I listen to music on two computers: one at work, one at home. Both run Linux and ssh, but neither is listening on an internet-accessible IP. Furthermore, syncing a lot of music across the internet would suck up bandwidth that my coworkers might not appreciate being sucked up. I often add new music to my collection at home, but I also sometimes rename or otherwise re-organize old music, and I would like all of those changes to be easily transferred to work.
The solution I've found is rdiffdir, which is part of duplicity. The man page describes how to use it pretty well, if a bit tersely:
rdiffdir [options] sig[nature] basis_dir signature_file
rdiffdir [options] delta signature_file new_dir delta_file
rdiffdir [options] patch basis_dir delta_file
So to copy my music collection changes from home to work, I would do the following:
1. Create a "signature" of the out-of-date directory at work. This generates a fairly large file—about 200MB for my music collection and not really compressible at all—but it's still much smaller than the actual directory contents.
work $ rdiffdir signature ~/Music music.sig
2. Bring the signature file home on a flash drive, and run the delta command. This will compare the signature with the contents of the up-to-date directory, and put all of the required changes into a delta file, which will be quite large.
home $ rdiffdir delta music.sig ~/Music music.delta
3. Put the delta file onto the flash drive and take it back to work. If it's too large, you can split it up into multiple chunks of a given size:
home $ split -b 7G music.delta
This creates files named xaa, xab, xac, etc., which you can then join back together with cat:
work $ cat xaa xab > music.delta
4. Now use the patch command to apply the delta to the out-of-date directory, updating its contents.
work $ rdiffdir patch ~/Music music.delta