Use case

You have to run your program on a remote server. However your favourite editor with your favourite configuration isn’t available over SSH. Or its over-SSH mode is very slow (looking at you, TRAMP). Also, you’d like to work online sometimes.

Unison is like Dropbox but without an intermediary server.

Setting it up

First, you need to have two¹ directories, one in your local machine and one in your remote machine, that will be synchronised. For example, /local/path/to/my/software in your local machine and /remote/path/software in your remote server. You must have SSH access to the server, which is for example at user@host.

Then you download and install Unison. Preferrably use your system’s package. You need two executables: unison and unison-fsmonitor. The second one is not always available with the unison package: it is not there, for example on Ubuntu or OS X. To install it:

• Ubuntu: install the ocaml package to compile Unison from source. Then just run make. It might give an error, but with a little bit of luck you will have compiled the unison-fsmonitor executable before that. Copy it to your $PATH.
• OS X: install UNOX
  

Now you have to set up a “profile” configuration file in your local machine. In $HOME/.unison/profile-name.prf, write something like:

root = /local/path/to/my/software
root = ssh://user@host//remote/path/software

ignore = Regex ^(.*/)*[^/]*\.[^/]*$
ignorenot = Regex ^(.*/)*[^/]*\.(py|txt|md|ipynb|bash|gitignore)$

This will synchronize all files ending in .py, .txt, .md… or having no “.” in their filename, between the two directories mentioned at the beginning. Read more about how the ignore and ignorenot rules work here and here.

Finally, in your local machine, run:

unison profile-name -repeat watch -times -logfile /var/log/unison.log

And the directories shall be synchronized.