grep prints lines that contain a match for one or more patterns.
This manual is for version 3.11 of GNU Grep.
This manual is for grep, a pattern matching engine.
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grep
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.
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.
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.
Print a usage message briefly summarizing the command-line options and the bug-reporting address, then exit.
Print the version number of grep to the standard output stream.
This version number should be included in all bug reports.
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.)
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. When file is ‘-’, read patterns from standard input. The empty file contains zero patterns, and therefore matches nothing. (-f is specified by POSIX.)
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.)
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.
Invert the sense of matching, to select non-matching lines. (-v is specified by POSIX.)
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 Special Backslash Expressions.
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.)
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.)
Surround 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.
See Environment Variables.
WHEN is ‘always’ to use colors, ‘never’ to not use
colors, or ‘auto’ to use colors if standard output is associated
with a terminal device and the TERM environment variable’s value
suggests that the terminal supports colors.
Plain --color is treated like --color=auto;
if no --color option is given, the default is --color=never.
Suppress normal output; instead print the name of each input file from which no output would normally have been printed.
Suppress normal output; instead print the name of each input file from which output would normally have been printed. Scanning each input file stops upon first match. (-l is specified by POSIX.)
Stop after the first num selected lines.
If num is zero, grep stops right away without reading input.
A num of −1 is treated as infinity and grep
does not stop; this is the default.
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.
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).
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.
Portability note: Solaris 10 grep lacks -q;
portable shell scripts typically can redirect standard output to
/dev/null instead of using -q.
(-q is specified by POSIX.)
Suppress error messages about nonexistent or unreadable files. (-s is specified by POSIX.)
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.
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.
Print the file name for each match. This is the default when there is more than one file to search.
Suppress the prefixing of file names on output. This is the default when there is only one file (or only standard input) to search.
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'
Prefix each line of output with the 1-based line number within its input file. (-n is specified by POSIX.)
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.
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 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.
Print num lines of trailing context after matching lines.
Print num lines of leading context before matching lines.
Print num lines of leading and trailing output context.
When -A, -B or -C are in use, print string instead of -- between groups of lines.
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:
Process a binary file as if it were text; this is equivalent to the ‘--binary-files=text’ option.
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 message to standard error 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.
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.
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.
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.
Skip files whose name matches any of the patterns read from file (using wildcard matching as described under --exclude).
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.
Process a binary file as if it did not contain matching data; this is equivalent to the ‘--binary-files=without-match’ option.
Search only files whose name matches glob, using wildcard matching as described under --exclude. If contradictory --include and --exclude options are given, the last matching one wins. If no --include or --exclude options match, a file is included unless the first such option is --include.
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.
For each directory operand, read and process all files in that directory, recursively, following all symbolic links.
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.
Use line buffering for standard output, regardless of output device. By default, standard output is line buffered for interactive devices, and is fully buffered otherwise. With full buffering, the output buffer is flushed when full; with line buffering, the buffer is also flushed after every output line. The buffer size is system dependent.
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.
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.
The behavior of grep is affected by several environment
variables, the most important of which control the locale, which
specifies how grep interprets characters in its patterns and
data.
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.UTF-8’,
then a 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.
The following environment variables affect the behavior of grep.
GREP_COLORThis obsolescent variable interacts with GREP_COLORS
confusingly, and grep warns if it is set and is not
overridden by GREP_COLORS. Instead of
‘GREP_COLOR='color'’, you can use
‘GREP_COLORS='mt=color'’.
GREP_COLORSThis variable controls how the --color option highlights 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).
The two-letter capability names
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.
Non-empty capability values
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.
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).
rvBoolean 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;31SGR 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;31SGR 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;31SGR 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=35SGR substring for file names prefixing any content line. The default is a magenta text foreground over the terminal’s default background.
ln=32SGR substring for line numbers prefixing any content line. The default is a green text foreground over the terminal’s default background.
bn=32SGR substring for byte offsets prefixing any content line. The default is a green text foreground over the terminal’s default background.
se=36SGR 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.
neBoolean 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_ALLLC_COLLATELANGThese variables specify the locale for the LC_COLLATE category,
which might affect how range expressions like ‘a-z’ are
interpreted.
LC_ALLLC_CTYPELANGThese 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.
See Character Encoding.
LANGUAGELC_ALLLC_MESSAGESLANGThese 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_CORRECTIf 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.
TERMThis variable specifies the output terminal type, which can affect what the --color option does. See General Output Control.
The GREP_OPTIONS environment variable of grep 2.20 and
earlier is no longer supported, as it caused problems when writing
portable scripts. To make arbitrary changes to how grep
works, you can 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
$HOME/bin/grep containing the following:
#! /bin/sh export PATH=/usr/bin exec grep --color=auto --devices=skip "$@"
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
-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.
