find . -type f -name "*.file" | xargs grep -l "STRING"

Find and List all the files with extension .file containing/not containing the given string "STRING"

The snippet is set to "containing", to switch it to "not containg" just replace "grep -l" with "grep -L"

containing : grep -l
not containing: grep -L

Find and List files containing/not containg a certain string

find

Bash