     [file version.texi not included]

grep
****

grep prints lines that contain a match for one or more patterns.

This manual is for version VERSION of GNU Grep.

This manual is for grep, a pattern matching engine.

Copyright © 1999--2002, 2005, 2008--2020 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 1.3 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''.

Introduction
************

Given one or more patterns, grep searches input files for matches to the patterns. When it finds a match in a line, it copies the line to standard output (by default), or produces whatever other sort of output you have requested with options.

Though grep expects to do the matching on text, it has no limits on input line length other than available memory, and it can match arbitrary characters within a line. If the final byte of an input file is not a newline, grep silently supplies one. Since newline is also a separator for the list of patterns, there is no way to match newline characters in a text.

Invoking grep
*************

The general synopsis of the grep command line is

     grep [OPTION...] [PATTERNS] [FILE...]

There can be zero or more OPTION arguments, and zero or more FILE arguments.  The PATTERNS argument contains one or more patterns separated by newlines, and is omitted when patterns are given via the -e PATTERNS or -f FILE options.  Typically PATTERNS should be quoted when grep is used in a shell command.

Command-line Options
====================

grep comes with a rich set of options: some from POSIX and some being GNU extensions. Long option names are always a GNU extension, even for options that are from POSIX specifications. Options that are specified by POSIX, under their short names, are explicitly marked as such to facilitate POSIX-portable programming. A few option names are provided for compatibility with older or more exotic implementations.

Several additional options control which variant of the grep matching engine is used. See grep Programs.

Generic Program Information
---------------------------

--help
     Print a usage message briefly summarizing the command-line options and the bug-reporting address, then exit.

-V
--version
     Print the version number of grep to the standard output stream. This version number should be included in all bug reports.

Matching Control
----------------

-e PATTERNS
--regexp=PATTERNS
     Use PATTERNS as one or more patterns; newlines within PATTERNS separate each pattern from the next. If this option is used multiple times or is combined with the -f (--file) option, search for all patterns given. Typically PATTERNS should be quoted when grep is used in a shell command. (-e is specified by POSIX.)

-f FILE
--file=FILE
     Obtain patterns from FILE, one per line. If this option is used multiple times or is combined with the -e (--regexp) option, search for all patterns given. The empty file contains zero patterns, and therefore matches nothing. (-f is specified by POSIX.)

-i
-y
--ignore-case
     Ignore case distinctions in patterns and input data, so that characters that differ only in case match each other.  Although this is straightforward when letters differ in case only via lowercase-uppercase pairs, the behavior is unspecified in other situations.  For example, uppercase ``S'' has an unusual lowercase counterpart ``ſ'' (Unicode character U+017F, LATIN SMALL LETTER LONG S) in many locales, and it is unspecified whether this unusual character matches ``S'' or ``s'' even though uppercasing it yields ``S''.  Another example: the lowercase German letter ``ß'' (U+00DF, LATIN SMALL LETTER SHARP S) is normally capitalized as the two-character string ``SS'' but it does not match ``SS'', and it might not match the uppercase letter ``ẞ'' (U+1E9E, LATIN CAPITAL LETTER SHARP S) even though lowercasing the latter yields the former.

     -y is an obsolete synonym that is provided for compatibility. (-i is specified by POSIX.)

--no-ignore-case
     Do not ignore case distinctions in patterns and input data.  This is the default.  This option is useful for passing to shell scripts that already use -i, in order to cancel its effects because the two options override each other.

-v
--invert-match
     Invert the sense of matching, to select non-matching lines. (-v is specified by POSIX.)

-w
--word-regexp
     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. This option has no effect if -x is also specified.

     Because the -w option can match a substring that does not begin and end with word constituents, it differs from surrounding a regular expression with \< and \>.  For example, although grep -w @ matches a line containing only @, grep '\<@\>' cannot match any line because @ is not a word constituent.  See The Backslash Character and Special Expressions.

-x
--line-regexp
     Select only those matches that exactly match the whole line. For regular expression patterns, this is like parenthesizing each pattern and then surrounding it with ^ and $. (-x is specified by POSIX.)

General Output Control
----------------------

-c
--count
     Suppress normal output; instead print a count of matching lines for each input file. With the -v (--invert-match) option, count non-matching lines. (-c is specified by POSIX.)

--color[=WHEN]
--colour[=WHEN]
     Surround the matched (non-empty) strings, matching lines, context lines, file names, line numbers, byte offsets, and separators (for fields and groups of context lines) with escape sequences to display them in color on the terminal. The colors are defined by the environment variable GREP_COLORS and default to ms=01;31:mc=01;31:sl=:cx=:fn=35:ln=32:bn=32:se=36 for bold red matched text, magenta file names, green line numbers, green byte offsets, cyan separators, and default terminal colors otherwise. The deprecated environment variable GREP_COLOR is still supported, but its setting does not have priority; it defaults to 01;31 (bold red) which only covers the color for matched text. WHEN is never, always, or auto.

