x11인지 wayland인지 확인

How to know whether Wayland or X11 is being used?

on X11 systems:

$ echo $XDG_SESSION_TYPE
x11

on some wayland system:

$ echo $XDG_SESSION_TYPE
wayland

https://unix.stackexchange.com/questions/202891/how-to-know-whether-wayland-or-x11-is-being-used