Unison

Unison is an open-source file synchronization tool designed for POSIX-compliant systems, including GNU/Linux, macOS, and BSD, as well as Windows. It enables the synchronization of two replicas of files and directories stored on different hosts or different disks on the same host, allowing changes made in one replica to be propagated to the other. This bidirectional approach ensures that both sides of a distributed directory structure remain up to date.

The software operates as a user-level program that utilizes standard system calls, removing the requirement for kernel modifications, FUSE implementations, or superuser privileges on either host. It is typically deployed as a command-line tool that can communicate between machines over SSH or directly via TCP. Because it copies data locally, synchronized files can be read and written while the system is offline, providing a different workflow than traditional network filesystems.

Key features

• Bidirectional synchronization of updates to both replicas
• Conflict detection and display for overlapping updates
• Cross-platform support between Windows and Unix-like systems
• Offline access to synchronized data via local copying
• Bandwidth optimization using a compression protocol for large files
• Resilience to abnormal termination and communication failures
• Repeat mode with a filesystem monitor for near-instant synchronization
• Operation without the need for FUSE or root access

Unison is built for users who need to maintain consistent data across multiple environments without relying on a constant network connection. It handles network latency by optimizing transfers and ensures that replicas remain in a sensible state even during unexpected communication failures. The tool is suitable for synchronizing a local workstation with a remote server or managing data across separate physical disks. It is designed to be careful with network bandwidth, making it functional over slow internet links.

This software provides a precise specification for bidirectional file synchronization across diverse operating systems and hardware configurations.