Question

I’m working on a nice long regular expression (for fun, yeah I know…) and I’d like to write it in a file so I can keep a better record of it. Can I call grep like this:

grep regex.txt fileToSearch.txt

I’ve tried it and it doesn’t work. Are there any flags I have to use or is this approach not possible?

Answer 1

from man grep:

   -f FILE, --file=FILE
          Obtain patterns from FILE, one per line.  The empty file contains zero patterns, and therefore matches nothing.  (-f is specified by POSIX.)

Example:

$> cat ./file.txt 
.*a

$> echo "abcabc" | grep -o -P -f ./file.txt 
abca