GRUB Bootloader

GRUB (Grand Unified Bootloader) is the first piece of software your OS controls after the firmware hands off. It finds your kernel on disk, lets you choose which one to boot, and passes configuration to it. When GRUB breaks, your system won't boot — so understanding it is essential.

What a Bootloader Does

Why does Linux need a bootloader at all? When the computer powers on, the firmware (BIOS/UEFI) only knows how to do very basic things. It can't read ext4 or understand Linux kernel formats. The bootloader bridges this gap — it knows enough about the filesystem to find the kernel, load it into RAM, and jump to its entry point.

GRUB Configuration

GRUB's main config file is generated automatically — you should never edit it directly:

# DO NOT edit /boot/grub/grub.cfg directly

# Edit the template instead:
sudo nano /etc/default/grub

# Then regenerate:
sudo update-grub          # Debian/Ubuntu
sudo grub2-mkconfig -o /boot/grub2/grub.cfg  # Fedora/RHEL

Key settings in /etc/default/grub:

GRUB_DEFAULT=0              # which entry to boot (0 = first)
GRUB_TIMEOUT=5              # seconds to show menu
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"  # kernel params
GRUB_CMDLINE_LINUX=""       # extra params always added

Inside grub.cfg — menuentry Format

menuentry 'Ubuntu' --class ubuntu {
    insmod gzio
    insmod part_gpt
    insmod ext2
    set root='hd0,gpt2'
    linux   /boot/vmlinuz-6.2.0 root=UUID=abc123 ro quiet splash
    initrd  /boot/initrd.img-6.2.0
}

set root — which disk/partition has /boot
linux — path to kernel + parameters
initrd — path to initramfs image

Kernel Parameters

The kernel command line lets you control kernel behavior at boot:

Parameter	Effect
`quiet`	Suppress most boot messages
`splash`	Show graphical boot screen
`nomodeset`	Disable GPU driver (use for graphics issues)
`single` / `1`	Boot into single-user mode
`root=UUID=...`	Which device is the root filesystem
`ro`	Mount root read-only initially
`init=/bin/bash`	Use bash as PID 1 (emergency)

GRUB Rescue Mode

I see "grub rescue>" — what happened? GRUB loaded its Stage 1 from MBR but couldn't find its Stage 2 or config file. Usually means the partition table changed, a disk was removed, or grub.cfg is missing.

# At grub rescue> prompt:
# Find where your boot partition is
grub rescue> ls
(hd0) (hd0,gpt1) (hd0,gpt2)

grub rescue> ls (hd0,gpt2)/
# Look for /boot/grub/ directory

grub rescue> set root=(hd0,gpt2)
grub rescue> set prefix=(hd0,gpt2)/boot/grub
grub rescue> insmod normal
grub rescue> normal
# Now you should see the GRUB menu

After booting, run sudo update-grub && sudo grub-install /dev/sda to permanently fix it.