Unison

Unison synchronizes files between directories on different hosts or disks. Unlike rsync or simple mirroring tools, Unison handles updates on both sides without a master or slave. It detects conflicts when files change in both locations simultaneously and lets you resolve them manually or with scripts. The tool works over SSH or a direct socket connection, making it useful for laptops, servers, and offline syncing.

Unison supports macOS, Linux, Windows, and many Unix variants. It uses a replica database to track file states, so it can sync incrementally and detect deletions. The current stable release is 2.53.7. Development happens on GitHub under bcpierce00/unison. The tool is written in OCaml and compiles on most POSIX systems. It does not have a built-in GUI on Linux, but a GTK+ interface is available on some platforms.

Unison is open source under the GPL. It is best for users who need two-way sync without a central server, like developers syncing code or sysadmins backing up configurations. It is not a real-time sync tool; you run it on demand or via cron. Notable for its resilience and decades of continuous development at the University of Pennsylvania.