Why have a dependency on Zenity instead of displaying the message in the terminal? Seems weirdly limiting to have a GUI dependency for a terminal application thus making this unusable on headless systems. I think you could make it optional and use STDERR if Zenity's not around.
I assume it's meant to work for programs that aren't being launched from the terminal
My question exactly, minus the optional part. If it's a command-line tool, it should not require any GUI elements at all.