Unison (file synchronizer)

Unison is a synchronization tool for Unix / Linux, Windows, Mac OS X, Solaris and some other operating systems. It balances between files from local or remote ( on other computers lying ) directories.


Unison has the following features:

  • Available cross-platform (Windows, Linux, Unix, Mac OS X etc.).
  • Can handle changes in the inventory file on both sides of the replicas; (As amended same file on both sides ) conflicts are displayed and can be solved manually.
  • Can save the modified files ( backup function)
  • Balance via computer networks ( LAN, Internet) by direct connection ( socket ) or tunneled over ssh
  • Bandwidth- optimized by using the rsync algorithm, so that only changed blocks of files to transfer.
  • Works as a user-level program and does not need root or. administrator rights
  • Is fault-tolerant to preserve even when the program crashes or network failures always a consistent state of the replicas.


Unison can be invoked from the command line with parameters or controlled by the profile files. It can be run interactively or in batch mode automatically. In batch mode, the unique changes are automatically synchronized, files with conflicts are skipped, however.

Unison scans after the start of the file stock per directory or computer and compares the timestamp with the list each stored locally. If it changes determines the appropriate files will be examined in more detail. Then Unison creates an overview of the two replicas with suggestions for synchronization and not automatically highlighted resolvable conflicts.

Examples of the use of

  • Balance of a directory to an external hard drive as a backup.
  • Direct comparison of directories between two networked computers.
  • Balance of directories between one or more computers (Linux, Apple, Windows, etc.) to a server.

User interfaces

For interactive use, there are GUI versions of Unison. On Windows and Linux, these are GTK -based, the corresponding component library has to be installed. The GUI versions allow for easier overview of the replicas and the proposed synchronization. All changes can be bookmarked using the keyboard or the mouse, and then carried out in a swing.


Unison works based on the local file systems. Come at the replicas different file systems for use can lead to the following problems:

  • Problems with attributes, which are not supported on both sides.
  • Problems with File Permissions (example: NTFS vs. FAT32. ).
  • Problems with file names with regard to uppercase or lowercase, for example: name.txt, name.txt or Name.txt denote ext3 under three different files with FAT32, but it refers to the same file.