} by 1,024 (the default),
@samp{M} by 1,048,576, @samp{G} by 1,073,741,824, and so on for
@samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}.  Appending
@samp{b} causes @var{size} to be interpreted as a byte count, with no
multiplication.

This option can improve the performance of @command{sort} by causing it
to start with a larger or smaller sort buffer than the default.
However, this option affects only the initial buffer size.  The buffer
grows beyond @var{size} if @command{sort} encounters input lines larger
than @var{size}.

@item -t @var{separator}
@opindex -t
@cindex field separator character
Use character @var{separator} as the field separator when finding the
sort keys in each line.  By default, fields are separated by the empty
string between a non-whitespace character and a whitespace character.
That is, given the input line @w{@samp{ foo bar}}, @code{sort} breaks it
into fields @w{@samp{ foo}} and @w{@samp{ bar}}.  The field separator is
not considered to be part of either the field preceding or the field
following.  But note that sort fields that extend to the end of the line,
as @samp{-k 2}, or sort fields consisting of a range, as @samp{-k 2,3},
retain the field separators present between the endpoints of the range.

@item -T @var{tempdir}
@opindex -T
@cindex temporary directory
@vindex TMPDIR
Use directory @var{tempdir} to store temporary files, overriding the
@env{TMPDIR} environment variable.  If this option is given more than
once, temporary files are stored in all the directories given.  If you
have a large sort or merge that is I/O-bound, you can often improve
performance by using this option to specify directories on different
disks and controllers.

@item -u
@opindex -u
@cindex uniquifying output
For the default case or the @samp{-m} option, only output the first
of a sequence of lines that compare equal.  For the @samp{-c} option,
check that no pair of consecutive lines compares equal.

@item -k @var{pos1}[,@var{pos2}]
@opindex -k
@cindex sort field
The recommended, @sc{posix}, option for specifying a sort field.  The field
consists of the part of the line between @var{pos1} and @var{pos2} (or the
end of the line, if @var{pos2} is omitted), @emph{inclusive}.
Fields and character positions are numbered starting with 1.
So to sort on the second field, you'd use @samp{-k 2,2}
See below for more examples.

@item -z
@opindex -z
@cindex sort zero-terminated lines
Treat the input as a set of lines, each terminated by a zero byte (@sc{ascii}
@sc{nul} (Null) character) instead of an @sc{ascii} @sc{lf} (Line Feed).
This option can be useful in conjunction with @samp{perl -0} or
@samp{find -print0} and @samp{xargs -0} which do the same in order to
reliably handle arbitrary pathnames (even those which contain Line Feed
characters.)

@item +@var{pos1}[-@var{pos2}]
The obsolete, traditional option for specifying a sort field.  The field
consists of the line between @var{pos1} and up to but @emph{not including}
@var{pos2} (or the end of the line if @var{pos2} is omitted).  Fields
and character positions are numbered starting with 0.  See below.

@end table

In addition, when @sc{gnu} @code{sort} is invoked with exactly one argument,
options @samp{--help} and @samp{--version} are recognized.  @xref{Common
options}.

Historical (BSD and System V) implementations of @code{sort} have
differed in their interpretation of some options, particularly
@samp{-b}, @samp{-f}, and @samp{-n}.  @sc{gnu} sort follows the @sc{posix}
behavior, which is usually (but not always!) like the System V behavior.
According to @sc{posix}, @samp{-n} no longer implies @samp{-b}.  For
consistency, @samp{-M} has been changed in the same way.  This may
affect the meaning of character positions in field specifications in
obscure cases.  The only fix is to add an explicit @samp{-b}.

A position in a sort field specified with the @samp{-k} or @samp{+}
option has the form @samp{@var{f}.@var{c}}, where @var{f} is the number
of the field to use and @var{c} is the number of the first character
from the beginning of the field (for @samp{+@var{pos}}) or from the end
of the previous field (for @samp{-@var{pos}}).  If the @samp{.@var{c}}
is omitted, it is taken to be the first character in the field.  If the
@samp{-b} option was specified, the @samp{.@var{c}} part of a field
specification is counted from the first nonblank character of the field
(for @samp{+@var{pos}}) or from the first nonblank character following
the previous field (for @samp{-@var{pos}}).

A sort key option may also have any of the option letters @samp{Mbdfinr}
appended to it, in which case the global ordering options are not used
for that particular field.  The @samp{-b} option may be independently
attached to either or both of the @samp{+@var{pos}} and
@samp{-@var{pos}} parts of a field specification, and if it is inherited
from the global options it will be attached to both.
Keys may span multiple fields.

Here are some examples to illustrate various combinations of options.
In them, the @sc{posix} @samp{-k} option is used to specify sort keys rather
than the obsolete @samp{+@var{pos1}-@var{pos2}} syntax.

@itemize @bullet

@item
Sort in descending (reverse) numeric order.

@example
sort -nr
@end example

@item
Sort alphabetically, omitting the first and second fields.
This uses a single key composed of the characters beginning
at the start of field three and extending to the end of each line.

@example
sort -k 3
@end example

@item
Sort numerically on the second field and resolve ties by sorting
alphabetically on the third and fourth characters of field five.
Use @samp{:} as the field delimiter.

@example
sort -t : -k 2,2n -k 5.3,5.4
@end example

Note that if you had written @samp{-k 2} instead of @samp{-k 2,2}
@command{sort} would have used all characters beginning in the second field
and extending to the end of the line as the primary @emph{numeric}
key.  For the large majority of applications, treating keys spanning
more than one field as numeric will not do what you expect.

Also note that the @samp{n} modifier was applied to the field-end
specifier for the first key.  It would have been equivalent to
specify @samp{-k 2n,2} or @samp{-k 2n,2n}.  All modifiers except
@samp{b} apply to the associated @emph{field}, regardless of whether
the modifier character is attached to the field-start and/or the
field-end part of the key specifier.

@item
Sort the password file on the fifth field and ignore any
leading white space.  Sort lines with equal values in field five
on the numeric user ID in field three.

@example
sort -t : -k 5b,5 -k 3,3n /etc/passwd
@end example

An alternative is to use the global numeric modifier @samp{-n}.

@example
sort -t : -n -k 5b,5 -k 3,3 /etc/passwd
@end example

@item
Generate a tags file in case-insensitive sorted order.

@smallexample
find src -type f -print0 | sort -t / -z -f | xargs -0 etags --append
@end smallexample

The use of @samp{-print0}, @samp{-z}, and @samp{-0} in this case means
that pathnames that contain Line Feed characters will not get broken up
by the sort operation.

Finally, to ignore both leading and trailing white space, you
could have applied the @samp{b} modifier to the field-end specifier
for the first key,

@example
sort -t : -n -k 5b,5b -k 3,3 /etc/passwd
@end example

or by using the global @samp{-b} modifier instead of @samp{-n}
and an explicit @samp{n} with the second key specifier.

@example
sort -t : -b -k 5,5 -k 3,3n /etc/passwd
@end example

@c This example is a bit contrived and needs more explanation.
@c @item
@c Sort records separated by an arbitrary string by using a pipe to convert
@c each record delimiter string to @samp{\0}, then using sort's -z option,
@c and converting each @samp{\0} back to the original record delimiter.
@c
@c @example
@c printf 'c\n\nb\n\na\n'|perl -0pe 's/\n\n/\n\0/g'|sort -z|perl -0pe 's/\0/\n/g'
@c @end example

@end itemize


@node uniq invocation
@section @code{uniq}: Uniquify files

@pindex uniq
@cindex uniquify files

@code{uniq} writes the unique lines in the given @file{input}, or
standard input if nothing is given or for an @var{input} name of
@samp{-}.  Synopsis:

@example
uniq [@var{option}]@dots{} [@var{input} [@var{output}]]
@end example

By default, @code{uniq} prints the unique lines in a sorted file, i.e.,
discards all but one of identical successive lines.  Optionally, it can
instead show only lines that appear exactly once, or lines that appear
more than once.

The input must be sorted.  If your input is not sorted, perhaps you want
to use @code{sort -u}.

If no @var{output} file is specified, @code{uniq} writes to standard
output.

The program accepts the following options.  Also see @ref{Common options}.

@table @samp

@item -@var{n}
@itemx -f @var{n}
@itemx --skip-fields=@var{n}
@opindex -@var{n}
@opindex -f
@opindex --skip-fields
Skip @var{n} fields on each line before checking for uniqueness.  Fields
are sequences of non-space non-tab characters that are separated from
each other by at least one space or tab.

