Unison

Unison synchronizes two replicas of files and directories across different hosts or disks, allowing independent modifications and then propagating changes to keep both sides up to date. It operates on POSIX‑compliant systems—including macOS, Linux, BSD—and Windows, and can connect via SSH or direct TCP, using a bandwidth‑efficient compression protocol similar to rsync. The tool detects and reports conflicting updates while automatically applying non‑conflicting ones, and it can run in repeat mode with a filesystem monitor to synchronize changes shortly after they occur.

Because Unison is a user‑level program, it requires no kernel modifications or superuser privileges, and it works without FUSE or other special filesystems. Its design emphasizes resilience: even abnormal termination or communication failures leave replicas and internal state in a consistent condition. The software is open‑source under GPL‑3.0, self‑hostable, and available as source code with binary packages provided by many Linux distributions. It is maintained by a small core team, with community support through mailing lists and a public GitHub repository.