grep

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.[6]

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.[7] 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.[1] He chose the name because in ed, the command g/re/p would print all lines matching a specified pattern.[8][9] 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.[10]

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.[11] Early variants included egrep and fgrep, introduced in Version 7 Unix.[10] The "egrep" variant supports an extended regular expression syntax added by Alfred Aho after Ken Thompson's original regular expression implementation.[12] The "fgrep" variant searches for any of a list of fixed strings using the Aho–Corasick string matching algorithm.[13] Binaries of these variants persist in most modern systems (although usually linking to grep), 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.[14]

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.[15]

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.[16] This higher-order function is typically named filter or where in other languages.

The pcregrep command is an implementation of grep that uses Perl regular expression syntax.[17] Similar functionality can be invoked in the GNU version of grep with the -P flag.[18]

Ports of grep (within Cygwin and GnuWin32, for example) also run under Microsoft Windows. Some versions of Windows feature the similar qgrep or findstr command.[19]

A grep command is also part of ASCII's MSX-DOS2 Tools for MSX-DOS version 2.[20]

The grep, egrep, and fgrep commands have also been ported to the IBM i operating system.[21]

The software Adobe InDesign has functions GREP (since CS3 version (2007)[22]), in the find/change dialog box[23] "GREP" tab, and introduced with InDesign CS4[24] in paragraph styles[25] "GREP styles".

agrep (approximate grep) matches even when the text only approximately fits the search pattern.[26]

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).[27] Compare with google.

• Boyer–Moore string search algorithm
• agrep, an approximate string-matching command
• find (Windows), a DOS and Windows command that performs text searches, similar to a simple grep
• find (Unix), a Unix command that finds files by attribute, very different from grep
• List of Unix commands
• vgrep, or "visual grep"

Notes

The Wikibook How To Search has a page on the topic of: grep

• GNU Grep official website
• GNU Grep manual
• grep(1) – Plan 9 Programmer's Manual, Volume 1
• grep(1) – Inferno General commands Manual
• "why GNU grep is fast" - implementation details from GNU grep's author.
• Network grep - A packet analyzer used to match patterns at the network layer
• Command Grep – 25 practical examples