rdup(1) is a utility inspired by rsync(1) and the Plan9 way of doing backups. rdup(1) 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.