grep/doc/grep.texi

\input texinfo  @c -*-texinfo-*-
@c %**start of header
@setfilename grep.info
@settitle grep, print lines matching a pattern
@c %**end of header

@c This file has the new style title page commands.
@c Run `makeinfo' rather than `texinfo-format-buffer'.

@c smallbook

@c tex
@c \overfullrule=0pt
@c end tex

@include version.texi

@c Combine indices.
@syncodeindex ky cp
@syncodeindex pg cp
@syncodeindex tp cp

@defcodeindex op
@syncodeindex op fn
@syncodeindex vr fn

@ifinfo
@direntry
* grep: (grep).                   print lines matching a pattern.
@end direntry
This file documents @command{grep}, a pattern matching engine.


Published by the Free Software Foundation,
59 Temple Place - Suite 330
Boston, MA 02111-1307, USA

Copyright 1999 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

@ignore
Permission is granted to process this file through TeX and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).

@end ignore
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that this permission notice may be stated in a translation approved
by the Foundation.
@end ifinfo

@setchapternewpage off

@titlepage
@title grep, searching for a pattern
@subtitle version @value{VERSION}, @value{UPDATED}
@author Alain Magloire et al.

@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1999 Free Software Foundation, Inc.

@sp 2
Published by the Free Software Foundation, @*
59 Temple Place - Suite 330, @*
Boston, MA 02111-1307, USA

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that this permission notice may be stated in a translation approved
by the Foundation.

@end titlepage
@page


@ifnottex
@node Top
@top Grep

@command{grep} searches for lines matching a pattern.

This document was produced for version @value{VERSION} of @sc{gnu}
@command{grep}.
@end ifnottex

@menu
* Introduction::                Introduction.
* Invoking::                    Invoking @command{grep}; description of options.
* Diagnostics::                 Exit status returned by @command{grep}.
* Grep Programs::               @command{grep} programs.
* Regular Expressions::         Regular Expressions.
* Usage::                       Examples.
* Reporting Bugs::              Reporting Bugs.
* Concept Index::               A menu with all the topics in this manual.
* Index::                       A menu with all @command{grep} commands
                                 and command-line options.
@end menu


@node Introduction
@chapter Introduction

@cindex Searching for a pattern.

@command{grep} searches the input files
for lines containing a match to a given
pattern list.  When it finds a match in a line, it copies the line to standard
output (by default), or does whatever other sort of output you have requested
with options.  @command{grep} expects to do the matching on text.
Since newline is also a separator for the list of patterns, there
is no way to match newline characters in a text.

@node Invoking
@chapter Invoking @command{grep}

@command{grep} comes with a rich set of options from @sc{posix.2} and @sc{gnu}
extensions.

@table @samp

@item -c
@itemx --count
@opindex -c
@opindex -count
@cindex counting lines
Suppress normal output; instead print a count of matching
lines for each input file.  With the @samp{-v}, @samp{--invert-match} option,
count non-matching lines.

@item -e @var{pattern}
@itemx --regexp=@var{pattern}
@opindex -e
@opindex --regexp=@var{pattern}
@cindex pattern list
Use @var{pattern} as the pattern; useful to protect patterns
beginning with a @samp{-}.

@item -f @var{file}
@itemx --file=@var{file}
@opindex -f
@opindex --file
@cindex pattern from file
Obtain patterns from @var{file}, one per line.  The empty
file contains zero patterns, and therefore matches nothing.

@item -i
@itemx --ignore-case
@opindex -i
@opindex --ignore-case
@cindex case insensitive search
Ignore case distinctions in both the pattern and the input files.

@item -l
@itemx --files-with-matches
@opindex -l
@opindex --files-with-matches
@cindex names of matching files
Suppress normal output; instead print the name of each input
file from which output would normally have been printed.
The scanning of every file will stop on the first match.

@item -n
@itemx --line-number
@opindex -n
@opindex --line-number
@cindex line numbering
Prefix each line of output with the line number within its input file.

