Unison

Unison is a file-synchronization tool for POSIX systems such as BSD, GNU/Linux and macOS, as well as Windows. It keeps two copies of a directory tree in step. The copies can sit on different machines or on separate disks of one machine, can each be modified on their own, and are reconciled later by carrying every side's changes across to the other.

It differs from plain mirroring or backup tools in handling edits made on both sides: changes that do not conflict are propagated automatically, while conflicting ones are detected and displayed. It runs as an ordinary user-level program, so no kernel changes, superuser rights or FUSE are needed, and data is copied so it can be read and written offline.

Machines are usually connected over ssh, though direct TCP also works, and the tool is careful with bandwidth so it performs reasonably over slow links. It can synchronize a Windows laptop with a Unix server, for example. Written in OCaml and licensed under GPL-3.0, it has been in use for more than twenty-five years.