|Original author(s)||Ken Thompson|
|Developer(s)||AT&T Bell Laboratories|
|Initial release||November 1974|
|Platform||Unix and Unix-like, OS-9, MSX-DOS|
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 a regular expression and print), which has the same effect: doing a global search with the regular expression and printing all matching lines. 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, it was a private utility written by Ken Thompson to search files. He was approached by his manager, Doug McIlroy, who said it would be great if we could look for things in files. Thompson said he would think about it overnight, but the "overnight think" was actually fixing bugs and doing things he had meant to do with it, about an hour's work. The next day Thompson presented McIlroy with grep, 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 overnight 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 (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. Thompson chose the name because in ed, the command g/re/p would print all lines matching a previously defined 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.
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, however their explicit usage has been deprecated and the functionalities of these variants are included in
grep as the command-line switches
-F; the use of the switches is therefore the recommended method of use.
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 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 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".
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 (i.e., one made from dead trees, paper). Compare with google.
- Boyer–Moore string search algorithm
- List of Unix commands
- agrep, an approximate string-matching command
- find (Windows), a DOS and Windows command that performs text searches, similar to a simple
- find (Unix), a Unix command that finds files by attribute, very different from
- vgrep, or "visual grep"
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QGREP.EXE[:] A similar tool to grep in UNIX, this tool can be used to search for a text string
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