vi /etc/lightdm/lightdm.conf
[SeatDefaults]
greeter-session=unity-greeter
user-session=ubuntu
greeter-show-manual-login=true #手工输入登陆系统的用户名和密码 allow-guest=false #不允许guest登录
sudo apt-get install gnome-session-fallback
vi /etc/lightdm/lightdm.conf
user-session=ubuntu ---> user-session=gnome-classic