1.系统安装；
1.set the password for root account:
sudo passwd root
2.edit file:
vi /usr/share/lightdm/lightdm.conf.d/50-ubuntu.confadd the following line to the end:greeter-show-manual-login=true3.restart.
原文：http://www.cnblogs.com/qiengo/p/4050369.html