@item -q
@itemx --quiet
@itemx --silent
@opindex -q
@opindex --quiet
@opindex --silent
@cindex quiet, silent
Quiet; suppress normal output.  The scanning of every file will stop on
the first match.  Also see the @samp{-s} or @samp{--no-messages} option.

@item -s
@itemx --no-messages
@opindex -s
@opindex --no-messages
@cindex suppress error messages
Suppress error messages about nonexistent or unreadable files.
Portability note: unlike @sc{gnu} @command{grep}, traditional
@command{grep} did not conform to @sc{posix.2}, because traditional
@command{grep} lacked a @samp{-q} option and its @samp{-s} option behaved
like @sc{gnu} @command{grep}'s @samp{-q} option.  Shell scripts intended
to be portable to traditional @command{grep} should avoid both
@samp{-q} and @samp{-s} and should redirect
output to @file{/dev/null} instead.

@item -v
@itemx --invert-match
@opindex -v
@opindex --invert-match
@cindex invert matching
@cindex print non-matching lines
Invert the sense of matching, to select non-matching lines.

@item -x
@itemx --line-regexp
@opindex -x
@opindex --line-regexp
@cindex match the whole line
Select only those matches that exactly match the whole line.

@end table

@section @sc{gnu} Extensions

@table @samp

@item -A @var{num}
@itemx --after-context=@var{num}
@opindex -A
@opindex --after-context
@cindex after context
@cindex context lines, after match
Print @var{num} lines of trailing context after matching lines.

@item -B @var{num}
@itemx --before-context=@var{num}
@opindex -B
@opindex --before-context
@cindex before context
@cindex context lines, before match
Print @var{num} lines of leading context before matching lines.

@item -C @var{num}
@itemx --context=[@var{num}]
@opindex -C
@opindex --context
@cindex context
Print @var{num} lines (default 2) of output context.


@item -@var{num}
@opindex -NUM
Same as @samp{--context=@var{num}} lines of leading and trailing
context.  However, grep will never print any given line more than once.


@item -V
@itemx --version
@opindex -V
@opindex --version
@cindex Version, printing
Print the version number of @command{grep} to the standard output stream.
This version number should be included in all bug reports.

@item --help
@opindex --help
@cindex Usage summary, printing
Print a usage message briefly summarizing these command-line options
and the bug-reporting address, then exit.

@itemx --binary-files=@var{type}
@opindex --binary-files
@cindex binary files
If the first few bytes of a file indicate that the file contains binary
data, assume that the file is of type @var{type}.  By default,
@var{type} is @samp{binary}, and @command{grep} normally outputs either
a one-line message saying that a binary file matches, or no message if
there is no match.  If @var{type} is @samp{without-match},
@command{grep} assumes that a binary file does not match.  If @var{type}
is @samp{text}, @command{grep} processes a binary file as if it were
text; this is equivalent to the @samp{-a} or @samp{--text} option.
@emph{Warning:} @samp{--binary-files=text} might output binary garbage,
which can have nasty side effects if the output is a terminal and if the
terminal driver interprets some of it as commands.

@item -b
@itemx --byte-offset
@opindex -b
@opindex --byte-offset
@cindex byte offset
Print the byte offset within the input file before each line of output.
When @command{grep} runs on @sc{ms-dos} or MS-Windows, the printed
byte offsets
depend on whether the @samp{-u} (@samp{--unix-byte-offsets}) option is
used; see below.

@item -d @var{action}
@itemx --directories=@var{action}
@opindex -d
@opindex --directories
@cindex directory search
If an input file is a directory, use @var{action} to process it.
By default, @var{action} is @samp{read}, which means that directories are
read just as if they were ordinary files (some operating systems
and filesystems disallow this, and will cause @command{grep} to print error
messages for every directory).  If @var{action} is @samp{skip},
directories are silently skipped.  If @var{action} is @samp{recurse},
@command{grep} reads all files under each directory, recursively; this is
equivalent to the @samp{-r} option.

