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Grub

Grub, the Grand Unified Bootloader, is the most common such bootloader on desktop Linux distributions. Its job is to get the machine from the BIOS or UEFI environment to being able to boot into Linux.

The boot process

Grub boot process

Grub has three stages:

  • Stage 1 is the earliest stage, designed to be small enough to fit in the 512k footprint of the MBR, the reserved space at the beginning of a hard drive for the bootloader. It contains the rescue> prompt.
  • Stage 1.5 is an intermediary stage that's loaded on systems where the hardware requires further work to access the second stage.
  • Stage 2 is the final stage of the Grub bootloader which provides the menu system, the grub> prompt and all of the code necessary to boot a Linux kernel with an initrd.

Entering the boot menu

Tap Esc or hold down Shift. If you end up at a grub> prompt, try running normal to get back to the boot menu. If you can't, the system doesn't have any boot menu entries and you're going to need to follow the rescue steps below.

Exploring filesystems

ls with no arguments will list detected filesystems:

grub> ls
(hd0) (hd0,msdos3) (hd0,msdos2) (hd0,msdos1)

You can then view filesystem properties:

grub> ls (hd0,msdos1)
        Partition hd0,msdos1: Filesystem type ext* - Last modification time 2020-02-17 13:10:55 Monday, UUID 00000000-0000-0000-0000-000000000000 - Partition start at 1024KiB - Total size 498688KiB

Or list contents:

grub> ls (hd0,msdos1)/
lost+found config-4.15.0-55-generic initrd.img-4.15.0-55-generic System.map4.15.0-55-generic vmlinuz-4.15.0-55-generic

Rescuing a system

First, tell the bootloader where it was installed to (usually the location of /boot/grub when the system is running):

grub> set root=hd0,1
grub> set prefix=(hd0,1)/boot/grub/

We can then set the location of the kernel (linux) and initrd images. Note that must provide the root= boot option to the kernel to a valid path to the root device according to udev. Once configured, tell Grub to try and boot this.

grub> linux /boot/vmlinuz-5.0.0-1031-azure root=/dev/disk/azure/root-part1
grub> initrd /boot/initrd.img-5.0.0-1031-azure
grub> boot