-L
--files-without-match
     Suppress normal output; instead print the name of each input file from which no output would normally have been printed. The scanning of each file stops on the first match.

-l
--files-with-matches
     Suppress normal output; instead print the name of each input file from which output would normally have been printed. The scanning of each file stops on the first match. (-l is specified by POSIX.)

-m NUM
--max-count=NUM
     Stop after the first NUM selected lines. If the input is standard input from a regular file, and NUM selected lines are output, grep ensures that the standard input is positioned just after the last selected 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:

          while grep -m 1 'PATTERN'
          do
            echo xxxx
          done < FILE

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

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

     When grep stops after NUM selected lines, it outputs any trailing context lines. When the -c or --count option is also used, grep does not output a count greater than NUM. When the -v or --invert-match option is also used, grep stops after outputting NUM non-matching lines.

-o
--only-matching
     Print only the matched (non-empty) parts of matching lines, with each such part on a separate output line. Output lines use the same delimiters as input, and delimiters are null bytes if -z (--null-data) is also used (see Other Options).

-q
--quiet
--silent
     Quiet; do not write anything to standard output. Exit immediately with zero status if any match is found, even if an error was detected. Also see the -s or --no-messages option. (-q is specified by POSIX.)

-s
--no-messages
     Suppress error messages about nonexistent or unreadable files. Portability note: unlike GNU grep, 7th Edition Unix grep did not conform to POSIX, because it lacked -q and its -s option behaved like GNU grep's -q option. (Of course, 7th Edition Unix predated POSIX by several years!) USG-style grep also lacked -q but its -s option behaved like GNU grep's. Portable shell scripts should avoid both -q and -s and should redirect standard and error output to /dev/null instead. (-s is specified by POSIX.)

Output Line Prefix Control
--------------------------

When several prefix fields are to be output, the order is always file name, line number, and byte offset, regardless of the order in which these options were specified.

-b
--byte-offset
     Print the 0-based byte offset within the input file before each line of output. If -o (--only-matching) is specified, print the offset of the matching part itself.

-H
--with-filename
     Print the file name for each match. This is the default when there is more than one file to search.

-h
--no-filename
     Suppress the prefixing of file names on output. This is the default when there is only one file (or only standard input) to search.

--label=LABEL
     Display input actually coming from standard input as input coming from file LABEL. This can be useful for commands that transform a file's contents before searching; e.g.:

          gzip -cd foo.gz | grep --label=foo -H 'some pattern'

-n
--line-number
     Prefix each line of output with the 1-based line number within its input file. (-n is specified by POSIX.)

-T
--initial-tab
     Make sure that the first character of actual line content lies on a tab stop, so that the alignment of tabs looks normal. This is useful with options that prefix their output to the actual content: -H, -n, and -b. This may also prepend spaces to output line numbers and byte offsets so that lines from a single file all start at the same column.

-Z
--null
     Output a zero byte (the ASCII NUL character) instead of the character that normally follows a file name. For example, 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 find -print0, perl -0, sort -z, and xargs -0 to process arbitrary file names, even those that contain newline characters.

Context Line Control
--------------------

Context lines are non-matching lines that are near a matching line. They are output only if one of the following options are used. Regardless of how these options are set, grep never outputs any given line more than once. If the -o (--only-matching) option is specified, these options have no effect and a warning is given upon their use.

-A NUM
--after-context=NUM
     Print NUM lines of trailing context after matching lines.

-B NUM
--before-context=NUM
     Print NUM lines of leading context before matching lines.

-C NUM
-NUM
--context=NUM
     Print NUM lines of leading and trailing output context.

--group-separator=STRING
     When -A, -B or -C are in use, print STRING instead of -- between groups of lines.

--no-group-separator
     When -A, -B or -C are in use, do not print a separator between groups of lines.

Here are some points about how grep chooses the separator to print between prefix fields and line content:

  * Matching lines normally use : as a separator between prefix fields and actual line content.

  * Context (i.e., non-matching) lines use - instead.

  * When context is not specified, matching lines are simply output one right after another.

  * When context is specified, lines that are adjacent in the input form a group and are output one right after another, while by default a separator appears between non-adjacent groups.

  * The default separator is a -- line; its presence and appearance can be changed with the options above.

  * Each group may contain several matching lines when they are close enough to each other that two adjacent groups connect and can merge into a single contiguous one.

File and Directory Selection
----------------------------

-a
--text
     Process a binary file as if it were text; this is equivalent to the --binary-files=text option.

--binary-files=TYPE
     If a file's data or metadata indicate that the file contains binary data, assume that the file is of type TYPE. Non-text bytes indicate binary data; these are either output bytes that are improperly encoded for the current locale (see Environment Variables), or null input bytes when the -z (--null-data) option is not given (see Other Options).

