Ubuntu 16.04
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GNOME Desktop Environment
Desktop Environment is not neccessary for Server usage, though. But Sometimes installation or using an application requires Desktop Environment, then build Desktop Environment as follwos.
[1] Install GNOME Desktop Environment.
apt-get -y install gnome
# select Gdm3 or Lightdm during installation (this example selects Lightdm)
[2] Click the icon which is located near the user name.
[3] Select "GNOME" like follows and login. If you'd like to use Classical style's GNOME like GNOME2, select "GNOME Classic".
[4] GNOME Desktop Environment starts like follows.