Ubuntu 24.10 GRUB Manual

From Zero‑to‑Hero – How GRUB Works, How to Configure It, and How to Fix Anything That Goes Wrong

Target audience – Anyone who installed Ubuntu 24.10 (or any later Ubuntu) on a BIOS‑ or UEFI‑based machine and wants to understand the bootloader that sits in front of the OS.
What you’ll get – A step‑by‑step guide, ready‑to‑copy command lines, a troubleshooting matrix, and placeholders for screenshots you can paste into your own documentation.

Table of Contents

1. What Is GRUB? – The Big Picture
2. GRUB’s Two Main Flavors: BIOS vs. UEFI
3. Boot Flow on Ubuntu 24.10
4. Where the Files Live
5. Installing / Re‑installing GRUB
6. The Core Configuration Files
7. Generating a New grub.cfg
8. Adding Custom Menu Entries
9. Changing the Default Kernel / Timeout
10. Themes, Backgrounds, and Fancy Tricks
11. Secure Boot & Shim
12. Dual‑Boot & Multi‑Boot Scenarios
13. Recovering From a Broken GRUB
14. Common Errors & How to Fix Them
15. Common Errors & How to Fix Them (Full Matrix)
16. Common Errors & How to Fix Them – Full Matrix (Expanded)
17. Tools: boot-repair, grub‑customizer, efibootmgr
18. Backup, Restore, and Version‑Control of GRUB
19. Cheat‑Sheet – One‑Page Command Summary
20. Final Thoughts

1. What Is GRUB? – The Big Picture

Key point: Never edit grub.cfg directly. It is regenerated every time you run update-grub (or grub-mkconfig). All persistent changes go in /etc/default/grub or in custom scripts under /etc/grub.d/.

2. GRUB’s Two Main Flavors: BIOS vs. UEFI

Ubuntu 24.10 defaults to UEFI on any system that supports it. If you force a legacy install (e.g., for an old BIOS box) the installer will set up the BIOS version automatically.

3. Boot Flow on Ubuntu 24.10

+-----------------+   BIOS/UEFI   +-----------------+   reads   +-----------------+
| Firmware (BIOS  |-------------> | GRUB Stage 1    |---------> | GRUB Stage 2    |
| or UEFI)        |   (MBR/EFI)  | (core.img)      |  grub.cfg  | (grub.cfg)      |
+-----------------+               +-----------------+           +-----------------+
                                                                  |
                                                                  v
                                                  +---------------------------+
                                                  | Linux kernel (vmlinuz)    |
                                                  | initramfs (initrd.img)    |
                                                  +---------------------------+
                                                                  |
                                                                  v
                                               +-------------------------------+
                                               | systemd (PID 1) → userspace   |
                                               +-------------------------------+

1. Firmware loads the appropriate GRUB binary.
2. GRUB Stage 2 parses grub.cfg and shows the menu.
3. When you pick an entry, GRUB loads the kernel (vmlinuz‑<version>) and the initramfs (initrd.img‑<version>).
4. The kernel mounts the root filesystem (using the UUID/label supplied by GRUB) and hands over to systemd.

Understanding this chain makes it easier to pinpoint where something went wrong.

4. Where the Files Live

5. Installing / Re‑installing GRUB

5.1 BIOS (Legacy)

sudo grub-install /dev/sda          # replace sda with your boot disk
sudo update-grub                    # generate grub.cfg

5.2 UEFI

# Ensure the ESP is mounted at /boot/efi (Ubuntu does this automatically)
sudo mount | grep efi               # sanity check
sudo grub-install \
    --target=x86_64-efi \
    --efi-directory=/boot/efi \
    --bootloader-id=ubuntu \
    --recheck
sudo update-grub

5.3 When the installer fails

6. The Core Configuration Files

6.1 /etc/default/grub – the “master switchboard”