     By default, TYPE is binary, and grep suppresses output after null input binary data is discovered, and suppresses output lines that contain improperly encoded data. When some output is suppressed, grep follows any output with a one-line message saying that a binary file matches.

     If TYPE is without-match, when grep discovers null input binary data it assumes that the rest of the file does not match; this is equivalent to the -I option.

     If TYPE is text, grep processes binary data as if it were text; this is equivalent to the -a option.

     When TYPE is binary, grep may treat non-text bytes as line terminators even without the -z (--null-data) option.  This means choosing binary versus text can affect whether a pattern matches a file.  For example, when TYPE is binary the pattern q$ might match q immediately followed by a null byte, even though this is not matched when TYPE is text.  Conversely, when TYPE is binary the pattern . (period) might not match a null byte.

     Warning: The -a (--binary-files=text) option 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.  On the other hand, when reading files whose text encodings are unknown, it can be helpful to use -a or to set LC_ALL='C' in the environment, in order to find more matches even if the matches are unsafe for direct display.

-D ACTION
--devices=ACTION
     If an input file is a device, FIFO, or socket, use ACTION to process it. If ACTION is read, all devices are read just as if they were ordinary files. If ACTION is skip, devices, FIFOs, and sockets are silently skipped. By default, devices are read if they are on the command line or if the -R (--dereference-recursive) option is used, and are skipped if they are encountered recursively and the -r (--recursive) option is used. This option has no effect on a file that is read via standard input.

-d ACTION
--directories=ACTION
     If an input file is a directory, use ACTION to process it. By default, ACTION is read, which means that directories are read just as if they were ordinary files (some operating systems and file systems disallow this, and will cause grep to print error messages for every directory or silently skip them). If ACTION is skip, directories are silently skipped. If ACTION is recurse, grep reads all files under each directory, recursively, following command-line symbolic links and skipping other symlinks; this is equivalent to the -r option.

--exclude=GLOB
     Skip any command-line file with a name suffix that matches the pattern GLOB, using wildcard matching; a name suffix is either the whole name, or a trailing part that starts with a non-slash character immediately after a slash (/) in the name. When searching recursively, skip any subfile whose base name matches GLOB; the base name is the part after the last slash.  A pattern can use *, ?, and [...] as wildcards, and \ to quote a wildcard or backslash character literally.

--exclude-from=FILE
     Skip files whose name matches any of the patterns read from FILE (using wildcard matching as described under --exclude).

--exclude-dir=GLOB
     Skip any command-line directory with a name suffix that matches the pattern GLOB.  When searching recursively, skip any subdirectory whose base name matches GLOB.  Ignore any redundant trailing slashes in GLOB.

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

--include=GLOB
     Search only files whose name matches GLOB, using wildcard matching as described under --exclude.

-r
--recursive
     For each directory operand, read and process all files in that directory, recursively. Follow symbolic links on the command line, but skip symlinks that are encountered recursively. Note that if no file operand is given, grep searches the working directory. This is the same as the --directories=recurse option.

-R
--dereference-recursive
     For each directory operand, read and process all files in that directory, recursively, following all symbolic links.

Other Options
-------------

--
     Delimit the option list.  Later arguments, if any, are treated as operands even if they begin with -.  For example, grep PAT -- -file1 file2 searches for the pattern PAT in the files named -file1 and file2.

--line-buffered
     Use line buffering on output. This can cause a performance penalty.

-U
--binary
     On platforms that distinguish between text and binary I/O, use the latter when reading and writing files other than the user's terminal, so that all input bytes are read and written as-is.  This overrides the default behavior where grep follows the operating system's advice whether to use text or binary I/O.  On MS-Windows when grep uses text I/O it reads a carriage return--newline pair as a newline and a Control-Z as end-of-file, and it writes a newline as a carriage return--newline pair.

     When using text I/O --byte-offset (-b) counts and --binary-files heuristics apply to input data after text-I/O processing.  Also, the --binary-files heuristics need not agree with the --binary option; that is, they may treat the data as text even if --binary is given, or vice versa. See File and Directory Selection.

     This option has no effect on GNU and other POSIX-compatible platforms, which do not distinguish text from binary I/O.

-z
--null-data
     Treat input and output data as sequences of lines, each terminated by a zero byte (the ASCII NUL character) instead of a newline. Like the -Z or --null option, this option can be used with commands like sort -z to process arbitrary file names.

Environment Variables
=====================

The behavior of grep is affected by the following environment variables.

The locale for category LC_FOO is specified by examining the three environment variables LC_ALL, LC_FOO, and LANG, in that order. The first of these variables that is set specifies the locale. For example, if LC_ALL is not set, but LC_COLLATE is set to pt_BR, then the Brazilian Portuguese locale is used for the LC_COLLATE category. As a special case for LC_MESSAGES only, the environment variable LANGUAGE can contain a colon-separated list of languages that overrides the three environment variables that ordinarily specify the LC_MESSAGES category. The C locale is used if none of these environment variables are set, if the locale catalog is not installed, or if grep was not compiled with national language support (NLS). The shell command locale -a lists locales that are currently available.

