#!/bin/bash

# This syntax is known as "process substitution".
# It's only available in Bash, not in plain sh.
comm -12 <(sort "${1}") <(sort "${2}")

# Note: Think carefully whether to pipe the 'sort' outputs through
# 'uniq'.  It depends on how 'comm' behaves, which I haven't read up
# on or thoroughly tested yet.
# 
# Also, this whole program could be written more interestingly in a
# real language to handle an arbitrary number of input files, and use
# hashes to identify lines (thus avoiding the need for sorted input).