AWS EC2 Ubuntu 18.04 LTS and export $DISPLAY to run graphical application

Question

If I SSH into a Raspberry Pi or into a local VM running Ubuntu 18.04 LTS, then I can run:

user:~$ export DISPLAY=:0
user:~$ gedit

And this will open a graphical window on the OS with the text editor, gedit. However, if I SSH into an AWS EC2 instance running Ubuntu 18.04 LTS, and I run "export DISPLAY=:0" followed by "gedit", I get these errors:

No protocol specified
Unable to init server: Could not connect: Connection refused

(gedit:<proces id>): Gtk-WARNING **: <time stamp>: cannot open display: :0

Is there any way to find the display number, or any other way to launch a graphical application? Note that I do not want to forward a GUI window back to my terminal by passing for instance the -X flag.

Answer 1

Make the graphical session write its display number somewhere at login time.

For example, put these in your ~/.Xprofile:

echo "$DISPLAY" > ~/tmp/current-display
echo "$XAUTHORITY" > ~/tmp/current-xauthority

To make sure everything works, you should also repeat this with $DBUS_SESSION_BUS_ADDRESS and $XDG_RUNTIME_DIR, although they are frequently static on systemd-based systems, but not always.

Later you will be able to run:

export DISPLAY=$(< ~/tmp/current-display)
export XAUTHORITY=$(< ~/tmp/current-xauthority)

Note that No protocol specified frequently means that the display number was correct but that it denied the connection due to missing Xauth data.