Boot Into GRUB menu
The first step is to reboot into the GRUB menu. If Ubuntu 18.04 Bionic Beaver is the only operating system installed you need to keep pressing SHIFT for the GRUB menu to show up.
Edit Grub Menu
Using arrows select the Ubuntu menu or the menu you use to boot your system and hit the e key. This will allow you to edit the menu:
Navigate down with the arrow-down key until you can see a line ending with:
ro quiet splash $vt_handoff
Using your keyboard replace the following keywords.FROM:
ro quiet splash $vt_handoff
TO:
rw init=/bin/bash
Once you have made the change press F10 or CTRL + x to perform boot into a single mode.
Reset password
Confirm that your system partition is mounted as read/write (rw):
# mount | grep -w /
If all good, reset your root password with the passwd command:
# passwd
Alternatively, you can reset your user password. The following linux command will reset the password for the linuxconfig user:
# passwd linuxconfig
Your root password is now reset. Reboot your system:
# exec /sbin/init
Troubleshooting
Enter new UNIX password: Retype new UNIX password: passwd: Authentication token manipulation error passwd: password unchanged
Your root partition is mounted read-only.
Try:
# mount -o remount,rw /
to resolve this issue.
end Kernel panic - not syncing: Attempted to kill init! exit code=0x0007f00
Make sure that you removed the splash boot option when editing the grub’s menu item.
When trying to reboot with the reboot command I get:Failed to connect to bus: No such file or directory
Failed to talk to init daemon.
Ignore, and reboot with:
# exec /sbin/init
Src: https://linuxconfig.org/how-to-reset-lost-root-password-on-ubuntu-18-04-bionic-beaver-linux