10.4. Using GRUB to Set Up the Boot Process

#### Linux From Scratch - Version 12.0-rc1

### Chapter 10. Making the LFS System Bootable

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# 10.4. Using GRUB to Set Up the Boot Process

### Note

If your system has UEFI support and you wish to boot LFS with UEFI, you should skip this page, and configure GRUB with UEFI support using the instructions provided in the BLFS page.

## 10.4.1. Introduction

### Warning

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

## 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:

### Warning

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

### Note

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.

## 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)

    menuentry "GNU/Linux, Linux 6.4.10-lfs-12.0-rc1" {
            linux   /boot/vmlinuz-6.4.10-lfs-12.0-rc1 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.

### Note

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.

### Note

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.

### Caution

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.

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