8.4. Using GRUB to Set Up the Boot Process

8.4.1. 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

8.4.2. 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 number starts from zero, but the partition number starts from one for normal partitions and 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).

8.4.3. 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 100 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.

Using 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

8.4.4. 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 ext2
set root=(hd0,2)

menuentry "GNU/Linux, Linux 4.20.12-lfs-8.4-systemd" {
        linux   /boot/vmlinuz-4.20.12-lfs-8.4-systemd root=/dev/sda2 ro
}
EOF

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.