X Windows authorization

Suppose you are user foo. Do this to execute a graphical application as another user bar.

Make foo’s magic cookie readable by bar. Environment variable XAUTHORITY holds the location of such cookie. So for example

foo$ sudo cp -av "$XAUTHORITY" /home/bar/.Xauthority
foo$ sudo chgrp bar /home/bar/.Xauthority
foo$ sudo chmod g+r  /home/bar/.Xauthority

The magic cookie is usually $HOME/.Xauthority or /var/run/<name of display manager>/foo.

Then get the value of foo’s display, in variable DISPLAY.

Login as bar and

bar$ export XAUTHORITY=<path to copy of foo's cookie>
bar$ export DISPLAY=<value of foo's DISPLAY environment variable>

Then just launch the application from the same terminal.