You can think of
cat as common terminal program. You can call blindly:
echo "hello world"
in bash for example and it will work. You will see the output.
I am looking for something like this for X11, something you can type (again blindly) and assuming user is working in X11 she/he will see visual output (so I am not asking how to detect X11, it works, this is assumption).
It could be even X11 command (like creating bare window with close button) if it is possible to call X11 subsystem from bash. Please note, I am not talking about problem like https://stackoverflow.com/questions/7035/how-to-show-a-message-box-from-a-bash-script-in-linux because tools like "zenity" (no question, it is useful) is not installed by default.
Important note: I need something persistent, something which will stay until power is off or user explicitly closes it.