Today I was moving all my static assets from static2 to static3 server, and after the change I needed to find if there still was an occurence of static2 string. To search all files recursively in all folders of the site, I used
grep -r "string" .
That command should be entered in the folder where you want to start the search. The leading dot, says grep to start "here" and the -r option to go recursively for all folders.
If you want to search for the string without caring about upper or lower case.
grep -ri "string" .
The -i option is for "ignore case"