@item +@var{n}
@itemx -s @var{n}
@itemx --skip-chars=@var{n}
@opindex +@var{n}
@opindex -s
@opindex --skip-chars
Skip @var{n} characters before checking for uniqueness.  If you use both
the field and character skipping options, fields are skipped over first.

@item -c
@itemx --count
@opindex -c
@opindex --count
Print the number of times each line occurred along with the line.

@item -i
@itemx --ignore-case
@opindex -i
@opindex --ignore-case
Ignore differences in case when comparing lines.

@item -d
@itemx --repeated
@opindex -d
@opindex --repeated
@cindex duplicate lines, outputting
Print only duplicate lines.

@item -D
@itemx --all-repeated
@opindex -D
@opindex --all-repeated
@cindex all duplicate lines, outputting
Print all duplicate lines and only duplicate lines.
This option is useful mainly in conjunction with other options e.g.,
to ignore case or to compare only selected fields.
This is a @sc{gnu} extension.
@c FIXME: give an example showing *how* it's useful

@item -u
@itemx --unique
@opindex -u
@opindex --unique
@cindex unique lines, outputting
Print only unique lines.

@item -w @var{n}
@itemx --check-chars=@var{n}
@opindex -w
@opindex --check-chars
Compare @var{n} characters on each line (after skipping any specified
fields and characters).  By default the entire rest of the lines are
compared.

@end table


@node comm invocation
@section @code{comm}: Compare two sorted files line by line

@pindex comm
@cindex line-by-line comparison
@cindex comparing sorted files

@code{comm} writes to standard output lines that are common, and lines
that are unique, to two input files; a file name of @samp{-} means
standard input.  Synopsis:

@example
comm [@var{option}]@dots{} @var{file1} @var{file2}
@end example

@vindex LC_COLLATE
Before @code{comm} can be used, the input files must be sorted using the
collating sequence specified by the @env{LC_COLLATE} locale.
If an input file ends in a non-newline
character, a newline is silently appended.  The @code{sort} command with
no options always outputs a file that is suitable input to @code{comm}.

@cindex differing lines
@cindex common lines
With no options, @code{comm} produces three column output.  Column one
contains lines unique to @var{file1}, column two contains lines unique
to @var{file2}, and column three contains lines common to both files.
Columns are separated by a single TAB character.
@c FIXME: when there's an option to supply an alternative separator
@c string, append `by default' to the above sentence.

@opindex -1
@opindex -2
@opindex -3
The options @samp{-1}, @samp{-2}, and @samp{-3} suppress printing of
the corresponding columns.  Also see @ref{Common options}.

Unlike some other comparison utilities, @code{comm} has an exit
status that does not depend on the result of the comparison.
Upon normal completion @code{comm} produces an exit code of zero.
If there is an error it exits with nonzero status.


@node tsort invocation
@section @code{tsort}: Topological sort

@pindex tsort
@cindex topological sort

@code{tsort} performs a topological sort on the given @var{file}, or
standard input if no input file is given or for a @var{file} of
@samp{-}.  Synopsis:

@example
tsort [@var{option}] [@var{file}]
@end example

@code{tsort} reads its input as pairs of strings, separated by blanks,
indicating a partial ordering.  The output is a total ordering that
corresponds to the given partial ordering.

For example

@example
tsort <<EOF
a b c
d
e f
b c d e
EOF
@end example

@noindent
will produce the output

@example
a
b
c
d
e
f
@end example

@code{tsort} will detect cycles in the input and writes the first cycle
encountered to standard error.

Note that for a given partial ordering, generally there is no unique
total ordering.

The only options are @samp{--help} and @samp{--version}.  @xref{Common
options}.


@node ptx invocation
@section @code{ptx}: Produce permuted indexes

@pindex ptx

@code{ptx} reads a text file and essentially produces a permuted index, with
each keyword in its context.  The calling sketch is either one of:

@example
ptx [@var{option} @dots{}] [@var{file} @dots{}]
ptx -G [@var{option} @dots{}] [@var{input} [@var{output}]]
@end example

The @samp{-G} (or its equivalent: @samp{--traditional}) option disables
all @sc{gnu} extensions and reverts to traditional mode, thus introducing some
limitations and changing several of the program's default option values.
When @samp{-G} is not specified, @sc{gnu} extensions are always enabled.
@sc{gnu} extensions to @code{ptx} are documented wherever appropriate in this
document.  For the full list, see @xref{Compatibility in ptx}.

Individual options are explained in the following sections.

When @sc{gnu} extensions are enabled, there may be zero, one or several
@var{file}s after the options.  If there is no @var{file}, the program
reads the standard input.  If there is one or several @var{file}s, they
give the name of input files which are all read in turn, as if all the
input files were concatenated.  However, there is a full contextual
break between each file and, when automatic referencing is requested,
file names and line numbers refer to individual text input files.  In
all cases, the program outputs the permuted index to the standard
output.

When @sc{gnu} extensions are @emph{not} enabled, that is, when the program
operates in traditional mode, there may be zero, one or two parameters
besides the options.  If there are no parameters, the program reads the
standard input and outputs the permuted index to the standard output.
If there is only one parameter, it names the text @var{input} to be read
instead of the standard input.  If two parameters are given, they give
respectively the name of the @var{input} file to read and the name of
the @var{output} file to produce.  @emph{Be very careful} to note that,
in this case, the contents of file given by the second parameter is
destroyed.  This behavior is dictated by System V @code{ptx}
compatibility; @sc{gnu} Standards normally discourage output parameters not
introduced by an option.

Note that for @emph{any} file named as the value of an option or as an
input text file, a single dash @kbd{-} may be used, in which case
standard input is assumed.  However, it would not make sense to use this
convention more than once per program invocation.

@menu
* General options in ptx::      Options which affect general program behavior.
* Charset selection in ptx::    Underlying character set considerations.
* Input processing in ptx::     Input fields, contexts, and keyword selection.
* Output formatting in ptx::    Types of output format, and sizing the fields.
* Compatibility in ptx::
@end menu


@node General options in ptx
@subsection General options

@table @samp

@item -C
@itemx --copyright
Print a short note about the copyright and copying conditions, then
exit without further processing.

@item -G
@itemx --traditional
As already explained, this option disables all @sc{gnu} extensions to
@code{ptx} and switches to traditional mode.

@item --help
Print a short help on standard output, then exit without further
processing.

@item --version
Print the program version on standard output, then exit without further
processing.

@end table


@node Charset selection in ptx
@subsection Charset selection

@c FIXME:  People don't necessarily know what an IBM-PC was these days.
As it is set up now, the program assumes that the input file is coded
using 8-bit ISO 8859-1 code, also known as Latin-1 character set,
@emph{unless} it is compiled for MS-DOS, in which case it uses the
character set of the IBM-PC.  (@sc{gnu} @code{ptx} is not known to work on
smaller MS-DOS machines anymore.)  Compared to 7-bit @sc{ascii}, the set
of characters which are letters is different; this alters the behavior
of regular expression matching.  Thus, the default regular expression
for a keyword allows foreign or diacriticized letters.  Keyword sorting,
however, is still crude; it obeys the underlying character set ordering
quite blindly.

@table @samp

@item -f
@itemx --ignore-case
Fold lower case letters to upper case for sorting.

@end table


@node Input processing in ptx
@subsection Word selection and input processing

@table @samp

@item -b @var{file}
@item --break-file=@var{file}

This option provides an alternative (to @samp{-W}) method of describing
which characters make up words.  It introduces the name of a
file which contains a list of characters which can@emph{not} be part of
one word; this file is called the @dfn{Break file}.  Any character which
is not part of the Break file is a word constituent.  If both options
@samp{-b} and @samp{-W} are specified, then @samp{-W} has precedence and
@samp{-b} is ignored.

When @sc{gnu} extensions are enabled, the only way to avoid newline as a
break character is to write all the break characters in the file with no
newline at all, not even at the end of the file.  When @sc{gnu} extensions
are disabled, spaces, tabs and newlines are always considered as break
characters even if not included in the Break file.

@item -i @var{file}
@itemx --ignore-file=@var{file}

The file associated with this option contains a list of words which will
never be taken as keywords in concordance output.  It is called the
@dfn{Ignore file}.  The file contains exactly one word in each line; the
end of line separation of words is not subject to the value of the
@samp{-S} option.

There is a default Ignore file used by @code{ptx} when this option is
not specified, usually found in @file{/usr/local/lib/eign} if this has
not been changed at installation time.  If you want to deactivate the
default Ignore file, specify @code{/dev/null} instead.

