Shell

[Hershel Fisch]

Hi, I'd greatly appreciate if some one can help me out on this one.
How do I write this command with the shell function, I would write it in
terminal as follows

Su -  myUsr
Password:
And another command.

Now the question is that every line above is a different entry
If I put 
Put shell("su - myUsr") in the message box this is the result
"Password's: Sorry"

And also if I understand correctly every command with the shell function is
a different terminal and does not correspond to the previous shell reply, is
that true? If so how do I write that every command should correspond to the
previously respond?

Thanks a million, Hershel