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grep

grepolis, grep command
grep is a command-line utility for searching plain-text data sets for lines matching 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 lines34 Grep was originally developed for the Unix operating system, but is available today for all Unix-like systems

Contents

  • 1 History
  • 2 Usage
  • 3 Variations
  • 4 Usage as a verb
  • 5 See also
  • 6 References
  • 7 External links

Historyedit

First appearing in Version 4 Unix, grep was created by Ken Thompson as a standalone application adapted from the regular expression parser he had written for ed which he also created1 In ed, the command g/re/p would print all lines matching a previously defined pattern56 Stating that it is "generally cited as the prototypical software tool", Doug McIlroy credited grep with "irrevocably ingraining" Thompson's tools philosophy in Unix7

Usageedit

grep searches files specified as arguments, or, if missing, the program's standard input By default, it reports matching lines on standard output, but specific modes of operation may be chosen with command line options

A simple example of a common usage of grep is the following, which searches the file fruitlisttxt for lines containing the text string apple:

$ grep "apple" fruitlisttxt

Matches occur when the specific sequence of characters is recognized, for example, lines containing pineapple or apples are printed regardless of word boundaries However, the search pattern specified as an argument 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 Case-insensitive matching occurs when the argument option -i ignore case is given

Multiple file names may be specified in the argument list For example, all files having the extension txt in a given directory may be searched if the shell supports globbing by using an asterisk as part of the filename:

$ grep "apple" txt

Regular expressions can be used to match more complicated text patterns The following prints all lines in the file that begin with the letter a, followed by any one character, followed by the letter sequence ple

$ grep ^aple fruitlisttxt

The name of grep derives from a usage in the Unix text editor ed and related programs Before grep existed as a separate command, the same effect might have been achieved in an editor:

$ ed fruitlisttxt g/^aple/p q

where the second line is the command given to ed to print the relevant lines, and the third line is the command to exit from the editor

Like most Unix commands, grep accepts options in the form of command-line arguments to change its behavior For example, the option flag l lower case L provides a list of the files which have matching lines, rather than listing the lines explicitly

Selecting all lines containing the self-standing word apple, ie surrounded by white space, punctuation or hyphens, may be accomplished with the option flag w

Exact line match is performed with the option flag x Lines only containing exactly and solely apple are selected with a line-regexp instead of word-regexp:

$ cat fruitlisttxt apple apples pineapple apple- apple-fruit fruit-apple banana pear peach orange $ grep -x "apple" fruitlisttxt apple

The v option reverses the sense of the match and prints all lines that do not contain apple, as in this example

$ grep -v "apple" fruitlisttxt banana pear peach orange

The i option in grep helps to match words that are case insensitive, as shown in below example

$ cat fruitlisttxt apple Pineapple apple- apple-FRUIT fruit-apple banana pear PEACH orange $ grep -i "fruit" fruitlisttxt apple-FRUIT fruit-apple

Variationsedit

A variety of grep implementations are available in many operating systems and software development environments8 Early variants included egrep and fgrep, introduced in Version 7 Unix7 The "egrep" variant applies an extended regular expression syntax that was added to Unix after Ken Thompson's original regular expression implementation by Alfred Aho9 The "fgrep" variant searches for any of a list of fixed strings using the Aho–Corasick string matching algorithm10 Binaries of these variants persist in most modern systems, however their explicit usage has depreciated and the functionalities of these variants are included in grep as the command-line switches -E and -F; the use of the switches is therefore the recommended method of use11

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 expression12

In the Perl programming language, grep is the name of the built-in function that finds elements in a list that satisfy a certain property13 This higher-order function is typically named filter in functional programming languages

The pcregrep command is an implementation of grep that uses Perl regular expression syntax14 Similar functionality can be invoked in the GNU version of grep with the -P flag15

Ports of grep within Cygwin and GnuWin32, for example also run under Microsoft Windows Some versions of Windows feature the similar qgrep or Findstr command16

The software Adobe InDesign has functions GREP since 2007 with CS3 version, in the search/change dialog box17 "GREP" tab, and in paragraph styles18 "GREP styles"

Usage as a verbedit

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 ie, one made from dead trees, paper19 Compare with google

See alsoedit

  • Free software portal
  • Boyer–Moore string search algorithm
  • List of Unix utilities
  • agrep
  • find
  • vgrep, or "visual grep"
  • FGREP DOS command, an external command in some versions of MS-DOS 2xx

Referencesedit

  1. ^ a b c Kernighan, Brian 1984 The Unix Programming Environment Prentice Hall p 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, By Benjamin Rualthanzauva, Published on Feb 5, 2014, Medium
  3. ^ Hauben et al 1997, Ch 9
  4. ^ Raymond, Eric "grep" Jargon File Retrieved 2006-06-29 
  5. ^ http://perlplovercom/classes/HoldSpace/samples/slide012html
  6. ^ http://robotsthoughtbotcom/how-grep-got-its-name
  7. ^ a b McIlroy, M D 1987 A Research Unix reader: annotated excerpts from the Programmer's Manual, 1971–1986 PDF Technical report CSTR Bell Labs 139 
  8. ^ Abou-Assaleh, Tony; Wei Ai March 2004 Survey of Global Regular Expression Print GREP Tools Technical report Dalhousie University 
  9. ^ Hume, Andrew 1988 "A Tale of Two Greps" Software—Practice & Experience 18 11: 1063  |access-date= requires |url= help
  10. ^ Meurant, Gerard 12 Sep 1990 Algorithms and Complexity Elsevier Science p 278 Retrieved 12 December 2015 
  11. ^ "grep" wwwpubsopengrouporg The Open Group Retrieved 12 December 2015 
  12. ^ "pgrep1" wwwlinuxdienet Retrieved 12 December 2015 
  13. ^ "grep" wwwperldocperlorg Retrieved 12 December 2015 
  14. ^ "pcregrep man page" wwwpcreorg University of Cambridge Retrieved 12 December 2015 
  15. ^ "grep1" wwwlinuxdienet Retrieved 12 December 2015 
  16. ^ Spalding, George 2000 Windows 2000 administration Network professional's library Osborne/McGraw-Hill p 634 ISBN 978-0-07-882582-8 Retrieved 2010-12-10 QGREPEXE: A similar tool to grep in UNIX, this tool can be used to search for a text string 
  17. ^ "InDesign Help: search/change" Retrieved 2016-08-12 
  18. ^ "InDesign Help: GREP stylus" Retrieved 2016-08-12 
  19. ^ 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 linksedit

  • "why GNU grep is fast" - implementation details from GNU grep's author
  • Network grep - A packet analyzer used to match patterns at the network layer

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