grep

Last updated: Apr 21, 2020

Name

grep, egrep, fgrep - print lines matching a pattern

Restrictions in NIOS Expert Mode

grep is not allowed to open any file except stdin. –f and FILE are not allowed. grep can only be used after the pipe symbol ( | ).

Synopsis

grep [OPTIONS] PATTERN
grep [OPTIONS] [-e PATTERN]

Description

grep searches the standard input for lines containing a match to the given PATTERN. By default,grep prints the matching lines.

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.

Options

Generic Program Information

--help Print a usage message briefly summarizing these 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 (see below).

Matcher Selection

-E, --extended-regexp

Interpret PATTERN as an extended regular expression (ERE, see below). (-E is specified by POSIX.)

-F, --fixed-strings

Interpret PATTERN as a list of fixed strings, separated by newlines, any of which is to be matched. (-F is specified by POSIX.)

-G, --basic-regexp

Interpret PATTERN as a basic regular expression (BRE, see below). This is the default.

-P, --perl-regexp

Interpret PATTERN as a Perl regular expression. This is highly experimental and grep -P may warn of unimplemented features.

Matching Control

-e PATTERN, --regexp=PATTERN

Use PATTERN as the pattern. This can be used to specify multiple search patterns, or to protect a pattern beginning with a hyphen (-). (-e is specified by POSIX.)

-i, --ignore-case

Ignore case distinctions in the PATTERN. (-i is specified by POSIX.)

-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.

-x, --line-regexp

Select only those matches that exactly match the whole line. (-x is specified by POSIX.)

-y Obsolete synonym for -i.

General Output Control

-c, --count

Suppress normal output. With the -v, --invert-match option (see below), count non-matching lines.(-c is specified by POSIX.)

--color[=WHEN], --colour[=WHEN]

Surround the matched (non-empty) strings, matching lines, context lines, 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. The deprecated environment variable GREP_COLOR is still supported, but its setting does not have priority. WHEN is never, always, or auto.

-m NUM, --max-count=NUM

If the input is standard input from a regular file, and NUM matching lines are output, grep ensures that the standard input is positioned to just after the last matching line before exiting, regardless of the presence of trailing context lines. This enables a calling process to resume a search. When grep stops after NUM matching
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 a matching line, with each such part on a separate output line.

-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. USG -style grep also lacked -q but its -s option behaved like GNU grep.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

-b, --byte-offset

If -o (--only-matching) is specified, print the offset of the matching part itself.

--label=LABEL

Display input actually coming from standard input as input coming from file LABEL. This is especially useful when implementing tools like zgrep, e.g.,gzip -cd foo.gz | grep --label=foo -H something. See also the -H option.

-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: -n, and -b. In order to improve the probability that lines from a single file will all start at the same column, this also causes the line number and byte offset (if present) to be printed in a minimum size field width.

-u, --unix-byte-offsets

Report Unix-style byte offsets. This will produce results identical to running grep on a Unix machine. This option has no effect unless -b option is also used; it has no
effect on platforms other than MS-DOS and MS-Windows.

Context Line Control

-A NUM, --after-context=NUM

Print NUM lines of trailing context after matching lines. Places a line containing a group separator (--) between contiguous groups of matches. With the -o or --only-matching option, this has no effect and a warning is given.

-B NUM, --before-context=NUM

Print NUM lines of leading context before matching lines. Places a line containing a group separator (--) between contiguous groups of matches. With the -o or --only-matching option, this has no effect and a warning is given.

-C NUM, -NUM, --context=NUM

Print NUM lines of output context. Places a line containing a group separator (--) between contiguous groups of matches. With the -o or --only-matching option, this has no effect and a warning is given.

Other Options

--line-buffered

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

--mmap If possible, use the mmap(2) system call to read input, instead of the default read(2) system call. In some situations,–mmap yields better performance.

