Unison is a tool that allows for the synchronization of files across servers
that also have it installed. It is like rsync but works both ways.

All options for this tool are included in a file stored in the .unison 
folder in your home directory, called a profile. For connecting to a different 
server, (tilde.club is an example of an exception to this.) a private key for 
that server is required in order to connect. To set that up, 
sshpref is the option to set to the value: '-i .ssh/ '.

To set an option the syntax is: option_name = value

Even if you don't have a server to connect to, unison is great for merging 
directories. Even on/to a Windows box.
Instead of following the common practice of using double hyphens for long 
flags on the comand line, a single is used for all.

For your personal machine, there is a graphical interface availiable as 
'unison-gtk' in APT, and very well could be in other packaging systems.
However, it is best if the profiles are edited manually as the user interface
does not do the best job of doing it for you.