Using the Unison File Synchronizer on macOS

For ages, I’ve been using the excellent Unison file synchronizer to synchronize my directories across several Linux machines, using the SSH protocol. I love it! 🙂

Unison gives you complete control over the synchronization, and, most of all, it’s a two-way synchronizer.

Quoting from its home page:

Unison is a file-synchronization tool for OSX, Unix, and Windows. It allows two replicas of a collection of files and directories to be stored on different hosts (or different disks on the same host), modified separately, and then brought up to date by propagating the changes in each replica to the other.

On Linux, I never experienced problems with Unison, especially from the installation point of view: it’s available on most distributions’ package managers. If that’s not the case, you can download a binary package from

However, I had never used Unison on a macOS computer, so today, I decided to try it.

Please, keep in mind that you must use the same version of Unison on all the computers you want to synchronize (at least, I seem to understand, the major.minor version numbers must be the same on all computers, and this also includes the version of OCaml, on which Unison relies).

For macOS, you go to, and you download the .app.tar.gz file according to the Unison (and OCaml) version you need. The other macOS .tar.gz archives, without the .app, contain the command-line binary and a GTK UI binary, which, however, requires the GTK libraries to be already installed on your system and, to be honest, I have no idea how to do that in a compatible way. On the contrary, the .app.tar.gz contains the macOS application, which, I seem to understand, it’s self-contained.

By the way, there’s also a brew package for Unison, but that’s only the command line application, so you won’t get any UI, which is quite helpful, especially when you want to have complete control over the elements to be synchronized and you want to have the last chance to select or unselect the files before the synchronization starts. Moreover, the UI is quite helpful when you have conflicts to solve.

Then, you extract the archive, and you need to run this command (assuming you have extracted it in the Downloads folder):

otherwise, macOS will complain (with an unhelpful error message about a damaged app) since it does not recognize the archive provider.

Move the into your Applications, and you’re good to go, assuming you already know how to use Unison.

The first time you run the app, it will ask you to install also the command-line version of Unison, which is also helpful:

And here’s a screenshot showing the files that are going to be synchronized in an example of mine (from the direction of the arrows, you can see that this is a two-way synchronization):

I find the Linux UI of Unison much simpler to understand and deal with, but maybe that’s because I’ve been using it for ages, and I still do.

Happy synchronization! 🙂

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