Reading grep patterns from a file

The -f option specifies a file where grep reads patterns. That's just like passing patterns on the command line (with the -e option if there's more than one), except that when you're calling from a shell you may need to quote the pattern to protect special characters in it from being expanded by the shell.

The argument -E or -F or -P, if any, tells grep which syntax the patterns are written in. With no argument, grep expects basic regular expressions; with -E, grep expects extended regular expressions; with -P (if supported), grep expects Perl regular expressions; and with -F, grep expects literal strings. Whether the patterns come from the command line or from a file doesn't matter.

Note that the strings are substrings: if you pass a+b as a pattern then a line containing a+b+c is matched. If you want to search for lines containing exactly one of the supplied strings and no more, then pass the -x option.