www.example.com

127.0.0.1 www.example.com www
[...]

hostnamectl set-hostname www

hostname -> www
hostname -f -> www.example.com

Static hostname: www
Icon name: computer-vm
Chassis: vm
Machine ID: [ID]
Boot ID: [ID]
Virtualization: microsoft
Operating System: Ubuntu 16.04.1 LTS
Kernel: Linux 4.4.0-59-generic
Architecture: x86-64

hostname -> XXX
hostname -f -> XXX.strato.net

Static hostname: www
Transient hostname: XXX
Icon name: computer-vm
Chassis: vm
Machine ID: [ID]
Boot ID: [ID]
Virtualization: microsoft
Operating System: Ubuntu 16.04.1 LTS
Kernel: Linux 4.4.0-59-generic
Architecture: x86-64

description     "set system hostname"

start on startup

task
exec hostname -b -F /etc/hostname.mod; full=$(cat /etc/hostname.mod);short=(${full//./ }); hostnamectl set-hostname $short