@item -H
@itemx --with-filename
@opindex -H
@opindex --With-filename
@cindex with filename prefix
Print the filename for each match.

@item -h
@itemx --no-filename
@opindex -h
@opindex --no-filename
@cindex no filename prefix
Suppress the prefixing of filenames on output when multiple files are searched.

@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 -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 --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{-d
recurse} option.

@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.
@cindex environment variables

@table @code

@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{--text
--directories=skip}, @command{grep} behaves as if the two options
@samp{--text} 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 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 locale is determined
by the first of these variables that is set.  American English is used
if none of these environment variables are set, or if the message
catalog is not installed, or if @command{grep} was not compiled with national
language support (@sc{nls}).

@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.  The locale is
determined by the first of these variables that is set.  The @sc{posix}
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}).

@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 matches were found, and 1 if no matches
were found (the @samp{-v} option inverts the sense of the exit status).
Exit status is 2 if there were syntax errors in the pattern,
inaccessible input files, or other system errors.

@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 three 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 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 pattern as an extended regular expression.


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

@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'' and ``extended''.  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 list of characters enclosed by @samp{[} and @samp{]} matches any
single character in that list; if the first character of the list is the
caret @samp{^}, then it
matches any character @strong{not} in the list.  For example, the regular
expression @samp{[0123456789]} matches any single digit.
A range of @sc{ascii} characters may be specified by giving the first
and last characters, separated by a hyphen.

Finally, certain named classes of characters are predefined, as follows.
Their interpretation depends on the @code{LC_CTYPE} locale; the
interpretation below is that of the @sc{posix} locale, which is the default
if no @code{LC_CTYPE} locale is specified.

@cindex classes of characters
@cindex character classes
@table @samp

@item [:alnum:]
@opindex alnum
@cindex alphanumeric characters
Any of @samp{[:digit:]} or @samp{[:alpha:]}

@item [:alpha:]
@opindex alpha
@cindex alphabetic characters
Any letter:@*
@code{a b c d e f g h i j k l m n o p q r s t u v w x y z},@*
@code{A B C D E F G H I J K L M N O P Q R S T U V W X Y Z}.

@item [:blank:]
@opindex blank
@cindex blank characters
Space or tab.

@item [:cntrl:]
@opindex cntrl
@cindex control characters
Any character with octal codes 000 through 037, or @code{DEL} (octal
code 177).

@item [:digit:]
@opindex digit
@cindex digit characters
@cindex numeric characters
Any one of @code{0 1 2 3 4 5 6 7 8 9}.

@item [:graph:]
@opindex graph
@cindex graphic characters
Anything that is not a @samp{[:alnum:]} or @samp{[:punct:]}.

@item [:lower:]
@opindex lower
@cindex lower-case alphabetic characters
Any one of @code{a b c d e f g h i j k l m n o p q r s t u v w x y z}.

@item [:print:]
@opindex print
@cindex printable characters
Any character from the @samp{[:space:]} class, and any character that is
@strong{not} in the @samp{[:graph:]} class.

@item [:punct:]
@opindex punct
@cindex punctuation characters
Any one of @code{!@: " # $ % & ' ( ) * + , - .@: / : ; < = > ?@: @@ [ \ ] ^ _ ` @{ | @} ~}.

@item [:space:]
@opindex space
@cindex space characters
@cindex whitespace characters
Any one of @code{CR FF HT NL VT SPACE}.

@item [:upper:]
@opindex upper
@cindex upper-case alphabetic characters
Any one of @code{A B C D E F G H I J K L M N O P Q R S T U V W X Y Z}.

@item [:xdigit:]
@opindex xdigit
@cindex xdigit class
@cindex hexadecimal digits
Any one of @code{a b c d e f A B C D E F 0 1 2 3 4 5 6 7 8 9}.

