Mac Geekery

Answer

You’ll need a machine acting as a server for this, and you’ll need Unison¹ installed on both machines.

Preparing

All good toys need a little setting up (at least the ones here) and this one is no different. Unison works a lot like rsync in that a local copy calls a remote copy and they chat it up over an SSH-encrypted pipe. Thus, you’ll need a copy of Unison on both machines unless you’re syncing with a second drive, in which case you obviously just need the one copy.

Your Mac

Ensure the Developer Tools are installed on your computer (this is essentially a prerequisite for answers on this site) and then download Darwin Ports and install it. Update it and then get Unison with it.

$ sudo /opt/local/bin/port -d selfupdate
$ sudo /opt/local/bin/port install unison

Darwin Ports will fetch the other utilities Unison depends upon and build them for your machine. I prefer this method for a variety of reasons, but if you would like a binary copy of the program, you can download one from this fellow here.

The Other Mac

If you have a monitor on it, just repeat the above install.

If you decide to build it from source and your remote machine is headless, don’t worry, there’s an answer. Just copy the DMG to the remote machine and then SSH in. Get to that folder and then:

$ hdiutil mount DarwinPorts-1.2.1.pkg
$ sudo installer -pkg /Volumes/DarwinPorts-1.2.1/DarwinPorts-1.2.1.pkg -target /

First Run

You can do this one of two ways: mount the remote volume on the desktop over AFP, or use Unison over SSH. The former is easier (and AFP works over SSH if you tell it to when connecting) and the latter is automatable.

If you chose to do it over AFP, mount the volume and then run something like the following:

/opt/local/bin/unison ~ /Volumes/AFPShare/MyHomeBackup

If you want to do this over SSH, then you need Unison on the remote machine as well and then run something like this:

/opt/local/bin/unison -servercmd /opt/local/bin/unison ~ ssh://mymac.local/Backup

The -servercmd part tells your local copy of Unison where its counterpart on the other machine is, much like rsync. Once it gets started, it’ll ask questions as it needs to and then begin the transfer and sync.

Latter Syncs

If you want this in your crontab, you’ll want passwordless SSH keys between the machines. Otherwise, give it a whirl now and again to keep it updated by using the same procedure as above.

Shell Commands for Non-Junkies

Don’t. That’s the short of it. Don’t learn the shell (“Terminal”) by using programs you’re unfamiliar with to do actual tasks you need done. Get a quick-start book and toy around in a dummy account for a day or two to learn it (some weekend, or pair of Saturdays) and then, once familiar, a lot of this will suddenly click.

There’s a lot of people pushing shell commands to solve any problem that’s hard to do in the GUI, but the vast majority of people don’t need to be using them at all. Those that do, should learn what it is before using it because it’s very likely that if you start to really use it before knowing what’s going on that you’ll roast yourself a few times on live data.

¹ No, not that Unison, this Unison. Yeah, had me for a while, too.