10.4. Using GRUB to Set Up the Boot Process



This section assume your system has UEFI support and you wish to boot LFS with UEFI and GRUB built following the instructions in Chapter 8.

If you've installed GRUB for UEFI with optional dependencies following BLFS, you should skip the instructions in this page but still learn the syntax of grub.cfg and the method to specify a partition in the file from this page, and configure GRUB with UEFI using the instructions provided in the BLFS page, but replace --target=x86_64-efi with --target=arm64-efi for the ARM64 system.

If your system does not support UEFI or you don't want to use it, you'll need to figure out how to configure the booting process of the system on your own.

10.4.1. Introduction



Configuring GRUB incorrectly can render your system inoperable without an alternate boot device such as a CD-ROM or bootable USB drive. This section is not required to boot your LFS system. You may just want to modify your current boot loader, e.g. Grub-Legacy, GRUB2, or LILO.

Ensure that an emergency boot disk is ready to rescue the computer if the computer becomes unusable (un-bootable). If you do not already have a boot device, you can create one. To create a emergency boot device for UEFI, consult section Create an Emergency Boot Disk in the BLFS page.

10.4.2. Turn off Secure Boot

LFS does not have the essential packages to support Secure Boot. To set up the boot process following the instructions in this section, Secure Boot must be turned off from the configuration interface of the firmware. Read the documentation provided by the manufacturer of your system to find out how.

10.4.3. GRUB Naming Conventions

GRUB uses its own naming structure for drives and partitions in the form of (hdn,m), where n is the hard drive number and m is the partition number. The hard drive numbers start from zero, but the partition numbers start from one for normal partitions (from five for extended partitions). Note that this is different from earlier versions where both numbers started from zero. For example, partition sda1 is (hd0,1) to GRUB and sdb3 is (hd1,3). In contrast to Linux, GRUB does not consider CD-ROM drives to be hard drives. For example, if using a CD on hdb and a second hard drive on hdc, that second hard drive would still be (hd1).

10.4.4. Setting Up the Configuration

GRUB works by creating an EFI executable in the EFI System Partition (ESP). You can find the ESP with:

fdisk -l | grep 'EFI System'

If no ESP exists on your hard drive (for example, you are building LFS on a fresh new system with a Live CD as the host distro), read the BLFS page for the instruction to create an ESP on your hard drive.

If the ESP is not mounted at /boot/efi (in the chroot), mount it now:

mkdir -pv /boot/efi
mount /boot/efi


The path to the device node is intentionally omitted in the command. We expect the entry for mounting the ESP to /boot/efi is already in /etc/fstab. Add the entry before running the command if you forgot to create an entry for the ESP in Section 10.2, “Creating the /etc/fstab File”.

The location of the boot partition is a choice of the user that affects the configuration. One recommendation is to have a separate small (suggested size is 200 MB) partition just for boot information. That way each build, whether LFS or some commercial distro, can access the same boot files and access can be made from any booted system. If you choose to do this, you will need to mount the separate partition, move all files in the current /boot directory (e.g. the Linux kernel you just built in the previous section) to the new partition. You will then need to unmount the partition and remount it as /boot. If you do this, be sure to update /etc/fstab.

Leaving /boot on the current LFS partition will also work, but configuration for multiple systems is more difficult.

Using the above information, determine the appropriate designator for the root partition (or boot partition, if a separate one is used). For the following example, it is assumed that the root (or separate boot) partition is sda2.

Install the GRUB files into /boot/grub and the GRUB EFI executable into /boot/efi/EFI/BOOT/BOOTAA64.EFI:



The following command will overwrite BOOTAA64.EFI. Do not run the command if this is not desired, for example, if it contains a third party boot manager. You can backup it with cp as it's a regular file.

grub-install --removable


--removable may seem strange here. The UEFI firmware searches EFI executables for boot loaders in a hardcoded path, EFI/BOOT/BOOTAA64.EFI in the ESP, and other boot loader paths listed in the EFI variables. We've not installed the utilities for manipulating EFI variables so we need to install the EFI executable into the hardcoded path. The hardcoded path is usually used by removable devices (for example, USB thumb devices) so the grub-install option for this purpose is named --removable.