Many of the environment variables in the following list let you control highlighting using Select Graphic Rendition (SGR) commands interpreted by the terminal or terminal emulator. (See the section in the documentation of your text terminal for permitted values and their meanings as character attributes.) These substring values are integers in decimal representation and can be concatenated with semicolons. grep takes care of assembling the result into a complete SGR sequence (\33[...m). Common values to concatenate include 1 for bold, 4 for underline, 5 for blink, 7 for inverse, 39 for default foreground color, 30 to 37 for foreground colors, 90 to 97 for 16-color mode foreground colors, 38;5;0 to 38;5;255 for 88-color and 256-color modes foreground colors, 49 for default background color, 40 to 47 for background colors, 100 to 107 for 16-color mode background colors, and 48;5;0 to 48;5;255 for 88-color and 256-color modes background colors.

The two-letter names used in the GREP_COLORS environment variable (and some of the others) refer to terminal ``capabilities,'' the ability of a terminal to highlight text, or change its color, and so on. These capabilities are stored in an online database and accessed by the terminfo library.

GREP_OPTIONS
     This variable specifies default options to be placed in front of any explicit options. As this causes problems when writing portable scripts, this feature will be removed in a future release of grep, and grep warns if it is used.  Please use an alias or script instead. For example, if grep is in the directory /usr/bin you can prepend $HOME/bin to your PATH and create an executable script $HOME/bin/grep containing the following:

          #! /bin/sh
          export PATH=/usr/bin
          exec grep --color=auto --devices=skip "$@"

GREP_COLOR
     This variable specifies the color used to highlight matched (non-empty) text. It is deprecated in favor of GREP_COLORS, but still supported. The mt, ms, and mc capabilities of GREP_COLORS have priority over it. It can only specify the color used to highlight the matching non-empty text in any matching line (a selected line when the -v command-line option is omitted, or a context line when -v is specified). The default is 01;31, which means a bold red foreground text on the terminal's default background.

GREP_COLORS
     This variable specifies the colors and other attributes used to highlight various parts of the output. Its value is a colon-separated list of terminfo capabilities that defaults to ms=01;31:mc=01;31:sl=:cx=:fn=35:ln=32:bn=32:se=36 with the rv and ne boolean capabilities omitted (i.e., false). Supported capabilities are as follows.

     sl=
          SGR substring for whole selected lines (i.e., matching lines when the -v command-line option is omitted, or non-matching lines when -v is specified). If however the boolean rv capability and the -v command-line option are both specified, it applies to context matching lines instead. The default is empty (i.e., the terminal's default color pair).

     cx=
          SGR substring for whole context lines (i.e., non-matching lines when the -v command-line option is omitted, or matching lines when -v is specified). If however the boolean rv capability and the -v command-line option are both specified, it applies to selected non-matching lines instead. The default is empty (i.e., the terminal's default color pair).

     rv
          Boolean value that reverses (swaps) the meanings of the sl= and cx= capabilities when the -v command-line option is specified. The default is false (i.e., the capability is omitted).

     mt=01;31
          SGR substring for matching non-empty text in any matching line (i.e., a selected line when the -v command-line option is omitted, or a context line when -v is specified). Setting this is equivalent to setting both ms= and mc= at once to the same value. The default is a bold red text foreground over the current line background.

     ms=01;31
          SGR substring for matching non-empty text in a selected line. (This is used only when the -v command-line option is omitted.) The effect of the sl= (or cx= if rv) capability remains active when this takes effect. The default is a bold red text foreground over the current line background.

     mc=01;31
          SGR substring for matching non-empty text in a context line. (This is used only when the -v command-line option is specified.) The effect of the cx= (or sl= if rv) capability remains active when this takes effect. The default is a bold red text foreground over the current line background.

     fn=35
          SGR substring for file names prefixing any content line. The default is a magenta text foreground over the terminal's default background.

     ln=32
          SGR substring for line numbers prefixing any content line. The default is a green text foreground over the terminal's default background.

     bn=32
          SGR substring for byte offsets prefixing any content line. The default is a green text foreground over the terminal's default background.

     se=36
          SGR substring for separators that are inserted between selected line fields (:), between context line fields (-), and between groups of adjacent lines when nonzero context is specified (--). The default is a cyan text foreground over the terminal's default background.

     ne
          Boolean value that prevents clearing to the end of line using Erase in Line (EL) to Right (\33[K) each time a colorized item ends. This is needed on terminals on which EL is not supported. It is otherwise useful on terminals for which the back_color_erase (bce) boolean terminfo capability does not apply, when the chosen highlight colors do not affect the background, or when EL is too slow or causes too much flicker. The default is false (i.e., the capability is omitted).

     Note that boolean capabilities have no =... part. They are omitted (i.e., false) by default and become true when specified.

