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Using unison just for backups

These are working notes. I am not guaranteeing that using unison for this purpose is a good idea

I consider synchronization an extension of the making backups problem, so it occurred to me that I could use unison to manage the backing up of my laptop to an external drive.

i.e. every week or so copy the files that have changed to a backup.

In my example case, I back up my temporary directory to my F: drive

C:\moreprgs\unison>unison.exe c:\moreprgs\unison\a.tmp f:\a-backup -backups

  • rather than delete old copies, -backups causes unison to move them to an different file. This means that you could end with up to -maxbackups (default=2) three copies of each file.

So I created a directory structure on my C: drive and repeatedly edited and synchronized my barsteward file:

C:\moreprgs\unison>dir a.tmp
 Volume in drive C is TPATMRJCDOTCOM_C
 Volume Serial Number is FCB2-E47D

 Directory of C:\moreprgs\unison\a.tmp

09/08/2005  22:17    <DIR>          .
09/08/2005  22:17    <DIR>          ..
22/07/2005  20:24                 7 b.txt
09/08/2005  22:21                35 barsteward
22/07/2005  20:44    <DIR>          d
09/08/2005  21:48                 0 foo
               3 File(s)             42 bytes
               3 Dir(s)  18,231,308,288 bytes free

After doing so I end up with:

C:\moreprgs\unison>dir f:\a-backup
 Volume in drive F has no label.
 Volume Serial Number is ECCF-391A

 Directory of f:\a-backup

09/08/2005  21:18    <DIR>          .
09/08/2005  21:18    <DIR>          ..
09/08/2005  21:18                 7 b.txt
09/08/2005  21:18    <DIR>          d
09/08/2005  21:48                 0 foo
09/08/2005  22:17                 0 barsteward..002..unison.bak
09/08/2005  22:19                17 barsteward..001..unison.bak
09/08/2005  22:21                35 barsteward
               5 File(s)             59 bytes
               3 Dir(s)     876,462,080 bytes free

C:\moreprgs\unison>unison.exe -ui text -batch c:\moreprgs\unison\a.tmp f:\a-bacup -backups
Contacting server...
Looking for changes
Reconciling changes

a.tmp          a-backup
changed  ---->            barsteward
a.tmp        : changed file       modified at 22:40:19 on  9 Aug, 2005  size 40
a-backup     : unchanged file     modified at 22:21:08 on  9 Aug, 2005  size 35
Propagating updates

UNISON started propagating changes at 22:40:44 on 09 Aug 2005
[BGN] Updating file barsteward
  from c:/moreprgs/unison/a.tmp
  to f:/a-backup
[END] Updating file barsteward
UNISON finished propagating changes at 22:40:45 on 09 Aug 2005

Saving synchronizer state
Synchronization complete  (1 item transferred, 0 skipped, 0 failures)


  1. Unlike the ReadWriteOfflineWiki possibility of ReplicationUsingUnison that requires multiple machines, backup typically doesn't. (If yours does, unison can handle that too).
  2. As long as I don't ever alter the contents of my external drive there are no changes coming back. Unison has a -prefer flag that could make this
  3. I have been doing my experiments without a set of unison profiles, instead specifying stuff on the command line. I expect to change my method as I understand more about unison.

-- MartinCleaver - 10 Aug 2005

See also SyncContrib

-- MartinCleaver - 26 Nov 2005

This looks pretty good - have been playing around getting my laptop to sync via OpenSSH to my Dreamhost webserver for backup purposes. As long as you use the backups option in the preferences, it's possible to keep old versions around, protecting against the problems of deleting or overwriting files by mistake, and then syncing (which will correctly delete the file or copy the overwritten one). Important in these days of Nyxem viruses (Google:nyxem+virus+overwrite)...

I now have Unison syncing working nicely between Windows and Linux (Dreamhost), including use of ssh-agent to enable unattended backups. The backup feature using centralised backups has a few issues with subdirectories but other than that everything's working well. Unison is a very useful tool, and once you have a working setup it's not hard to create profiles to back up additional sets of files. I'm also using it to sync data files between a laptop and desktop, via the web host as recommended and to create an off-site backup.

-- RichardDonkin - 05 Feb 2006

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Topic revision: r6 - 2006-02-06 - RichardDonkin
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