Read backup-diff.sh.

What does it do?

Now, make a copy for yourself,

cp ~cs50/examples/backup-diff.sh .

or, to work on your laptop,

scp username@flume.cs.dartmouth.edu:~cs50/examples/backup-diff.sh .

and modify your copy so it exits with zero status when there are no differences, and non-zero status when there are any missing backups or differing files.