Use Unison on Webfaction

in Geek Inside

Unison is a great little tool to synchronize files between your local machine and a remote server. It’s free, open-source, and cross-platform. I use it every day to develop Jetpack. However, until very recently I couldn’t use it to work with files on this very site. I’m hosted with Webfaction, a great hosting provider for developers who want flexibility without having to deal with server administration. A lot of packages are already installed and configured on my server for me. Unison is not available by default, so I had to install it myself. Luckily, Webfaction lets you do that!

Before we get started, there are 2 important things to remember when setting up Unison:
– It has to be installed both locally and on the remote server.
– Both instances should use the same version of Unison.

Installing Unison locally is as easy as brew install unison if you’re using Mac OSX. On your Webfaction server, it’s a bit trickier.

Since Unison was built using the OCaml language, you’ll need to install OCaml first.

I did everything in the ~/lib directory on my server.

$ wget http://caml.inria.fr/pub/distrib/ocaml-4.04/ocaml-4.04.0.tar.gz
$ tar -zxvf ocaml-4.04.0.tar.gz 
$ rm ocaml-4.04.0.tar.gz 
$ cd ocaml-4.04.0 
$ ./configure -prefix $HOME
$ make world.opt 
$ make install

Once this was done, I checked the version of Unison running on my local machine:

$ unison -version
unison version 2.48.4

I then had to find and install that same version on Webfaction. You can find a full list of releases here, and the GitHub repository here.

$ cd ~/lib
$ wget https://www.seas.upenn.edu/\~bcpierce/unison//download/releases/unison-2.48.4/unison-2.48.4.tar.gz
$ tar -zxvf unison-2.48.4.tar.gz 
$ rm unison-2.48.4.tar.gz 
$ cd src
$ make
$ chmod a+x unison 
$ cp unison ~/bin

That’s it!

Try checking if Unison is installed by running unison -version. It should return unison version 2.48.4.

I then ran a few tests and synchronized 2 directories on the server; all worked fine. To do that quick test, create 2 folders, test1 and test2. Add a random file in test1. Then, run unison test1 test2. You should see the random file you created get copied over to test2 right away. Success!

But wait a second… When trying to synchronize a local directory with a directory on the server, Unison didn’t work. I kept getting this error:

zsh:1: command not found: unison
Fatal error: Lost connection with the server

After some research, I found that since I installed Unison myself on Webfaction, the executable isn’t in the PATH for a non-interactive shell session. Luckily, Unison gives you 2 options to work around that:
– You can use the servercmd parameter when launching Unison to pass the location of unison on to the remote server, like so: $ unison -servercmd /home/username/bin/unison
– You can include that parameter in your Unison configuration file, like so:
servercmd = /home/username/bin/unison

And just like that, it works!

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