# /etc/default/grub
GRUB_DEFAULT=0                     # default entry (0 = first)
GRUB_TIMEOUT_STYLE=menu           # show menu (instead of countdown)
GRUB_TIMEOUT=5                     # seconds before auto‑boot
GRUB_DISTRIBUTOR=`lsb_release -i -s`  # usually "Ubuntu"
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX=""              # extra kernel parameters
GRUB_DISABLE_SUBMENU=true         # flat list instead of submenu
GRUB_GFXMODE=1920x1080,auto        # graphics mode for the menu
GRUB_GFXPAYLOAD_LINUX=keep         # keep the same mode for the kernel
GRUB_THEME="/usr/share/grub/themes/starfield/theme.txt"

Key variables you’ll change often

6.2 /etc/grub.d/* – the generators

Never edit 10_linux or 30_os-prober directly – they get regenerated each time you run update-grub. Put your own items in 40_custom.

7. Generating a New grub.cfg

# The canonical command (run as root)
sudo update-grub                # wrapper → grub-mkconfig -o /boot/grub/grub.cfg

Under the hood:

sudo grub-mkconfig -o /boot/grub/grub.cfg

If you suspect a broken script, you can run it manually to see the output:

sudo /etc/grub.d/10_linux

The output is a chunk of the future grub.cfg.

8. Adding Custom Menu Entries

8.1 The simplest way – 40_custom

Edit the file:

sudo vim /etc/grub.d/40_custom

Add a block like this:

#!/bin/sh
exec tail -n +3 $0
# This file provides an easy way to add custom menu entries.  Simply
# add any menu entries you want below, and run "update-grub" after
# saving this file.

menuentry "My Old Ubuntu (3.10 kernel)" {
    set root=(hd0,1)               # adjust to your /boot partition
    linux /boot/vmlinuz-3.10.0-112-generic root=UUID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx ro quiet splash
    initrd /boot/initrd.img-3.10.0-112-generic
}

Make sure the file is executable (it already is on a fresh install, but verify):

sudo chmod +x /etc/grub.d/40_custom

Run sudo update-grub and you’ll see the new entry appear at the bottom of the menu.

8.2 Advanced – Full‑screen rescue entry

menuentry "Rescue mode – Drop to root shell" {
    set root=(hd0,msdos1)
    linux /boot/vmlinuz-$(uname -r) root=UUID=$rootuuid ro recovery nomodeset
    initrd /boot/initrd.img-$(uname -r)
}

9. Changing the Default Kernel / Timeout

9.1 Pick a specific kernel as default

1. Find the entry number (grub-mkconfig prints numbers in comments).
2. Edit /etc/default/grub:

GRUB_DEFAULT="Advanced options for Ubuntu>Ubuntu, with Linux 6.5.0-28-generic"

You can also use the saved mode:

GRUB_DEFAULT=saved
sudo grub-set-default "Advanced options for Ubuntu>Ubuntu, with Linux 6.5.0-28-generic"

Now the next boot will use that kernel, but you can still pick another one from the menu.

9.2 Adjust the timeout

GRUB_TIMEOUT=3                # 3 seconds
GRUB_TIMEOUT_STYLE=hidden    # hide the menu unless SHIFT is held

Run sudo update-grub after each change.

10. Themes, Backgrounds, and Fancy Tricks

Ubuntu ships a basic theme, but you can install many community ones.

10.1 Install a theme package

sudo apt install grub-theme-starfield  # example from the repos

10.2 Use a custom theme you downloaded

Assume you unpacked mytheme.tar.gz into /usr/share/grub/themes/mytheme.

sudo cp -r mytheme /usr/share/grub/themes/
sudo vim /etc/default/grub
# change/add:
GRUB_THEME="/usr/share/grub/themes/mytheme/theme.txt"

Run sudo update-grub and reboot.

