0040_Linux_GRUB

grub rescue> shows if GRUB can`t find its configuration (grub.cfg) or its modules.

It happens if:

  • Move or delete the /boot partition
  • Disk`s UUID changed
  • Trouble with MBR or UFI

Common commands grub rescue>:

  • ls – shows disks and partitions (hd0) (hd0,msdos1)
  • set – shows or changes (prefix, root)
  • set root=(hd0,msdos1) – there are must be folders /var, /opt, /etc, /proc ….
  • set prefix=(hd0,msdos1)/boot/grub – grub config and modules
  • insmod normal – load a module normal
  • normal – go to the grub menu
#Example
ls
set root=(hd0,msdos1)
set prefix=(hd0,msdos1)/boot/grub
insmod normal
normal

If it’s correct, then the grub menu shows up.

grub> it is a CLI GRUB and it shows:

  • Then “c” was pressed
  • The grub menu can`t be found, but modules were loaded

Here are more commands than in the grub rescue mode. You can manually load a kernel. 

#Example
ls
set root=(hd0,msdos1)
linux /boot/vmlinuz-6.8.0 root=/dev/sda1 ro
initrd /boot/initrd.img-6.8.0
boot

GRUB: BIOS and UEFI recovery

General algorithm:

  1. Boot from LiveCD/LiveUSB Linux (Ubuntu, Debian, etc.).
  2. Determine where the system root and EFI partition are (for UEFI): lsblk,fdisk -l
  3. Mount the system.
  4. Make a chroot into the installed system.
  5. Reinstall GRUB and update the configuration.
  6. Reboot.

 

#Example for BIOS (Legacy)
# Use LiveCD/LiveUSB Linux 
# Find the "/" 
lsblk
fdisk -l
sudo mount /dev/sdXY /mnt
sudo mount --bind /dev /mnt/dev
sudo mount --bind /proc /mnt/proc
sudo mount --bind /sys /mnt/sys

#
sudo chroot /mnt

#
grub-install /dev/sdX
update-grub

#
exit
#
sudo umount /mnt/dev /mnt/proc /mnt/sys
sudo umount /mnt
reboot

#Example for UEFI
# Use LiveCD/LiveUSB Linux 
# Find the "/" 
lsblk
fdisk -l
sudo mount /dev/sdXY /mnt

# 100–500 MB, type EFI System Partition
sudo mount /dev/sdXZ /mnt/boot/efi

#
sudo mount --bind /dev /mnt/dev
sudo mount --bind /proc /mnt/proc
sudo mount --bind /sys /mnt/sys

#
sudo chroot /mnt

#
grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=GRUB
update-grub

#
exit

#
sudo umount /mnt/dev /mnt/proc /mnt/sys
sudo umount /mnt/boot/efi
sudo umount /mnt
reboot

#Auto-repair (boot-repair)

sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt update
sudo apt install -y boot-repair
boot-repair