The Korn shell is pretty standard on all Unix systems. You usually have a choice between selecting from the Bourne shell, Korn shell (not to be confused w/ the musical group), and C shells. Most software vendors, including Oracle, assume that the person installing the software on a platform at least know the basics of that platform. This isn't a knock at all - I just want to suggest that if you are running Oracle on the platform and if you don't have any experience with it (which I am assuming because you didn't install all of the shells) that you pick up a book on the OS basics and read it carefully.
You shouldn't modify the Oracle supplied scripts in bin unless you have a good reason to.
I didn't even realize that you had a choice to not install the basic shells w/ Linux. Are you sure they aren't installed?
Senior Database Administrator