Reset the Linux Root User Password, If you forget the password
With Screen Shots:
Follow these steps to reset the password when using GRUB
1) Reboot the system, and when you are at the selection prompt, highlight the line of Enterprise Linux and press ' e ' from keyboard.
2) This will take you to another screen where you should select the entry that begins with ' kernel /boot/vmlinuz-2.6.18-164.el5 ro root=LABLE=/ rhgb quiet' and press ' e ' again.
3 ) Append ' single' to the end of that line (without the quotes). Make sure that there is a space between what's there and ' single ' and press enter. If your system requires you to enter your root password to log into single-user mode, then append init=/bin/bash after 'single'. Hit 'Enter' to save the changes.
4) Press 'b' to boot into Single User Mode.
5) Once the system finishes booting, you will be logged in as root. Use passwd and choose a new password for root.
6) Type reboot to reboot the system, and you can login with the new password you just selected.