rdup is a utility inspired by rsync and the plan9 way of doing
backups.  rdup itself does not backup anything. It only prints a
list of files that are changed, or all files in case of a null
dump, to standard output.

Subsequent programs in a pipe line can be used to actually implement
to backup scheme. After a run a new filelist is written. No warning
is given when filelist is an existing file, it just gets overwritten
by rdup. New runs will print out only those files that have actually
changed since the last run, thereby making incremental backups
possible.