UEFI implementation usually prefers the boot loaders with paths recorded in an EFI variable, to the boot loader with the hardcoded search path. You may need to invoke the boot device selection menu or setting interface of your EFI firmware on next boot to explicitly select the bootloader.

Some UEFI implementation may completely skip the hardcoded path if there are other boot loaders in the same hard drive with paths recorded in an EFI variable. Then you need to create an EFI variable for the newly installed boot loader. Install efibootmgr, then run the following commands:

mount -v -t efivarfs efivarfs /sys/firmware/efi/efivars
efibootmgr -B -L LFS || true
efibootmgr -c -L LFS -l '\EFI\BOOT\BOOTAA64.EFI' -d /dev/sda
umount /sys/firmware/efi/efivars

Replace /dev/sda with the device node of the hard drive where you are installing GRUB into. For some UEFI firmwares, -e 3 option may be needed for the efibootmgr -c command.

10.4.5. Creating the GRUB Configuration File

Generate /boot/grub/grub.cfg:

cat > /boot/grub/grub.cfg << "EOF"
# Begin /boot/grub/grub.cfg
set default=0
set timeout=5

insmod part_gpt
insmod ext2
set root=(hd0,2)

insmod all_video

menuentry "GNU/Linux, Linux 6.7.4-lfs-arm64-r12.0-319-systemd" {
        linux   /boot/vmlinuz-6.7.4-lfs-arm64-r12.0-319-systemd root=/dev/sda2 ro

The insmod commands load the GRUB modules named part_gpt and ext2. Despite the naming, ext2 actually supports ext2, ext3, and ext4 filesystems. The grub-install command has embedded some modules into the main GRUB image (installed into the MBR or the GRUB BIOS partition) to access the other modules (in /boot/grub/i386-pc) without a chicken-or-egg issue, so with a typical configuration these two modules are already embedded and those two insmod commands will do nothing. But they do no harm anyway, and they may be needed with some rare configurations.



From GRUB's perspective, the kernel files are relative to the partition used. If you used a separate /boot partition, remove /boot from the above linux line. You will also need to change the set root line to point to the boot partition.



The GRUB designator for a partition may change if you added or removed some disks (including removable disks like USB thumb devices). The change may cause boot failure because grub.cfg refers to some old designators. If you wish to avoid such a problem, you may use the UUID of a partition and the UUID of a filesystem instead of a GRUB designator to specify a device. Run lsblk -o UUID,PARTUUID,PATH,MOUNTPOINT to show the UUIDs of your filesystems (in the UUID column) and partitions (in the PARTUUID column). Then replace set root=(hdx,y) with search --set=root --fs-uuid <UUID of the filesystem where the kernel is installed>, and replace root=/dev/sda2 with root=PARTUUID=<UUID of the partition where LFS is built>.

Note that the UUID of a partition is completely different from the UUID of the filesystem in this partition. Some online resources may instruct you to use root=UUID=<filesystem UUID> instead of root=PARTUUID=<partition UUID>, but doing so will require an initramfs, which is beyond the scope of LFS.

The name of the device node for a partition in /dev may also change (this is less likely than a GRUB designator change). You can also replace paths to device nodes like /dev/sda1 with PARTUUID=<partition UUID>, in /etc/fstab, to avoid a potential boot failure in case the device node name has changed.

GRUB is an extremely powerful program and it provides a tremendous number of options for booting from a wide variety of devices, operating systems, and partition types. There are also many options for customization such as graphical splash screens, playing sounds, mouse input, etc. The details of these options are beyond the scope of this introduction.



There is a command, grub-mkconfig, that can write a configuration file automatically. It uses a set of scripts in /etc/grub.d/ and will destroy any customizations that you make. These scripts are designed primarily for non-source distributions and are not recommended for LFS. If you install a commercial Linux distribution, there is a good chance that this program will be run. Be sure to back up your grub.cfg file.