It looks like a good idea to always use
fgrep is equivalent to
grep -F 1. It interprets the pattern as a list of fixed strings, separated by new lines, any of which is to be matched.
For example, if you have a file
users.txt with a list of users (one user on each line) and want to search the group file to see if any of the users are in it, you would use:
$ grep -F -f users.txt /etc/group