How to Use Grub Rescue to Fix Linux Boot Failure

December 2, 2021

Introduction

The GRUB (Grand Unified Bootloader) is a tool for booting and loading operating system kernels and the default bootloader for systems based on the Linux kernel. Although it runs first when a machine is turned on, regular users rarely see GRUB in action. It functions automatically and requires no user input.

However, when attempting to boot another operating system alongside Linux on the same machine, the other system's bootloader may overwrite GRUB, resulting in the inability of the Linux system to boot up.

This article will show you how to fix Linux boot failure using GRUB Rescue commands and the Boot Repair tool.

How to use Grub Rescue to fix Linux boot failure

Prerequisites

  • An account with sudo privileges.
  • Access to the command line.

Note: The tutorial below is written for GRUB 2, the current iteration of the GRUB bootloader.

GRUB Boot Issues

The most common reason for GRUB not booting into the operating system is another OS's bootloader overwriting GRUB boot configuration. The problem occurs during an attempt a dual boot with an existing Linux installation. Another reason is the accidental removal of GRUB configuration files.

When GRUB is not able to boot the system, the GRUB Rescue prompt appears.

GRUB displaying "no such partition" error and the GRUB Rescue prompt.

The example above shows GRUB displaying the "no such partition" error before displaying the grub rescue prompt. Another common GRUB error is "unknown filesystem", followed by the same prompt.

GRUB displaying "unknown filesystem" error and the GRUB Rescue prompt.

Sometimes, the screen may show the grub prompt only.

GRUB displaying only the GRUB prompt.

GRUB Rescue Commands

Below is the list of the commonly used GRUB Rescue commands. Use the commands in the prompts mentioned in the previous section.

CommandDescriptionExample
bootStart booting (shortcuts: F10, CTRL + x).The command is issued without arguments.
catWrite the contents of a file to standard output.cat (hd0,1)/boot/grub/grub.cfg
configfileLoad a configuration file.configfile (hd0,1)/boot/grub/grub.cfg
initrdLoad the initrd.img file.initrd (hd0,1)/initrd.img
insmodLoad a module.insmod (hd0,1)/boot/grub/normal.mod
loopbackMount an image file as a device.loopback loop0 (hd0,1)/iso/image.iso
lsDisplay the contents of a directory or partition.ls (hd0,1)
lsmodDisplay a list of loaded modules.The command is issued without arguments.
normalActivate the normal module.The command is issued without arguments.
searchSearch for devices. Option --file searches for files, --label searches for labels, --fs-uuid searches for filesystem UUID.search -file [filename]
setSet an environment variable. If issued with no arguments, the command prints the list of all environment variables and their values.set [variable-name]=[value]

Fixing Boot Failure

This tutorial covers two ways to resolve GRUB boot issues, using the GRUB Rescue prompt, and the Boot Repair tool.

Via Grub Terminal

1. Use the set command with no arguments to view the environment variables:

set

The example output shows that GRUB is set up to boot from (hd0,msdos3) partition:

The output of the set command in GRUB.

2. The ls command lists the available partitions on the disk.

ls

The output shows the partition list.

The output of the ls command in GRUB.

Use the ls command to find the partition containing the boot directory.

ls [partition-name]

The example shows the boot directory in the (hd0,msdos1) partition.

Finding the partition containing the boot folder in GRUB.

3. Set the boot partition as the value of the root variable. The example uses the partition named (hd0,msdos1).

set root=(hd0,msdos1)

4. Load the normal boot mode.

insmod normal

5. Start the normal boot mode.

normal

The normal mode enables you to issue more complex commands.

6. Load the Linux kernel using the linux command.

linux /boot/vmlinuz-4.2.0-16-generic root=/dev/sda1 ro

7. Issue the boot command.

boot

The system now boots properly.

Via Live image

Another way to fix your GRUB boot issues is to use a Linux live image to boot from an external device.

1. Download a live Linux installer. This example uses the Ubuntu 20.04 ISO image.

2. Use a tool such as Etcher to write the Linux image to an SD card or a USB flash drive.

3. Insert the bootable device and start the computer.

4. Select Try Ubuntu on the welcome screen.

The Ubuntu welcome screen.

5. When the live system boots up, connect to the internet.

6. Open the terminal and type the following command to add the repository for the Boot Repair tool.

sudo add-apt-repository ppa:yannubuntu/boot-repair
Adding the repository for the Boot Repair tool.

Press Enter and wait for the repository to be added.

7. Update the repositories.

sudo apt update

8. Install the Boot Repair tool.

sudo apt install boot-repair

9. Start the Boot Repair tool via the terminal.

boot-repair

10. Select Recommended repair.

Selecting Recommended repair on the Boot Repair main screen.

Wait for the tool to finish repairing the bootloader.

Note: The Boot Repair tool is available as a live image, so you can boot it from an external drive without using another live OS.

Updating GRUB config file

When the system successfully boots up, make sure the GRUB configuration is up to date.

Run this command:

update-grub
Updating GRUB in the Linux terminal.

Reinstalling GRUB

Follow the steps below to reinstall GRUB on your Linux system.

1. Mount the partition containing the OS installation. The example mounts the /dev/sda1 partition to the /mnt directory.

sudo mount /dev/sda1 /mnt

2. Bind the /dev, /dev/pts, /proc, and /sys directories to the corresponding directories in the /mnt folder.

sudo mount --bind /dev /mnt/dev &&
sudo mount --bind /dev/pts /mnt/dev/pts &&
sudo mount --bind /proc /mnt/proc &&
sudo mount --bind /sys /mnt/sys

3. Install GRUB.

sudo grub-install -root-directory=/mnt/ /dev/sda
Reinstalling GRUB in the Linux terminal.

4. Unmount the directories when the installation completes successfully.

sudo umount /mnt/sys &&
sudo umount /mnt/proc &&
sudo umount /mnt/dev/pts &&
sudo umount /mnt/dev &&
sudo umount /mnt

Conclusion

After reading this article, you should be able to fix your Linux boot failure using GRUB Rescue or Boot Repair utilities. For another way to fix your boot-related issues, read How to Use fsck Command.

Was this article helpful?
YesNo
Marko Aleksic
Marko Aleksić is a Technical Writer at phoenixNAP. His innate curiosity regarding all things IT, combined with over a decade long background in writing, teaching and working in IT-related fields, led him to technical writing, where he has an opportunity to employ his skills and make technology less daunting to everyone.
Next you should read
How to Build Linux Kernel From Scratch
November 12, 2020

In this step-by-step guide, you will learn how to build and compile a Linux kernel from scratch, for disabling drivers or trying experimental patches.
Read more
How to Reset or Change the Root Password in Linux
October 22, 2018

This guide will help you change your Linux root password in Ubuntu or CentOS, or reset the password.
Read more
How to Use fsck Command
May 14, 2020

The fsck (File System Consistency Check) Linux utility checks filesystems for errors or outstanding issues. Follow this guide to learn how to use fsck.
Read more
How to Update Linux Kernel In Ubuntu
October 22, 2018

If you’re running a Linux operating system (like Ubuntu), it’s a good idea to check and update the kernel regularly.
Read more
  • © 2022 Copyright phoenixNAP | Global IT Services. All Rights Reserved.