-z, --null-data

Treat the input as a set of lines, each terminated by a zero byte (the ASCII NUL character) instead of a newline.

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:

available functionality between 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 regular expressions give additional functionality, and are documented in pcresyntax(3) and pcrepattern(3), but may not be available on every system.

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.

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.

For example, the regular expression [0123456789] matches any single digit.

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, using the locale’s collating sequence and character set.

For example, in the default C locale, [a-d] is equivalent to [abcd]. Many locales sort characters in
dictionary order, and in these locales [a-d] is typically not equivalent to [abcd]; it might be equivalent to [aBbCcDd], for example. 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 names are self explanatory, and they are [:alnum:],[:alpha:], [:cntrl:],[:digit:], [:graph:], [:lower:],[:print:], [:punct:], [:space:], [:upper:], and [:xdigit:].

For example, [[:alnum:]] means [0-9A-Za-z], except the latter form depends upon the C locale and the ASCII character encoding, whereas the former is independent of locale and character set. (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.) Most meta-characters lose their special meaning inside
bracket expressions. To include a literal ] place it first in the list. Similarly, to include a literal ^ place it anywhere but first. Finally, to include a literal-place it last.

Anchoring

The caret ^ and the dollar sign $ are meta-characters that respectively match the empty string at the beginning and end of a line.

The Backslash Character and Special Expressions.

The symbols \< and \> respectively match the empty string at the beginning and end of a word. The symbol \b matches the empty string at the edge of a word, and \B matches the empty string provided it’s not at the edge of a word.The symbol \w is a synonym for [[:alnum:]] and \W is a synonym for [^[:alnum:]].

Repetition

A regular expression may be followed by one of several repetition operators:
?   The preceding item is optional and 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.
{n,m} The preceding item is matched at least n times, but not more than m times.

Concatenation

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

Alternation

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

Precedence

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.

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.

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.2 allows this behavior as an extension, but portable scripts should avoid it.

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, 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_MESSAGES is set to pt_BR, then the Brazilian Portuguese locale is used for 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).

GREP_OPTIONS

This variable specifies default options to be placed in front of any explicit options. Option specifications are separated by whitespace. A backslash escapes the next character, so it can be used to specify an option containing whitespace or a backslash.

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

Specifies the colors and other attributes used to highlight various parts of the output. Its value is a colon-separated list of 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 only used when the -v command-line option is omitted.) The effect of the sl=(or cx= if rv) capability remains active when this kicks in. 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 only used when the -v command-line option is specified.) The effect of the cx= (or sl= if rv)
capability remains active when this kicks in. The default is a bold red text foreground over the current line 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 term info 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.

See the Select Graphic Rendition (SGR) section in the documentation of the text terminal that is used for permitted values and their meaning 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 SGRsequence (\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.

LC_ALL, LC_COLLATE, LANG

These variables specify the locale for the LC_COLLATE category, which determines the collating sequence used to interpret range expressions like [a-z].

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.

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.2 requires; otherwise, grep behaves more like other GNU programs. Also, POSIX.2 requires that are not really against the law the default is to diagnose them

_N_GNU_nonoption_argv_flags_, described below.

_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. 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 selected lines are found and 1 otherwise. But the exit status is 2 if an error occurred, unless the -q or --quiet or --silent option is used and a selected line is found. Note, however, that POSIX only mandates, for programs such as grep, cmp, and diff, that the exit status in case of error be greater than 1; it is therefore advisable, for the sake of portability, to use logic that tests for this general condition instead of strict equality with 2.

Copyright

This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Bugs

Reporting Bugs

Email bug reports to <bug-grep@gnu.org>, a mailing list whose web page is <http://lists.gnu.org/mailman/listinfo/bug-grep>. grep’s Savannah bug tracker is located at <http://savannah.gnu.org/bugs/?group=grep>.

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 are very slow, and may require exponential time.

Notes

GNU’s not Unix, but Unix is a beast; its plural form is Unixen.

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