GRUB

GRUB is a bootloader responsible for loading vmlinuz and initrd.img into RAM at booting.

vmlinuz is the linux kernel.
initrd.img contains tools and drivers to mount root filesystem.

Configuration

Configuration files.

/etc/default/grub
- User settings.
/etc/grub.d
- Helper files.
/boot/grub2/grub.cfg
- Read by grub at boot time.
- Generated by grub-mkconfig.

Parameters.

Parameters	Meaning	Examples
GRUB_TIMEOUT	Timeout for grub menu	-1, 0, 5
GRUB_TIMEOUT_STYLE	Menu style	menu, countdown, hidden
GRUB_DEFAULT	Default entry to boot	Index or ID of meny entry
GRUB_CMDLINE_LINUX	Kernel parameters	console=ttyS0
GRUB_DISABLE_OS_PROBER	Menu entries for other OS	true, false

Change configuration.

# edit config
$ vi /etc/default/grub

# validate config
$ grub-script-check -v /etc/default/grub

# apply change
$ grub-mkconfig -o /boot/grub/grub.cfg

Add menu entry.

$ vi /etc/grub.d/40_custom

menuentry 'Ubuntu' {
    insmod gzio
    insmod part_gpt
    insmod ext2

    linux /vmlinuz-5.15.0-164-generic /dev/mapper/ubuntu--vg-ubuntu--lv ro
    initrd /initrd.img-5.15.0-164-generic
}

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

Installation

Install grub to a device.

# install grub
$ grub-install /dev/sdb

# verify
$ xxd -l 512 /dev/sdb

Master Boot Record.

Master Boot Record (MBR) is the first 512 bytes of a disk.
Command grub-install writes boot code to MBR.
When power on, BIOS executes the boot code to start the boot process.

Offset	Size	Purpose
0x0000	446 B	Boot code
0x01be	64 B	Partition table (4 entries × 16B)
0x01fe	2 B	Boot signature (0x55aa)

Configuration

Installation

Reference