Ubuntu: how to reset lost administrative password

After an extended vacation, I came home to discover to my horror that I could no longer login to my seldom-used Ubuntu laptop. The reason was embarrassingly simple: I forgot my password. My muscle memory (or finger memory) did not help while I was frantically typing my usual passwords but to no avail. I own the administrative account on that system. So, I had no one else to turn to for help. Luckily, I was able to login another machine and google how to reset the administrative password on Ubuntu.

Prerequisites

There are 2 preconditions for using the procedure to reset the administrative password.

1. Physical access to machine.
   You need to access the system console in order to interrupt loading of the OS.
2. The root password was disabled.
   By default, Ubuntu disables the password of the root account by assigning it a value which cannot possibly match any encrypted value. During the installation of Ubuntu, you were asked to create the first user. That initial user, by default, belongs to the sudo group which means that the user can be elevated to perform system administration functions. The administrative password which we are going to reset refers to the password of the initial user.
   The procedure assumes that you did not manually assign root a valid password. If root has a valid password and you know it, then you can simply sign on as root and reset the administrative password using the password command. If the root password was also forgotten, this procedure does not apply because you need to enter that password to drop to root shell prompt in this procedure.

How to reset administrative password

1. Power recycle.
   After the BIOS screen appears, press down the left shift key to enter GRUB. This step can be quite finicky, and you may need to repeat it several times until you get the timing just right.
   
2. Scroll down to Advanced options for Ubuntu, and press Enter.
   
3. In the ensuing screen, scroll down to the top Recovery mode line, and press Enter.
   If the Linux kernel image had been upgraded on the machine before, you would see multiple recover mode lines on the screen. Select the recovery mode line that corresponds to the latest Linux image(that is nearest to the top).
   
4. In the Recovery Menu screen, scroll down to root, and press Enter.
   
5. Press Enter again.
   If you have previously assigned root a password, you would be prompted to enter it at this step. Otherwise, just press Enter to continue.
   
6. Remount filesystem.
   After all the hard work, you are now at the root shell prompt. The filesystem at this point is read-only. Remount the file system to add write permission.

   # mount -o rw,remount /
   

7. Reset administrative password.
   Use the passwd command to change the password for the administrative user.

   # passwd <adminuser>
   

   Press Control-D to return to the recovery menu.
   
8. Select resume.
   
9. Exit recovery.
   Press OK to exit recovery mode and continue booting.