LC_ALL
LC_COLLATE
LANG
     These variables specify the locale for the LC_COLLATE category, which might affect how range expressions like [a-z] are interpreted.

LC_ALL
LC_CTYPE
LANG
     These variables specify the locale for the LC_CTYPE category, which determines the type of characters, e.g., which characters are whitespace. This category also determines the character encoding, that is, whether text is encoded in UTF-8, ASCII, or some other encoding.  In the C or POSIX locale, all characters are encoded as a single byte and every byte is a valid character. In more-complex encodings such as UTF-8, a sequence of multiple bytes may be needed to represent a character, and some bytes may be encoding errors that do not contribute to the representation of any character. POSIX does not specify the behavior of grep when patterns or input data contain encoding errors or null characters, so portable scripts should avoid such usage.  As an extension to POSIX, GNU grep treats null characters like any other character. However, unless the -a (--binary-files=text) option is used, the presence of null characters in input or of encoding errors in output causes GNU grep to treat the file as binary and suppress details about matches.  See File and Directory Selection.

LANGUAGE
LC_ALL
LC_MESSAGES
LANG
     These variables specify the locale for the LC_MESSAGES category, which determines the language that grep uses for messages. The default C locale uses American English messages.

POSIXLY_CORRECT
     If set, grep behaves as POSIX requires; otherwise, grep behaves more like other GNU programs. POSIX 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, POSIXLY_CORRECT disables special handling of an invalid bracket expression.  See invalid-bracket-expr.

_N_GNU_nonoption_argv_flags_
     (Here N is grep's numeric process ID.) If the Ith character of this environment variable's value is 1, do not consider the Ith operand of 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 GNU C library, and only when POSIXLY_CORRECT is not set.

Exit Status
===========

Normally the exit status is 0 if a line is selected, 1 if no lines were selected, and 2 if an error occurred.  However, if the -L or --files-without-match is used, the exit status is 0 if a file is listed, 1 if no files were listed, and 2 if an error occurred.  Also, if the -q or --quiet or --silent option is used and a line is selected, the exit status is 0 even if an error occurred.  Other grep implementations may exit with status greater than 2 on error.

grep Programs
=============

grep searches the named input files for lines containing a match to the given patterns. By default, grep prints the matching lines. A file named - stands for standard input. If no input is specified, grep searches the working directory . if given a command-line option specifying recursion; otherwise, grep searches standard input. There are four major variants of grep, controlled by the following options.

-G
--basic-regexp
     Interpret patterns as basic regular expressions (BREs). This is the default.

-E
--extended-regexp
     Interpret patterns as extended regular expressions (EREs). (-E is specified by POSIX.)

-F
--fixed-strings
     Interpret patterns as fixed strings, not regular expressions. (-F is specified by POSIX.)

-P
--perl-regexp
     Interpret patterns as Perl-compatible regular expressions (PCREs). PCRE support is here to stay, but consider this option experimental when combined with the -z (--null-data) option, and note that grep -P may warn of unimplemented features. See Other Options.

In addition, two variant programs egrep and fgrep are available. egrep is the same as grep -E. fgrep is the same as grep -F. Direct invocation as either egrep or fgrep is deprecated, but is provided to allow historical applications that rely on them to run unmodified.

Regular Expressions
*******************

A 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. grep understands three different versions of regular expression syntax: basic (BRE), extended (ERE), and Perl-compatible (PCRE). In GNU grep, there is no difference in available functionality between the basic and extended syntaxes. 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. Perl-compatible regular expressions give additional functionality, and are documented in the pcresyntax(3) and pcrepattern(3) manual pages, but work only if PCRE is available in the system.

Fundamental Structure
=====================

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 meta-character with special meaning may be quoted by preceding it with a backslash.

The period . matches any single character. It is unspecified whether . matches an encoding error.

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

?
     The preceding item is optional and will be matched at most once.

*
     The preceding item will be matched zero or more times.

+
     The preceding item will be matched one or more times.

{N}
     The preceding item is matched exactly N times.

{N,}
     The preceding item is matched N or more times.

{,M}
     The preceding item is matched at most M times. This is a GNU extension.

{N,M}
     The preceding item is matched at least N times, but not more than M times.

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

Two regular expressions may be joined by the infix operator |; the resulting regular expression matches any string matching either alternate expression.

Repetition takes precedence over concatenation, which in turn takes precedence over alternation. A whole expression may be enclosed in parentheses to override these precedence rules and form a subexpression. An unmatched ) matches just itself.

Character Classes and Bracket Expressions
=========================================

A bracket expression is a list of characters enclosed by [ and ]. It matches any single character in that list. If the first character of the list is the caret ^, then it matches any character not in the list, and it is unspecified whether it matches an encoding error. For example, the regular expression [0123456789] matches any single digit, whereas [^()] matches any single character that is not an opening or closing parenthesis, and might or might not match an encoding error.

