If you have forgotten your Linux system root password it can be reset either by taking action at boot or via a live Linux CD.
Reset Root Password from Grub Menu
To reset the root password the boot sequence will be interrupted with a change made to the status of the boot drive. This will enable access to run the passwd command from the command line applying the new password.
Start by restarting the computer
When the screen shows the GRUB boot loader list of boot options click on the e key to edit the grub configuration.
The change to be made is to put the boot into read-write mode.
Look for the line beginning with linux, similar to the below:
linux /boot/vmlinuz-5.3.0-1-amd64 root=UUID=b1386117-8b4b-4d54-acfb-4fedcef5ff01 ro quiet
Edit this line changing the text ro to rw. also append with the text init=/bin/bash. Thus giving:
linux /boot/vmlinuz-5.3.0-1-amd64 root=UUID=b1386117-8b4b-4d54-acfb-4fedcef5ff01 rw quiet init=/bin/bash
Press f10 to continue booting into single user mode. A command prompt will be shown with the user set as root.
The filesystem is to be mounted in read-write mode. To mount the hard drive type
mount -n -o remount, rw /
The password can now be changed using the password command. Simply type passwd and enter the updated password twice when prompted.
With the password set the computer can be rebooted. The changes made previously to the boot menu are a one off, no action will be needed regards these. To restart the computer type
exec /sbin/init
Reset Root Password with Live CD
The root password can also be modified using a Linux live CD.
Boot the computer from the live CD.
Note you may need to edit the BIOS of the computer to set the CD/DVD player as the primary boot device.
Start a command prompt and as root user (or with su) the fdisk command will be used to determine the hard disk partitioning.
fdisk -l
Often the boot disk is assigned to /dev/sda1, but requires checking.
The hard drive is mounted to a partition within the live CD. First create the partition and then mount it
mkdir /mnt/bootdisc mount /dev/sda1 /mnt/bootdisk
This boot disk is now chrooted. This will enable the root of the system to be set within the temporary environment of the boot disk within which the root password command will be run.
chroot /mnt/bootdisk
And now we can run the password command to set the lost root password
passwd root
Having entered the password twice all can be tidied up.
First by unmounting the boot drive
umount /mnt/bootdrive
and by exit from root
exit
Finally remove the live CD and reboot.
