> So your not supposed to just type ls at a prompt?
>

You shouldn't have '.' (the current) directory in the PATH where the shell
looks for programs to execute, because someone could replace the "real"
program with one in the current directory.

To the original problem:

a.) either ruby is not in /usr/local/bin
-> use `#!/usr/bin/env ruby` instead

b.) '.' is not in the PATH
-> don't change PATH, call your script using `./ruby_test`

c.) you're script is not executable
-> call `chmod +x ruby_test`

or any combination of the above.

   -tim