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, 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. He was approached by his manager, Doug McIlroy, who expressed interest in having a similar functionality implemented. Thompson responded that he would think about such a utility overnight, but what he did was actually fixing bugs and making improvements, which took about an hour. The next day Thompson presented grep to McIlroy, who said it was exactly what he wanted. Thompson's account may explain why it is believed that grep was written overnight.
The first version of grep was written by Ken Thompson 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 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", Doug McIlroy credited grep with "irrevocably ingraining" Thompson's tools philosophy in Unix.

Sample usage

The following example demonstrates the output of the grep command given different arguments

$ grep root /etc/passwd
$ grep -n root /etc/passwd
$ grep -c false /etc/passwd


A variety of grep implementations are available in many operating systems and software development environments. Early variants included egrep and 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, however their explicit usage has been deprecated and the functionalities of these variants are included in grep as the command-line switches -E and -F; the use of the switches is therefore the recommended method of use.
Other commands contain the word "grep" to indicate that they search. 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 filter in functional programming languages.
The pcregrep command is an implementation of grep that uses Perl regular expression syntax. Similar functionality can be invoked in the GNU version of grep with the -P flag.
Ports of grep also run under Microsoft Windows. Some versions of Windows feature the similar qgrep or findstr command.
A grep command is also part of ASCII's MSX-DOS2 Tools for MSX-DOS version 2.
The software Adobe InDesign has functions GREP, in the find/change dialog box "GREP" tab, and introduced with InDesign CS4 in paragraph styles "GREP styles".

Usage as a verb

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. Compare with google.