"You appear to be running an X server"的解决方法：
在安装过程中出现了这个问题，一位大神的解决方法：
Hit CTRL+ALT+F1 and login using your credentials.
kill your current X server session by typing sudo service lightdm stop or sudo stop lightdm
Enter runlevel 3 by typing sudo init 3 and install your *.run file.
You might be required to reboot when the installation finishes. If not, run sudo service start lightdm or sudo start lightdm to start your X server again.
先到NVIDIA 官网上下载驱动文件（.run 格式）
1.按住CTRL+ALT+F1 进入命令行
2.sudo service lightdm stop 或者 sudo stop lightdm
3.sudo init 3
4.安装驱动程序：＃: sudo ./NVIDIA-Linux-x86_64-177.67-pkg2.run //当前目录下执行NVIDIA驱动程序
5.按照提示安装完成，简单方法重启就好了。命令：sudo reboot