Interpret patterns as basic regular expressions (BREs). This is the default.
Interpret patterns as extended regular expressions (EREs). (-E is specified by POSIX.)
Interpret patterns as fixed strings, not regular expressions. (-F is specified by POSIX.)
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.
For documentation, refer to https://www.pcre.org/, with these caveats:
grep applies each regexp to a line at a time,
so the ‘(?s)’ directive (making ‘.’ match line breaks)
is generally ineffective.
However, with -z (--null-data) it can work:
$ printf 'a\nb\n' |grep -zP '(?s)a.b' a b
But beware: with the -z (--null-data) and a file containing no NUL byte, grep must read the entire file into memory before processing any of it. Thus, it will exhaust memory and fail for some large files.
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,
basic and extended regular expressions are merely different notations
for the same pattern-matching functionality.
In other implementations, basic regular expressions are ordinarily
less powerful than extended, though occasionally it is the other way around.
The following description applies to extended regular expressions;
differences for basic regular expressions are summarized afterwards.
Perl-compatible regular expressions have different functionality, and
are documented in the pcre2syntax(3) and pcre2pattern(3) manual
pages, but work only if PCRE is available in the system.
In regular expressions, the characters ‘.?*+{|()[\^$’ are special characters and have uses described below. All other characters are ordinary characters, and each ordinary character is a regular expression that matches itself.
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 operators beginning with ‘{’ are called interval expressions.
The preceding item is optional and is matched at most once.
The preceding item is matched zero or more times.
The preceding item is matched one or more times.
The preceding item is matched exactly n times.
The preceding item is matched n or more times.
The preceding item is matched at most m times. This is a GNU extension.
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 of the two expressions, which are called alternatives.
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.
Not every character string is a valid regular expression. See Problematic Regular 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 be erratic, or it might be invalid.
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.
Alphanumeric characters: ‘[:alpha:]’ and ‘[:digit:]’; in the ‘C’ locale and ASCII character encoding, this is the same as ‘[0-9A-Za-z]’.
Alphabetic characters: ‘[:lower:]’ and ‘[:upper:]’; in the ‘C’ locale and ASCII character encoding, this is the same as ‘[A-Za-z]’.
Blank characters: space and tab.
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.
Digits: 0 1 2 3 4 5 6 7 8 9.
Graphical characters: ‘[:alnum:]’ and ‘[:punct:]’.
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.
Printable characters: ‘[:alnum:]’, ‘[:punct:]’, and space.
Punctuation characters; in the ‘C’ locale and ASCII character
encoding, this is
! " # $ % & ' ( ) * + , - . / : ; < = > ? @ [ \ ] ^ _ ` { | } ~.
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-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.
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
regular expression ‘[:epru]’.
Special 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. To make the ‘-’ a list item, it is best to put it last.
represents the characters not in the list. If you want to make the ‘^’ character a list item, place it anywhere but first.
The ‘\’ character followed by a special character is a regular expression that matches the special character. The ‘\’ character, when followed by certain ordinary characters, takes a special meaning:
Match the empty string at the edge of a word.
Match the empty string provided it’s not at the edge of a word.
Match the empty string at the beginning of a word.
Match the empty string at the end of a word.
Match word constituent, it is a synonym for ‘[_[:alnum:]]’.
Match non-word constituent, it is a synonym for ‘[^_[:alnum:]]’.
Match whitespace, it is a synonym for ‘[[:space:]]’.
Match non-whitespace, it is a synonym for ‘[^[:space:]]’.
Match ‘]’.
Match ‘}’.
For example, ‘\brat\b’ matches the separate word ‘rat’, ‘\Brat\B’ matches ‘crate’ but not ‘furry rat’.
The behavior of grep is unspecified if a unescaped backslash
is not followed by a special character, a nonzero digit, or a
character in the above list. Although grep might issue a
diagnostic and/or give the backslash an interpretation now, its
behavior may change if the syntax of regular expressions is extended
in future versions.
The caret ‘^’ and the dollar sign ‘$’ are special characters that respectively match the empty string at the beginning and end of a line. They are termed anchors, since they force the match to be “anchored” to beginning or end of a line, respectively.
The back-reference ‘\n’, where n is a single nonzero digit, matches the substring previously matched by the nth parenthesized subexpression of the regular expression. For example, ‘(a)\1’ matches ‘aa’. If the parenthesized subexpression does not participate in the match, the back-reference makes the whole match fail; for example, ‘(a)*\1’ fails to match ‘a’. If the parenthesized subexpression matches more than one substring, the back-reference refers to the last matched substring; for example, ‘^(ab*)*\1$’ matches ‘ababbabb’ but not ‘ababbab’. 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 regular expressions differ from extended regular expressions in the following ways:
Some strings are invalid regular expressions and cause
grep to issue a diagnostic and fail. For example, ‘xy\1’
is invalid because there is no parenthesized subexpression for the
back-reference ‘\1’ to refer to.
