Not able to 'su' to other user when rebooting the server

Hi All,

I have created one script with starting letter 'S' (Example: Start) and stored in the path "/etc/rc.d/".

This script was called automatically when we reboot the server.
In this script i am using 'su' command to switch other user but it is not working, it giving some error.

su sas -c "pwd ; whoami"

Note:

After done the reboot, same command is working fine but it is not working when we rebooting the server.

Please help me to resolve this issue.

What was the OS. Is it SUN microsystem?

I am using AIX