Within a bracket expression, a range expression consists of two characters separated by a hyphen. It matches any single character that sorts between the two characters, inclusive. In the default C locale, the sorting sequence is the native character order; for example, [a-d] is equivalent to [abcd]. In other locales, the sorting sequence is not specified, and [a-d] might be equivalent to [abcd] or to [aBbCcDd], or it might fail to match any character, or the set of characters that it matches might even be erratic. To obtain the traditional interpretation of bracket expressions, you can use the C locale by setting the LC_ALL environment variable to the value C.

Finally, certain named classes of characters are predefined within bracket expressions, as follows. Their interpretation depends on the LC_CTYPE locale; for example, [[:alnum:]] means the character class of numbers and letters in the current locale.

[:alnum:]
     Alphanumeric characters: [:alpha:] and [:digit:]; in the C locale and ASCII character encoding, this is the same as [0-9A-Za-z].

[:alpha:]
     Alphabetic characters: [:lower:] and [:upper:]; in the C locale and ASCII character encoding, this is the same as [A-Za-z].

[:blank:]
     Blank characters: space and tab.

[:cntrl:]
     Control characters. In ASCII, these characters have octal codes 000 through 037, and 177 (DEL). In other character sets, these are the equivalent characters, if any.

[:digit:]
     Digits: 0 1 2 3 4 5 6 7 8 9.

[:graph:]
     Graphical characters: [:alnum:] and [:punct:].

[:lower:]
     Lower-case letters; in the C locale and ASCII character encoding, this is 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.

[:print:]
     Printable characters: [:alnum:], [:punct:], and space.

[:punct:]
     Punctuation characters; in the C locale and ASCII character encoding, this is ! " # $ % & ' ( ) * + , - . / : ; < = > ? @ [ \ ] ^ _ ` { | } ~.

[:space:]
     Space characters: in the C locale, this is tab, newline, vertical tab, form feed, carriage return, and space. See Usage, for more discussion of matching newlines.

[:upper:]
     Upper-case letters: in the C locale and ASCII character encoding, this is 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.

[:xdigit:]
     Hexadecimal digits: 0 1 2 3 4 5 6 7 8 9 A B C D E F a b c d e f.

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 expression.

If you mistakenly omit the outer brackets, and search for say, [:upper:], GNU grep prints a diagnostic and exits with status 2, on the assumption that you did not intend to search for the nominally equivalent regular expression: [:epru]. Set the POSIXLY_CORRECT environment variable to disable this feature.

Most meta-characters lose their special meaning inside bracket expressions.

]
     ends the bracket expression if it's not the first list item. So, if you want to make the ] character a list item, you must put it first.

[.
     represents the open collating symbol.

.]
     represents the close collating symbol.

[=
     represents the open equivalence class.

=]
     represents the close equivalence class.

[:
     represents the open character class symbol, and should be followed by a valid character class name.

:]
     represents the close character class symbol.

-
     represents the range if it's not first or last in a list or the ending point of a range.

^
     represents the characters not in the list. If you want to make the ^ character a list item, place it anywhere but first.

The Backslash Character and Special Expressions
===============================================

The \ character, when followed by certain ordinary characters, takes a special meaning:

\b
     Match the empty string at the edge of a word.

\B
     Match the empty string provided it's not at the edge of a word.

\<
     Match the empty string at the beginning of word.

\>
     Match the empty string at the end of word.

\w
     Match word constituent, it is a synonym for [_[:alnum:]].

\W
     Match non-word constituent, it is a synonym for [^_[:alnum:]].

\s
     Match whitespace, it is a synonym for [[:space:]].

\S
     Match non-whitespace, it is a synonym for [^[:space:]].

For example, \brat\b matches the separate word rat, \Brat\B matches crate but not furry rat.

Anchoring
=========

The caret ^ and the dollar sign $ are meta-characters that respectively match the empty string at the beginning and end of a line. They are termed anchors, since they force the match to be ``anchored'' to beginning or end of a line, respectively.

Back-references and Subexpressions
==================================

The back-reference \N, where N is a single digit, matches the substring previously matched by the Nth parenthesized subexpression of the regular expression. For example, (a)\1 matches aa. When used with alternation, if the group does not participate in the match then the back-reference makes the whole match fail. For example, a(.)|b\1 will not match ba. When multiple regular expressions are given with -e or from a file (-f FILE), back-references are local to each expression.

See Known Bugs, for some known problems with back-references.

Basic vs Extended Regular Expressions
=====================================

In basic regular expressions the meta-characters ?, +, {, |, (, and ) lose their special meaning; instead use the backslashed versions \?, \+, \{, \|, \(, and \).

Traditional egrep did not support the { meta-character, and some egrep implementations support \{ instead, so portable scripts should avoid { in grep -E patterns and should use [{] to match a literal {.

