installing the GUI

The packages we need is:

1. Desktop environment: xfce

   pacman -S xfce4 xfce4-goodies 

    

2. A X Window System: xorg-server

   pacman -S xorg-server xorg-server-utils xorg-apps 

    

3. display manager:lightDM

   pacman -S lightdm  lightdm-gtk-greeter xorg-server-xephyr

   'xorg-server-xephyr' is for debuging use.  After installed lightDM, we need to enable it:

   $ systemctl enable lightdm

   then reboot, you get an GUI automatically!