checksum bash script in ubuntu

I have two folders with same file names inside. How can I do a checksum in bash to evaluate which files are different or identical? Currently, I've written a bash script below which builds corresponding file names and does "cksum" on them, which generates two numbers per checksum. But I have to somehow save these two numbers for each record and subtract them to see which one's a non-match.

#!/bin/bash
folderOld="home/OldFiles/"
folderNew="home/NewFiles/"
for ((fileNumber=1;fileNumber<1000000;fileNumber++))
do
FileName="file${fileNumber}.dat"
OldFile=$folderOld$FileName
NewFile=$folderNew$FileName
cksum $OldFile
cksum $NewFile
done

Solution

You don't need to subtract the numbers, you just need to check whether they are equal:

OldSum=$(cksum $OldFile | cut -d' ' -f1)
NewSum=$(cksum $NewFile | cut -d' ' -f1)
if [[ $OldSum != $NewSum ]]; then
    # Checksum mismatch, do something useful here
fi

I'm using cut -d' ' -f1 here to split the line on spaces, and take the first field. So file size and file name are ignored.

By the way, cksum uses CRC32 by default, which has a fair risk of false negatives on so many files. Better to use, for example, sha256sum.