Understanding GRUB Rescue and How to Fix It

(A Guide to Diagnosing and Resolving Boot Errors)

Introduction

GRUB (Grand Unified Bootloader) is a critical component of Linux systems, responsible for loading the operating system. However, encountering the GRUB rescue prompt can be alarming. This blog explains why this happens and provides two practical methods to resolve it.

Why Does GRUB Rescue Appear?

The GRUB rescue prompt appears when the bootloader fails to locate critical files or configurations. Common causes include:

  1. Corrupted or Broken GRUB Installation: Occurs after interrupted updates or failed OS installations.
  2. Incorrect Partition Changes: Modifying disk partitions (e.g., deleting/resizing) can break GRUB’s path.
  3. Missing OS or Boot Files: Accidental deletion of /boot or misconfigured kernel updates.
  4. Disk Errors: Hardware failures or filesystem corruption.

Method 1: On-the-Spot Fix Using GRUB Rescue Commands

If you have access to the GRUB rescue prompt, try these steps:

Step 1: Identify Partitions

List available partitions using:

    ls

Step 2: Locate the Boot Partition

Check each partition for the /boot/grub directory:

ls (hdX,msdosY)/boot/grub

Continue this process until you locate the /boot/grub directory. Then, replace X (disk number) and Y (partition number) based on your system.

Step 3: Set the Correct Root and Prefix

Set the correct root and prefix:

set root=(hdX,msdosY)  

set prefix=(hdX,msdosY)/boot/grub

Step 4: Load the Normal Modules

insmod normal

normal

This should bring up the standard GRUB menu, allowing you to boot into your operating system.

Step 5: Reinstall GRUB

Once booted, it’s crucial to reinstall GRUB to prevent future issues:

sudo grub-mkconfig -o /boot/grub/grub.cfg

It generated grub configuration file for arch-based distros, for debian based distros, you can use the following command:

sudo grub-update

Now reinstall the GRUB Bootloader to the specified disk:

sudo grub-install /dev/sdaX

It is recommended to regenerate the grub configuration after this step.

Method 2: GRUB Setup using a Live USB

This method involves booting into a Live USB environment, mounting the necessary partitions, and reinstalling GRUB manually. Follow these steps carefully:

Step 1: Boot into Live USB

  1. Create a Live USB using tools like Rufus or BalenaEtcher.
  2. Boot into the Live environment by selecting the USB drive in your BIOS/UEFI menu.

Step 2: Identify Partitions

Use lsblk to list all disks and partitions:

lsblk

Step3: Mount the root partition

Mounts the root partition (/dev/nvme0n1p4) to the /mnt directory. Replace /dev/nvme0n1p4 with your actual root partition identifier.

sudo mount  /dev/nvme0n1p4 /mnt

Step 4: Mount the boot partition

Mounts the boot partition (/dev/nvme0n1p3) to /mnt/boot. Adjust /dev/nvme0n1p3 to match your boot partition.

sudo mount  /dev/nvme0n1p3 /mnt/boot

Step 5: Bind Mount System Directories

Bind /dev, /proc, /sys, and EFI variables to the chroot environment:

i: /dev directory

Bind the /dev directory to /mnt/dev for device access in the chroot environment.

sudo mount  - -bind  /dev  /mnt/dev

ii: /proc directory

Bind the /proc directory to /mnt/proc for process information.

sudo mount --bind /proc /mnt/proc

iii: /sys directory

Bind the /sys directory to /mnt/sys for system information.

sudo mount  - -bind  /sys  /mnt/sys

iv: efi variables

Binds EFI variables to /mnt/sys/firmware/efi/efivars for UEFI systems.

sudo mount  - -bind  /sys/firmware/efi/efivars  /mnt/sys/firmware/efi/efivars

v: /dev/pts (optional for nvme or gpt file systems)

If your system uses the pts filesystem or you want a pseudo-terminal support.

sudo mount --bind /dev/pts /mnt/dev/pts

Step 6: Chroot into the System

Switches the root directory to /mnt, allowing you to work on the installed system. Enter the chroot environment:

sudo chroot /mnt

Step 7: Reinstall GRUB

Install the GRUB bootloader for UEFI systems, specifying the target architecture, EFI directory, and a bootloader identifier.

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

Step 8: Generate GRUB Configuration file

Generate a new GRUB configuration file, detecting available operating systems and kernels.

sudo grub-mkconfig -o /boot/grub/grub.cfg

Step 9: Exit chroot Environment

Exits the chroot environment, returning to the Live USB session.

exit

Step 9: Unmount Filesystems

Recursively unmounts all filesystems mounted under /mnt.

sudo umount -R /mnt

Step 10: Reboot the System

Restarts the system. Remove the Live USB to boot into your repaired system.

reboot

Step 11: Regenerate GRUB Configuration File

After booting into your system, it’s advisable to regenerate the GRUB configuration file to ensure all operating systems are detected:

sudo grub-mkconfig -o /boot/grub/grub.cfg

Step 12: Reboot

sudo reboot now

Facing the “grub rescue” prompt can be resolved using methods like insmod or a Live USB. Utilize the Arch Linux community, forums, and the Arch Wiki for support. Proactively addressing boot issues and leveraging resources ensures a stable system. Seek help or consult documentation for further assistance.