@item -o @var{file}
@itemx --only-file=@var{file}

The file associated with this option contains a list of words which will
be retained in concordance output; any word not mentioned in this file
is ignored.  The file is called the @dfn{Only file}.  The file contains
exactly one word in each line; the end of line separation of words is
not subject to the value of the @samp{-S} option.

There is no default for the Only file.  When both an Only file and an
Ignore file are specified, a word is considered a keyword only
if it is listed in the Only file and not in the Ignore file.

@item -r
@itemx --references

On each input line, the leading sequence of non-white space characters will be
taken to be a reference that has the purpose of identifying this input
line in the resulting permuted index.  For more information about reference
production, see @xref{Output formatting in ptx}.
Using this option changes the default value for option @samp{-S}.

Using this option, the program does not try very hard to remove
references from contexts in output, but it succeeds in doing so
@emph{when} the context ends exactly at the newline.  If option
@samp{-r} is used with @samp{-S} default value, or when @sc{gnu} extensions
are disabled, this condition is always met and references are completely
excluded from the output contexts.

@item -S @var{regexp}
@itemx --sentence-regexp=@var{regexp}

This option selects which regular expression will describe the end of a
line or the end of a sentence.  In fact, this regular expression is not
the only distinction between end of lines or end of sentences, and input
line boundaries have no special significance outside this option.  By
default, when @sc{gnu} extensions are enabled and if @samp{-r} option is not
used, end of sentences are used.  In this case, this @var{regex} is
imported from @sc{gnu} Emacs:

@example
[.?!][]\"')@}]*\\($\\|\t\\|  \\)[ \t\n]*
@end example

Whenever @sc{gnu} extensions are disabled or if @samp{-r} option is used, end
of lines are used; in this case, the default @var{regexp} is just:

@example
\n
@end example

Using an empty @var{regexp} is equivalent to completely disabling end of
line or end of sentence recognition.  In this case, the whole file is
considered to be a single big line or sentence.  The user might want to
disallow all truncation flag generation as well, through option @samp{-F
""}.  @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
Manual}.

When the keywords happen to be near the beginning of the input line or
sentence, this often creates an unused area at the beginning of the
output context line; when the keywords happen to be near the end of the
input line or sentence, this often creates an unused area at the end of
the output context line.  The program tries to fill those unused areas
by wrapping around context in them; the tail of the input line or
sentence is used to fill the unused area on the left of the output line;
the head of the input line or sentence is used to fill the unused area
on the right of the output line.

As a matter of convenience to the user, many usual backslashed escape
sequences from the C language are recognized and converted to the
corresponding characters by @code{ptx} itself.

@item -W @var{regexp}
@itemx --word-regexp=@var{regexp}

This option selects which regular expression will describe each keyword.
By default, if @sc{gnu} extensions are enabled, a word is a sequence of
letters; the @var{regexp} used is @samp{\w+}.  When @sc{gnu} extensions are
disabled, a word is by default anything which ends with a space, a tab
or a newline; the @var{regexp} used is @samp{[^ \t\n]+}.

An empty @var{regexp} is equivalent to not using this option.
@xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
Manual}.

As a matter of convenience to the user, many usual backslashed escape
sequences, as found in the C language, are recognized and converted to
the corresponding characters by @code{ptx} itself.

@end table


@node Output formatting in ptx
@subsection Output formatting

Output format is mainly controlled by the @samp{-O} and @samp{-T} options
described in the table below.  When neither @samp{-O} nor @samp{-T} are
selected, and if @sc{gnu} extensions are enabled, the program chooses an
output format suitable for a dumb terminal.  Each keyword occurrence is
output to the center of one line, surrounded by its left and right
contexts.  Each field is properly justified, so the concordance output
can be readily observed.  As a special feature, if automatic
references are selected by option @samp{-A} and are output before the
left context, that is, if option @samp{-R} is @emph{not} selected, then
a colon is added after the reference; this nicely interfaces with @sc{gnu}
Emacs @code{next-error} processing.  In this default output format, each
white space character, like newline and tab, is merely changed to
exactly one space, with no special attempt to compress consecutive
spaces.  This might change in the future.  Except for those white space
characters, every other character of the underlying set of 256
characters is transmitted verbatim.

Output format is further controlled by the following options.

@table @samp

@item -g @var{number}
@itemx --gap-size=@var{number}

Select the size of the minimum white space gap between the fields on the
output line.

@item -w @var{number}
@itemx --width=@var{number}

Select the maximum output width of each final line.  If references are
used, they are included or excluded from the maximum output width
depending on the value of option @samp{-R}.  If this option is not
selected, that is, when references are output before the left context,
the maximum output width takes into account the maximum length of all
references.  If this option is selected, that is, when references are
output after the right context, the maximum output width does not take
into account the space taken by references, nor the gap that precedes
them.

@item -A
@itemx --auto-reference

Select automatic references.  Each input line will have an automatic
reference made up of the file name and the line ordinal, with a single
colon between them.  However, the file name will be empty when standard
input is being read.  If both @samp{-A} and @samp{-r} are selected, then
the input reference is still read and skipped, but the automatic
reference is used at output time, overriding the input reference.

@item -R
@itemx --right-side-refs

In the default output format, when option @samp{-R} is not used, any
references produced by the effect of options @samp{-r} or @samp{-A} are
placed to the far right of output lines, after the right context.  With
default output format, when the @samp{-R} option is specified, references
are rather placed at the beginning of each output line, before the left
context.  For any other output format, option @samp{-R} is
ignored, with one exception:  with @samp{-R} the width of references
is @emph{not} taken into account in total output width given by @samp{-w}.

This option is automatically selected whenever @sc{gnu} extensions are
disabled.

@item -F @var{string}
@itemx --flac-truncation=@var{string}

This option will request that any truncation in the output be reported
using the string @var{string}.  Most output fields theoretically extend
towards the beginning or the end of the current line, or current
sentence, as selected with option @samp{-S}.  But there is a maximum
allowed output line width, changeable through option @samp{-w}, which is
further divided into space for various output fields.  When a field has
to be truncated because it cannot extend beyond the beginning or the end of
the current line to fit in, then a truncation occurs.  By default,
the string used is a single slash, as in @samp{-F /}.

@var{string} may have more than one character, as in @samp{-F ...}.
Also, in the particular case when @var{string} is empty (@samp{-F ""}),
truncation flagging is disabled, and no truncation marks are appended in
this case.

As a matter of convenience to the user, many usual backslashed escape
sequences, as found in the C language, are recognized and converted to
the corresponding characters by @code{ptx} itself.

@item -M @var{string}
@itemx --macro-name=@var{string}

Select another @var{string} to be used instead of @samp{xx}, while
generating output suitable for @code{nroff}, @code{troff} or @TeX{}.

@item -O
@itemx --format=roff

Choose an output format suitable for @code{nroff} or @code{troff}
processing.  Each output line will look like:

@smallexample
.xx "@var{tail}" "@var{before}" "@var{keyword_and_after}" "@var{head}" "@var{ref}"
@end smallexample

so it will be possible to write a @samp{.xx} roff macro to take care of
the output typesetting.  This is the default output format when @sc{gnu}
extensions are disabled.  Option @samp{-M} can be used to change
@samp{xx} to another macro name.

In this output format, each non-graphical character, like newline and
tab, is merely changed to exactly one space, with no special attempt to
compress consecutive spaces.  Each quote character: @kbd{"} is doubled
so it will be correctly processed by @code{nroff} or @code{troff}.

@item -T
@itemx --format=tex

Choose an output format suitable for @TeX{} processing.  Each output
line will look like:

@smallexample
\xx @{@var{tail}@}@{@var{before}@}@{@var{keyword}@}@{@var{after}@}@{@var{head}@}@{@var{ref}@}
@end smallexample

@noindent
so it will be possible to write a @code{\xx} definition to take care of
the output typesetting.  Note that when references are not being
produced, that is, neither option @samp{-A} nor option @samp{-r} is
selected, the last parameter of each @code{\xx} call is inhibited.
Option @samp{-M} can be used to change @samp{xx} to another macro
name.

