grep -lniR 'STRING' *

-l

Suppress normal output; instead print the name of each input file from which output would normally have been printed. The scanning will stop on the first match.

-n

Prefix each line of output with the line number within its input file

-i

Ignore case distinctions in both the PATTERN and the input files

*

searches for any file in this folder

-R

searches in all subfolders