Grep

grep

Example of grep command
Original author(s)	Ken Thompson[1][2]
Developer(s)	AT&T Bell Laboratories
Initial release	November 1973; 51 years ago (1973-11)[1]
Written in	C
Operating system	Unix, Unix-like, Plan 9, Inferno, OS-9, MSX-DOS, IBM i
Platform	Cross-platform
Type	Command

grep is a command-line utility for searching plaintext datasets for lines that match a regular expression. Its name comes from the ed command g/re/p (global regular expression search and print), which has the same effect.^[3][4] grep was originally developed for the Unix operating system, but later became available for all Unix-like systems and some others such as OS-9.^[5]

History

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 on his own program called s (short for "search"). 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 to search through such a large amount of text, as it loaded the entire file into memory to enable random access editing, so Thompson excerpted that regexp code into a standalone tool which would instead process arbitrarily long files sequentially without buffering too much into memory.^[1] He chose the name because in ed, the command g/re/p would print all lines featuring a specified pattern match.^[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]

Implementations

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 exist in modern systems, usually linking to grep or calling grep as a shell script with the appropriate flag added, e.g. exec grep -E "$@". egrep and fgrep, while commonly deployed on POSIX systems, to the point the POSIX specification mentions their widespread existence, are actually not part of POSIX.^[14]

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.^[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

Main article: agrep

agrep (approximate grep) is an open-source approximate string matching program, developed by Udi Manber and Sun Wu between 1988 and 1991,^[26] for use with the Unix operating system. It was later ported to OS/2, DOS, and Windows.

agrep (approximate grep) matches even when the text only approximately fits the search pattern.^[27]

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

Usage as a verb

In December 2003, the Oxford English Dictionary Online added "grep" as both a noun and a verb.^[28]

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", which in this context is a dysphemism for paper).^[29]

See also

• Boyer–Moore string-search algorithm
• agrep, an approximate string-matching command
• find (Windows) or Findstr, 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"
• ngrep, the network grep

References

1. Kernighan, Brian (1984). The Unix Programming Environment. Prentice Hall. pp. 102. ISBN 0-13-937681-X.
2. “grep was a private command of mine for quite a while before i made it public.” -Ken Thompson Archived 2015-05-26 at the Wayback Machine, By Benjamin Rualthanzauva, Published on Feb 5, 2014, Medium
3. Hauben et al. 1997, Ch. 9
4. Raymond, Eric. "grep". Jargon File. Archived from the original on 2006-06-17. Retrieved 2006-06-29.
5. Paul S. Dayan (1992). The OS-9 Guru - 1 : The Facts. Galactic Industrial Limited. ISBN 0-9519228-0-7.
6. VCF East 2019 -- Brian Kernighan interviews Ken Thompson (video). YouTube. 6 May 2019. Archived from the original on 2021-12-11. (35 mins)
7. Computerphile, Where GREP Came From, interview with Brian Kernighan
8. "ed regexes". perl.plover.com. Archived from the original on 20 October 2017. Retrieved 24 April 2018.
9. "How Grep Got its Name". robots.thoughtbot.com. Archived from the original on 9 August 2017. Retrieved 24 April 2018.
10. McIlroy, M. D. (1987). A Research Unix reader: annotated excerpts from the Programmer's Manual, 1971–1986 (PDF) (Technical report). CSTR. Bell Labs. 139. Archived (PDF) from the original on 2017-11-11.
11. Abou-Assaleh, Tony; Wei Ai (March 2004). Survey of Global Regular Expression Print (GREP) Tools (Technical report). Dalhousie University.
12. Hume, Andrew (1988). "A Tale of Two Greps". Software: Practice and Experience. 18 (11): 1063. doi:10.1002/spe.4380181105. S2CID 6395770.
13. Meurant, Gerard (12 Sep 1990). Algorithms and Complexity. Elsevier Science. p. 278. ISBN 9780080933917. Archived from the original on 4 March 2016. Retrieved 12 December 2015.
14. "grep". www.pubs.opengroup.org. The Open Group. Archived from the original on 28 November 2015. Retrieved 12 December 2015.
15. "pgrep(1)". www.linux.die.net. Archived from the original on 22 December 2015. Retrieved 12 December 2015.
16. "grep". www.perldoc.perl.org. Archived from the original on 7 December 2015. Retrieved 12 December 2015.
17. "pcregrep man page". www.pcre.org. University of Cambridge. Archived from the original on 23 December 2015. Retrieved 12 December 2015.
18. "grep(1)". www.linux.die.net. Archived from the original on 10 December 2015. Retrieved 12 December 2015.
19. Spalding, George (2000). Windows 2000 administration. Network professional's library. Osborne/McGraw-Hill. pp. 634. ISBN 978-0-07-882582-8. Retrieved 2010-12-10. QGREP.EXE[:] A similar tool to grep in UNIX, this tool can be used to search for a text string
20. "MSX-DOS2 Tools User's Manual by ASCII Corporation". April 1993.
21. IBM. "IBM System i Version 7.2 Programming Qshell" (PDF). IBM. Retrieved 2020-09-05.
22. "Review: Adobe InDesign CS3 - CreativePro.com". creativepro.com. 20 April 2007. Archived from the original on 5 January 2018. Retrieved 24 April 2018.
23. "InDesign Help: find/change". Archived from the original on 2016-08-28. Retrieved 2016-08-12.
24. "InDesign: GREP Styles (1) Setting text between parentheses in Italic". Archived from the original on 2017-09-24. Retrieved 2018-01-05.
25. "InDesign Help: GREP styles". Archived from the original on 2016-08-28. Retrieved 2016-08-12.
26. Wu, Sun; Manber, Udi (20–24 January 1992). Agrep -- a fast approximate pattern-matching tool. 1992 Winter USENIX Conference. San Francisco, California. CiteSeerX 10.1.1.89.5424.
27. S. Lee Henry (June 1998). "Proper Searching". Sun Expert. pp. 35–26.
28. "New words list December 2003". Oxford English Dictionary. Retrieved 2021-12-06.
29. Jargon File, article "Documentation"

Notes

• Alain Magloire (August 2000). Grep: Searching for a Pattern. Iuniverse Inc. ISBN 0-595-10039-2.
• 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.
• Michael Hauben; et al. (April 1997). Netizens: On the History and Impact of Usenet and the Internet (Perspectives). Wiley-IEEE Computer Society Press. ISBN 978-0-8186-7706-9.

External links

• 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.
• Command Grep – 25 practical examples