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

Usage
*****

Here is an example command that invokes GNU grep:

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

This lists all lines in the files menu.h and main.c that contain the string hello followed by the string world; this is because .* matches zero or more characters within a line. See Regular Expressions. The -i option causes grep to ignore case, causing it to match the line Hello, world!, which it would not otherwise match.

Here is a more complex example session, showing the location and contents of any line containing f and ending in .c, within all files in the current directory whose names contain g and end in .h. The -n option outputs line numbers, the -- argument treats any later arguments starting with - as file names not options, and the empty file /dev/null causes file names to be output even if only one file name happens to be of the form *g*.h.

     $ grep -n -- 'f.*\.c$' *g*.h /dev/null
     argmatch.h:1:/* definitions and prototypes for argmatch.c

The only line that contains a match is line 1 of argmatch.h. Note that the regular expression syntax used in the pattern differs from the globbing syntax that the shell uses to match file names.

See Invoking, for more details about how to invoke grep.

Here are some common questions and answers about grep usage.

 1. How can I list just the names of matching files?

          grep -l 'main' test-*.c

     lists names of test-*.c files in the current directory whose contents mention main.

 2. How do I search directories recursively?

          grep -r 'hello' /home/gigi

     searches for hello in all files under the /home/gigi directory. For more control over which files are searched, use find and grep. For example, the following command searches only C files:

          find /home/gigi -name '*.c' ! -type d \
            -exec grep -H 'hello' '{}' +

     This differs from the command:

          grep -H 'hello' /home/gigi/*.c

     which merely looks for hello in non-hidden C files in /home/gigi whose names end in .c. The find command line above is more similar to the command:

          grep -r --include='*.c' 'hello' /home/gigi

 3. What if a pattern or file has a leading -?

          grep -- '--cut here--' *

     searches for all lines matching --cut here--. Without --, grep would attempt to parse --cut here-- as a list of options, and there would be similar problems with any file names beginning with -.

     Alternatively, you can prevent misinterpretation of leading - by using -e for patterns and leading ./ for files:

          grep -e '--cut here--' ./*

 4. Suppose I want to search for a whole word, not a part of a word?

          grep -w 'hello' test*.log

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

          grep 'hello\>' test*.log

     searches only for words ending in hello, so it matches the word Othello.

 5. How do I output context around the matching lines?

          grep -C 2 'hello' test*.log

     prints two lines of context around each matching line.

 6. How do I force grep to print the name of the file?

     Append /dev/null:

          grep 'eli' /etc/passwd /dev/null

     gets you:

          /etc/passwd:eli:x:2098:1000:Eli Smith:/home/eli:/bin/bash

     Alternatively, use -H, which is a GNU extension:

          grep -H 'eli' /etc/passwd

 7. Why do people use strange regular expressions on ps output?

          ps -ef | grep '[c]ron'

     If the pattern had been written without the square brackets, it would have matched not only the ps output line for cron, but also the ps output line for grep. Note that on some platforms, ps limits the output to the width of the screen; grep does not have any limit on the length of a line except the available memory.

 8. Why does grep report ``Binary file matches''?

     If 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 GNU grep suppresses output from files that appear to be binary files. To force GNU grep to output lines even from files that appear to be binary, use the -a or --binary-files=text option. To eliminate the ``Binary file matches'' messages, use the -I or --binary-files=without-match option.

 9. Why doesn't grep -lv print non-matching file names?

     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 -L or --files-without-match option.

 10. I can do ``OR'' with |, but what about ``AND''?

          grep 'paul' /etc/motd | grep 'franc,ois'

     finds all lines that contain both paul and franc,ois.

 11. Why does the empty pattern match every input line?

     The grep command searches for lines that contain strings that match a pattern.  Every line contains the empty string, so an empty pattern causes grep to find a match on each line.  It is not the only such pattern: ^, $, and many other patterns cause grep to match every line.

     To match empty lines, use the pattern ^$.  To match blank lines, use the pattern ^[[:blank:]]*$.  To match no lines at all, use the command grep -f /dev/null.

 12. How can I search in both standard input and in files?

     Use the special file name -:

          cat /etc/passwd | grep 'alain' - /etc/motd

 13. Why is this back-reference failing?

          echo 'ba' | grep -E '(a)\1|b\1'

     This gives no output, because the first alternate (a)\1 does not match, as there is no aa in the input, so the \1 in the second alternate has nothing to refer back to, meaning it will never match anything. (The second alternate in this example can only match if the first alternate has matched---making the second one superfluous.)

 14. How can I match across lines?

     Standard grep cannot do this, as it is fundamentally line-based. Therefore, merely using the [:space:] character class does not match newlines in the way you might expect.

     With the GNU grep option -z (--null-data), each input and output ``line'' is null-terminated; see Other Options.  Thus, you can match newlines in the input, but typically if there is a match the entire input is output, so this usage is often combined with output-suppressing options like -q, e.g.:

          printf 'foo\nbar\n' | grep -z -q 'foo[[:space:]]\+bar'

     If this does not suffice, you can transform the input before giving it to grep, or turn to awk, sed, perl, or many other utilities that are designed to operate across lines.

