How to use grep to search for strings in files on the Linux shell

The grep command, which means global regular expression print, remains amongst the most versatile commands in a Linux terminal environment. It happens to be an immensely powerful program that lends users the ability to sort input based on complex rules, thus rendering it a fairly popular link across numerous command chains. The grep command is primarily used to search text or search any given file for lines containing a match to the supplied words/strings.

Source: Howtoforge Linux Howtos und Tutorials