In this output format, some special characters, like @kbd{$}, @kbd{%},
@kbd{&}, @kbd{#} and @kbd{_} are automatically protected with a
backslash.  Curly brackets @kbd{@{}, @kbd{@}} are protected with a
backslash and a pair of dollar signs (to force mathematical mode).  The
backslash itself produces the sequence @code{\backslash@{@}}.
Circumflex and tilde diacritics produce the sequence @code{^\@{ @}} and
@code{~\@{ @}} respectively.  Other diacriticized characters of the
underlying character set produce an appropriate @TeX{} sequence as far
as possible.  The other non-graphical characters, like newline and tab,
and all other characters which are not part of @sc{ascii}, are merely
changed to exactly one space, with no special attempt to compress
consecutive spaces.  Let me know how to improve this special character
processing for @TeX{}.

@end table


@node Compatibility in ptx
@subsection The @sc{gnu} extensions to @code{ptx}

This version of @code{ptx} contains a few features which do not exist in
System V @code{ptx}.  These extra features are suppressed by using the
@samp{-G} command line option, unless overridden by other command line
options.  Some @sc{gnu} extensions cannot be recovered by overriding, so the
simple rule is to avoid @samp{-G} if you care about @sc{gnu} extensions.
Here are the differences between this program and System V @code{ptx}.

@itemize @bullet

@item
This program can read many input files at once, it always writes the
resulting concordance on standard output.  On the other hand, System V
@code{ptx} reads only one file and sends the result to standard output
or, if a second @var{file} parameter is given on the command, to that
@var{file}.

Having output parameters not introduced by options is a dangerous
practice which @sc{gnu} avoids as far as possible.  So, for using @code{ptx}
portably between @sc{gnu} and System V, you should always use it with a
single input file, and always expect the result on standard output.  You
might also want to automatically configure in a @samp{-G} option to
@code{ptx} calls in products using @code{ptx}, if the configurator finds
that the installed @code{ptx} accepts @samp{-G}.

@item
The only options available in System V @code{ptx} are options @samp{-b},
@samp{-f}, @samp{-g}, @samp{-i}, @samp{-o}, @samp{-r}, @samp{-t} and
@samp{-w}.  All other options are @sc{gnu} extensions and are not repeated in
this enumeration.  Moreover, some options have a slightly different
meaning when @sc{gnu} extensions are enabled, as explained below.

@item
By default, concordance output is not formatted for @code{troff} or
@code{nroff}.  It is rather formatted for a dumb terminal.  @code{troff}
or @code{nroff} output may still be selected through option @samp{-O}.

@item
Unless @samp{-R} option is used, the maximum reference width is
subtracted from the total output line width.  With @sc{gnu} extensions
disabled, width of references is not taken into account in the output
line width computations.

@item
All 256 characters, even @kbd{NUL}s, are always read and processed from
input file with no adverse effect, even if @sc{gnu} extensions are disabled.
However, System V @code{ptx} does not accept 8-bit characters, a few
control characters are rejected, and the tilde @kbd{~} is also rejected.

@item
Input line length is only limited by available memory, even if @sc{gnu}
extensions are disabled.  However, System V @code{ptx} processes only
the first 200 characters in each line.

@item
The break (non-word) characters default to be every character except all
letters of the underlying character set, diacriticized or not.  When @sc{gnu}
extensions are disabled, the break characters default to space, tab and
newline only.

@item
The program makes better use of output line width.  If @sc{gnu} extensions
are disabled, the program rather tries to imitate System V @code{ptx},
but still, there are some slight disposition glitches this program does
not completely reproduce.

@item
The user can specify both an Ignore file and an Only file.  This is not
allowed with System V @code{ptx}.

@end itemize


@node Operating on fields within a line
@chapter Operating on fields within a line

@menu
* cut invocation::              Print selected parts of lines.
* paste invocation::            Merge lines of files.
* join invocation::             Join lines on a common field.
@end menu


@node cut invocation
@section @code{cut}: Print selected parts of lines

@pindex cut
@code{cut} writes to standard output selected parts of each line of each
input file, or standard input if no files are given or for a file name of
@samp{-}.  Synopsis:

@example
cut [@var{option}]@dots{} [@var{file}]@dots{}
@end example

In the table which follows, the @var{byte-list}, @var{character-list},
and @var{field-list} are one or more numbers or ranges (two numbers
separated by a dash) separated by commas.  Bytes, characters, and
fields are numbered starting at 1.  Incomplete ranges may be
given: @samp{-@var{m}} means @samp{1-@var{m}}; @samp{@var{n}-} means
@samp{@var{n}} through end of line or last field.

The program accepts the following options.  Also see @ref{Common
options}.

@table @samp

@item -b @var{byte-list}
@itemx --bytes=@var{byte-list}
@opindex -b
@opindex --bytes
Print only the bytes in positions listed in @var{byte-list}.  Tabs and
backspaces are treated like any other character; they take up 1 byte.

@item -c @var{character-list}
@itemx --characters=@var{character-list}
@opindex -c
@opindex --characters
Print only characters in positions listed in @var{character-list}.
The same as @samp{-b} for now, but internationalization will change
that.  Tabs and backspaces are treated like any other character; they
take up 1 character.

@item -f @var{field-list}
@itemx --fields=@var{field-list}
@opindex -f
@opindex --fields
Print only the fields listed in @var{field-list}.  Fields are
separated by a TAB character by default.
Also print any line that contains no delimiter character, unless
the @samp{--only-delimited} (@samp{-s}) option is specified

@item -d @var{input_delim_byte}
@itemx --delimiter=@var{input_delim_byte}
@opindex -d
@opindex --delimiter
For @samp{-f}, fields are separated in the input by the first character
in @var{input_delim_byte} (default is TAB).

@item -n
@opindex -n
Do not split multi-byte characters (no-op for now).

@item -s
@itemx --only-delimited
@opindex -s
@opindex --only-delimited
For @samp{-f}, do not print lines that do not contain the field separator
character.

@itemx --output-delimiter=@var{output_delim_string}
@opindex --output-delimiter
For @samp{-f}, output fields are separated by @var{output_delim_string}.
The default is to use the input delimiter.


@end table


@node paste invocation
@section @code{paste}: Merge lines of files

@pindex paste
@cindex merging files

@code{paste} writes to standard output lines consisting of sequentially
corresponding lines of each given file, separated by a TAB character.
Standard input is used for a file name of @samp{-} or if no input files
are given.

Synopsis:

@example
paste [@var{option}]@dots{} [@var{file}]@dots{}
@end example

The program accepts the following options.  Also see @ref{Common options}.

@table @samp

@item -s
@itemx --serial
@opindex -s
@opindex --serial
Paste the lines of one file at a time rather than one line from each
file.

@item -d @var{delim-list}
@itemx --delimiters=@var{delim-list}
@opindex -d
@opindex --delimiters
Consecutively use the characters in @var{delim-list} instead of
TAB to separate merged lines.  When @var{delim-list} is
exhausted, start again at its beginning.

@end table


@node join invocation
@section @code{join}: Join lines on a common field

@pindex join
@cindex common field, joining on

@code{join} writes to standard output a line for each pair of input
lines that have identical join fields.  Synopsis:

@example
join [@var{option}]@dots{} @var{file1} @var{file2}
@end example

@vindex LC_COLLATE
Either @var{file1} or @var{file2} (but not both) can be @samp{-},
meaning standard input.  @var{file1} and @var{file2} should be already
sorted in increasing textual order on the join fields, using the
collating sequence specified by the @env{LC_COLLATE} locale.  Unless
the @samp{-t} option is given, the input should be sorted ignoring blanks at
the start of the join field, as in @code{sort -b}.  If the
@samp{--ignore-case} option is given, lines should be sorted without
regard to the case of characters in the join field, as in @code{sort -f}.

The defaults are: the join field is the first field in each line;
fields in the input are separated by one or more blanks, with leading
blanks on the line ignored; fields in the output are separated by a
space; each output line consists of the join field, the remaining
fields from @var{file1}, then the remaining fields from @var{file2}.

The program accepts the following options.  Also see @ref{Common options}.

@table @samp

@item -a @var{file-number}
@opindex -a
Print a line for each unpairable line in file @var{file-number} (either
@samp{1} or @samp{2}), in addition to the normal output.

@item -e @var{string}
@opindex -e
Replace those output fields that are missing in the input with
@var{string}.

@item -i
@itemx --ignore-case
@opindex -i
@opindex --ignore-case
Ignore differences in case when comparing keys.
With this option, the lines of the input files must be ordered in the same way.
Use @samp{sort -f} to produce this ordering.

@item -1 @var{field}
@itemx -j1 @var{field}
@opindex -1
@opindex -j1
Join on field @var{field} (a positive integer) of file 1.

