How can I show a GUI message box from a Bash script in Linux?

I'm writing a few little Bash scripts under [Ubuntu][1] Linux. I want to be able to run them from the GUI without needing a terminal window to enter any input or view any output. So far, the only input required is a password for [sudo][2], and gksudo handles that fine. But I haven't found an easy way to show a message box yet. Is there some kind of 'gkmessage' command available? I'd prefer something present in a default Ubuntu install, but I don't mind installing a new package if necessary. [1]: https://en.wikipedia.org/wiki/Ubuntu_%28operating_system%29 [2]: https://en.wikipedia.org/wiki/Sudo