grep(1) -r "^deb .*gitlab" /etc/apt/sources.list*
print lines matching a pattern
-R, -r, --recursive
       Read all files under each directory, recursively; this is equivalent to the -d recurse option.
grep searches the named input FILEs (or standard input if no files are named, or if a single hyphen-minus
(-) is given as file name) for lines containing a match to the given PATTERN.  By  default,  grep  prints
the matching lines.
source manpages: grep