grep.texi

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@cindex changing name of standard input
Displays input actually coming from standard input as input coming from file
@var{LABEL}. This is especially useful for tools like zgrep, e.g.
@command{gzip -cd foo.gz |grep --label=foo something}

@item -L
@itemx --files-without-match
@opindex -L
@opindex --files-without-match
@cindex files which don't match
Suppress normal output; instead print the name of each input
file from which no output would normally have been printed.
The scanning of every file will stop on the first match.

@item -a
@itemx --text
@opindex -a
@opindex --text
@cindex suppress binary data
@cindex binary files
Process a binary file as if it were text; this is equivalent to the
@samp{--binary-files=text} option.

@item -I
Process a binary file as if it did not contain matching data; this is
equivalent to the @samp{--binary-files=without-match} option.

@item -w
@itemx --word-regexp
@opindex -w
@opindex --word-regexp
@cindex matching whole words
Select only those lines containing matches that form
whole words.  The test is that the matching substring
must either be at the beginning of the line, or preceded
by a non-word constituent character.  Similarly,
it must be either at the end of the line or followed by
a non-word constituent character.  Word-constituent
characters are letters, digits, and the underscore.

@item -r
@itemx -R
@itemx --recursive
@opindex -r
@opindex --recursive
@cindex recursive search
@cindex searching directory trees
For each directory mentioned in the command line, read and process all
files in that directory, recursively.  This is the same as the
@samp{--directories=recurse} option.

@item --include=@var{file_pattern}
@opindex --include
@cindex include files
@cindex searching directory trees
When processing directories recursively, only files matching @var{file_pattern}
will be search.

@item --exclude=@var{file_pattern}
@opindex --exclude
@cindex exclude files
@cindex searching directory trees
When processing directories recursively, skip files matching @var{file_pattern}.

@item -m @var{num}
@itemx --max-count=@var{num}
@opindex -m
@opindex --max-count
@cindex max-count
Stop reading a file after @var{num} matching lines.  If the input is
standard input from a regular file, and @var{num} matching lines are
output, @command{grep} ensures that the standard input is positioned to
just after the last matching line before exiting, regardless of the
presence of trailing context lines.  This enables a calling process
to resume a search.  For example, the following shell script makes use
of it:

@example
while grep -m 1 PATTERN
do
  echo xxxx
done < FILE
@end example

But the following probably will not work because a pipe is not a regular
file:

@example
# This probably will not work.
cat FILE |
while grep -m 1 PATTERN
do
  echo xxxx
done
@end example

When @command{grep} stops after NUM matching lines, it outputs
any trailing context lines. Since context does not include matching
lines, @command{grep} will stop when it encounters another matching line.
When the @samp{-c} or @samp{--count} option is also used,
@command{grep} does not output a count greater than @var{num}.
When the @samp{-v} or @samp{--invert-match} option is
also used, @command{grep} stops after outputting @var{num}
non-matching lines.

@item -y
@opindex -y
@cindex case insensitive search, obsolete option
Obsolete synonym for @samp{-i}.

@item -U
@itemx --binary
@opindex -U
@opindex --binary
@cindex DOS/Windows binary files
@cindex binary files, DOS/Windows
Treat the file(s) as binary.  By default, under @sc{ms-dos}
and MS-Windows, @command{grep} guesses the file type by looking
at the contents of the first 32kB read from the file.
If @command{grep} decides the file is a text file, it strips the
@code{CR} characters from the original file contents (to make
regular expressions with @code{^} and @code{$} work correctly).
Specifying @samp{-U} overrules this guesswork, causing all
files to be read and passed to the matching mechanism
verbatim; if the file is a text file with @code{CR/LF} pairs
at the end of each line, this will cause some regular
expressions to fail.  This option has no effect on platforms other than
@sc{ms-dos} and MS-Windows.

