grep files
find content
look for ‘foo’
grep -r 'foo' /path/where/to/look
look for ‘foo’ in files finishing in ‘.bar’
grep -r --include="*.bar" 'foo' /path/where/to/look
look for excactly ‘foo’ in files and show where it appears
grep -nrw /path/whre/to/look -e 'foo'