Unison (file synchronizer)

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For other uses, see Unison (disambiguation).
Unison Logo.png
Screenshot of Unison
Unison on MacOS showing the native Mac UI (macuinew)
Original author(s) Benjamin C. Pierce
Stable release
2.48.3[1] / 12 February 2015; 19 months ago (2015-02-12)
Development status Active (stable, maintained[2])
Written in OCaml
Operating system Unix-like, Microsoft Windows, Mac OS
Available in English
Type File synchronization
License GNU-GPLv3
Website www.cis.upenn.edu/~bcpierce/unison/

Unison is a file synchronization program. It is used for synchronizing files between two directories, either on one computer, or between a computer and another storage device (e.g. another computer, or a removable disc). It runs on Unix-like operating systems (including Linux, Mac OS X, Android, and Solaris) and Windows.


Unison allows the same version of files to be maintained on multiple computing devices. In other words, when two devices are synchronized, the user can be sure that the most current version of a file is available on both devices, regardless of where it was last modified.

  • It runs on many operating systems, and can synchronize files across platforms, so that for instance a Windows laptop may be synchronized with a Unix server.
  • It detects 'conflicts' where a file has been modified on both sources, and displays these to the user
  • It communicates over the TCP/IP protocol so that any two machines with an internet connection can be synchronized. This also means that the data transferred can be secured by tunneling over an encrypted SSH connection.
  • It uses the rsync algorithm developed by Andrew Tridgell. This algorithm transfers only the parts of a file that have changed, and so is faster than copying the whole file.
  • It is designed to be robust in the event of a program or system crash or a communication failure.
  • It is open-source.
  • It is written in the OCaml language.

File synchronization tools such as Unison are similar to version control tools (git, CVS, Subversion, etc.), distributed filesystems (Coda, Lustre, etc.), and mirroring utilities (rsync, etc.), in that all these attempt to keep sets of files synchronized. However, file synchronization tools can deal with modifications to both versions of the directory structure, without the overhead of version control.

Development status[edit]

According to the developers, Unison has reached "a mature and stable state" and new features are not actively being developed.[2] It is, however, maintained and supported, and ongoing releases contain bug fixes, improvements, and contributed patches. Consulting the developers for desired improvements is possible.[2]

Support for Unison is provided by third parties for specific operating systems.[3]

The day job of the original developers began focusing on a project called 'Harmony', which they considered a natural progression from Unison, to synchronize any hierarchical structure expressed in XML.[4] However, Harmony has been abandoned, and the developers are now focusing on the Boomerang project which creates 'lenses' for transforming string data.[5]

See also[edit]


External links[edit]

Advice on using Unison[edit]