grep - search a file for a pattern
grep [-E| -F][-c| -l| -q][-insvx]
grep [-E| -F][-c| -l| -q][-insvx][-e
grep [-E| -F][-c| -l| -q][-insvx]
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>s; the pattern_file's contents shall consist of one or more patterns terminated by <newline>. 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 IEEE Std 1003.1-2001, 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>s) of the input, there is no way for a pattern to match a <newline> found in the input.
The grep utility shall conform to the Base Definitions volume of IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines.
The following options shall be supported:
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>s 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 IEEE Std 1003.1-2001, 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.
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 IEEE Std 1003.1-2001, Section 9.3, Basic Regular Expressions.
The following operands shall be supported:
The standard input shall be used only if no file operands are specified. See the INPUT FILES section.
The input files shall be text files.
The following environment variables shall affect the execution of grep:
Determine the locale for the behavior of ranges, equivalence classes, and multi-character collating elements within regular expressions.
If the -l option is in effect, and the -q option is not, the following shall be written for each file containing at least one selected input line:
Otherwise, if more than one file argument appears, and -q is not specified, the grep utility shall prefix each output line by:
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:
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>
The standard error shall be used only for diagnostic messages.
The following exit values shall be returned:
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.
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 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).
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
To find all empty lines in the standard input:
grep -v .
Both of the following commands print all lines containing strings "abc" or "def" or both:
grep -E 'abc|def' grep -F 'abc def'
Both of the following commands print all lines matching exactly "abc" or "def" :
grep -E '^abc$|^def$' grep -F -x 'abc def'
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.
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Created 1996-2023 by Maxim Chirkov
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