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NAME
grep — search a file for a pattern
SYNOPSIS
grep [-E|-F] [-c|-l|-q] [-insvx] -e pattern_list
[-e pattern_list]... [-f pattern_file]... [file...]
grep [-E|-F] [-c|-l|-q] [-insvx] [-e pattern_list]...
-f pattern_file [-f pattern_file]... [file...]
grep [-E|-F] [-c|-l|-q] [-insvx] pattern_list [file...]
DESCRIPTION
The grep utility shall search the input files, selecting lines matching one or more
patterns; the types of patterns are controlled by the options specified. The patterns are
specified by the -e option, -f option, or the pattern_list operand. The pattern_list's
value shall consist of one or more patterns separated by <newline> characters; the
pattern_file's contents shall consist of one or more patterns terminated by a <newline>
character. By default, an input line shall be selected if any pattern, treated as an
entire basic regular expression (BRE) as described in the Base Definitions volume of
POSIX.1‐2017, Section 9.3, Basic Regular Expressions, matches any part of the line
excluding the terminating <newline>; a null BRE shall match every line. By default, each
selected input line shall be written to the standard output.
Regular expression matching shall be based on text lines. Since a <newline> separates or
terminates patterns (see the -e and -f options below), regular expressions cannot contain
a <newline>. Similarly, since patterns are matched against individual lines (excluding
the terminating <newline> characters) of the input, there is no way for a pattern to match
a <newline> found in the input.
OPTIONS
The grep utility shall conform to the Base Definitions volume of POSIX.1‐2017, Section
12.2, Utility Syntax Guidelines.
The following options shall be supported:
-E Match using extended regular expressions. Treat each pattern specified as an
ERE, as described in the Base Definitions volume of POSIX.1‐2017, Section 9.4,
Extended Regular Expressions. If any entire ERE pattern matches some part of an
input line excluding the terminating <newline>, the line shall be matched. A
null ERE shall match every line.
-F Match using fixed strings. Treat each pattern specified as a string instead of a
regular expression. If an input line contains any of the patterns as a
contiguous sequence of bytes, the line shall be matched. A null string shall
match every line.
-c Write only a count of selected lines to standard output.
-e pattern_list
Specify one or more patterns to be used during the search for input. The
application shall ensure that patterns in pattern_list are separated by a
<newline>. A null pattern can be specified by two adjacent <newline> characters
in pattern_list. Unless the -E or -F option is also specified, each pattern
shall be treated as a BRE, as described in the Base Definitions volume of
POSIX.1‐2017, Section 9.3, Basic Regular Expressions. Multiple -e and -f
options shall be accepted by the grep utility. All of the specified patterns
shall be used when matching lines, but the order of evaluation is unspecified.
-f pattern_file
Read one or more patterns from the file named by the pathname pattern_file.
Patterns in pattern_file shall be terminated by a <newline>. A null pattern can
be specified by an empty line in pattern_file. Unless the -E or -F option is
also specified, each pattern shall be treated as a BRE, as described in the Base
Definitions volume of POSIX.1‐2017, Section 9.3, Basic Regular Expressions.
-i Perform pattern matching in searches without regard to case; see the Base
Definitions volume of POSIX.1‐2017, Section 9.2, Regular Expression General
Requirements.
-l (The letter ell.) Write only the names of files containing selected lines to
standard output. Pathnames shall be written once per file searched. If the
standard input is searched, a pathname of "(standardinput)" shall be written, in
the POSIX locale. In other locales, "standardinput" may be replaced by something
more appropriate in those locales.
-n Precede each output line by its relative line number in the file, each file
starting at line 1. The line number counter shall be reset for each file
processed.
-q Quiet. Nothing shall be written to the standard output, regardless of matching
lines. Exit with zero status if an input line is selected.
-s Suppress the error messages ordinarily written for nonexistent or unreadable
files. Other error messages shall not be suppressed.
-v Select lines not matching any of the specified patterns. If the -v option is not
specified, selected lines shall be those that match any of the specified
patterns.
-x Consider only input lines that use all characters in the line excluding the
terminating <newline> to match an entire fixed string or regular expression to
be matching lines.
OPERANDS
The following operands shall be supported:
pattern_list
Specify one or more patterns to be used during the search for input. This
operand shall be treated as if it were specified as -e pattern_list.
file A pathname of a file to be searched for the patterns. If no file operands are
specified, the standard input shall be used.
STDIN
The standard input shall be used if no file operands are specified, and shall be used if a
file operand is '-' and the implementation treats the '-' as meaning standard input.
Otherwise, the standard input shall not be used. See the INPUT FILES section.
INPUT FILES
The input files shall be text files.
ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of grep:
LANG Provide a default value for the internationalization variables that are unset or
null. (See the Base Definitions volume of POSIX.1‐2017, Section 8.2,
Internationalization Variables for the precedence of internationalization
variables used to determine the values of locale categories.)
LC_ALL If set to a non-empty string value, override the values of all the other
internationalization variables.
