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GRUB-configuration.md

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GRUB Bootloader Configuration

When doing kernel development, you’ll often work with multiple kernel builds: you may be testing different implementations/features, and ideally you also have at least one pristine fallback kernel in case you (almost inevitably) really mess up and build an unbootable kernel.

We use a bootloader to manage these kernels (see Boot Process); we use it to choose which kernel we boot into, and what parameters we pass to that kernel. GRUB is one of the most common bootloaders.

Installing GRUB

Usually, it is pre-installed.

Example

sudo apt install grub2

Important Directories and Files

  • /boot/:
    The /boot/ directory contains everything your machine needs during the boot process, including the kernel image that contains your operating system (that’s why you need to copy them to /boot/ to boot into them). This is also where GRUB reads its configuration file from at boot time, and where update-grub looks to generate the bootloader configuration file.
  • /boot/grub/grub.cfg:
    This is the configuration file that GRUB reads from during the boot process. It is written as a shell script, and defines the menu entries that appear on your bootloader menu. This configuration file is usually generated from /etc/default/grub, so any changes made here will be lost next time you generate a new configuration file (by running update-grub). While we don't usually edit this file, you can read it to make sure update-grub did what you expected it to.
  • /etc/default/grub:
    This GRUB file is what you’ll be editing in order to set up your bootloader. When you run update-grub, it will generate the /boot/grub/grub.cfg configuration file from the options you set in your grub file and the kernels it finds in your /boot/ directory.

GRUB File Syntax

Menu Entries in grub.cfg

The grub.cfg configuration file contains one or more menuentry blocks, each representing a single GRUB boot menu entry. These blocks always start with the menuentry keyword followed by a title, list of options, and an opening curly bracket, and end with a closing curly bracket. Anything between the opening and closing bracket should be indented.

You can use the this script to list all entries in the boot menu. You can pass a parameter to adjust column heading format dynamically based on short or long. After running this script by ./grub_menu.sh short, you will see something like this:

GRUB Menu

If you want to reboot into Kernel 5.10.0-9-amd64 you need to set GRUB_DEFAULT="1>2" in /etc/default/grub, then run sudo update-grub.

Configuring /etc/default/grub

The syntax of this file is declarative. Each option is identified by a key, which you set with KEY=value. Here are some useful options:

  • GRUB_DEFAULT: the default menu entry your bootloader will boot into. The value assigned to this option can be one of three types:
    • Some integer N: set the default to be Nth menu entry.
    • Some string ID: set the default to be the kernel identified by the menu entry ID.
    • saved: if GRUB_SAVEDEFAULT is true, your bootloader will boot into the last kernel it booted into by default; or if user sets the default one by grub-set-default, your bootloader will boot into the user selected one.
  • GRUB_TIMEOUT: how long your bootloader menu waits before booting into the menu entry.