| grep | |
|---|---|
 Example of grep command | |
| Original author | Ken Thompson[1][2] |
| Developer | 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 acommand-line utility for searchingtext for lines that match aregular expression. Its name comes from theed commandg/re/p (global,regularexpression,print), which has the same effect.[3][4] grep was originally developed for theUnix operating system, and is commonly available onUnix-like and some other systems such asOS-9.[5] Theshellcommand thatruns the utility has the same name:grep.
Before it was named, grep was a private utility written byKen 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 calleds (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 inPDP-11assembly language to helpLee E. McMahon analyze the text ofThe Federalist Papers to determine authorship of the individual papers.[7] Theed text editor (also authored by Thompson) hadregular 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 enablerandom 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 commandg/re/p, where there is theregularexpression to match, would print all lines featuring a specified pattern match.[8][9] grep was first included inVersion 4 Unix. Stating that it is "generally cited asthe prototypical software tool", McIlroy credited grep with "irrevocably ingraining" Thompson'stools philosophy in Unix.[10]
A variety of grep implementations are available in many operating systems and software development environments.[11] Early variants includedegrep andfgrep, introduced inVersion 7 Unix.[10] Theegrep variant supports anextended regular expression syntax added byAlfred Aho afterKen Thompson's original regular expression implementation.[12] Thefgrep variant searches for any of a list offixed strings using theAho–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.execgrep-E"$@". Commandsegrep andfgrep, 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. Thepgrep utility, for instance, displays the processes whose names match a given regular expression.[15]
In thePerl programming language,grep is a built-in function that finds elements in a list that satisfy a certain property.[16] Thishigher-order function is typically namedfilter orwhere in other languages.
Thepcregrep command is an implementation of grep that usesPerl regular expression syntax.[17] Similar functionality can be invoked in the GNU version of grep with the-P flag.[18]
Ports of grep (withinCygwin andGnuWin32, for example) also run underMicrosoft Windows. Some versions of Windows feature the similarqgrep orfindstr command.[19]
A grep command is also part ofASCII'sMSX-DOS2 Tools forMSX-DOS version 2.[20]
Thegrep,egrep, andfgrep commands have also been ported to theIBM i operating system.[21]
The softwareAdobe InDesign has functions GREP (since CS3 version (2007)[22]), in thefind/change dialog box[23] "GREP" tab, and introduced with InDesign CS4[24] inparagraph styles[25] "GREP styles".
agrep (approximate grep) is anopen-sourceapproximate string matching program, developed byUdi Manber and Sun Wu between 1988 and 1991,[26] for use with theUnix operating system. It was later ported toOS/2,DOS, andWindows.
agrep matches even when the text onlyapproximately fits the search pattern.[27]
This following invocation findsnetmasks in filemyfile, 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
In December 2003, theOxford 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 adysphemism for paper).[29]
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).
QGREP.EXE[:] A similar tool to grep in UNIX, this tool can be used to search for a text string
grep(1) – Plan 9 Programmer's Manual, Volume 1grep(1) – Inferno General commandsManual| File system | |
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| Processes | |
| User environment | |
| Text processing | |
| Shell builtins | |
| Searching | |
| Documentation | |
| Software development | |
| Miscellaneous | |
| File system | |
|---|---|
| Processes | |
| User environment | |
| Text processing | |
| Shell builtins | |
| Networking | |
| Searching | |
| Software development | |
| Miscellaneous | |
