Comparing two files in a bash script

Inspect the exit value of cmp --silent.

I wanted to write a bash script that compares the contents of two files, and takes an action if the two files are different. I also wanted this script to be compatible with my usual bash flags – I run usually write scripts with errexit and nounset.

I discovered I can use the cmp command. Here’s a simple example:

#!/usr/bin/env bash

set -o errexit
set -o nounset

if cmp --silent "1.txt" "2.txt"
then
    echo "the files are the same!"
else
    echo "the files are different!"
fi

Note that if one or both of the files don’t exist, then cmp returns a non-zero exit code. In my use case, that was fine, but in other cases you might want to distinguish between “the files are different” and “the files don’t exist”.

You can invert this if you only want to do something when the files are different:

#!/usr/bin/env bash

set -o errexit
set -o nounset

if ! cmp --silent "1.txt" "2.txt"
then
    echo "the files are different!"
fi