How to use a UNIX console app
First of all, I assume we're referring to the executable downloaded at http://www.zeroflux.org/proj/knock/files/knock-macos.tar.gz.

Most command-line executables like knock aren't designed to be double-clicked on in the Finder. Doing so simply executes the executable with no options, so this isn't what you want.

Instead, open a new Terminal window (using Spotlight or in the Finder from /Applications/Utilities/) and type in something like the following and press return:

/Users/me/Desktop/knock --version

Doing that should print out:

knock 0.5
Copyright (C) 2004-2005 Judd Vinet &lt;jvinet@zeroflux.org&gt;


Basically, when you type that stuff into the Terminal prompt, the Terminal executes /Users/me/Desktop/knock and passes it the --version option.

If you were to type in only the following and press return, you'd get the same "unwanted" output as before:

/Users/me/Desktop/knock