Grub

calendar Dec 13, 2023 · 2 min read · Linux  ·
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Configuring grub for a multi OS system

Main grub configuration file is /etc/default/grub

Default Entry = Last Saved

1GRUB_SAVEDEFAULT=true
2GRUB_DEFAULT=saved

Note :

  1. Comment out entries such as GRUB_DEFAULT=0 for this to take effect
  2. This does not work if /boot partition is in btrfs or lvm filesystem

Detect other bootable OSes

Activate os-prober

1GRUB_DISABLE_OS_PROBER=false

Console

For a TUI,

1GRUB_TERMINAL=console

Update grub.cfg

On Debian

1sudo update-grub

On Arch/Fedora (Even Debian)

1sudo grub-mkconfig -o /boot/grub/grub.cfg

Add custom entry

Note : Place custom grub entries in /etc/grub.d/ not in /etc/default/grub.d/

To add custom menus and submenus edit the file /etc/grub.d/40_custom

Examples

Standard

 1# "Shutdown" menu entry
 2menuentry "System shutdown" {
 3	echo "System shutting down..."
 4	halt
 5}
 6
 7# "Restart" menu entry
 8menuentry "System restart" {
 9	echo "System rebooting..."
10	reboot
11}
12
13# "UEFI Firmware Settings" menu entry
14if [ ${grub_platform} == "efi" ]; then
15	menuentry 'UEFI Firmware Settings' --id 'uefi-firmware' {
16		fwsetup
17	}
18fi

Chainloading a different EFI paritition

 1if [ "${grub_platform}" == "efi" ]; then
 2	menuentry "Debian on Different SSD" {
 3		savedefault
 4		insmod part_gpt
 5		insmod fat
 6		insmod chain
 7		search --no-floppy --fs-uuid --set=root AB12-CD34
 8		chainloader /EFI/debian/grubx64.efi
 9	}
10fi

Windows grub entry

1menuentry 'Windows Boot Manager (on /dev/nvme0n1p1)' --class windows --class os $menuentry_id_option 'osprober-efi-541A-B5FD' {
2	savedefault
3	insmod part_gpt
4	insmod fat
5	search --no-floppy --fs-uuid --set=root 541A-B5FD
6	chainloader /EFI/Microsoft/Boot/bootmgfw.efi
7}