> Ah ?
>
> ~ $> which cd
> /usr/bin/cd
>
> Under Leopard
>

That may be the case under Leopard, but for many users of Linux, 'cd'
is a command that is built into the 'bash' shell.

As a result, a call to the command 'cd' using exec wont work. This is
probably one of the reasons that languages such as Ruby include
functions for precisely this operation.

The message

into.rb:1:in `exec': No such file or directory - cd /tmp
(Errno::ENOENT)

is complaining that there is 'No such file or directory' as 'cd'...
not '/tmp'. You can confirm this:

$ cat into.rb
exec "cd"
$ ruby into.rb
into.rb:1:in `exec': No such file or directory - cd (Errno::ENOENT)
	from into.rb:1

Dan