Also, some regular expressions have unspecified behavior and
should be avoided even if grep does not currently diagnose
them. For example, ‘xy\0’ has unspecified behavior because
‘0’ is not a special character and ‘\0’ is not a special
backslash expression (see Special Backslash Expressions).
Unspecified behavior can be particularly problematic because the set
of matched strings might be only partially specified, or not be
specified at all, or the expression might even be invalid.
The following regular expression constructs are invalid on all
platforms conforming to POSIX, so portable scripts can assume that
grep rejects these constructs:
GNU grep treats the following constructs as invalid.
However, other grep implementations might allow them, so
portable scripts should not rely on their being invalid:
grep might treat it as
a valid range that never matches.
grep might treat
‘[:alpha:]’ like ‘[[:alpha:]]’, or like ‘[:ahlp]’.
The following constructs have well-defined behavior in GNU
grep. However, they have unspecified behavior elsewhere, so
portable scripts should avoid them:
grep treats the ‘{’ as an ordinary character.
grep silently supplies a newline.
The following constructs have unspecified behavior, in both GNU
and other grep implementations. Scripts should avoid
them whenever possible.
grep might specify ‘\x’ to have a new behavior.
grep it is not documented whether
these range expressions use native code points, or use the collating
sequence specified by the LC_COLLATE category, or have some
other interpretation. Outside the POSIX locale, it is portable to use
‘[[:lower:]]’ to match a lower-case letter, or
‘[abcdefghijklmnopqrstuvwxyz]’ to match an ASCII lower-case
letter.
The LC_CTYPE locale specifies the encoding of characters in
patterns and data, that is, whether text is encoded in UTF-8, ASCII,
or some other encoding. See Environment Variables.
In the ‘C’ or ‘POSIX’ locale, every character is 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.
Regardless of locale, the 103 characters in the POSIX Portable Character Set (a subset of ASCII) are always encoded as a single byte, and the 128 ASCII characters have their usual single-byte encodings on all but oddball platforms.
In a regular expression, non-ASCII and non-printable characters other than newline are not special, and represent themselves. For example, in a locale using UTF-8 the command ‘grep 'Λ ω'’ (where the white space between ‘Λ’ and the ‘ω’ is a tab character) searches for ‘Λ’ (Unicode character U+039B GREEK CAPITAL LETTER LAMBDA), followed by a tab (U+0009 TAB), followed by ‘ω’ (U+03C9 GREEK SMALL LETTER OMEGA).
Suppose you want to limit your pattern to only printable characters
(or even only printable ASCII characters) to keep your script readable
or portable, but you also want to match specific non-ASCII or non-null
non-printable characters. If you are using the -P
(--perl-regexp) option, PCREs give you several ways to do
this. Otherwise, if you are using Bash, the GNU project’s shell, you
can represent these characters via ANSI-C quoting. For example, the
Bash commands ‘grep $'Λ\tω'’ and ‘grep $'\u039B\t\u03C9'’
both search for the same three-character string ‘Λ ω’
mentioned earlier. However, because Bash translates ANSI-C quoting
before grep sees the pattern, this technique should not be
used to match printable ASCII characters; for example, ‘grep
$'\u005E'’ is equivalent to ‘grep '^'’ and matches any line, not
just lines containing the character ‘^’ (U+005E CIRCUMFLEX
ACCENT).
Since PCREs and ANSI-C quoting are GNU extensions to POSIX, portable shell scripts written in ASCII should use other methods to match specific non-ASCII characters. For example, in a UTF-8 locale the command ‘grep "$(printf '\316\233\t\317\211\n')"’ is a portable albeit hard-to-read alternative to Bash’s ‘grep $'Λ\tω'’. However, none of these techniques will let you put a null character directly into a command-line pattern; null characters can appear only in a pattern specified via the -f (--file) option.
Here is an example command that invokes GNU grep:
grep -i 'hello.*world' menu.h main.c
This lists all lines in the files menu.h and main.c that
contain the string ‘hello’ followed by the string ‘world’;
this is because ‘.*’ matches zero or more characters within a line.
See Regular Expressions.
The -i option causes grep
to ignore case, causing it to match the line ‘Hello, world!’, which
it would not otherwise match.
Here is a more complex example,
showing the location and contents of any line
containing ‘f’ and ending in ‘.c’,
within all files in the current directory whose names
start with non-‘.’, contain ‘g’, and end in ‘.h’.
The -n option outputs line numbers, the -- argument
treats any later arguments as file names not options even if
*g*.h expands to a file name that starts with ‘-’,
and the empty file /dev/null causes file names to be output
even if only one file name happens to be of the form ‘*g*.h’.
grep -n -- 'f.*\.c$' *g*.h /dev/null
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 grep, for more details about
how to invoke grep.
