====== Systemd ======

  * http://www.enricozini.org/blog/2017/debian/systemd-03-units/
  * http://www.enricozini.org/blog/2017/debian/systemd-02-cmdline/
  * http://www.enricozini.org/blog/2017/debian/systemd-01-intro/

  * https://www.digitalocean.com/community/tutorials/how-to-use-systemctl-to-manage-systemd-services-and-units