Bi-directional file sync with Unison

I heard about Unison a long time ago, but never tried it for real. Two-way file synchronization just sounds too fragile, almost too good to be true. Still, for some specific use cases, it might make sense. I wanted to sync source code from remote machines to my laptop in a way that allows me to edit the files locally without worrying about downloading and uploading them manually each time. Given that the files will only change on one side at a time, it seemed like a reasonable way to try out Unison. There’s a greater chance it will work when concurrent writes are not involved.

Getting the unison package is just one apt or brew install away, but, to my surprise, it doesn’t support watching directories by default. Whenever it is started with -repeat watch (or repeat = watch in the config file), it stops with:

Error: No file monitoring helper program found

It requires the unison-fsmonitor command, which isn’t included in the Debian package (there has been an open bug since 2016), and it doesn’t support macOS at all. To work around this, on Debian, it can be compiled from source (it only requires the ocaml package in addition to build-essential), and on macOS, there’s an unofficial Rust-based implementation. Be sure to install it on both nodes, otherwise it will stop with a similar error:

Error: Server: No file monitoring helper program found