@end table
For example, @samp{[[:alnum:]]} means @samp{[0-9A-Za-z]}, except the latter
form is dependent upon the @sc{ascii} character encoding, whereas the
former is portable.  (Note that the brackets in these class names are
part of the symbolic names, and must be included in addition to
the brackets delimiting the bracket list.)  Most metacharacters lose
their special meaning inside lists.  To include a literal @samp{]}, place it
first in the list.  Similarly, to include a literal @samp{^}, place it anywhere
but first.  Finally, to include a literal @samp{-}, place it last.

The period @samp{.} matches any single character.  The symbol @samp{\w}
is a synonym for @samp{[[:alnum:]]} and @samp{\W} is a synonym for
@samp{[^[:alnum]]}.

The caret @samp{^} and the dollar sign @samp{$} are metacharacters that
respectively match the empty string at the beginning and end
of a line.  The symbols @samp{\<} and @samp{\>} respectively match the
empty string at the beginning and end of a word.  The symbol
@samp{\b} matches the empty string at the edge of a word, and @samp{\B}
matches the empty string provided it's not at the edge of a word.

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


@table @samp

@item ?
@opindex ?
@cindex question mark
@cindex match sub-expression at most once
The preceding item is optional and will be matched at most once.

@item *
@opindex *
@cindex asterisk
@cindex match sub-expression zero or more times
The preceding item will be matched zero or more times.

@item +
@opindex +
@cindex plus sign
The preceding item will be matched one or more times.

@item @{@var{n}@}
@opindex @{n@}
@cindex braces, one argument
@cindex match sub-expression n times
The preceding item is matched exactly @var{n} times.

@item @{@var{n},@}
@opindex @{n,@}
@cindex braces, second argument omitted
@cindex match sub-expression n or more times
The preceding item is matched n or more times.

@item @{@var{n},@var{m}@}
@opindex @{n,m@}
@cindex braces, two arguments
The preceding item is matched at least @var{n} times, but not more than
@var{m} times.

@end table

Two regular expressions may be concatenated; the resulting regular
expression matches any string formed by concatenating two substrings
that respectively match the concatenated subexpressions.

Two regular expressions may be joined by the infix operator @samp{|}; the
resulting regular expression matches any string matching either
subexpression.

Repetition takes precedence over concatenation, which in turn
takes precedence over alternation.  A whole subexpression may be
enclosed in parentheses to override these precedence rules.

The backreference @samp{\@var{n}}, where @var{n} is a single digit, matches the
substring previously matched by the @var{n}th parenthesized subexpression
of the regular expression.

@cindex basic regular expressions
In basic regular expressions the metacharacters @samp{?}, @samp{+},
@samp{@{}, @samp{|}, @samp{(}, and @samp{)} lose their special meaning;
instead use the backslashed versions @samp{\?}, @samp{\+}, @samp{\@{},
@samp{\|}, @samp{\(}, and @samp{\)}.

@cindex interval specifications
Traditional @command{egrep} did not support the @samp{@{} metacharacter,
and some @command{egrep} implementations support @samp{\@{} instead, so
portable scripts should avoid @samp{@{} in @samp{egrep} patterns and
should use @samp{[@{]} to match a literal @samp{@{}.

@sc{gnu} @command{egrep} attempts to support traditional usage by
assuming that @samp{@{} is not special if it would be the start of an
invalid interval specification.  For example, the shell command
@samp{egrep '@{1'} searches for the two-character string @samp{@{1}
instead of reporting a syntax error in the regular expression.
@sc{posix.2} allows this behavior as an extension, but portable scripts
should avoid it.

@node Usage
@chapter Usage

@cindex Usage, examples
Here is an example shell command that invokes @sc{gnu} @command{grep}:

@example
grep -i 'hello.*world' menu.h main.c
@end example

@noindent
This lists all lines in the files @file{menu.h} and @file{main.c} that
contain the string @samp{hello} followed by the string @samp{world};
this is because @samp{.*} matches zero or more characters within a line.
@xref{Regular Expressions}.  The @samp{-i} option causes @command{grep}
to ignore case, causing it to match the line @samp{Hello, world!}, which
it would not otherwise match.  @xref{Invoking}, for more details about
how to invoke @command{grep}.

