|Original author(s)||Ken Thompson|
|Developer(s)||AT&T Bell Laboratories|
|Initial release||November 1973|
|Operating system||Unix, Unix-like, Plan 9, Inferno, OS-9, MSX-DOS, IBM i|
grep is a command-line utility for searching plain-text data sets for lines that match a regular expression. Its name comes from the ed command g/re/p (globally search for a regular expression and print matching lines), which has the same effect.
grep was originally developed for the Unix operating system, but later available for all Unix-like systems and some others such as OS-9.
Before it was named, grep was a private utility written by Ken Thompson to search files for certain patterns. Doug McIlroy, unaware of its existence, asked Thompson to write such a program. Responding that he would think about such a utility overnight, Thompson actually corrected bugs and made improvements for about an hour. The next day he presented the program to McIlroy, who said it was exactly what he wanted. Thompson's account may explain the belief that grep was written overnight.
Thompson wrote the first version in PDP-11 assembly language to help Lee E. McMahon analyze the text of the Federalist Papers to determine authorship of the individual papers. The ed text editor (also authored by Thompson) had regular expression support but could not be used on such a large amount of text, so Thompson excerpted that code into a standalone tool. He chose the name because in ed, the command g/re/p would print all lines matching a specified pattern.
grep was first included in Version 4 Unix. Stating that it is "generally cited as the prototypical software tool", McIlroy credited
grep with "irrevocably ingraining" Thompson's tools philosophy in Unix.
The following example demonstrates the output of the
grep command given different arguments
$ grep root /etc/passwd root:x:0:0:root:/root:/bin/bash operator:x:11:0:operator:/root:/sbin/nologin $ grep -n root /etc/passwd 1:root:x:0:0:root:/root:/bin/bash 12:operator:x:11:0:operator:/root:/sbin/nologin $ grep -c false /etc/passwd 7
A variety of
grep implementations are available in many operating systems and software development environments. Early variants included
fgrep, introduced in Version 7 Unix. The "
egrep" variant supports an extended regular expression syntax added by Alfred Aho after Ken Thompson's original regular expression implementation. The "
fgrep" variant searches for any of a list of fixed strings using the Aho-Corasick string matching algorithm. Binaries of these variants persist in most modern systems, usually linking to
grep; however, using these binaries has been deprecated in favor of using the
-F command-line switches of
Other commands contain the word "grep" to indicate they are search tools, typically ones that rely on regular expression matches. The
pgrep utility, for instance, displays the processes whose names match a given regular expression.
In the Perl programming language, 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
where in other languages.
The software Adobe InDesign has functions GREP (since CS3 version (2007)), in the find/change dialog box "GREP" tab, and introduced with InDesign CS4 in paragraph styles "GREP styles".
agrep (approximate grep) matches even when the text only approximately fits the search pattern.
This following invocation finds netmasks in file myfile, but also any other word that can be derived from it, given no more than two substitutions.
agrep -2 netmasks myfile
This example generates a list of matches with the closest, that is those with the fewest, substitutions listed first. The command flag B means best:
agrep -B netmasks myfile
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). Compare with google.
QGREP.EXE[:] A similar tool to grep in UNIX, this tool can be used to search for a text string