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. 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
To boot LFS on host systems that have UEFI enabled, the kernel needs to have been built with the CONFIG_EFI_STUB capability described in the previous section. However, LFS can be booted using GRUB2 without such an addition. To do this, the UEFI Mode and Secure Boot capabilities in the host system's BIOS need to be turned off. For details, see the lfs-uefi.txt hint at http://www.linuxfromscratch.org/hints/downloads/files/lfs-uefi.txt.
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).
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
.
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:
The following command will overwrite the current boot loader. Do not run the command if this is not desired, for example, if using a third party boot manager to manage the Master Boot Record (MBR).
grub-install /dev/sda
If the system has been booted using UEFI, grub-install will try to
install files for the x86_64-efi target, but those files
have not been installed in chapter 6. If this is the case, add
--target i386-pc
to the command
above.
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 5.10.17-lfs-10.1" {
linux /boot/vmlinuz-5.10.17-lfs-10.1 root=/dev/sda2 ro
}
EOF
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.
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.