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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 ...
Like the Aho–Corasick string matching algorithm, it can search for multiple patterns at once. It combines ideas from Aho–Corasick with the fast matching of the Boyer–Moore string-search algorithm. For a text of length n and maximum pattern length of m, its worst-case running time is O(mn), though the average case is often much better. [2]
[drive:][path]filename Specifies a file or files to search. Flags: /B Matches pattern if at the beginning of a line. /E Matches pattern if at the end of a line. /L Uses search strings literally. /R Uses search strings as regular expressions. /S Searches for matching files in the current directory and all subdirectories.
A regex pattern matches a target string. The pattern is composed of a sequence of atoms. An atom is a single point within the regex pattern which it tries to match to the target string. The simplest atom is a literal, but grouping parts of the pattern to match an atom will require using ( ) as metacharacters.
cat file | grep pattern cat file | less can instead be written as grep pattern file less file A common interactive use of cat for a single file is to output the content of a file to standard output. However, if the output is piped or redirected, cat is unnecessary.
A screenshot of the original 1971 Unix reference page for glob – the owner is dmr, short for Dennis Ritchie.. glob() (/ ɡ l ɒ b /) is a libc function for globbing, which is the archetypal use of pattern matching against the names in a filesystem directory such that a name pattern is expanded into a list of names matching that pattern.
AWK (/ ɔː k / [4]) is a domain-specific language designed for text processing and typically used as a data extraction and reporting tool. Like sed and grep, it is a filter, [4] and it is a standard feature of most Unix-like operating systems.
NOTE: An application using a library for regular expression support does not necessarily support the full set of features of the library, e.g., GNU grep uses PCRE, but supports no lookahead, though PCRE does.