@item -2 @var{field}
@itemx -j2 @var{field}
@opindex -2
@opindex -j2
Join on field @var{field} (a positive integer) of file 2.

@item -j @var{field}
Equivalent to @samp{-1 @var{field} -2 @var{field}}.

@item -o @var{field-list}@dots{}
Construct each output line according to the format in @var{field-list}.
Each element in @var{field-list} is either the single character @samp{0} or
has the form @var{m.n} where the file number, @var{m}, is @samp{1} or
@samp{2} and @var{n} is a positive field number.

A field specification of @samp{0} denotes the join field.
In most cases, the functionality of the @samp{0} field spec
may be reproduced using the explicit @var{m.n} that corresponds
to the join field.  However, when printing unpairable lines
(using either of the @samp{-a} or @samp{-v} options), there is no way
to specify the join field using @var{m.n} in @var{field-list}
if there are unpairable lines in both files.
To give @code{join} that functionality, @sc{posix} invented the @samp{0}
field specification notation.

The elements in @var{field-list}
are separated by commas or blanks.  Multiple @var{field-list}
arguments can be given after a single @samp{-o} option; the values
of all lists given with @samp{-o} are concatenated together.
All output lines -- including those printed because of any -a or -v
option -- are subject to the specified @var{field-list}.

@item -t @var{char}
Use character @var{char} as the input and output field separator.

@item -v @var{file-number}
Print a line for each unpairable line in file @var{file-number}
(either @samp{1} or @samp{2}), instead of the normal output.

@end table

In addition, when @sc{gnu} @code{join} is invoked with exactly one argument,
options @samp{--help} and @samp{--version} are recognized.  @xref{Common
options}.


@node Operating on characters
@chapter Operating on characters

@cindex operating on characters

This commands operate on individual characters.

@menu
* tr invocation::               Translate, squeeze, and/or delete characters.
* expand invocation::           Convert tabs to spaces.
* unexpand invocation::         Convert spaces to tabs.
@end menu


@node tr invocation
@section @code{tr}: Translate, squeeze, and/or delete characters

@pindex tr

Synopsis:

@example
tr [@var{option}]@dots{} @var{set1} [@var{set2}]
@end example

@code{tr} copies standard input to standard output, performing
one of the following operations:

@itemize @bullet
@item
translate, and optionally squeeze repeated characters in the result,
@item
squeeze repeated characters,
@item
delete characters,
@item
delete characters, then squeeze repeated characters from the result.
@end itemize

The @var{set1} and (if given) @var{set2} arguments define ordered
sets of characters, referred to below as @var{set1} and @var{set2}.  These
sets are the characters of the input that @code{tr} operates on.
The @samp{--complement} (@samp{-c}) option replaces @var{set1} with its
complement (all of the characters that are not in @var{set1}).

@menu
* Character sets::              Specifying sets of characters.
* Translating::                 Changing one characters to another.
* Squeezing::                   Squeezing repeats and deleting.
* Warnings in tr::              Warning messages.
@end menu


@node Character sets
@subsection Specifying sets of characters

@cindex specifying sets of characters

The format of the @var{set1} and @var{set2} arguments resembles
the format of regular expressions; however, they are not regular
expressions, only lists of characters.  Most characters simply
represent themselves in these strings, but the strings can contain
the shorthands listed below, for convenience.  Some of them can be
used only in @var{set1} or @var{set2}, as noted below.

@table @asis

@item Backslash escapes
@cindex backslash escapes

A backslash followed by a character not listed below causes an error
message.

@table @samp
@item \a
Control-G.
@item \b
Control-H.
@item \f
Control-L.
@item \n
Control-J.
@item \r
Control-M.
@item \t
Control-I.
@item \v
Control-K.
@item \@var{ooo}
The character with the value given by @var{ooo}, which is 1 to 3
octal digits,
@item \\
A backslash.
@end table

@item Ranges
@cindex ranges

The notation @samp{@var{m}-@var{n}} expands to all of the characters
from @var{m} through @var{n}, in ascending order.  @var{m} should
collate before @var{n}; if it doesn't, an error results.  As an example,
@samp{0-9} is the same as @samp{0123456789}.

@sc{gnu} @code{tr} does not support the System V syntax that uses square
brackets to enclose ranges.  Translations specified in that format
sometimes work as expected, since the brackets are often transliterated
to themselves.  However, they should be avoided because they sometimes
behave unexpectedly.  For example, @samp{tr -d '[0-9]'} deletes brackets
as well as digits.

Many historically common and even accepted uses of ranges are not
portable.  For example, on @sc{ebcdic} hosts using the @samp{A-Z}
range will not do what most would expect because @samp{A} through @samp{Z}
are not contiguous as they are in @sc{ascii}.
If you can rely on a @sc{posix} compliant version of @code{tr}, then
the best way to work around this is to use character classes (see below).
Otherwise, it is most portable (and most ugly) to enumerate the members
of the ranges.

@item Repeated characters
@cindex repeated characters

The notation @samp{[@var{c}*@var{n}]} in @var{set2} expands to @var{n}
copies of character @var{c}.  Thus, @samp{[y*6]} is the same as
@samp{yyyyyy}.  The notation @samp{[@var{c}*]} in @var{string2} expands
to as many copies of @var{c} as are needed to make @var{set2} as long as
@var{set1}.  If @var{n} begins with @samp{0}, it is interpreted in
octal, otherwise in decimal.

@item Character classes
@cindex characters classes

The notation @samp{[:@var{class}:]} expands to all of the characters in
the (predefined) class @var{class}.  The characters expand in no
particular order, except for the @code{upper} and @code{lower} classes,
which expand in ascending order.  When the @samp{--delete} (@samp{-d})
and @samp{--squeeze-repeats} (@samp{-s}) options are both given, any
character class can be used in @var{set2}.  Otherwise, only the
character classes @code{lower} and @code{upper} are accepted in
@var{set2}, and then only if the corresponding character class
(@code{upper} and @code{lower}, respectively) is specified in the same
relative position in @var{set1}.  Doing this specifies case conversion.
The class names are given below; an error results when an invalid class
name is given.

@table @code
@item alnum
@opindex alnum
Letters and digits.
@item alpha
@opindex alpha
Letters.
@item blank
@opindex blank
Horizontal whitespace.
@item cntrl
@opindex cntrl
Control characters.
@item digit
@opindex digit
Digits.
@item graph
@opindex graph
Printable characters, not including space.
@item lower
@opindex lower
Lowercase letters.
@item print
@opindex print
Printable characters, including space.
@item punct
@opindex punct
Punctuation characters.
@item space
@opindex space
Horizontal or vertical whitespace.
@item upper
@opindex upper
Uppercase letters.
@item xdigit
@opindex xdigit
Hexadecimal digits.
@end table

@item Equivalence classes
@cindex equivalence classes

The syntax @samp{[=@var{c}=]} expands to all of the characters that are
equivalent to @var{c}, in no particular order.  Equivalence classes are
a relatively recent invention intended to support non-English alphabets.
But there seems to be no standard way to define them or determine their
contents.  Therefore, they are not fully implemented in @sc{gnu} @code{tr};
each character's equivalence class consists only of that character,
which is of no particular use.

@end table


@node Translating
@subsection Translating

@cindex translating characters

@code{tr} performs translation when @var{set1} and @var{set2} are
both given and the @samp{--delete} (@samp{-d}) option is not given.
@code{tr} translates each character of its input that is in @var{set1}
to the corresponding character in @var{set2}.  Characters not in
@var{set1} are passed through unchanged.  When a character appears more
than once in @var{set1} and the corresponding characters in @var{set2}
are not all the same, only the final one is used.  For example, these
two commands are equivalent:

@example
tr aaa xyz
tr a z
@end example

A common use of @code{tr} is to convert lowercase characters to
uppercase.  This can be done in many ways.  Here are three of them:

@example
tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
tr a-z A-Z
tr '[:lower:]' '[:upper:]'
@end example

@noindent
But note that using ranges like @code{a-z} above is not portable.

When @code{tr} is performing translation, @var{set1} and @var{set2}
typically have the same length.  If @var{set1} is shorter than
@var{set2}, the extra characters at the end of @var{set2} are ignored.

On the other hand, making @var{set1} longer than @var{set2} is not
portable; @sc{posix.2} says that the result is undefined.  In this situation,
BSD @code{tr} pads @var{set2} to the length of @var{set1} by repeating
the last character of @var{set2} as many times as necessary.  System V
@code{tr} truncates @var{set1} to the length of @var{set2}.

