Unison is good software. It’s a command line program to synchronize filesystems, to keep a directory tree identical on multiple computers. I use it to sync about 40G of files across two Macs, to keep my home directory and source code and various applications in sync. The neat trick is I sync those two Macs through a portable hard drive so I don’t have to wait for hours for files to go over the Internet. Unison can also work online so changes are propagated automatically.

Unison is a lot like rsync. But Unison is designed to be bidirectional. Rsync always syncs one way: copy A to B. Unison will look at the differences between A and B and merge them, including a limited UI for conflict resolution. This protects me from the case where I modify something on both machines without syncing beforehand.

The main drawback with Unison is it’s slow, it takes many minutes to decide what files to sync. I also hate the interactive UI; it doesn’t work well when you have lots of files that changed in both places. I’m also a bit concerned that it’s no longer under active development but Unison is the rare software that’s a complete product, it’s not clear it needs many changes.

There are other tools solving similar file sync problems, none perfect. Dropbox is phenomenal but doesn’t have offline syncing of large files. Camlistore is promising but not quite ready for civilian use. git can be used to keep stuff in sync but is better suited for text files whose history you want to keepl. And CrashPlan is great for online backup but doesn’t really provide a second live copy.