 15. What do grep, fgrep, and egrep stand for?

     The name grep comes from the way line editing was done on Unix. For example, ed uses the following syntax to print a list of matching lines on the screen:

          global/regular expression/print
          g/re/p

     fgrep stands for Fixed grep; egrep stands for Extended grep.

Performance
***********

Typically grep is an efficient way to search text.  However, it can be quite slow in some cases, and it can search large files where even minor performance tweaking can help significantly. Although the algorithm used by grep is an implementation detail that can change from release to release, understanding its basic strengths and weaknesses can help you improve its performance.

The grep command operates partly via a set of automata that are designed for efficiency, and partly via a slower matcher that takes over when the fast matchers run into unusual features like back-references.  When feasible, the Boyer--Moore fast string searching algorithm is used to match a single fixed pattern, and the Aho--Corasick algorithm is used to match multiple fixed patterns.

Generally speaking grep operates more efficiently in single-byte locales, since it can avoid the special processing needed for multi-byte characters.  If your patterns will work just as well that way, setting LC_ALL to a single-byte locale can help performance considerably.  Setting LC_ALL='C' can be particularly efficient, as grep is tuned for that locale.

Outside the C locale, case-insensitive search, and search for bracket expressions like [a-z] and [[=a=]b], can be surprisingly inefficient due to difficulties in fast portable access to concepts like multi-character collating elements.

A back-reference such as \1 can hurt performance significantly in some cases, since back-references cannot in general be implemented via a finite state automaton, and instead trigger a backtracking algorithm that can be quite inefficient.  For example, although the pattern ^(.*)\1{14}(.*)\2{13}$ matches only lines whose lengths can be written as a sum 15x + 14y for nonnegative integers x and y, the pattern matcher does not perform linear Diophantine analysis and instead backtracks through all possible matching strings, using an algorithm that is exponential in the worst case.

On some operating systems that support files with holes---large regions of zeros that are not physically present on secondary storage---grep can skip over the holes efficiently without needing to read the zeros.  This optimization is not available if the -a (--binary-files=text) option is used (see File and Directory Selection), unless the -z (--null-data) option is also used (see Other Options).

For more about the algorithms used by grep and about related string matching algorithms, see:

  * Aho AV. Algorithms for finding patterns in strings. In: van Leeuwen J. Handbook of Theoretical Computer Science, vol. A. New York: Elsevier; 1990. p. 255--300. This surveys classic string matching algorithms, some of which are used by grep.

  * Aho AV, Corasick MJ. Efficient string matching: an aid to bibliographic search. CACM. 1975;18(6):333--40. https://dx.doi.org/10.1145/360825.360855. This introduces the Aho--Corasick algorithm.

  * Boyer RS, Moore JS. A fast string searching algorithm. CACM. 1977;20(10):762--72. https://dx.doi.org/10.1145/359842.359859. This introduces the Boyer--Moore algorithm.

  * Faro S, Lecroq T. The exact online string matching problem: a review of the most recent results. ACM Comput Surv. 2013;45(2):13. https://dx.doi.org/10.1145/2431211.2431212. This surveys string matching algorithms that might help improve the performance of grep in the future.

Reporting bugs
**************

Bug reports can be found at the GNU bug report logs for grep (https://debbugs.gnu.org/cgi/pkgreport.cgi?package=grep). If you find a bug not listed there, please email it to bug-grep@gnu.org to create a new bug report.

Known Bugs
==========

Large repetition counts in the {n,m} construct may cause 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.

Back-references can greatly slow down matching, as they can generate exponentially many matching possibilities that can consume both time and memory to explore.  Also, the POSIX specification for back-references is at times unclear.  Furthermore, many regular expression implementations have back-reference bugs that can cause programs to return incorrect answers or even crash, and fixing these bugs has often been low-priority---for example, as of 2019 the GNU C library bug database contained back-reference bugs 52, 10844, 11053, and 25322, with little sign of forthcoming fixes.  Luckily, back-references are rarely useful and it should be little trouble to avoid them in practical applications.

Copying
*******

GNU grep is licensed under the GNU GPL, which makes it free software.

The ``free'' in ``free software'' refers to liberty, not price.  As some GNU project advocates like to point out, think of ``free speech'' rather than ``free beer''.  In short, you have the right (freedom) to run and change grep and distribute it to other people, and---if you want---charge money for doing either.  The important restriction is that you have to grant your recipients the same rights and impose the same restrictions.

This general method of licensing software is sometimes called open source.  The GNU project prefers the term ``free software'' for reasons outlined at https://www.gnu.org/philosophy/open-source-misses-the-point.html.

This manual is free documentation in the same sense.  The documentation license is included below.  The license for the program is available with the source code, or at https://www.gnu.org/licenses/gpl.html.

GNU Free Documentation License
==============================

     [file fdl.texi not included]