grep is a Unix command line utility (well most Unix utilities are command line) that searches the input files for pattern and prints lines that contain the pattern. If you are reading this you, you are probably no stranger to grep.
grep was written by Ken Thompson, the same guy who wrote Unix. grep first appeared in Unix v4 with limited features as compared to today’s grep.
I’ve used grep so much over the years. One of the countless little utilities that’s the staple of all UNIX-like systems that you never really think about, but use all the time.