Today at work I had a pair of files containing account numbers, one per line, and I needed to determine which numbers were in both files, and which were in each file individually. I had a bad case of brain-lock trying to get the right output from diff, and instead wrote a quick script to give the results in the format I wanted.

The script uses a single hash whose keys are account numbers, and values determine which files the account numbers were seen in: 1 for the first file, 2 for the second, 3 for both. Here I'm using bitwise OR instead of addition in case a duplicate account number appears in the same file (a | b | b == a | b).

(Also, I wouldn't turn down a way to get similar output from diff.)

#!/usr/bin/perl -w
use strict;

$\ = "\n";
my %h;

open FA, "file_a.txt"; 
for(<FA>){chomp; $h{$_} |= 1}; 
close FA;

open FB, "file_b.txt"; 
for(<FB>){chomp; $h{$_} |= 2}; 
close FB;

my @keys = sort keys %h;

print"Only File a:"; 
for (@keys) {print if $h{$_} == 1}; 

print "\nOnly in File b:"; 
for (@keys) {print if $h{$_} == 2};

print "\nIn both files:";
for(@keys){print if $h{$_} == 3};
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