Running GUI application as another (non-root) user

su vs. su -

When becoming another user you generally want to use su - user2. The dash will force user2's .bash_profile to get sourced.

xhost

Additionally you'll need to grant users access to your display. This is governed by X. You can use the command xhost + to allow other users permission to display GUI's to user1's desktop.

NOTE: When running xhost + you'll want to run this while still in a shell that belongs to user1.

$DISPLAY

When you become user2 you may need to set the environment variable $DISPLAY.

$ export DISPLAY=:0.0

You could use X11 forwarding:

ssh -XY otheruser@localhost your-gui-program-name-here

You need to share the authentication token from the user1 (assuming ~is home of user1):

cat ~/.Xauthority | sudo -u user2 -i tee .Xauthority > /dev/null