By default, @sc{gnu} @code{tr} handles this case like BSD @code{tr}.  When
the @samp{--truncate-set1} (@samp{-t}) option is given, @sc{gnu} @code{tr}
handles this case like the System V @code{tr} instead.  This option is
ignored for operations other than translation.

Acting like System V @code{tr} in this case breaks the relatively common
BSD idiom:

@example
tr -cs A-Za-z0-9 '\012'
@end example

@noindent
because it converts only zero bytes (the first element in the
complement of @var{set1}), rather than all non-alphanumerics, to
newlines.

@noindent
By the way, the above idiom is not portable because it uses ranges.
Assuming a @sc{posix} compliant @code{tr}, here is a better way to write it:

@example
tr -cs '[:alnum:]' '[\n*]'
@end example


@node Squeezing
@subsection Squeezing repeats and deleting

@cindex squeezing repeat characters
@cindex deleting characters

When given just the @samp{--delete} (@samp{-d}) option, @code{tr}
removes any input characters that are in @var{set1}.

When given just the @samp{--squeeze-repeats} (@samp{-s}) option,
@code{tr} replaces each input sequence of a repeated character that
is in @var{set1} with a single occurrence of that character.

When given both @samp{--delete} and @samp{--squeeze-repeats}, @code{tr}
first performs any deletions using @var{set1}, then squeezes repeats
from any remaining characters using @var{set2}.

The @samp{--squeeze-repeats} option may also be used when translating,
in which case @code{tr} first performs translation, then squeezes
repeats from any remaining characters using @var{set2}.

Here are some examples to illustrate various combinations of options:

@itemize @bullet

@item
Remove all zero bytes:

@example
tr -d '\000'
@end example

@item
Put all words on lines by themselves.  This converts all
non-alphanumeric characters to newlines, then squeezes each string
of repeated newlines into a single newline:

@example
tr -cs '[:alnum:]' '[\n*]'
@end example

@item
Convert each sequence of repeated newlines to a single newline:

@example
tr -s '\n'
@end example

@item
Find doubled occurrences of words in a document.
For example, people often write ``the the'' with the duplicated words
separated by a newline.  The bourne shell script below works first
by converting each sequence of punctuation and blank characters to a
single newline.  That puts each ``word'' on a line by itself.
Next it maps all uppercase characters to lower case, and finally it
runs @code{uniq} with the @samp{-d} option to print out only the words
that were adjacent duplicates.

@example
#!/bin/sh
cat "$@@" \
  | tr -s '[:punct:][:blank:]' '\n' \
  | tr '[:upper:]' '[:lower:]' \
  | uniq -d
@end example

@item
Deleting a small set of characters is usually straightforward.  For example,
to remove all @samp{a}s, @samp{x}s, and @samp{M}s you would do this:

@example
tr -d axM
@end example

However, when @samp{-} is one of those characters, it can be tricky because
@samp{-} has special meanings.  Performing the same task as above but also
removing all @samp{-} characters, we might try @code{tr -d -axM}, but
that would fail because @code{tr} would try to interpret @samp{-a} as
a command-line option.  Alternatively, we could try putting the hyphen
inside the string, @code{tr -d a-xM}, but that wouldn't work either because
it would make @code{tr} interpret @code{a-x} as the range of characters
@samp{a}@dots{}@samp{x} rather than the three.
One way to solve the problem is to put the hyphen at the end of the list
of characters:

@example
tr -d axM-
@end example

More generally, use the character class notation @code{[=c=]}
with @samp{-} (or any other character) in place of the @samp{c}:

@example
tr -d '[=-=]axM'
@end example

Note how single quotes are used in the above example to protect the
square brackets from interpretation by a shell.

@end itemize


@node Warnings in tr
@subsection Warning messages

@vindex POSIXLY_CORRECT
Setting the environment variable @env{POSIXLY_CORRECT} turns off the
following warning and error messages, for strict compliance with
@sc{posix.2}.  Otherwise, the following diagnostics are issued:

@enumerate

@item
When the @samp{--delete} option is given but @samp{--squeeze-repeats}
is not, and @var{set2} is given, @sc{gnu} @code{tr} by default prints
a usage message and exits, because @var{set2} would not be used.
The @sc{posix} specification says that @var{set2} must be ignored in
this case. Silently ignoring arguments is a bad idea.

@item
When an ambiguous octal escape is given.  For example, @samp{\400}
is actually @samp{\40} followed by the digit @samp{0}, because the
value 400 octal does not fit into a single byte.

@end enumerate

@sc{gnu} @code{tr} does not provide complete BSD or System V compatibility.
For example, it is impossible to disable interpretation of the @sc{posix}
constructs @samp{[:alpha:]}, @samp{[=c=]}, and @samp{[c*10]}.  Also, @sc{gnu}
@code{tr} does not delete zero bytes automatically, unlike traditional
Unix versions, which provide no way to preserve zero bytes.


@node expand invocation
@section @code{expand}: Convert tabs to spaces

@pindex expand
@cindex tabs to spaces, converting
@cindex converting tabs to spaces

@code{expand} writes the contents of each given @var{file}, or standard
input if none are given or for a @var{file} of @samp{-}, to standard
output, with tab characters converted to the appropriate number of
spaces.  Synopsis:

@example
expand [@var{option}]@dots{} [@var{file}]@dots{}
@end example

By default, @code{expand} converts all tabs to spaces.  It preserves
backspace characters in the output; they decrement the column count for
tab calculations.  The default action is equivalent to @samp{-8} (set
tabs every 8 columns).

The program accepts the following options.  Also see @ref{Common options}.

@table @samp

@item -@var{tab1}[,@var{tab2}]@dots{}
@itemx -t @var{tab1}[,@var{tab2}]@dots{}
@itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
@opindex -@var{tab}
@opindex -t
@opindex --tabs
@cindex tabstops, setting
If only one tab stop is given, set the tabs @var{tab1} spaces apart
(default is 8).  Otherwise, set the tabs at columns @var{tab1},
@var{tab2}, @dots{} (numbered from 0), and replace any tabs beyond the
last tabstop given with single spaces.  If the tabstops are specified
with the @samp{-t} or @samp{--tabs} option, they can be separated by
blanks as well as by commas.

@item -i
@itemx --initial
@opindex -i
@opindex --initial
@cindex initial tabs, converting
Only convert initial tabs (those that precede all non-space or non-tab
characters) on each line to spaces.

@end table


@node unexpand invocation
@section @code{unexpand}: Convert spaces to tabs

@pindex unexpand

@code{unexpand} writes the contents of each given @var{file}, or
standard input if none are given or for a @var{file} of @samp{-}, to
standard output, with strings of two or more space or tab characters
converted to as many tabs as possible followed by as many spaces as are
needed.  Synopsis:

@example
unexpand [@var{option}]@dots{} [@var{file}]@dots{}
@end example

By default, @code{unexpand} converts only initial spaces and tabs (those
that precede all non space or tab characters) on each line.  It
preserves backspace characters in the output; they decrement the column
count for tab calculations.  By default, tabs are set at every 8th
column.

The program accepts the following options.  Also see @ref{Common options}.

@table @samp

@item -@var{tab1}[,@var{tab2}]@dots{}
@itemx -t @var{tab1}[,@var{tab2}]@dots{}
@itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
@opindex -@var{tab}
@opindex -t
@opindex --tabs
If only one tab stop is given, set the tabs @var{tab1} spaces apart
instead of the default 8.  Otherwise, set the tabs at columns
@var{tab1}, @var{tab2}, @dots{} (numbered from 0), and leave spaces and
tabs beyond the tabstops given unchanged.  If the tabstops are specified
with the @samp{-t} or @samp{--tabs} option, they can be separated by
blanks as well as by commas.  This option implies the @samp{-a} option.

@item -a
@itemx --all
@opindex -a
@opindex --all
Convert all strings of two or more spaces or tabs, not just initial
ones, to tabs.

@end table

@c              What's GNU?
@c              Arnold Robbins
@node Opening the software toolbox
@chapter Opening the software toolbox

This chapter originally appeared in @cite{Linux Journal}, volume 1,
number 2, in the @cite{What's GNU?} column. It was written by Arnold
Robbins.

@menu
* Toolbox introduction::        Toolbox introduction
* I/O redirection::             I/O redirection
* The who command::             The @code{who} command
* The cut command::             The @code{cut} command
* The sort command::            The @code{sort} command
* The uniq command::            The @code{uniq} command
* Putting the tools together::  Putting the tools together
@end menu


@node Toolbox introduction
@unnumberedsec Toolbox introduction

This month's column is only peripherally related to the @sc{gnu} Project, in
that it describes a number of the @sc{gnu} tools on your Linux system and how
they might be used.  What it's really about is the ``Software Tools'' philosophy
of program development and usage.

