Unison

Unison is a bidirectional file synchronization tool that runs on Unix-like operating systems (including Linux, Mac OS X, and Solaris) and Windows. It allows two replicas of a collection of files and directories to be stored on different hosts (or different disks on the same host), modified separately, and then brought up to date by propagating the changes in each replica to the other. It also unrestricted in terms of which system can be the host.

Installation

Installing unison from the official repositories, which provides CLI, GTK+ and GTK+ 2.0 interfaces. For offline documentation install unison-doc^AUR from the AUR.

Configuration

In order to use Unison, you need to create a profile.

GUI

To configure Unison with the GUI run:

$ unison-gtk2

Manual

Alternatively, manually create a profile in ~/.unison and add the following lines to the default configuration file, ~/.unison/profilename.prf.

Define the root directory to be synchronized.

root=/home/user/

Define the remote directory where the files should be sychronized to.

root=ssh://example.com//path/to/server/storags

Optionally, provide arguments to SSH.

sshargs=-p 4000

Define which directories and files should be synchronized:

# dirs
path=Documents
path=Photos
path=Study
# files
path=.bashrc
path=.vimrc

You can also define which files to ignore:

ignore=Name temp.*
ignore=Name .*~
ignore=Name *.tmp

Note: For more information see the Sample profiles in the User Manual and Reference Guide.

Usage

Once your profile is set up, you can start syncing:

$ unison profilename

or using the GUI tool:

$ unison-gtk2

and select the profile. Unison has a nice interface where you can view the progress and changes.

Version incompatibility

For Unison to function properly, the same version must be installed on all clients. If, for example, one computer has version 2.40 and the other has 2.32, they will not be able to sync with each other. This applies to all computers that share a directory to be synchronized across your machines.

Due to the staggered releases with varying Linux distros, you might be stuck with older versions of Unison, while Arch Linux has the latest upstream version in the Extra repository. There are unofficial PKGBUILDs for versions 2.32 (unison-232^AUR) and 2.27 (unison-227^AUR) on the AUR that allow users of multiple distros to continue using Unison with their existing systems.

Tips and tricks

Save human time and keystrokes

If one runs unison within a VDT emulator capable of maintaining a suitable scrollback buffer, there is no purpose in having to confirm every non-conflicting change; set the auto option to true to avoid these prompts.

Common config sync

When syncing configuration files which would vary (e.g., due to endemic applications, security-sensitive configuration) among systems (servers, workstations, laptops, smartphones, etc.) but nevertheless contain common constructs (e.g., key bindings, basic shell aliases), it would be apt to separate such content into separate config files (e.g., .bashrc_common), and sync only these.

Warning: Bidirectional syncing of config files can lend itself to become an avenue for an attack, by enabling the peer syncing system to receive malicious changes to config files (and perhaps even other peers the system syncs with). This is an attractive option for adversaries, especially when the conceptual security levels of the two systems differ (e.g., public shell server vs. personal workstation), since it would likely be simpler to compromise a system of lower security. Always use the noupdate option when bidirectional syncing between two particular systems is deemed unnecessary; when necessary, verify each change when syncing. Automatic bidirectional syncs should be done with extreme caution.

See also

• Official website
• Yahoo! user group
• Liberation through data replication by Philip Guo
• Setting up Unison for your mom by Philip Guo
• Replication using Unison on TWiki