10.4. Using GRUB to Set Up the Boot Process

10.4.1. Introduction

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. In order for the procedure below to work, you need to jump ahead to BLFS and install xorriso from the libisoburn package.

cd /tmp
grub-mkrescue --output=grub-img.iso
xorriso -as cdrecord -v dev=/dev/cdrw blank=as_needed grub-img.iso

10.4.2. 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).

10.4.3. Setting Up the Configuration

GRUB works by writing data to the first physical track of the hard disk. This area is not part of any file system. The programs there access GRUB modules in the boot partition. The default location is /boot/grub/.

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 set up the boot track:

grub-install /dev/sda

10.4.4. 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)
set gfxpayload=1024x768x32

menuentry "GNU/Linux, Linux 6.12.5-lfs-r12.2-51-systemd" {
        linux   /boot/vmlinuz-6.12.5-lfs-r12.2-51-systemd root=/dev/sda2 ro
}
EOF

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.

The set gfxpayload=1024x768x32 command sets the resolution and color depth of the VESA framebuffer to be passed to the kernel. It's necessary for the kernel SimpleDRM driver to use the VESA framebuffer. You can use a different resolution or color depth value which better suits for your monitor.

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.