The software tools philosophy was an important and integral concept
in the initial design and development of Unix (of which Linux and @sc{gnu} are
essentially clones).  Unfortunately, in the modern day press of
Internetworking and flashy GUIs, it seems to have fallen by the
wayside.  This is a shame, since it provides a powerful mental model
for solving many kinds of problems.

Many people carry a Swiss Army knife around in their pants pockets (or
purse).  A Swiss Army knife is a handy tool to have: it has several knife
blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps
a number of other things on it.  For the everyday, small miscellaneous jobs
where you need a simple, general purpose tool, it's just the thing.

On the other hand, an experienced carpenter doesn't build a house using
a Swiss Army knife.  Instead, he has a toolbox chock full of specialized
tools---a saw, a hammer, a screwdriver, a plane, and so on.  And he knows
exactly when and where to use each tool; you won't catch him hammering nails
with the handle of his screwdriver.

The Unix developers at Bell Labs were all professional programmers and trained
computer scientists.  They had found that while a one-size-fits-all program
might appeal to a user because there's only one program to use, in practice
such programs are

@enumerate a
@item
difficult to write,

@item
difficult to maintain and
debug, and

@item
difficult to extend to meet new situations.
@end enumerate

Instead, they felt that programs should be specialized tools.  In short, each
program ``should do one thing well.''  No more and no less.  Such programs are
simpler to design, write, and get right---they only do one thing.

Furthermore, they found that with the right machinery for hooking programs
together, that the whole was greater than the sum of the parts.  By combining
several special purpose programs, you could accomplish a specific task
that none of the programs was designed for, and accomplish it much more
quickly and easily than if you had to write a special purpose program.
We will see some (classic) examples of this further on in the column.
(An important additional point was that, if necessary, take a detour
and build any software tools you may need first, if you don't already
have something appropriate in the toolbox.)

@node I/O redirection
@unnumberedsec I/O redirection

Hopefully, you are familiar with the basics of I/O redirection in the
shell, in particular the concepts of ``standard input,'' ``standard output,''
and ``standard error''.  Briefly, ``standard input'' is a data source, where
data comes from.  A program should not need to either know or care if the
data source is a disk file, a keyboard, a magnetic tape, or even a punched
card reader.  Similarly, ``standard output'' is a data sink, where data goes
to.  The program should neither know nor care where this might be.
Programs that only read their standard input, do something to the data,
and then send it on, are called ``filters'', by analogy to filters in a
water pipeline.

With the Unix shell, it's very easy to set up data pipelines:

@smallexample
program_to_create_data | filter1 | .... | filterN > final.pretty.data
@end smallexample

We start out by creating the raw data; each filter applies some successive
transformation to the data, until by the time it comes out of the pipeline,
it is in the desired form.

This is fine and good for standard input and standard output.  Where does the
standard error come in to play?  Well, think about @code{filter1} in
the pipeline above.  What happens if it encounters an error in the data it
sees?  If it writes an error message to standard output, it will just
disappear down the pipeline into @code{filter2}'s input, and the
user will probably never see it.  So programs need a place where they can send
error messages so that the user will notice them.  This is standard error,
and it is usually connected to your console or window, even if you have
redirected standard output of your program away from your screen.

For filter programs to work together, the format of the data has to be
agreed upon.  The most straightforward and easiest format to use is simply
lines of text.  Unix data files are generally just streams of bytes, with
lines delimited by the @sc{ascii} @sc{lf} (Line Feed) character,
conventionally called a ``newline'' in the Unix literature. (This is
@code{'\n'} if you're a C programmer.)  This is the format used by all
the traditional filtering programs.  (Many earlier operating systems
had elaborate facilities and special purpose programs for managing
binary data.  Unix has always shied away from such things, under the
philosophy that it's easiest to simply be able to view and edit your
data with a text editor.)

OK, enough introduction. Let's take a look at some of the tools, and then
we'll see how to hook them together in interesting ways.   In the following
discussion, we will only present those command line options that interest
us.  As you should always do, double check your system documentation
for the full story.

@node The who command
@unnumberedsec The @code{who} command

The first program is the @code{who} command.  By itself, it generates a
list of the users who are currently logged in.  Although I'm writing
this on a single-user system, we'll pretend that several people are
logged in:

@example
$ who
arnold   console Jan 22 19:57
miriam   ttyp0   Jan 23 14:19(:0.0)
bill     ttyp1   Jan 21 09:32(:0.0)
arnold   ttyp2   Jan 23 20:48(:0.0)
@end example

Here, the @samp{$} is the usual shell prompt, at which I typed @code{who}.
There are three people logged in, and I am logged in twice.  On traditional
Unix systems, user names are never more than eight characters long.  This
little bit of trivia will be useful later.  The output of @code{who} is nice,
but the data is not all that exciting.

@node The cut command
@unnumberedsec The @code{cut} command

The next program we'll look at is the @code{cut} command.  This program
cuts out columns or fields of input data.  For example, we can tell it
to print just the login name and full name from the @file{/etc/passwd
file}.  The @file{/etc/passwd} file has seven fields, separated by
colons:

@example
arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/ksh
@end example

To get the first and fifth fields, we would use cut like this:

@example
$ cut -d: -f1,5 /etc/passwd
root:Operator
@dots{}
arnold:Arnold D. Robbins
miriam:Miriam A. Robbins
@dots{}
@end example

With the @samp{-c} option, @code{cut} will cut out specific characters
(i.e., columns) in the input lines.  This command looks like it might be
useful for data filtering.


@node The sort command
@unnumberedsec The @code{sort} command

Next we'll look at the @code{sort} command.  This is one of the most
powerful commands on a Unix-style system; one that you will often find
yourself using when setting up fancy data plumbing. The @code{sort}
command reads and sorts each file named on the command line.  It then
merges the sorted data and writes it to standard output.  It will read
standard input if no files are given on the command line (thus
making it into a filter).  The sort is based on the character collating
sequence or based on user-supplied ordering criteria.


@node The uniq command
@unnumberedsec The @code{uniq} command

Finally (at least for now), we'll look at the @code{uniq} program.  When
sorting data, you will often end up with duplicate lines, lines that
are identical.  Usually, all you need is one instance of each line.
This is where @code{uniq} comes in. The @code{uniq} program reads its
standard input, which it expects to be sorted.  It only prints out one
copy of each duplicated line.  It does have several options.  Later on,
we'll use the @samp{-c} option, which prints each unique line, preceded
by a count of the number of times that line occurred in the input.


@node Putting the tools together
@unnumberedsec Putting the tools together

Now, let's suppose this is a large BBS system with dozens of users
logged in.  The management wants the SysOp to write a program that will
generate a sorted list of logged in users.  Furthermore, even if a user
is logged in multiple times, his or her name should only show up in the
output once.

The SysOp could sit down with the system documentation and write a C
program that did this. It would take perhaps a couple of hundred lines
of code and about two hours to write it, test it, and debug it.
However, knowing the software toolbox, the SysOp can instead start out
by generating just a list of logged on users:

@example
$ who | cut -c1-8
arnold
miriam
bill
arnold
@end example

Next, sort the list:

@example
$ who | cut -c1-8 | sort
arnold
arnold
bill
miriam
@end example

Finally, run the sorted list through @code{uniq}, to weed out duplicates:

@example
$ who | cut -c1-8 | sort | uniq
arnold
bill
miriam
@end example

The @code{sort} command actually has a @samp{-u} option that does what
@code{uniq} does. However, @code{uniq} has other uses for which one
cannot substitute @samp{sort -u}.

The SysOp puts this pipeline into a shell script, and makes it available for
all the users on the system:

@example
# cat > /usr/local/bin/listusers
who | cut -c1-8 | sort | uniq
^D
# chmod +x /usr/local/bin/listusers
@end example

There are four major points to note here.  First, with just four
programs, on one command line, the SysOp was able to save about two
hours worth of work.  Furthermore, the shell pipeline is just about as
efficient as the C program would be, and it is much more efficient in
terms of programmer time.  People time is much more expensive than
computer time, and in our modern ``there's never enough time to do
everything'' society, saving two hours of programmer time is no mean
feat.

