Look through files for instances of a pattern. I find I use this often in a build environment to see what's been used where.
Here, we look for all instances of 'pattern' in the current directory (and sub directories), print line, and line number
grep -rn . -e 'pattern'-r for recursive, -n for print line number, . is the dir to search in -e specifies next arg as pattern (can be basic regex)