System startup halts at a GRUB rescue prompt or skips directly to Windows after a failed update, BIOS change, or dual-boot adjustment. This typically means the GRUB bootloader is missing, misconfigured, or overwritten, leaving Linux partitions inaccessible. Restoring GRUB restores access to your Linux installation and restores the multi-boot menu, resolving the boot issue and eliminating the need for a full OS reinstall.

Repairing GRUB Using a Live Linux USB

Step 1: Boot from a live Linux USB image that matches your installed distribution (for example, Fedora Live if you use Fedora, Ubuntu Live for Ubuntu). Insert the USB, access your system’s boot menu (often via F12, Esc, or Del), and select the USB device. Choose the “Try” or “Live” session option without installing.

Step 2: Open a terminal in the live session. Identify your Linux root, boot, and EFI partitions. Use lsblk -f or fdisk -l to list available block devices and their mount points. For Btrfs systems, note subvolumes (e.g., root or @).

Step 3: Mount your Linux partitions. For typical Btrfs setups:


sudo su
mount -o subvol=root /dev/nvme0n1p7 /mnt
mount /dev/nvme0n1p6 /mnt/boot
mount /dev/nvme0n1p1 /mnt/boot/efi

Adjust device names and subvolumes to match your system. For LUKS-encrypted partitions, unlock with cryptsetup luksOpen first, then mount the resulting /dev/mapper/ device.

Step 4: Bind essential filesystems to enable chroot operations:


mount -o bind /dev /mnt/dev
mount -o bind /sys /mnt/sys
mount -o bind /proc /mnt/proc
mount -o bind /run /mnt/run
mount -o bind /sys/firmware/efi/efivars /mnt/sys/firmware/efi/efivars

Step 5: Change root into your installed system:


chroot /mnt

This gives you a shell inside your installed Linux environment. If you see errors mounting partitions, double-check your device names and /etc/fstab entries for accuracy. For Btrfs or complex setups, confirm subvolume names and options.

Step 6: Reinstall GRUB and related boot components. On most modern systems with UEFI, use:


dnf reinstall shim-* grub2-*

or for Ubuntu/Debian:


apt-get install --reinstall grub-efi-amd64

This reinstalls the bootloader and its signed shims, which are critical for Secure Boot compatibility.

Step 7: Regenerate the GRUB configuration file to detect all installed systems and kernels:


grub2-mkconfig -o /boot/grub2/grub.cfg

On Ubuntu or Debian, use:


update-grub

Step 8: Register the bootloader with UEFI firmware if necessary. This step is important if the UEFI boot menu does not show a Linux entry:


efibootmgr -c -d /dev/nvme0n1 -p 1 -L Fedora -l '\EFI\fedora\shimx64.efi'

Change device paths and labels as needed for your distribution and partition layout.

Step 9: Exit the chroot, unmount all partitions in reverse order, and safely reboot:


exit
umount /mnt/boot/efi
umount /mnt/boot
umount /mnt/dev
umount /mnt/sys
umount /mnt/proc
umount /mnt/run
umount /mnt
reboot

Remove the live USB when prompted. On reboot, your system should present the GRUB menu, allowing you to select between Linux and other installed operating systems.

Restoring GRUB Using the Boot Repair Tool

The Boot Repair tool offers a graphical interface for repairing GRUB without manual chroot steps. This is especially useful for Ubuntu and derivatives, or when manual steps fail.

Step 1: Boot from a live Linux USB session as described above.

Step 2: Open a terminal and add the Boot Repair repository (for Ubuntu):


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

Step 3: Launch Boot Repair:


boot-repair

Follow the on-screen prompts and select “Recommended repair.” Boot Repair will attempt to detect your partitions, reinstall GRUB, and update the UEFI boot order automatically.

Step 4: Once the process completes, reboot and check if the GRUB menu appears. If the repair fails, Boot Repair provides a detailed report you can reference for further troubleshooting.

Manual Recovery from the GRUB Rescue Prompt

If your system drops to a grub rescue> prompt, you can attempt temporary recovery without external media. This method is most useful for quick fixes or diagnostics.

Step 1: At the rescue prompt, list available partitions:


ls

Identify the partition containing your /boot directory, for example (hd0,gpt2) or (hd0,msdos1).

Step 2: Set the root and prefix variables accordingly:


set root=(hd0,gpt2)
set prefix=(hd0,gpt2)/boot/grub
insmod normal
normal

This should load the full GRUB menu. If successful, you can boot into Linux and then run the full GRUB reinstall process from within your OS.

Additional Tips and Troubleshooting

  • For dual-boot systems with Windows, disabling Fast Startup in Windows settings prevents Windows from locking the EFI partition, which can block GRUB updates.
  • Secure Boot may block unsigned GRUB binaries. If you see Secure Boot errors, ensure Secure Boot is disabled in UEFI firmware, or reinstall signed shims as shown above.
  • If mounting the EFI partition fails with I/O errors, check the partition for corruption and repair or reformat if needed.
  • Always double-check device names and UUIDs before mounting or running grub-install to avoid writing to the wrong disk.
  • For distributions using systemd-boot instead of GRUB, use bootctl install and reinstall-kernels after mounting the ESP at /efi or /boot/efi.

Following these steps restores Linux boot access and the GRUB menu after updates or configuration changes disrupt startup. Keeping backups of critical configuration files and noting your partition layout before updates can further reduce recovery time in the future.