grep

Example of grep command

Original author(s)| Ken Thompson[1][2]
Developer(s)| AT&T Bell Laboratories
Initial release| November 1973; 52 years ago (1973-11)[1]
Written in| C
Operating system| Unix, Unix-like, Plan 9, Inferno, OS-9, MSX-DOS, IBM i
Type| Command

grep is a command-line utility for searching text for lines that match a regular expression. Its name comes from the ed command `g/re/p` (global, regular expression, print), which has the same effect.[3][4] grep was originally developed for the Unix operating system, and is commonly available on Unix-like and some other systems such as OS-9.[5] The shell command that runs the utility has the same name: `grep`.

## Contents

* 1 History
* 2 Implementations
* 2.1 agrep
* 3 Usage as a verb
* 4 See also
* 5 References
* 6 External links

## 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`, where the `re` is the regular expression to match, 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 "$@"`. Commands `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 a 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

`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 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

* agrep – an approximate string-matching command
* Boyer–Moore string-search algorithm – search algorithm used in grep
* find (Unix) – a Unix command that finds files instead of text
* find (Windows) or findstr – a DOS/Windows command that searches for text in files, similarly to grep
* List of POSIX commands
* ngrep – the network grep
* vgrep – a humorous term for visual inspection

## References

1. ↑ 1.0 1.1 1.2 Kernighan, Brian (1984). The Unix Programming Environment. Prentice Hall. pp. 102. ISBN 0-13-937681-X. https://archive.org/details/unixprogramminge0000kern/page/102.
2. ↑ “grep was a private command of mine for quite a while before i made it public.” -Ken Thompson , By Benjamin Rualthanzauva, Published on Feb 5, 2014, Medium
3. ↑ "On the Early History and Impact of Unix Tools to Build the Tools for a New Millennium". Netizens: On the History and Impact of Usenet and the Internet. Los Alamitos, California: IEEE Computer Society Press. 1997. ISBN 978-0-8186-7706-9. http://www.columbia.edu/~rh120. "One afternoon I asked Ken Thompson if he could lift the regular expression recognizer out of the editor and make a one-pass program to do it. He said yes. The next morning I found a note in my mail announcing a program named grep. It worked like a charm. When asked what that funny name meant, Ken said it was obvious. It stood for the editor command that it simulated, g/re/p (global regular expression print)."
4. ↑ Raymond, Eric. "grep". Jargon File. http://www.catb.org/~esr/jargon/html/G/grep.html.
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". http://perl.plover.com/classes/HoldSpace/samples/slide012.html.
9. ↑ "How Grep Got its Name". http://robots.thoughtbot.com/how-grep-got-its-name.
10. ↑ 10.0 10.1 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.
13. ↑ Meurant, Gerard (12 Sep 1990). Algorithms and Complexity. Elsevier Science. p. 278. ISBN 9780080933917. https://books.google.com/books?id=6WriBQAAQBAJ. Retrieved 12 December 2015.
14. ↑ "grep". The Open Group. http://pubs.opengroup.org/onlinepubs/009695399/utilities/grep.html.
15. ↑ "pgrep(1)". http://linux.die.net/man/1/pgrep.
16. ↑ "grep". http://perldoc.perl.org/functions/grep.html.
17. ↑ "pcregrep man page". University of Cambridge. http://www.pcre.org/original/doc/html/pcregrep.html.
18. ↑ "grep(1)". http://linux.die.net/man/1/grep.
19. ↑ Spalding, George (2000). Windows 2000 administration. Network professional's library. Osborne/McGraw-Hill. pp. 634. ISBN 978-0-07-882582-8. https://archive.org/details/windows2000admin0000spal. 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. https://archive.org/details/MSXDOS2TOOLS.
21. ↑ IBM. "IBM System i Version 7.2 Programming Qshell" (in en). https://www.ibm.com/support/knowledgecenter/ssw_ibm_i_74/rzahz/rzahzpdf.pdf?view=kc.
22. ↑ "Review: Adobe InDesign CS3 - CreativePro.com". 20 April 2007. https://creativepro.com/review-adobe-indesign-cs3/.
23. ↑ "InDesign Help: find/change". https://helpx.adobe.com/indesign/using/find-change.html.
24. ↑ "InDesign: GREP Styles (1) Setting text between parentheses in Italic". http://carijansen.com/tip-088/.
25. ↑ "InDesign Help: GREP styles". https://helpx.adobe.com/indesign/using/drop-caps-nested-styles.html#create_grep_styles.
26. ↑ Wu, Sun; Manber, Udi (20–24 January 1992). "Agrep -- a fast approximate pattern-matching tool". 1992 Winter USENIX Conference. San Francisco, California.
27. ↑ S. Lee Henry (June 1998). "Proper Searching". Sun Expert: 35–26.
28. ↑ "New words list December 2003". https://www.oed.com/information/updates/previous-updates/2003-2/december-2003/.
29. ↑ Jargon File, article "Documentation"

Notes

* Alain Magloire (August 2000). Grep: Searching for a Pattern. Iuniverse Inc. ISBN 0-595-10039-2.
* Hume, Andrew (1988). "Grep wars: The strategic search initiative". Proceedings of the EUUG Spring 88 Conference (Buntingford, UK: European UNIX User Group): 237–245.

## External links

| 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.
* Command Grep – 25 practical examples

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