Sometimes it might be necessary to start in single-user mode to do some administration work, or even reset an existing password.

Normally this can be achieved via the GRUB boot loader.

CentOS / RedHat (with root account enabled)

Switch on your system.

Press Esc until the GRUB menu shows up.

This will bring up the GNU GRUB menu. If the CentOS/RedHat logo/boot messages show up you will need to restart (Ctrl-Alt-Del) and try again.

Select the OS/boot you want to edit. Normally the first line. Press e to edit it.

CentOS Linux (3.10.0-1160.53.1.el7.x86_64) 7 (Core)
CentOS Linux (3.10.0-1160.45.1.el7.x86_64) 7 (Core)
CentOS Linux (3.10.0-1160.42.2.el7.x86_64) 7 (Core)
CentOS Linux (3.10.0-1160.41.1.el7.x86_64) 7 (Core)
CentOS Linux (3.10.0-1160.36.2.el7.x86_64) 7 (Core)
CentOS Linux (0-rescue-d0401f7cdedb4955a0a262b3e0054323) 7 (Core)


Use the ↑ and ↓ keys to change the selection.
Press 'e' to edit the selected item, or 'c' for command prompt.

You will need to find the entry for the kernel. Normally starts with linux16.

[...]
linux16 /vmlinuz-3.10.0-1160.53.1.el7.x86_64 root=/dev/mapper/centos_centos7-root ro rd.lvm.lv=centos_centos7/root rd.lvm.lv=centos_centos7/swap rhgb quite LANG=en_US.UTF-8
[..]

At the end of the line you can choose to add one of the following:

single
systemd.unit=rescue.target
systemd.unit=emergency.target

So that it looks like this:

[...]
linux16 /vmlinuz-3.10.0-1160.53.1.el7.x86_64 root=/dev/mapper/centos_centos7-root ro rd.lvm.lv=centos_centos7/root rd.lvm.lv=centos_centos7/swap rhgb quite LANG=en_US.UTF-8 single
[..]

Type Ctrl-X to exit and the system will start in single- user mode.

If required, remount the root filesystem:

# mount -o remount,rw /

[If there are other filesystems you need to mount from fstab:]

# mount --all

CentOS / RedHat (without root account enabled)

It might be that your system didn’t have a root account enabled, in which case the above steps will fail. There is a workaround.

Switch on your system.

Press Esc until the GRUB menu shows up.

This will bring up the GNU GRUB menu. If the CentOS/RedHat logo/boot messages show up you will need to restart (Ctrl-Alt-Del) and try again.

Select the OS/boot you want to edit. Normally the first line. Press e to edit it.

CentOS Linux (3.10.0-1160.53.1.el7.x86_64) 7 (Core)
CentOS Linux (3.10.0-1160.45.1.el7.x86_64) 7 (Core)
CentOS Linux (3.10.0-1160.42.2.el7.x86_64) 7 (Core)
CentOS Linux (3.10.0-1160.41.1.el7.x86_64) 7 (Core)
CentOS Linux (3.10.0-1160.36.2.el7.x86_64) 7 (Core)
CentOS Linux (0-rescue-d0401f7cdedb4955a0a262b3e0054323) 7 (Core)


Use the ↑ and ↓ keys to change the selection.
Press 'e' to edit the selected item, or 'c' for command prompt.

You will need to find the entry for the kernel. Normally starts with linux16.

[...]
linux16 /vmlinuz-3.10.0-1160.53.1.el7.x86_64 root=/dev/mapper/centos_centos7-root ro rd.lvm.lv=centos_centos7/root rd.lvm.lv=centos_centos7/swap rhgb quite LANG=en_US.UTF-8
[..]

At the end of the line add this:

rd.break

So that it looks like this:

[...]
linux16 /vmlinuz-3.10.0-1160.53.1.el7.x86_64 root=/dev/mapper/centos_centos7-root ro rd.lvm.lv=centos_centos7/root rd.lvm.lv=centos_centos7/swap rhgb quite LANG=en_US.UTF-8 rd.break
[..]

Type Ctrl-X to exit and the system will start emergency mode in read-only mode.

Remount the root filesystem as read/write:

# mount -o remount,rw /sysroot

Switch to the sysroot jail.

# chroot /sysroot

Reset the password, or do any required tasks.

If you have SELinux enforcing mode enabled it will protect the OS from any changes. After you change the password type the following to overcome this:

# touch /.autorelabel

Restart.

# reboot -f

Ubuntu / Debian

Switch on your system.

Press and hold the Shift key.