Second, it is also important to emphasize that with the
@emph{combination} of the tools, it is possible to do a special
purpose job never imagined by the authors of the individual programs.

Third, it is also valuable to build up your pipeline in stages, as we did here.
This allows you to view the data at each stage in the pipeline, which helps
you acquire the confidence that you are indeed using these tools correctly.

Finally, by bundling the pipeline in a shell script, other users can use
your command, without having to remember the fancy plumbing you set up for
them. In terms of how you run them, shell scripts and compiled programs are
indistinguishable.

After the previous warm-up exercise, we'll look at two additional, more
complicated pipelines.  For them, we need to introduce two more tools.

The first is the @code{tr} command, which stands for ``transliterate.''
The @code{tr} command works on a character-by-character basis, changing
characters. Normally it is used for things like mapping upper case to
lower case:

@example
$ echo ThIs ExAmPlE HaS MIXED case! | tr '[:upper:]' '[:lower:]'
this example has mixed case!
@end example

There are several options of interest:

@table @samp
@item -c
work on the complement of the listed characters, i.e.,
operations apply to characters not in the given set

@item -d
delete characters in the first set from the output

@item -s
squeeze repeated characters in the output into just one character.
@end table

We will be using all three options in a moment.

The other command we'll look at is @code{comm}.  The @code{comm}
command takes two sorted input files as input data, and prints out the
files' lines in three columns.  The output columns are the data lines
unique to the first file, the data lines unique to the second file, and
the data lines that are common to both.  The @samp{-1}, @samp{-2}, and
@samp{-3} command line options omit the respective columns. (This is
non-intuitive and takes a little getting used to.)  For example:

@example
$ cat f1
11111
22222
33333
44444
$ cat f2
00000
22222
33333
55555
$ comm f1 f2
        00000
11111
                22222
                33333
44444
        55555
@end example

The single dash as a filename tells @code{comm} to read standard input
instead of a regular file.

Now we're ready to build a fancy pipeline.  The first application is a word
frequency counter.  This helps an author determine if he or she is over-using
certain words.

The first step is to change the case of all the letters in our input file
to one case.  ``The'' and ``the'' are the same word when doing counting.

@example
$ tr '[:upper:]' '[:lower:]' < whats.gnu | ...
@end example

The next step is to get rid of punctuation.  Quoted words and unquoted words
should be treated identically; it's easiest to just get the punctuation out of
the way.

@smallexample
$ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \012' | ...
@end smallexample

The second @code{tr} command operates on the complement of the listed
characters, which are all the letters, the digits, the underscore, and
the blank.  The @samp{\012} represents the newline character; it has to
be left alone.  (The @sc{ascii} tab character should also be included for
good measure in a production script.)

At this point, we have data consisting of words separated by blank space.
The words only contain alphanumeric characters (and the underscore).  The
next step is break the data apart so that we have one word per line. This
makes the counting operation much easier, as we will see shortly.

@smallexample
$ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \012' |
> tr -s ' ' '\012' | ...
@end smallexample

This command turns blanks into newlines.  The @samp{-s} option squeezes
multiple newline characters in the output into just one.  This helps us
avoid blank lines. (The @samp{>} is the shell's ``secondary prompt.''
This is what the shell prints when it notices you haven't finished
typing in all of a command.)

We now have data consisting of one word per line, no punctuation, all one
case.  We're ready to count each word:

@smallexample
$ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \012' |
> tr -s ' ' '\012' | sort | uniq -c | ...
@end smallexample

At this point, the data might look something like this:

@example
  60 a
   2 able
   6 about
   1 above
   2 accomplish
   1 acquire
   1 actually
   2 additional
@end example

The output is sorted by word, not by count!  What we want is the most
frequently used words first.  Fortunately, this is easy to accomplish,
with the help of two more @code{sort} options:

@table @samp
@item -n
do a numeric sort, not a textual one

@item -r
reverse the order of the sort
@end table

The final pipeline looks like this:

@smallexample
$ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \012' |
> tr -s ' ' '\012' | sort | uniq -c | sort -nr
 156 the
  60 a
  58 to
  51 of
  51 and
 ...
@end smallexample

Whew!  That's a lot to digest.  Yet, the same principles apply. With six
commands, on two lines (really one long one split for convenience), we've
created a program that does something interesting and useful, in much
less time than we could have written a C program to do the same thing.

A minor modification to the above pipeline can give us a simple spelling
checker!  To determine if you've spelled a word correctly, all you have to
do is look it up in a dictionary.  If it is not there, then chances are
that your spelling is incorrect.  So, we need a dictionary.  If you
have the Slackware Linux distribution, you have the file
@file{/usr/lib/ispell/ispell.words}, which is a sorted, 38,400 word
dictionary.

Now, how to compare our file with the dictionary?  As before, we generate
a sorted list of words, one per line:

@smallexample
$ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \012' |
> tr -s ' ' '\012' | sort -u | ...
@end smallexample

Now, all we need is a list of words that are @emph{not} in the
dictionary.  Here is where the @code{comm} command comes in.

@smallexample
$ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \012' |
> tr -s ' ' '\012' | sort -u |
> comm -23 - /usr/lib/ispell/ispell.words
@end smallexample

The @samp{-2} and @samp{-3} options eliminate lines that are only in the
dictionary (the second file), and lines that are in both files.  Lines
only in the first file (standard input, our stream of words), are
words that are not in the dictionary.  These are likely candidates for
spelling errors.  This pipeline was the first cut at a production
spelling checker on Unix.

There are some other tools that deserve brief mention.

@table @code
@item grep
search files for text that matches a regular expression

@item egrep
like @code{grep}, but with more powerful regular expressions

@item wc
count lines, words, characters

@item tee
a T-fitting for data pipes, copies data to files and to standard output

@item sed
the stream editor, an advanced tool

@item awk
a data manipulation language, another advanced tool
@end table

The software tools philosophy also espoused the following bit of
advice: ``Let someone else do the hard part.'' This means, take
something that gives you most of what you need, and then massage it the
rest of the way until it's in the form that you want.

To summarize:

@enumerate 1
@item
Each program should do one thing well. No more, no less.

@item
Combining programs with appropriate plumbing leads to results where
the whole is greater than the sum of the parts.  It also leads to novel
uses of programs that the authors might never have imagined.

@item
Programs should never print extraneous header or trailer data, since these
could get sent on down a pipeline. (A point we didn't mention earlier.)

@item
Let someone else do the hard part.

@item
Know your toolbox! Use each program appropriately. If you don't have an
appropriate tool, build one.
@end enumerate

As of this writing, all the programs we've discussed are available via
anonymous @code{ftp} from @code{prep.ai.mit.edu} as
@file{/pub/gnu/textutils-1.9.tar.gz}.@footnote{Version 1.9 was current
when this column was written. Check the nearest @sc{gnu} archive for the
current version.  The main @sc{gnu} FTP site is now @code{ftp.gnu.org}.}

None of what I have presented in this column is new. The Software Tools
philosophy was first introduced in the book @cite{Software Tools},
by Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN
0-201-03669-X).   This book showed how to write and use software
tools.   It was written in 1976, using a preprocessor for FORTRAN named
@code{ratfor} (RATional FORtran).  At the time, C was not as ubiquitous
as it is now; FORTRAN was.  The last chapter presented a @code{ratfor}
to FORTRAN processor, written in @code{ratfor}. @code{ratfor} looks an
awful lot like C; if you know C, you won't have any problem following
the code.

In 1981, the book was updated and made available as @cite{Software
Tools in Pascal} (Addison-Wesley, ISBN 0-201-10342-7).  Both books
remain in print, and are well worth reading if you're a programmer.
They certainly made a major change in how I view programming.

Initially, the programs in both books were available (on 9-track tape)
from Addison-Wesley.  Unfortunately, this is no longer the case,
although you might be able to find copies floating around the Internet.
For a number of years, there was an active Software Tools Users Group,
whose members had ported the original @code{ratfor} programs to essentially
every computer system with a FORTRAN compiler.  The popularity of the
group waned in the middle '80s as Unix began to spread beyond universities.

With the current proliferation of @sc{gnu} code and other clones of Unix
programs, these programs now receive little attention; modern C versions are
much more efficient and do more than these programs do.  Nevertheless, as
exposition of good programming style, and evangelism for a still-valuable
philosophy, these books are unparalleled, and I recommend them highly.

Acknowledgment: I would like to express my gratitude to Brian Kernighan
of Bell Labs, the original Software Toolsmith, for reviewing this column.


@node Index
@unnumbered Index

@printindex cp

@contents
@bye

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