Grep

History

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 created). In ed, the command g/re/p would print all lines matching a previously defined pattern. Stating that it is "generally cited as the prototypical software tool", Doug McIlroy credited grep with "irrevocably ingraining" Thompson's tools philosophy in Unix.

Usage

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 fruitlist.txt for lines containing the text string apple:

$ grep "apple" fruitlist.txt

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 ^a.ple fruitlist.txt

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:

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, i.e. 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:

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

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

Variations

A variety of grep implementations are available in many operating systems and software development environments. Early variants included egrep and fgrep, introduced in Version 7 Unix. The "egrep" variant applies an extended regular expression syntax that was added to Unix after Ken Thompson's original regular expression implementation by Alfred Aho. The "fgrep" variant searches for any of a list of fixed strings using the Aho–Corasick string matching algorithm. 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 use.

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 expression.

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. 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 syntax. Similar functionality can be invoked in the GNU version of grep with the -P flag.

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.

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

Usage as a verb

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 (i.e., one made from dead trees, paper). Compare with google.