From LQWiki

Unison is a backup application that allows for comparing directory contents.

Web Site

http://www.cis.upenn.edu/~bcpierce/unison/

Run it

To run Unison to compare two directories on the same computer, start it like this:

unison /directory1 /directory2

To run Unison to compare a directory on computer earth with one on computer mars, run unison as daemon on one computer and connect to its network socket from the other computer like this:

earth:/ # cd /
earth:/ # unison -socket 1807
mars:/ # unison /srv/www/htdocs/mediawiki socket://earth:1807//srv/www/htdocs/mediawiki

In this example, we are using port 1807.