File Synchronization with Unison over SSH

Posted by admin on April 9, 2009 under Tech Tips | Be the First to Comment

Previously, I posted on using rsync over SSH for file synchronization. While this works very well when pushing data in one direction, it’s not well suited for synchronizing modifications that are made on both sides. An excellent bidirectional utility for that type of job is Unison, which sports many of the same benefits as rsync, but has some distinct advantages for more complex synchronization scenarios.

A basic example would be to synchronize a local directory called “MyDocs” with a remote SSH server. From the following output, you can see that this directory contains four text files.

ls -ld ~/MyDocs
drwxr-xr-x 2 gmendoza gmendoza 4096 2009-04-09 16:05 /home/gmendoza/MyDocs

ls -l ~/MyDocs
total 12
-rw-r--r-- 1 gmendoza gmendoza 31 2009-04-09 16:09 file1.txt
-rw-r--r-- 1 gmendoza gmendoza 31 2009-04-09 16:09 file2.txt
-rw-r--r-- 1 gmendoza gmendoza 31 2009-04-09 16:09 file3.txt
-rw-r--r-- 1 gmendoza gmendoza 31 2009-04-09 16:09 file4.txt

The first time you run Unison for this particular directory structure, both sides will create a local index and hash table. You’ll get a warning and will be prompted with a message, asking you to hit the space bar if you accept. If the root directory on the remote side does not exist yet, you’ll also be prompted to accept the changes.

unison MyDocs ssh://host2/MyDocs
Contacting server...
Connected [//host1//home/gmendoza/MyDocs -> //host2//home/gmendoza/MyDocs]
Looking for changes
Warning: No archive files were found for these roots, whose canonical names are:
/home/gmendoza/MyDocs
//host2//home/gmendoza/MyDocs
(snipped for brevity...)
Press return to continue.[] Waiting for changes from server
Reconciling changes

local host2
dir ----> / [f]

Proceed with propagating updates? [] y
Propagating updates

UNISON 2.27.57 started propagating changes at 16:14:30 on 09 Apr 2009
[BGN] Copying from /home/gmendoza/MyDocs to //host2//home/gmendoza/MyDocs
[END] Copying
UNISON 2.27.57 finished propagating changes at 16:14:30 on 09 Apr 2009

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

Subsequent synchronizations are shown as the following.

unison MyDocs ssh://host2/MyDocs
Contacting server...
Connected [//host1//home/gmendoza/MyDocs -> //host2//home/gmendoza/MyDocs]
Looking for changes
Waiting for changes from server
Reconciling changes
Nothing to do: replicas have not changed since last sync.

For the following example, I have modified file1.txt on host1, and file2.txt on host2. Both file3.txt and file4.txt have been modified on each side. The great thing about unison is that when there is a conflict, you have the opportunity to view the differences and select which direction you wish to synchronize. Pressing the “x” key displays some basic information about the files that differ. In this case, I have chosen the files with the most recent timestamp. You choose the file direction by pressing the greater and less-than symbols, “>” and “<".

unison MyDocs ssh://host2/MyDocs
(snipped)
local host2
changed <-?-> changed file3.txt [] x
local : changed file modified on 2009-04-09 at 16:16:29 size 50
host2 : changed file modified on 2009-04-09 at 16:16:43 size 55
changed <==== changed file3.txt [] <
changed <-?-> changed file4.txt [] x
local : changed file modified on 2009-04-09 at 16:17:20 size 56
host2 : changed file modified on 2009-04-09 at 16:16:59 size 41
changed ====> changed file4.txt [] >
changed ----> file1.txt [f]
<---- changed file2.txt [f]

Proceed with propagating updates? [] y
Propagating updates

UNISON 2.27.57 started propagating changes at 16:18:27 on 09 Apr 2009
[BGN] Updating file file3.txt from //host2//home/gmendoza/MyDocs to /home/gmendoza/MyDocs
[BGN] Updating file file4.txt from /home/gmendoza/MyDocs to //host2//home/gmendoza/MyDocs
[BGN] Updating file file1.txt from /home/gmendoza/MyDocs to //host2//home/gmendoza/MyDocs
[BGN] Updating file file2.txt from //host2//home/gmendoza/MyDocs to /home/gmendoza/MyDocs
[END] Updating file file3.txt
[END] Updating file file2.txt
[END] Updating file file4.txt
[END] Updating file file1.txt
UNISON 2.27.57 finished propagating changes at 16:18:27 on 09 Apr 2009

Saving synchronizer state
Synchronization complete (4 items transferred, 0 skipped, 0 failures)

Unison also has a GTK front end for the graphically inclined. Be sure to check out all the documentation for a full understanding of syntax.

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