Changing user with su does not allow you to run X apps

It happens whenever you su to a different account on the same machine. The X session details are not inherited by the new user, so you do not have access to the X session.

The solution is to use xhost like this:

> xhost
access control enabled, only authorized clients can connect
SI:localuser:nickapos

activate xhost, and then add the new user that needs to run xapps:

>xhost +si:localuser:nickapostolakis

localuser:nickapostolakis being added to access control list

verify by re-running xhost:

> xhost

access control enabled, only authorized clients can connect

SI:localuser:nickapostolakis

SI:localuser:nickapos