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 (globally search for a regular expression and print matching lines), which has the same effect. was originally developed for the Unix operating system, but later available for all Unix-like systems and some others such as OS-9.

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  (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.

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. The ed text editor (also authored by Thompson) had regular expression support but could not be used on such a large amount of text, so Thompson excerpted that code into a standalone tool. He chose the name because in ed, the command g/re/p would print all lines matching a specified pattern. was first included in Version 4 Unix. Stating that it is "generally cited as the prototypical software tool", McIlroy credited  with "irrevocably ingraining" Thompson's tools philosophy in Unix.

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, than one could with a hard copy (i.e. one made from "dead trees", which in this context is a dysphemism for paper).