rdup is an utility inspired by rsync and the Plan9 way of doing backups.

rdup itself does not backup anything; it only prints a list of the names
of files that have changed since the last backup.  It also handles files
that are removed, allowing for correct incremental backups.

An example script named rdup-simple is included to show how to implement
a simple snapshot-style backup strategy.

WWW: http://archive.miek.nl/projects/rdup/index.html