shell script: test for graphical session

[John Crawley <john@bunsenlabs.org>]

A (bash) script wants to put up a GUI (yad) if it can, or otherwise try  
to interact with the user on the terminal.

Previously I was using loginctl:
loginctl show-session -p Type $XDG_SESSION_ID
and looking for 'Type=x11' or 'Type=wayland'

However, if a user logs in on a tty and then runs 'startx' (I don't know  
if Wayland has an equivalent) then the above command returns 'Type=tty'  
even when X is running.
($XDG_SESSION_TYPE gives the same results.)

I've replaced the test with
[[ -n $DISPLAY || -n $WAYLAND_DISPLAY ]] && <use GUI>
but environment variables are a bit fragile and I was wondering if there  
might be a better way.

For example, is there a scriptable GTK test for whether widgets can be  
drawn?

--
John