Debian 9: Install Xfce for desktop environment

The following command will install Xfce.

$ sudo apt install -y expect
$ cat <<EOF | expect
set timeout -1
spawn sudo apt install -y task-xfce-desktop
expect "Default display manager: "
send "2\n"
expect eof
EOF
$ sudo reboot

After reboot, lightdm-gtk-greeter will be started. You can select other desktop environment.

Xfce is displayed.

The following command will uninstall Xfce.

$ sudo apt remove -y task-xfce-desktop xfdesktop4 xfce4-* libxfce4-*
$ sudo apt autoremove -y
$ sudo apt install --reinstall -y gdm3
$ sudo reboot