Here are some common questions and answers about grep usage.
grep -l 'main' test-*.c
lists names of ‘test-*.c’ files in the current directory whose contents mention ‘main’.
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
grep "$pattern" *
can behave unexpectedly if the value of ‘pattern’ begins with ‘-’, or if the ‘*’ expands to a file name with leading ‘-’. To avoid the problem, you can use -e for patterns and leading ‘./’ for files:
grep -e "$pattern" ./*
searches for all lines matching the pattern in all the working
directory’s files whose names do not begin with ‘.’.
Without the -e, grep might treat the pattern as an
option if it begins with ‘-’. Without the ‘./’, there might
be similar problems with file names beginning with ‘-’.
Alternatively, you can use ‘--’ before the pattern and file names:
grep -- "$pattern" *
This also fixes the problem, except that if there is a file named ‘-’,
grep misinterprets the ‘-’ as standard input.
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’.
grep -C 2 'hello' test*.log
prints two lines of context around each matching line.
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
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.
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.
‘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.
grep 'paul' /etc/motd | grep 'franc,ois'
finds all lines that contain both ‘paul’ and ‘franc,ois’.
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 an extended regular expression like ‘a^’ or ‘$a’. To match every line, a portable script should use a pattern like ‘^’ instead of the empty pattern, as POSIX does not specify the behavior of the empty pattern.
Use the special file name ‘-’:
cat /etc/passwd | grep 'alain' - /etc/motd
grep?
The grep command follows the convention of programs like
cmp and diff where an exit status of 1 is not an
error. The shell command ‘set -e’ causes the shell to exit if
any subcommand exits with nonzero status, and this will cause the
shell to exit merely because grep selected no lines,
which is ordinarily not what you want.
There is a related problem with Bash’s set -e -o pipefail.
Since grep does not always read all its input, a command
outputting to a pipe read by grep can fail when
grep exits before reading all its input, and the command’s
failure can cause Bash to exit.
echo 'ba' | grep -E '(a)\1|b\1'
This outputs an error message, because the second ‘\1’ has nothing to refer back to, meaning it will never match anything.
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.
grep, -E, and -F 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
The -E option stands for Extended grep.
The -F option stands for Fixed grep;
egrep and fgrep?
7th Edition Unix had commands egrep and fgrep
that were the counterparts of the modern ‘grep -E’ and ‘grep -F’.
Although breaking up grep into three programs was perhaps
useful on the small computers of the 1970s, egrep and
fgrep were deemed obsolescent by POSIX in 1992,
removed from POSIX in 2001, deprecated by GNU Grep 2.5.3 in 2007,
and changed to issue obsolescence warnings by GNU Grep 3.8 in 2022;
eventually, they are planned to be removed entirely.
If you prefer the old names, you can use your own substitutes,
such as a shell script named egrep with the following
contents:
#!/bin/sh exec grep -E "$@"
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.
Interval expressions may be implemented internally via repetition. For example, ‘^(a|bc){2,4}$’ might be implemented as ‘^(a|bc)(a|bc)((a|bc)(a|bc)?)?$’. A large repetition count may exhaust memory or greatly slow matching. Even small counts can cause problems if cascaded; for example, ‘grep -E ".*{10,}{10,}{10,}{10,}{10,}"’ is likely to overflow a stack. Fortunately, regular expressions like these are typically artificial, and cascaded repetitions do not conform to POSIX so cannot be used in portable programs anyway.
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}$’ matches only lines whose lengths can be written as a sum 15x + 14y for nonnegative integers x and y, the pattern matcher does not perform linear Diophantine analysis and instead backtracks through all possible matching strings, using an algorithm that is exponential in the worst case.
On some operating systems that support files with holes—large
regions of zeros that are not physically present on secondary
storage—grep can skip over the holes efficiently without
needing to read the zeros. This optimization is not available if the
-a (--binary-files=text) option is used (see File and Directory Selection), unless the -z (--null-data)
option is also used (see Other Options).
For efficiency grep does not always read all its input.
For example, the shell command ‘sed '/^...$/d' | grep -q X’ can
cause grep to exit immediately after reading a line
containing ‘X’, without bothering to read the rest of its input data.
This in turn can cause sed to exit with a nonzero status because
sed cannot write to its output pipe after grep exits.
For more about the algorithms used by grep and about
related string matching algorithms, see:
grep.
grep in the future.
grep.
Bug reports can be found at the
GNU bug report logs for grep.
If you find a bug not listed there, please email it to
bug-grep@gnu.org to create a new bug report.
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 2021 the GNU C library bug database contained back-reference bugs 52, 10844, 11053, 24269 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.
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.
Copyright © 2000–2002, 2007–2008, 2023 Free Software Foundation, Inc. https://fsf.org/ Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
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