Zenity is free software and a cross-platform program that allows the execution of GTK dialog boxes in command-line and shell scripts.

[3] Other scripting languages such as Perl and Python can be used to construct full-scale GUI applications, but the zenity program enables a shell script to interact with a GUI user.... [The] user interface is not as refined as one that could be provided by a full-featured GUI application, but it is perfectly suitable for simple interactions.

[5] A Zenity port to Mac OS X is available in MacPorts and Homebrew.

As of 2018, Zenity ports for Windows are available: zenity-windows (based on version 3.20.0) and winzenity (based on 3.8.0 / statically linked) Zenity does not possess any built-in scripting capabilities and it must, therefore, rely on an interpreter for processing.

To create a script that runs on more than one platform without extensive modifications, it would be best to use an interpreter that is available on the widest range of operating systems.