Grep

grep is a command line text search utility originally written for Unix. The name is taken from the first letters in global / regular expression / parser, a series of instructions for the ed text editor. The grep command searches files or standard input globally for lines matching a given regular expression, and prints them to the program's standard output.

Usage

This is an example of a common grep usage:

grep apple fruitlist.txt

In this case, grep prints all lines containing 'apple' from the file fruitlist.txt, regardless of word boundaries; therefore lines containing 'pineapple' or 'apples' are also printed. The grep command 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'.

Like most Unix commands, grep accepts command line arguments to change this and many other behaviors. For example:

grep -i apple fruitlist.txt

This prints all lines containing 'apple' regardless of capitalization. The '-i' argument tells grep to be case insensitive, or to ignore case.

To print all lines containing 'apple' as a word ('pineapple' and 'apples' will not match):

grep -w apple fruitlist.txt

Regular expressions can be used to match more complicated queries.

Variations

There are countless implementations and derivatives of grep available for many operating systems. Early variants of grep included egrep and fgrep. The former applies an extended regular expression syntax that was added to Unix after Ken Thompson's original regular expression implementation. The latter searches for any of a list of 'fixed' strings using the Aho-Corasick algorithm. These variants are embodied in most modern grep implementations as command-line switches (and standardized as -E and -F in POSIX). 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.

pcregrep is an implementation of grep that uses Perl regular expression syntax.

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 a built-in function that finds elements in a list. In functional programming languages, this higher-order function is typically named "filter" instead.

The DOS, OS/2 and Microsoft Windows platforms provide the find command for simple string searches. Windows includes the "findstr" command which approximates much of the functionality of “grep”. Ports of grep (Cygwin and GnuWin32, for example) are also available for Windows.

Usage as a conversational 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). Compare with google. Visual grep is used as a term meaning to look through text searching for something, in the manner of the grep program.

Applications such as integrated development environments, text editors, and word processors often feature regular expression search, and sometimes refer to it as “grep”.

See also

• Boyer–Moore string search algorithm
• List of Unix utilities

References

• Alain Magloire (2000). Grep: Searching for a Pattern. Iuniverse Inc. ISBN 0-595-10039-2. {{cite book}}: Unknown parameter |month= ignored (help)
• Hume, Andrew A tale of two greps, Software—Practice and Experience 18, ( 11 ), 1063–1072 ( 1988).
• Hume, Andrew Grep wars: The strategic search initiative. In Peter Collinson, editor, Proceedings of the EUUG Spring 88 Conference, pages 237–245, Buntingford, UK, 1988. European UNIX User Group.

External links

• grep: search a file for a pattern – Shell and Utilities Reference, The Single UNIX Specification, Version 4 from The Open Group
• The grep command tutorial for Linux / UNIX.
• The grep Command - by The Linux Information Project (LINFO)
• "The Treacherous Optimization" - article on tradeoffs in grep to favor best-case over worst-case scenarios
• Egrep for linguists An introduction to egrep
• Tony Abou-Assaleh's list of Greps
• Greppin' in the GNU World Lab Google Code University
• Top 'grep' commands - Usage examples
• Agent Ransack for Windows