Cloud-free folder syncing with Unison

Syncing folders between computers can be surprisingly complicated if you work for an academic institution that takes a dim view of Dropbox and other cloud-based solutions. You are also sure to run out of space fast if you use it to sync e.g. neuroimaging data. Unison is a free command-line solution for direct computer-to-computer sync. It is basically a clever wrapper around rsync, which takes care of all the niggly details of getting two-way sync working properly. It also provides a nice command-line interface for resolving sync conflicts (far better than e.g. Evernote's GUI for the same thing).

Getting Unison up and running for your setup can be complicated. Here I present a simple shell script that takes care of the standard case of syncing two computers over SSH.

In general, my solution assumes that one computer is used to issue the unison command (the client, which is probably your laptop or home computer) which then connects and syncs with another computer (the remote, which is probably a server or a desktop computer at your place of work). You could set this up to run automatically as a cron or launchd, but I prefer to issue the command manually at the end of the workday to make sure that I have saved all changes first.

Installing unison

Unison is available through various package managers, e.g. macports and brew. The main stumbling point here is to ensure unison is available on the remote (the computer you are syncing to). On my setup (brew with OS X Yosemite), unison returned bash: unison: command not found, even though unison was clearly availabe when connecting to the remote over SSH. By contrast, ssh [client] unison returned an error, thus revealing that unison was only getting added to the path in interactive shells (which are not the same as login shells used by scripts). In the end I manually added /usr/local/bin to my path in .bashrc to get around this issue (.bash_profile wouldn't work since this isn't sourced by non-interactive shells).

Configuring unison

The unisync script below provides the basic sync functionality for syncing between two folders named unibox in the user's home directory. The first input argument specifies the hostname, while the second provides the username for the remote. By default, the username for client and remote as assumed to be the same.

So for instance, my standard usage is unisync ucl j.carlin, where ucl is an alias or shortcut for my full SSH connection stored in ssh/.config, and j.carlin is my username which is necessary here because it is different from my username on the client.