I was trying to install a NVIDIA graphics driver on a machine. The installer failed with this error:
ERROR: You appear to be running an X server; please exit X before installing. For further details, please see the section INSTALLING THE NVIDIA DRIVER in the README available on the Linux driver download page at www.nvidia.com
This is a common error when installing the graphics driver on Linux. You just need to exit X before you install the driver.
This can be done on Ubuntu with this command:
$ sudo service lightdm stop
Tried with: Ubuntu 14.04