Most curious. Didn't know the "XYZZY" string was so widely spreaded around the world... 😎

<rant mode="useless">

The page wrote:

'Nothing happens...'

Data General's AOS/VS operating system had an undocumented command named "XYZZY." In the original 16-bit version, the response was: "Nothing happens." In a later 32-bit version, this was amended to: "Twice as much nothing happens."

Well, it was not the operating system per se that had a hidden command - it was the CLI (Command Line Interpreter).

Like with most CLI's you could create your own command scripts, but you could not override the built-in commands (likes FILES), unless you explicitely wrote out the full name, e.g. FILES.CLI.

Except for the hidden built-in XYZZY command where the CLI would first look for script file called XYZZY.CLI, and only then reply with the "Nothing happens..." message.