@cindex Using @command{grep}, Q&A
@cindex FAQ about @command{grep} usage
Here are some common questions and answers about @command{grep} usage.

@enumerate

@item
How can I list just the names of matching files?

@example
grep -l 'main' *.c
@end example

@noindent
lists the names of all C files in the current directory whose contents
mention @samp{main}.

@item
How do I search directories recursively?

@example
grep -r 'hello' /home/gigi
@end example

@noindent
searches for @samp{hello} in all files under the directory
@file{/home/gigi}.  For more control of which files are searched, use
@command{find}, @command{grep} and @command{xargs}.  For example,
the following command searches only C files:

@smallexample
find /home/gigi -name '*.c' -print | xargs grep 'hello' /dev/null
@end smallexample

@item
What if a pattern has a leading @samp{-}?

@example
grep -e '--cut here--' *
@end example

@noindent
searches for all lines matching @samp{--cut here--}.  Without @samp{-e},
@command{grep} would attempt to parse @samp{--cut here--} as a list of
options.

@item
Suppose I want to search for a whole word, not a part of a word?

@example
grep -w 'hello' *
@end example

@noindent
searches only for instances of @samp{hello} that are entire words; it
does not match @samp{Othello}.  For more control, use @samp{\<} and
@samp{\>} to match the start and end of words.  For example:

@example
grep 'hello\>' *
@end example

@noindent
searches only for words ending in @samp{hello}, so it matches the word
@samp{Othello}.

@item
How do I output context around the matching lines?

@example
grep -C 2 'hello' *
@end example

@noindent
prints two lines of context around each matching line.

@item
How do I force grep to print the name of the file?

Append @file{/dev/null}:

@example
grep 'eli' /etc/passwd /dev/null
@end example

@item
Why do people use strange regular expressions on @command{ps} output?

@example
ps -ef | grep '[c]ron'
@end example

If the pattern had been written without the square brackets, it would
have matched not only the @command{ps} output line for @command{cron},
but also the @command{ps} output line for @command{grep}.

@item
Why does @command{grep} report ``Binary file matches''?

If @command{grep} listed all matching ``lines'' from a binary file, it
would probably generate output that is not useful, and it might even
muck up your display.  So @sc{gnu} @command{grep} suppresses output from
files that appear to be binary files.  To force @sc{gnu} @command{grep}
to output lines even from files that appear to be binary, use the
@samp{-a} or @samp{--text} option.

@item
Why doesn't @samp{grep -lv} print nonmatching file names?

@samp{grep -lv} lists the names of all files containing one or more
lines that do not match.  To list the names of all files that contain no
matching lines, use the @samp{-L} or @samp{--files-without-match}
option.

@item
I can do @sc{or} with @samp{|}, but what about @sc{and}?

@example
grep 'paul' /etc/motd | grep 'franc,ois'
@end example

@noindent
finds all lines that contain both @samp{paul} and @samp{franc,ois}.

@item
How can I search in both standard input and in files?

Use the special file name @samp{-}:

@example
cat /etc/passwd | grep 'alain' - /etc/motd
@end example
@end enumerate

@node Reporting Bugs
@chapter Reporting bugs

@cindex Bugs, reporting
Email bug reports to @email{bug-gnu-utils@@gnu.org}.
Be sure to include the word ``grep'' somewhere in the ``Subject:'' field.

Large repetition counts in the @samp{@{m,n@}} construct may cause
@command{grep} to use lots of memory.  In addition, certain other
obscure regular expressions require exponential time and
space, and may cause grep to run out of memory.
Backreferences are very slow, and may require exponential time.

@page
@node Concept Index
@unnumbered Concept Index

This is a general index of all issues discussed in this manual, with the
exception of the @command{grep} commands and command-line options.

@printindex cp

@page
@node Index
@unnumbered Index

This is an alphabetical list of all @command{grep} commands, command-line
options, and environment variables.

@printindex fn

@contents
@bye