@item -u
@itemx --unix-byte-offsets
@opindex -u
@opindex --unix-byte-offsets
@cindex DOS byte offsets
@cindex byte offsets, on DOS/Windows
Report Unix-style byte offsets.  This switch causes
@command{grep} to report byte offsets as if the file were Unix style
text file, i.e., the byte offsets ignore the @code{CR} characters which were
stripped.  This will produce results identical to running @command{grep} on
a Unix machine.  This option has no effect unless @samp{-b}
option is also used; it has no effect on platforms other than @sc{ms-dos} and
MS-Windows.

@item --mmap
@opindex --mmap
@cindex memory mapped input
If possible, use the @code{mmap} system call to read input, instead of
the default @code{read} system call.  In some situations, @samp{--mmap}
yields better performance.  However, @samp{--mmap} can cause undefined
behavior (including core dumps) if an input file shrinks while
@command{grep} is operating, or if an I/O error occurs.

@item -Z
@itemx --null
@opindex -Z
@opindex --null
@cindex zero-terminated file names
Output a zero byte (the @sc{ascii} @code{NUL} character) instead of the
character that normally follows a file name.  For example, @samp{grep
-lZ} outputs a zero byte after each file name instead of the usual
newline.  This option makes the output unambiguous, even in the presence
of file names containing unusual characters like newlines.  This option
can be used with commands like @samp{find -print0}, @samp{perl -0},
@samp{sort -z}, and @samp{xargs -0} to process arbitrary file names,
even those that contain newline characters.

@item -z
@itemx --null-data
@opindex -z
@opindex --null-data
@cindex zero-terminated lines
Treat the input as a set of lines, each terminated by a zero byte (the
@sc{ascii} @code{NUL} character) instead of a newline.  Like the @samp{-Z}
or @samp{--null} option, this option can be used with commands like
@samp{sort -z} to process arbitrary file names.

@end table

Several additional options control which variant of the @command{grep}
matching engine is used.  @xref{Grep Programs}.

@section Environment Variables

Grep's behavior is affected by the following environment variables.

A locale @code{LC_@var{foo}} is specified by examining the three
environment variables @env{LC_ALL}, @env{LC_@var{foo}}, and @env{LANG},
in that order.  The first of these variables that is set specifies the
locale.  For example, if @env{LC_ALL} is not set, but @env{LC_MESSAGES}
is set to @samp{pt_BR}, then Brazilian Portuguese is used for the
@code{LC_MESSAGES} locale.  The C locale is used if none of these
environment variables are set, or if the locale catalog is not
installed, or if @command{grep} was not compiled with national language
support (@sc{nls}).

@cindex environment variables

@table @env

@item GREP_OPTIONS
@vindex GREP_OPTIONS
@cindex default options environment variable
This variable specifies default options to be placed in front of any
explicit options.  For example, if @code{GREP_OPTIONS} is
@samp{--binary-files=without-match --directories=skip}, @command{grep}
behaves as if the two options @samp{--binary-files=without-match} and
@samp{--directories=skip} had been specified before
any explicit options.  Option specifications are separated by
whitespace.  A backslash escapes the next character, so it can be used to
specify an option containing whitespace or a backslash.

@item GREP_COLOR
@vindex GREP_COLOR
@cindex highlight markers
This variable specifies the surrounding markers use to highlight the matching
text.  The default is control ascii red.

@item LC_ALL
@itemx LC_COLLATE
@itemx LANG
@vindex LC_ALL
@vindex LC_COLLATE
@vindex LANG
@cindex character type
@cindex national language support
@cindex NLS
These variables specify the @code{LC_COLLATE} locale, which determines
the collating sequence used to interpret range expressions like
@samp{[a-z]}.

@item LC_ALL
@itemx LC_CTYPE
@itemx LANG
@vindex LC_ALL
@vindex LC_CTYPE
@vindex LANG
@cindex character type
@cindex national language support
@cindex NLS
These variables specify the @code{LC_CTYPE} locale, which determines the
type of characters, e.g., which characters are whitespace.