10.3 Change the background image only

If you don’t want a full theme, just set GRUB_BACKGROUND:

GRUB_BACKGROUND="/usr/share/backgrounds/ubuntu/planet.jpg"

11. Secure Boot & Shim

Ubuntu 24.10 ships with Signed GRUB (shim.efi). If Secure Boot is enabled in the firmware:

1. Boot → shimx64.efi (signed by Microsoft) loads grubx64.efi.
2. If you install a custom kernel that isn’t signed, you must sign it with your own Machine Owner Key (MOK) or disable Secure Boot.

11.1 Enroll your own MOK (if you need custom kernels)

# Install the signing tools
sudo apt install mokutil

# Generate a key pair
openssl req -new -x509 -newkey rsa:2048 -keyout MOK.priv -outform DER -out MOK.der -nodes -days 36500 -subj "/CN=My Ubuntu MOK/"

# Enroll the key
sudo mokutil --import MOK.der
# Reboot – you’ll be prompted to confirm the key enrollment.

After enrollment, you can sign a kernel:

sudo sbsign --key MOK.priv --cert MOK.der \
    --output /boot/vmlinuz-5.19.0-custom-signed \
    /boot/vmlinuz-5.19.0-custom

Then point your custom GRUB entry to the signed kernel.

12. Dual‑Boot & Multi‑Boot Scenarios

12.1 Adding Windows to the menu

If os-prober is enabled (default), Windows will appear automatically.

sudo vim /etc/default/grub
GRUB_DISABLE_OS_PROBER=false   # ensure it’s not disabled

Run sudo update-grub.

12.2 Manual Windows entry (when os-prober fails)

menuentry "Windows 11 (loader)" {
    insmod part_gpt
    insmod fat
    set root='hd0,gpt1'   # ESP partition
    chainloader /EFI/Microsoft/Boot/bootmgfw.efi
}

12.3 Keeping a separate GRUB for each OS (advanced)

You can install GRUB on each disk’s MBR/EFI and use the firmware’s boot‑order list (efibootmgr) to pick which OS boots by default.

sudo efibootmgr -c -d /dev/sda -p 1 -L "Ubuntu" -l '\EFI\ubuntu\shimx64.efi'
sudo efibootmgr -c -d /dev/sdb -p 1 -L "Windows" -l '\EFI\Microsoft\Boot\bootmgfw.efi'

Now you can change the default with:

sudo efibootmgr -o 0002,0001   # Windows (0002) first, then Ubuntu (0001)

13. Recovering From a Broken GRUB

13.1 The “GRUB rescue>” prompt

Typical messages:

grub rescue> unknown command `linux'
grub rescue> ls

Step‑by‑step rescue (BIOS)

Boot into Ubuntu and reinstall GRUB from the OS:

sudo grub-install /dev/sda
sudo update-grub

Set the root and prefix

grub rescue> set root=(hd0,msdos1)
grub rescue> set prefix=(hd0,msdos1)/boot/grub
grub rescue> insmod normal
grub rescue> normal

This should drop you into the normal GRUB menu.

Find the one that contains /boot/grub

grub rescue> ls (hd0,msdos1)/

Look for grub directory.

Identify your partitions

grub rescue> ls
(hd0) (hd0,msdos1) (hd0,msdos2) …

Step‑by‑step rescue (UEFI)

If you land in grub> (not grub rescue>), the same set root/prefix trick works, but you may need to specify the ESP:

grub> ls (hd0,gpt1)/
grub> set root=(hd0,gpt1)
grub> set prefix=(hd0,gpt1)/EFI/ubuntu
grub> insmod normal
grub> normal

13.2 Using a Live USB (the “safest” method)

1. Boot from an Ubuntu 24.10 live USB (choose “Try Ubuntu”).
2. Mount your system partitions:

sudo mount /dev/sda2 /mnt               # root partition
sudo mount /dev/sda1 /mnt/boot/efi      # ESP (if UEFI)
sudo mount --bind /dev /mnt/dev
sudo mount --bind /proc /mnt/proc
sudo mount --bind /sys /mnt/sys

3. Chroot into the installed system:

sudo chroot /mnt

4. Reinstall GRUB:

# BIOS:
grub-install /dev/sda
# UEFI:
grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=ubuntu --recheck
update-grub

5. Exit and reboot.

14. Common Errors & How to Fix Them

15. Common Errors & How to Fix Them (Full Matrix)

Below is the full matrix of the most common GRUB problems you’ll encounter on Ubuntu 24.10. Each row includes a short description, the command that usually triggers it, the diagnostic steps, and the fix.

