[ALUG] basic question about scripts, change user in midstream

How do I change user in a script, and then carry on with the task at hand? What I need to do is get the user to su to root, give the password of course, then carry on. For instance: su [and this then prompts for his password of course] cd /home/fred/Desktop [ now I would get him to enter a file name of a folder on his desktop and then...] chown -R fred filename What happens when I try this is that after changing accounts, the script stops. Very basic question.........., sorry. Peter