If you forgot your Linux root password, you can use this technique:

1. Boot the system and when you see the following message “Press any key to enter the menu”,press any key. (You will see the list of available kernel versions.)

2. Press e in order to edit commands before booting.

3. Highlight the list item with vmlinuz in it by using the arrow keys and press e.

4. Now type single or init 1 at the end of the line (you must enter SPACE first).

5. Then press enter and b to boot the system with the new argument. (The system will boot into single user mode and you will see bash prompt)

Now it’s time to change the password:

6. Type passwd

### Shell Commands ###
passwd
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
Type reboot to restart the system.

Source: Java