If you have a vast number of files and you want to do something with them, then use something like the following:

grep -l "mystring" * | xargs -I{} echo {}

this will echo each filename that contains mystring in the current working directory. You could replace echo linux command with any other command, for instance with rm -f to remove those files (be careful!). The -I{} switch to xargs prevents having problems with filenames that have spaces. If you want to search in a different directory than the working directory replace * with it.

So lets say you want to find files that contain DNS in /etc. You can do this with:

grep -r -l "DNS" /etc | xargs -I{} echo {}

If you want to find files that contain any form of some string in /home/user. You can do this with:

grep -i -r -l "some string" /home/user | xargs -I{} echo {}

where -i (or –ignore-case) does a case-insensitive search (i.e. it will match any some string, or Some String, etc.) and -r stands for recursive.

If you have a complex scenario where you only want to process specific type of files or file patterns, you may want to use find instead of grep directly.