A profile is how a particular replication pair is defined and configured. Once you have Unison installed on all the machines you wish to sync, you need to add a profile. If you just want to sync to an external hard drive, you don’t need any of the above just point it to the source folder on your machine and the folder to sync to on the external drive and it will handle the rest, just as if you were syncing data between two folders on the hard drive of a single computer. I use my Linux workstation in the office as my master node.
This will not in general be a computer you have sitting on your home network behind your cable/DSL router. Obviously you also need to have a machine that you can connect to via SSH and which will act as the hub or master machine.
UNISON FILE SYNCH INSTALL
Also note that the file formats used in replicating data between machines has changed a in recent versions of Unison, so be careful to install the same version on all your machines.
UNISON FILE SYNCH WINDOWS
It isn’t quite so easy to set those things up if all you have are Windows boxes you need to sync too, but still doable. On Linux, those things come as standard or are easily added via the package manager. It’s a little trickier to install on other OSes, but mainly because you need an SSH client on each machine you want to sync, and an SSH server daemon running on the hub (master) machine. On Linux, Unison is pretty easy to install as it tends to be available through your distro’s package manager ( yum in my case on Fedora/RHEL/Centos). It’s all rather neat and I keep my work folder which contains all my work since about 2002 (plus some earlier things) in sync over my cable/DSL connection at home, despite it being about 25GB in size! To keep network usage to a minimum, Unison uses an rsync-like algorithm to transfer over the network only the parts of files that have changed, reconstituting the file at the other end from the bits that didn’t change and the transfered bits that did. Unison connects pairs of machines either via a socket or over SSH. For me, the master is my Linux workstation in the office and I sync the other two computers with it. In that situation you need to assign one computer as the hub or master machine with which all the others are synchronised. In multi-machine setups, like mine, you don’t have a single pair of machines to keep in sync, but many pairs. It is designed to keep pairs of computers in sync by replicating changes made on one of the machines in the pair to the other and vice versa, at the same time.
Unison is a file synchronisation tool for flavours of Unix (Linux and OS X included) and Windows. Unison a multi-OS file synchronisation tool