Print duplicate lines found in files using sort and uniq commands.
#!/usr/bin/env bash
##
# Print duplicate lines in files.
#
# sort opts
# (-f) to ignore the case of a letter when ordering lines.
#
# uniq opts
# (-i) to ignore case differences.
# (-c) to count the number of occurrences.
# (-d) to only output lines that are repeated.
##
cat <file_1> <file_2> | sort -f | uniq -i -d