According to Timothy Martens:
"whereis ruby" returns nothing
"whereis [ruby]" returns nothing
Forget whereis(1), it is not really doing the right way.
For bash users, use « type <command> ». In zsh, one can use ruby<ESC> ? to find the path for a given command (provided it is in the PATH). type is also supported in zsh of course. csh/tcsh has the same key binding as zsh too. which(1) is also a csh/tcsh builtin.