Linux Bootloader Configuration Guide

Introduction to Linux Bootloaders

A bootloader is a crucial component in the Linux boot process. It's responsible for loading the operating system kernel into memory and transferring control to it. The two most common bootloaders in Linux systems are:

GRUB2 (GRand Unified Bootloader version 2): The default bootloader for most modern Linux distributions.
LILO (LInux LOader): An older bootloader, still used in some systems but largely replaced by GRUB2.

This guide will primarily focus on GRUB2 configuration, as it's the most widely used bootloader in modern Linux systems.

GRUB2 Configuration Files

GRUB2 uses several configuration files:

/boot/grub/grub.cfg: The main configuration file (don't edit this directly)
/etc/default/grub: The primary configuration file for users
/etc/grub.d/: Directory containing scripts that generate the grub.cfg file

Editing /etc/default/grub

This file contains the main user-configurable settings for GRUB2. Here are some key parameters:

GRUB_DEFAULT=0
GRUB_TIMEOUT=5
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX=""

GRUB_DEFAULT: Sets the default menu entry to boot
GRUB_TIMEOUT: Sets the time (in seconds) to display the menu
GRUB_CMDLINE_LINUX_DEFAULT: Kernel parameters for normal mode
GRUB_CMDLINE_LINUX: Kernel parameters for all modes

Customizing GRUB2

1. Changing the Default Boot Entry

To change the default boot entry, edit the GRUB_DEFAULT line in /etc/default/grub. You can use a number (starting from 0) or the menu entry title.

GRUB_DEFAULT="Advanced options for Ubuntu"

2. Adjusting Boot Timeout

Modify the GRUB_TIMEOUT value to change how long the menu is displayed:

GRUB_TIMEOUT=10

3. Adding Kernel Parameters

To add kernel parameters, edit the GRUB_CMDLINE_LINUX_DEFAULT line:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash nomodeset"

4. Changing GRUB2 Resolution

Add the following line to set a specific resolution:

GRUB_GFXMODE=1024x768

Pro Tip: After making changes to /etc/default/grub, always run sudo update-grub to apply the changes.

Advanced GRUB2 Configuration

1. Custom Menu Entries

To add custom menu entries, create a new file in /etc/grub.d/ (e.g., 40_custom) and add your entries:

#!/bin/sh
exec tail -n +3 $0
menuentry "My Custom Linux" {
    set root=(hd0,1)
    linux /vmlinuz root=/dev/sda1
    initrd /initrd.img
}

2. GRUB2 Theming

You can customize GRUB2's appearance by setting a theme. Add this line to /etc/default/grub:

GRUB_THEME="/path/to/your/theme.txt"

3. Dual-Booting Configuration

GRUB2 usually detects other operating systems automatically. If it doesn't, you can manually add entries for other OSes in /etc/grub.d/40_custom.

Troubleshooting GRUB2

If GRUB2 fails to load, use a live USB to chroot into your system and reinstall GRUB2
For boot parameter issues, try editing the GRUB menu at boot time (press 'e' on the desired entry)
If you can't see the GRUB menu, try holding Shift during boot