1. Install unison on both servers

apt-get install unison

2. Create a script to add to crontab to call unison periodically

unison -numericids -owner -silent -times -contactquietly -batch -auto -log -logfile /var/log/unison.log profile_file 2>&1

3. Create the above mentioned profile file on ~/.unison/profile_file.prf

root = ssh://server_ip//path/to/root/dir
root = /path/to/root/dir
ignore = Regex .*(cache|Cache|te?mp|logs?).* # ignore log and cache files

That’s it! Now add the following line to your crontab to execute it every 5min

*/5 * * * * /path/to/my_unison_script.sh

After a few days of running this I found that sometimes one unison execution would overlap another. To resolve this I found a little tool called lockrun

To get it to work I did:

1. Download the source code from the official lockrun website

wget http://unixwiz.net/tools/lockrun.c

2. Build/install as shown in http://unixwiz.net/tools/lockrun.html

gcc lockrun.c -o lockrun
sudo cp lockrun /usr/local/bin/

3. Change the previous crontab line to use lockrun

*/5 * * * * /usr/local/bin/lockrun --lockfile=/tmp/hosting.lockrun -- /path/to/my_unison_script.sh

All done!!!