The command "ruby" does nothing on my Mac
What are you trying to do, exactly? The ruby command expects input, in most cases a file that contains Ruby code that you want it to run. In that case you have to specify the name of the file:
> ruby my_ruby_file.rbIf instead you want to run the interactive Ruby shell, i.e. the REPL console that you can type Ruby code into and have it executed each time you press enter, the command you want is irb.