grep -rnw '/path/to/somewhere/' -e 'pattern' -r or -R is recursive, -n is line number, and -w stands for match the whole word. -l (lower-case L) can be added to just give the file name of matching files. Along with these, --exclude, --include, --exclude-dir or --include-dir flags could be used for efficient searching:
This will only search through those files which have .c or .h extensions: grep --include=\*.{c,h} -rnw '/path/to/somewhere/' -e "pattern" |