How to send a message to the user's screen in a graphical shell?

Question

Graphic shells

Good day!
I connect via ssh to another machine. The machine has different DEs installed: lxde, kde and gnome. Users sit behind the machines.
How to display messages to users?
Tried to use xmessage and notify-send, but either nothing is shown, or errors are flying:
No protocol specified
Error: Can't open display :0.0

Answer 1

You need to somehow find out what socket the user is sitting on, then do so. so that the user allows anyone to connect to his X socket, and then do his job by specifying this same socket:
DISPLAY=:0 notify-send ...
In the example, the socket is specified: 0