| Developer(s) | GNU Project |
|---|---|
| Stable release | 2.10 / November 16, 2011 |
| Development status | Active |
| Operating system | Linux, Mac OS, Solaris and Windows |
| Platform | IA-32, x86-64, PowerPC and others |
| Available in | English and others |
grep is a command-line text-search utility originally written for Unix. The name comes from the ed command g/re/p (global / regular expression / print).[1][2] The grep command searches files or standard input globally for lines matching a given regular expression, and prints the lines to the program's standard output.
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Grep was created by Ken Thompson as a standalone application adapted from the regular expression parser he had written for ed (which he also created). Its official creation date is given as March 3, 1973, in the Manual for Unix Version 4.
This is an example of a common usage of grep:
grep apple fruitlist.txt
In this case, grep prints all lines containing the sequence of characters apple from the file fruitlist.txt; therefore lines containing pineapple or apples are also printed. The pattern specified as a grep argument is case sensitive by default, so this example's output does not include lines containing Apple (with a capital A) unless they also contain apple.
More than one file name may be specified as arguments to grep. For, example all files having the extension '.txt' in a given directory may be searched if the shell supports globbing by using an asterisk in place of the file name:
grep apple *.txt
Regular expressions can be used to match more complicated text patterns. The following prints all lines in the file that begin with the letter a, followed by any one character, followed by the letter sequence ple.
grep ^a.ple fruitlist.txt
As noted above, the name of grep derives from a usage in ed and related text editors. Before grep existed as a separate command, the same effect might have been achieved in an editor:
ed fruitlist.txt g/^a.ple/p q
where the second line is the command given to ed to print the relevant lines, and the third line is the command to exit from ed.
Like most Unix commands, grep accepts options in the form of command-line arguments, to change many of its behaviors. For example, the option flag -i enables case-insensitive search (ignore case).
grep -i apple fruitlist.txt
This prints all lines containing apple regardless of capitalization.
Selecting all lines containing apple as a word, i.e. surrounded' by white space (pineapple and apples do not match) may be accomplished with the -w option flag:
grep -w apple fruitlist.txt
But if fruitlist.txt contains apple as a word followed by hyphen (-) character, it is also matched.
cat fruitlist.txt apple apples pineapple apple- apple-fruit fruit-apple grep -w apple fruitlist.txt apple apple- apple-fruit fruit-apple
Exact line match is performed with the -x option. Lines only containing exactly and solely apple are selected with a line-regexp instead of word-regexp:
cat fruitlist.txt apple apples pineapple apple- apple-fruit fruit-apple grep -x apple fruitlist.txt apple
The -v (lower-case v) option reverses the sense of the match and prints all lines that do not contain apple, as in this example.
grep -v apple fruitlist.txt banana pear peach orange
There are countless implementations and derivatives of grep available for many operating systems. Early variants of grep included egrep and fgrep. egrep applies an extended regular expression syntax that was added to Unix after Ken Thompson's original regular expression implementation. fgrep searches for any of a list of fixed strings using the Aho–Corasick string matching algorithm. These variants of grep persist in most modern grep implementations as command-line switches (and standardized as -E and -F in POSIX[3]). In such combined implementations, grep may also behave differently depending on the name by which it is invoked, allowing fgrep, egrep, and grep to be links to the same program.
Other commands contain the word "grep" to indicate that they search (usually for regular expression matches). The pgrep utility, for instance, displays the processes whose names match a given regular expression.
In Perl, grep is the name of the built-in function that finds elements in a list that satisfy a certain property. This higher-order function is typically named filter in functional programming languages.
pcregrep is an implementation of grep that uses Perl regular expression syntax.
Ports of grep (within Cygwin and GnuWin32, for example) also run under Microsoft Windows. Some versions of Windows feature the similar qgrep command.[4]
In December 2003, the Oxford English Dictionary Online added draft entries for "grep" as both a noun and a verb.
A common verb usage is the phrase "You can't grep dead trees"—meaning one can more easily search through digital media, using tools such as grep, than one could with a hard copy (i.e., one made from dead trees, paper).[5] Compare with google.
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