In some instances pressing the Esc key several times (instead of holding it) achieves the same result. Just be aware that if you press it too many times it will bring you to the GRUB CLI. You can type normal and you will get to the menu described below.

This will bring up the GNU GRUB menu. If the Ubuntu logo/boot messages show up you will need to restart (ctrl-alt-del) and try again.

Select Advanced Options on the GRUB menu.

			GNU GRUB version 2.04

 Ubuntu 20.04.4 LTS
*Advanced options for Ubuntu 20.04.4 LTS
 History for Ubuntu 20.04.4 LTS
 UEFI Firmware Settings

And select the recovery mode option. Normally the latest kernel installed on your system.

			GNU GRUB version 2.04

 * Ubuntu 20.04.4 LTS, with Linux 5.13.0-37-generic
** Ubuntu 20.04.4 LTS, with Linux 5.13.0-37-generic (recovery mode)
 Ubuntu 20.04.4 LTS, with Linux 5.13.0-35-generic
 Ubuntu 20.04.4 LTS, with Linux 5.13.0-35-generic (recovery mode)

This will boot the system and show a series of options. Select root.

Recovery Menu (filesystem state: read only)

	resume			Resume normal boot
	clean			Try to make free space
	dpkg			Repair broken packages
	fsck			Check all file systems
	grub			Update grub bootloader
	network			Enable networking
	root			Drop to root shell prompt
	system-summary	System summary

					<OK>

This message will show. Press Enter.

Press Enter for maintenance
(or press Ctrl-D to continue)

If your / volume is ZFS it will be already read and write. Other filesystems might start in read only mode. If so, remount:

# mount -o remount,rw /

[If there are other filesystems you need to mount from fstab:]

# mount --all

Changing an user’s password

# passwrd <username>

Adding a new user

In the rare event of not having an user, you can add one and give it sudo privileges.

# useradd <username>

# usermod -a -G sudo <username>

# passwd <username>

Other OS

You can find description on how to get to single-user mode for other Linux distros in this Microsoft’s article.

https://docs.microsoft.com/en-us/troubleshoot/azure/virtual-machines/serial-console-grub-single-user-mode

After a recent system update I got the following error message:

Error: invalid environment block

Press any key to continue...

Luckily the system would boot up but ignoring errors isn’t best practice. This error is caused by a faulty GRUB2 environment block. This is a file located in /boot/grub/grubenv.

You can easily regenerate it with the following commands. It is advisable to make a backup copy the file just in case you need to revert.

# grub-editenv grubenv create
# grub-editenv grubenv set default=
# grub-editenv grubenv list
# update-grub

After rebooting the message should have disappeared.

If you can’t boot from your system drive you can use a Live CD and then mount your system’s boot partition and apply the same commands.

I haven’t tested this part personally but maybe the commands will help as a reference. Details are scarce on purpose, check what the commands do before doing anything.

# mount /dev/sda1 /mnt/boot/efi
or
# mount /dev/sda1 /mnt/boot/

# grub-editenv /mnt/boot/grub/grubenv grubenv create 
# grub-editenv /mnt/boot/grub/grubenv grubenv set default=
# grub-editenv /mnt/boot/grub/grubenv grubenv list
# grub-mkconfig  -o /mnt/boot/grub/grub.cfg

The other approach, also untested by me, could involve chroot.

# mount /dev/sda2 /target
# mount --bind /dev /target/dev
# mount --bind /dev/pts /target/dev/pts
# mount --bind /sys /target/sys
# mount --bind /proc /target/proc
# mount /dev/sda1 /target/boot

chroot /target

# grub-editenv grubenv create
# grub-editenv grubenv set default=
# grub-editenv grubenv list
# update-grub

During boot of your system and just after the BIOS messages Ubuntu might display a black screen with an error: no video mode activated message. The system boots normally but it is annoying.

GRUB by default hides the boot menu and this message can show up if there is no video mode set. It seems to be a known bug when the hidden option is active. I think you can just dismiss is it with ESC or ignore it as it has no adverse effect.

If you find it annoying you can get rid of it with the following steps.

Activate the menu option in GRUB.

vim /etc/default/grub

Make the following changes. Adjust your timeout to the number of seconds you want the boot menu to be displayed. Values below 5 seconds are difficult to catch.

#GRUB_TIMEOUT_STYLE=hidden
GRUB_TIMEOUT_STYLE=menu
GRUB_TIMEOUT=10

Update the changes:

$ sudo update-grub

Reboot and check that the changes have been applied correctly.