File Synchronization with Unison
Once users become familiar with Unison, a common thought is to use it for keeping one's home directory in sync between machines, say, your laptop and desktop. This can be realized pretty easily. Listing 2 has a simple profile that does the job, but you probably want to extend it. Listing 2, for example, ignores MP3 files and Unison's own files and demonstrates the use of include for having common settings applied to all profiles.
Listing 2. .unison/home.prf
# Unison preferences file root = /home/erik root = ssh://remotehost/home/erik # exactly two or none "root" lines ignore = Name *.mp3 # ignore all .mp3 files anywhere ignore = Path .unison # ignore all files with .unison somewhere in their full path include default # imports settings from default.prf
Test our new profile like this:
$ unison home -testserver
And invoke it like this:
$ unison home -batch $ unison home
The -batch run takes care of the easy cases without asking, backing up and logging as needed, and the second run asks you about any tricky business—like merging.
The root = lines can be omitted if you want to specify the files to be synchronized on the command line instead. The lines are equivalent to this invocation:
$ unison home /home/erik ssh://remotehost/home/erik
In order to do a three-way merge, backups must be enabled. By default, with backups disabled, Unison keeps only a checksum and metadata, such as permissions, so it has no unmodified file to reference.
In version 2.9.1 of Unison, if you choose merge for a conflict and the merge is successful without manual intervention, the changes are propagated immediately, which doesn't give you a chance to back out. So, if you have the space, I suggest leaving maxbackups at 5 or so, instead of the default 2, to leave yourself the chance of recovering from automatic mismerges. Contents of the backup directory after a merge look like this:
$ ls -1 .unison/backup/ shared.txt merged version ("NEW") shared.txt.1.unibck changed remotely ("CURRENT2") shared.txt.2.unibck changed locally ("CURRENT1") shared.txt.3.unibck old version ("OLD")
As of the newest beta, 2.10.3 at the time of this writing, Unison can invoke different merge programs for different files. You might want to use 3DM to merge XML files, for example, or a database merge tool for your Berkeley databases. This functionality still is new and subject to change. It has been noted by the project leader that the merge functionality was in need of a rewrite and didn't really work too well in 2.9.1 and 2.9.20. Thus, if you intend to do much merging, you will be better off tracking the bleeding edge.
Resources for this article: /article/8059.
Erik Inge Bolsø is a UNIX consultant and épée fencer who lives in Molde, Norway, and has been running Linux since 1996. Another of his hobbies can be found by doing a Google search for “balrog genealogy”, and he can be reached at email@example.com.