switching to another user in shell script...

Hi,
I have a shell script in which I need to switch to another user and execute some commands and then come back to the original user.
To make it more clear -
I have to log in as user root then 'su' to jag - execute a script called backup.sh and then logout and come back to root again..

rgds,
Jagadeesh

su - jag -c "backup.sh"

this may help you.