http://askubuntu.com/questions/413990/want-to-convert-ubuntu-12-04-desktop-to-server
sudo apt-get install tasksel
sudo tasksel remove ubuntu-desktop
sudo tasksel install server
apt-get install linux-server linux-image-server
apt-get –purge remove lightdm
vim /etc/default/grub：
GRUB_TIMEOUT=5
( Comment out ‘GRUB_HIDDEN_TIMEOUT’ )
GRUB_CMDLINE_LINUX_DEFAULT=”"
GRUB_TERMINAL=console ( only for PC )
sudo update-grub