This post is just to point to the page where the great Rene mantains the .deb packages for Forticlient SSLVPN Linux client (instead of .tar.gz provided by Fortinet).
You can find the .deb files built by Rene in his blog Bits and Bites.
Kudos to him!
[UPDATE: 9th Dec 2017]
If you want to use the FortiClient from command line, this is the command (for 64bit, same for 32bit with the right path)
$ yes | /opt/forticlient-sslvpn/64bit/forticlientsslvpn_cli --server <YOUR SERVER IP/FQDN HERE>:<YOUR SERVER PORT HERE> --vpnuser <YOUR USERNAME HERE> > /dev/null
[UPDATE: 19th of November 2018]
Since Ubuntu 18.10 I start using the OpenFortiGUI and it works well, so I suggest to give it a try