LC_COLLATE
Determine the locale for the behavior of ranges, equivalence classes, and multi-
character collating elements within regular expressions.
LC_CTYPE Determine the locale for the interpretation of sequences of bytes of text data
as characters (for example, single-byte as opposed to multi-byte characters in
arguments and input files) and the behavior of character classes within regular
expressions.
LC_MESSAGES
Determine the locale that should be used to affect the format and contents of
diagnostic messages written to standard error.
NLSPATH Determine the location of message catalogs for the processing of LC_MESSAGES.
ASYNCHRONOUS EVENTS
Default.
STDOUT
If the -l option is in effect, the following shall be written for each file containing at
least one selected input line:
"%s\n", <file>
Otherwise, if more than one file argument appears, and -q is not specified, the grep
utility shall prefix each output line by:
"%s:", <file>
The remainder of each output line shall depend on the other options specified:
* If the -c option is in effect, the remainder of each output line shall contain:
"%d\n", <count>
* Otherwise, if -c is not in effect and the -n option is in effect, the following shall
be written to standard output:
"%d:", <line number>
* Finally, the following shall be written to standard output:
"%s", <selected-line contents>
STDERR
The standard error shall be used only for diagnostic messages.
OUTPUT FILES
None.
EXTENDED DESCRIPTION
None.
EXIT STATUS
The following exit values shall be returned:
0 One or more lines were selected.
1 No lines were selected.
>1 An error occurred.
CONSEQUENCES OF ERRORS
If the -q option is specified, the exit status shall be zero if an input line is selected,
even if an error was detected. Otherwise, default actions shall be performed.
The following sections are informative.
APPLICATION USAGE
Care should be taken when using characters in pattern_list that may also be meaningful to
the command interpreter. It is safest to enclose the entire pattern_list argument in
single-quotes:
'...'
The -e pattern_list option has the same effect as the pattern_list operand, but is useful
when pattern_list begins with the <hyphen-minus> delimiter. It is also useful when it is
more convenient to provide multiple patterns as separate arguments.
Multiple -e and -f options are accepted and grep uses all of the patterns it is given
while matching input text lines. (Note that the order of evaluation is not specified. If
an implementation finds a null string as a pattern, it is allowed to use that pattern
first, matching every line, and effectively ignore any other patterns.)
The -q option provides a means of easily determining whether or not a pattern (or string)
exists in a group of files. When searching several files, it provides a performance
improvement (because it can quit as soon as it finds the first match) and requires less
care by the user in choosing the set of files to supply as arguments (because it exits
zero if it finds a match even if grep detected an access or read error on earlier file
operands).
When using grep to process pathnames, it is recommended that LC_ALL, or at least LC_CTYPE
and LC_COLLATE, are set to POSIX or C in the environment, since pathnames can contain byte
sequences that do not form valid characters in some locales, in which case the utility's
behavior would be undefined. In the POSIX locale each byte is a valid single-byte
character, and therefore this problem is avoided.
EXAMPLES
1. To find all uses of the word "Posix" (in any case) in file text.mm and write with line
numbers:
grep -i -n posix text.mm
2. To find all empty lines in the standard input:
grep ^$
or:
grep -v .
3. Both of the following commands print all lines containing strings "abc" or "def" or
both:
grep -E 'abc|def'
grep -F 'abc
def'
4. Both of the following commands print all lines matching exactly "abc" or "def":
grep -E '^abc$|^def$'
grep -F -x 'abc
def'
RATIONALE
This grep has been enhanced in an upwards-compatible way to provide the exact
functionality of the historical egrep and fgrep commands as well. It was the clear
intention of the standard developers to consolidate the three greps into a single command.
The old egrep and fgrep commands are likely to be supported for many years to come as
implementation extensions, allowing historical applications to operate unmodified.
Historical implementations usually silently ignored all but one of multiply-specified -e
and -f options, but were not consistent as to which specification was actually used.
The -b option was omitted from the OPTIONS section because block numbers are
implementation-defined.
The System V restriction on using - to mean standard input was omitted.
A definition of action taken when given a null BRE or ERE is specified. This is an error
condition in some historical implementations.
The -l option previously indicated that its use was undefined when no files were
explicitly named. This behavior was historical and placed an unnecessary restriction on
future implementations. It has been removed.
The historical BSD grep -s option practice is easily duplicated by redirecting standard
output to /dev/null. The -s option required here is from System V.
The -x option, historically available only with fgrep, is available here for all of the
non-obsolescent versions.
FUTURE DIRECTIONS
None.
SEE ALSO
sed
The Base Definitions volume of POSIX.1‐2017, Chapter 8, Environment Variables, Chapter 9,
Regular Expressions, Section 12.2, Utility Syntax Guidelines
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1-2017, Standard for Information Technology -- Portable Operating System Interface
(POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by
the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and The Open Group
Standard, the original IEEE and The Open Group Standard is the referee document. The
original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have
been introduced during the conversion of the source files to man page format. To report
such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .