Grep

Short description: Unix command line utility for text search

grep

Example of `grep` command

Original author(s)| Ken Thompson[1][2]
Developer(s)| AT&T Bell Laboratories
Initial release| November 1973; 50 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 plain-text data sets 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 available for all Unix-like systems and some others such as OS-9.[5]

## 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 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 page: Software:Agrep

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

## Usage as a verb

In December 2003, the Oxford English Dictionary Online added "grep" as both a noun and a verb.[27]

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).[28]

## 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. ↑ 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. ↑ Hauben et al. 1997, Ch. 9
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 Template:Cite tech report
11. ↑ Template:Cite tech report
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. ↑ S. Lee Henry (June 1998). "Proper Searching". Sun Expert: 35–26.
27. ↑ "New words list December 2003". https://public.oed.com/updates/new-words-list-december-2003/.
28. ↑ 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 (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

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