Set the system’s hostname in /etc/myname. My test system is called caddis.blackhelicopters.org.
$ cat /etc/myname caddis.blackhelicopters.org
To change the hostname, edit /etc/myname. The new hostname takes effect after the next reboot.
To change the hostname until the next boot, use
hostname -s and the new hostname, like this:
# hostname -s treble.blackhelicopters.org # hostname treble.blackhelicopters.org
You can edit /etc/myname and run
hostname -s to make a change take effect immediately and persist after the next boot.