@item LC_ALL
@itemx LC_MESSAGES
@itemx LANG
@vindex LC_ALL
@vindex LC_MESSAGES
@vindex LANG
@cindex language of messages
@cindex message language
@cindex national language support
@cindex NLS
@cindex translation of message language
These variables specify the @code{LC_MESSAGES} locale, which determines
the language that @command{grep} uses for messages.  The default C
locale uses American English messages.

@item POSIXLY_CORRECT
@vindex POSIXLY_CORRECT
If set, @command{grep} behaves as @sc{posix.2} requires; otherwise,
@command{grep} behaves more like other @sc{gnu} programs.  @sc{posix.2}
requires that options that
follow file names must be treated as file names; by default, such
options are permuted to the front of the operand list and are treated as
options.  Also, @sc{posix.2} requires that unrecognized options be
diagnosed as
``illegal'', but since they are not really against the law the default
is to diagnose them as ``invalid''.  @code{POSIXLY_CORRECT} also
disables @code{_@var{N}_GNU_nonoption_argv_flags_}, described below.

@item _@var{N}_GNU_nonoption_argv_flags_
@vindex _@var{N}_GNU_nonoption_argv_flags_
(Here @code{@var{N}} is @command{grep}'s numeric process ID.)  If the
@var{i}th character of this environment variable's value is @samp{1}, do
not consider the @var{i}th operand of @command{grep} to be an option, even if
it appears to be one.  A shell can put this variable in the environment
for each command it runs, specifying which operands are the results of
file name wildcard expansion and therefore should not be treated as
options.  This behavior is available only with the @sc{gnu} C library, and
only when @code{POSIXLY_CORRECT} is not set.

@end table

@node Diagnostics
@chapter Diagnostics

Normally, exit status is 0 if selected lines are found and 1 otherwise.
But the exit status is 2 if an error occurred, unless the @option{-q} or
@option{--quiet} or @option{--silent} option is used and a selected line
is found.

@node Grep Programs
@chapter @command{grep} programs

@command{grep} searches the named input files (or standard input if no
files are named, or the file name @file{-} is given) for lines containing
a match to the given pattern.  By default, @command{grep} prints the
matching lines.  There are four major variants of @command{grep},
controlled by the following options.

@table @samp

@item -G
@itemx --basic-regexp
@opindex -G
@opindex --basic-regexp
@cindex matching basic regular expressions
Interpret the pattern as a basic regular expression.  This is the default.

@item -E
@itemx --extended-regexp
@opindex -E
@opindex --extended-regexp
@cindex matching extended regular expressions
Interpret the pattern as an extended regular expression.

@item -F
@itemx --fixed-strings
@opindex -F
@opindex --fixed-strings
@cindex matching fixed strings
Interpret the pattern as a list of fixed strings, separated
by newlines, any of which is to be matched.

@item -P
@itemx --perl-regexp
@opindex -P
@opindex --perl-regexp
@cindex matching Perl regular expressions
Interpret the pattern as a Perl regular expression.

@end table

In addition, two variant programs @sc{egrep} and @sc{fgrep} are available.
@sc{egrep} is the same as @samp{grep -E}.  @sc{fgrep} is the
same as @samp{grep -F}.

@node Regular Expressions
@chapter Regular Expressions
@cindex regular expressions

A @dfn{regular expression} is a pattern that describes a set of strings.
Regular expressions are constructed analogously to arithmetic expressions,
by using various operators to combine smaller expressions.
@command{grep} understands two different versions of regular expression
syntax: ``basic''(BRE) and ``extended''(ERE).  In @sc{gnu} @command{grep},
there is no difference in available functionality using either syntax.
In other implementations, basic regular expressions are less powerful.
The following description applies to extended regular expressions;
differences for basic regular expressions are summarized afterwards.

The fundamental building blocks are the regular expressions that match
a single character.  Most characters, including all letters and digits,
are regular expressions that match themselves.  Any metacharacter
with special meaning may be quoted by preceding it with a backslash.

A regular expression may be followed by one of several
repetition operators:

@table @samp

@item .
@opindex .