Kernel -> X -> DisplayManager -> Desktop
LightDM is the display manager running in Ubuntu up to version 16.04 LTS. While it has been replaced by GDM in later Ubuntu releases
systemctl restart lightdm
and see the result:yum /apt
commands. Next some modifications in configuration files might be needed example /etc/lightdm/lightdm.conf
.And finally we should enable previous DM and enable the new one with systemctl enable/disable lightdm
commands and reboot
.telinit
command or set default target via systemctl set-default
command.