Unison

Unison is a file-synchronization tool for POSIX systems (Linux, BSD, macOS) and Windows. It keeps two replicas of a set of files and directories synchronized, allowing changes to be made independently on each side and then propagated in both directions.

Key Features

• Bidirectional synchronization (not one-way mirroring) with automatic propagation of non-conflicting changes
• Conflict detection and presentation when both replicas are modified
• Works locally (different disks) or across networks between hosts
• Communication over SSH or direct TCP connections
• Efficient transfers over slow links, optimizing small updates to large files with an rsync-like delta/compression approach
• Offline-friendly behavior since data is copied rather than accessed via a network filesystem
• Resilient to failures, aiming to keep replicas and internal state consistent after interruptions
• Repeat mode with filesystem monitoring to sync changes soon after they happen

Use Cases

• Keeping a laptop and a server directory synchronized across SSH
• Syncing personal work folders between multiple computers without relying on a centralized cloud drive
• Maintaining consistent configuration or project directories across machines while handling occasional conflicts

Limitations and Considerations

• Designed for synchronizing exactly two replicas at a time; it is not a multi-node distributed filesystem
• Conflicts require user resolution when both sides change the same file or structure

Unison is a mature, long-used synchronizer focused on correctness, cross-platform support, and reliable two-way syncing. It fits best where you want direct control over replication behavior and a robust tool that works well over real-world networks.