16. Common Errors & How to Fix Them – Full Matrix (Expanded)

17. Tools

18. Backup, Restore, and Version‑Control of GRUB

18.1 Simple backup

sudo cp -a /etc/default/grub /etc/default/grub.bak
sudo cp -a /etc/grub.d/ /etc/grub.d.bak/
sudo cp -a /boot/efi/EFI/ubuntu/ /boot/efi/EFI/ubuntu.bak/

18.2 Using git (nice for experimentation)

cd /etc
sudo git init
sudo git add grub.d/ default/grub
sudo git commit -m "Initial GRUB config"
# After each change:
sudo git add -A
sudo git commit -m "Custom Windows entry"

Now you have a history of every change and can revert easily:

sudo git checkout HEAD~1 /etc/default/grub
sudo update-grub

18.3 Full system backup (recommended)

sudo rsync -aAXv / /mnt/backup --exclude={"/dev/*","/proc/*","/sys/*","/tmp/*","/run/*","/mnt/*","/media/*","/lost+found"}

This captures /etc, /boot, and the ESP.

18.4 Restoring GRUB after a disaster

1. Boot live USB.
2. Mount and chroot (see § 13.2).
3. Restore files from backup (/boot/grub/, /etc/default/grub, /etc/grub.d/).
4. Run sudo update-grub.

18.5 Using Git for incremental changes

cd /etc
sudo git init
sudo git add default/grub grub.d/
sudo git commit -m "Initial GRUB config"
# After each edit:
sudo git commit -am "Added custom entry for X"

Now you can git log to see every tweak you made over time.

19. Cheat‑Sheet – One‑Page Command Summary

# Install GRUB (BIOS)
sudo grub-install /dev/sda
sudo update-grub

# Install GRUB (UEFI)
sudo grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=ubuntu
sudo update-grub

# Edit main config
sudo vim /etc/default/grub
# → change variables, then:
sudo update-grub

# Add custom entry
sudo vim /etc/grub.d/40_custom
# (add menuentry block)
sudo update-grub

# Rescue from "grub rescue>"
grub rescue> ls
grub rescue> set root=(hd0,msdos1)
grub rescue> set prefix=(hd0,msdos1)/boot/grub
grub rescue> insmod normal
grub rescue> normal

# Live‑USB chroot (UEFI)
sudo mount /dev/sda2 /mnt
sudo mount /dev/sda1 /mnt/boot/efi
for d in /dev /proc /sys; do sudo mount --bind $d /mnt$d; done
sudo chroot /mnt
# now reinstall:
grub-install /dev/sda
update-grub
exit
sudo reboot

# Secure‑Boot: enroll MOK
sudo mokutil --import MOK.der
# Sign a kernel
sudo sbsign --key MOK.priv --cert MOK.der --output /boot/vmlinuz‑signed /boot/vmlinuz‑<ver>

# EFI boot order
sudo efibootmgr -v
sudo efibootmgr -o 0002,0001   # Windows first, then Ubuntu

Final Thoughts

GRUB is a critical piece of the boot chain, but it is also highly configurable. By keeping all changes in /etc/default/grub, /etc/grub.d/, and using tools like boot-repair or grub‑customizer, you can tailor the boot experience to your exact needs while preserving a safety net via backups and version control.

Whenever you modify partitions, install new kernels, or switch between BIOS and UEFI, remember to regenerate the GRUB configuration (sudo update-grub) and, if necessary, re‑install the appropriate GRUB flavour. With the troubleshooting matrix above, you should be able to recover from most boot‑related mishaps on Ubuntu 22.04 LTS and newer.

Happy booting!