If your system has UEFI support and you wish to boot LFS with UEFI,
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 support using the instructions provided in
the BLFS page.
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
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).
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:
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 8.
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 part_gpt
insmod ext2
set root=(hd0,2)
set gfxpayload=1024x768x32
menuentry "GNU/Linux, Linux 6.12.5-lfs-r12.2-833" {
linux /boot/vmlinuz-6.12.5-lfs-r12.2-833 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.
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
, and replace <UUID of the filesystem where the kernel
is installed>
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=
instead of <filesystem
UUID>
root=PARTUUID=
,
but doing so will require an initramfs, which is beyond the scope
of LFS.
<partition UUID>
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=
, in
<partition UUID>
/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.