grep is used to search for input the matches a given regular expression.
grep [options] <pattern> [file]I don’t know what all the options are and I don’t tend to use them anyway… Where no file is specified, grep reads from standard input (stdin). In this way it is particularly useful for searching through the output of a command; a command can be run and its output piped to grep.
ls -R / | grep "found"
# Don't run this, it will take forever...