The [grep](http://en.wikipedia.org/wiki/Grep) utility searches a pattern (regular expression) in a text file. There are many options to control how matches are performed and how to display matches.

### External references

* [grep specification (Single UNIX v3)](http://pubs.opengroup.org/onlinepubs/009695399/utilities/grep.html)
* [GNU grep manual](http://www.gnu.org/software/grep/manual/grep.html)

### Further reading

* http://unix.stackexchange.com/questions/366/convince-grep-to-output-all-lines-not-just-those-with-matches
* http://unix.stackexchange.com/questions/6979/grep-count-total-number-of-occurrences
* http://unix.stackexchange.com/questions/11067/is-there-a-way-to-modify-a-file-in-place
* http://unix.stackexchange.com/questions/3514/how-to-grep-standard-error-stream-stderr
* http://unix.stackexchange.com/questions/11305/grep-show-all-the-file-up-to-the-match
* http://unix.stackexchange.com/questions/3229/grep-and-tail-f