A bootloader is a small program that acts as a bridge between the BIOS/UEFI firmware and the operating system kernel.

Think of it as: The "middle manager" that receives the system from hardware initialization and prepares it for the operating system.

Location:

• BIOS/MBR systems: First 512 bytes of the disk (MBR)

• UEFI/GPT systems: EFI System Partition (ESP) at /boot/efi

The BIOS/UEFI doesn't know how to load an operating system directly. The bootloader:

✅ Provides a menu to choose between multiple operating systems

✅ Loads the Linux kernel into memory

✅ Loads the initial RAM disk (initrd/initramfs)

✅ Passes boot parameters to the kernel

✅ Handles boot-time configuration

1. GRUB (GRand Unified Bootloader)

• Original version, now called "GRUB Legacy"

• Rarely used today

• Configuration: /boot/grub/menu.lst or /boot/grub/grub.conf

2. GRUB 2 ⭐ (Most Common)

• Modern replacement for GRUB

• Used by: Ubuntu, Fedora, Debian, CentOS, RHEL, and most distributions

• Much more powerful and flexible

• Configuration: /boot/grub/grub.cfg (auto-generated, don't edit directly!)

3. LILO (Linux Loader)

• One of the oldest bootloaders

• Largely obsolete

• Configuration: /etc/lilo.conf

4. systemd-boot (formerly gummiboot)

• Simple UEFI-only bootloader

• Used by: Some Arch Linux installations

• Configuration: /boot/loader/

5. SYSLINUX

• Lightweight bootloader

• Often used for: USB drives, rescue disks

• Multiple variants: ISOLINUX (CDs), PXELINUX (network boot)

Boot Process with GRUB 2:

1. BIOS/UEFI → Loads GRUB 2

2. GRUB 2 reads configuration (/boot/grub/grub.cfg)

3. Displays boot menu (if multiple options exist)

4. User selects OS or waits for timeout

5. GRUB 2 loads kernel image (vmlinuz-*)

6. GRUB 2 loads initramfs (initrd.img-* or initramfs-*)

7. Passes control to kernel with boot parameters

8. Kernel takes over

Configuration Files:

This is the file you SHOULD edit to customize GRUB.

# Default boot entry (0 = first, 1 = second, etc.)

GRUB_DEFAULT=0

# Wait time before auto-booting (seconds)

GRUB_TIMEOUT=5

# Show/hide GRUB menu (true/false)

GRUB_TIMEOUT_STYLE=menu

# Kernel boot parameters

GRUB_CMDLINE_LINUX="quiet splash"

# Additional kernel parameters for normal boot

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

# Resolution for GRUB menu

GRUB_GFXMODE=640x480

# Terminal output

GRUB_TERMINAL=console

# Disable submenu grouping

GRUB_DISABLE_SUBMENU=true

# Remember last boot choice

GRUB_SAVEDEFAULT=false

Scripts that build the final grub.cfg:

Scripts are executed in numerical order.

After editing /etc/default/grub, you MUST regenerate grub.cfg:

Debian/Ubuntu:

sudo update-grub # OR sudo grub-mkconfig -o /boot/grub/grub.cfg

RHEL/CentOS/Fedora:

sudo grub2-mkconfig -o /boot/grub2/grub.cfg # For UEFI systems: sudo grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg

If GRUB is corrupted or missing:

BIOS/MBR systems:

sudo grub-install /dev/sda # Where /dev/sda is your disk (not partition!)

UEFI systems:

sudo grub-install --target=x86_64-efi --efi-directory=/boot/efi

# Method 1

ls /boot/vmlinuz-*

# Method 2

dpkg --list | grep linux-image # Debian/Ubuntu

rpm -qa | grep kernel # RHEL/CentOS/Fedora

# List all boot entries with numbers

sudo grep menuentry /boot/grub/grub.cfg

# Set default (e.g., entry #2)

sudo grub-set-default 2

# OR edit /etc/default/grub

GRUB_DEFAULT=2

grub-install --version

# OR

grub2-install --version

When you see the GRUB menu at boot:

Press 'e' at GRUB menu to temporarily edit boot parameters:

Common Kernel Parameters:

# Boot into single-user mode (rescue/recovery)

single

# OR

systemd.unit=rescue.target

# Boot into multi-user mode (no GUI)

systemd.unit=multi-user.target

# OR

3

# Boot into graphical mode s

ystemd.unit=graphical.target

# OR

5

# Boot with different root partition

root=/dev/sda2

# Boot in verbose mode (remove quiet)

# Remove: quiet splash

# See all boot messages

# Disable graphics mode

nomodeset

# Boot with previous kernel (useful if new kernel fails)

# Select "Advanced options" → older kernel

To boot with edited parameters:

Press Ctrl + X or F10 after editing

Press 'c' at GRUB menu to access command line.

Useful GRUB Commands:

# List all partitions

ls

# View partition contents

ls (hd0,1)/

# Set root partition

set root=(hd0,1)

# Load kernel

linux /vmlinuz-5.4.0-42-generic root=/dev/sda1

# Load initrd

initrd /initrd.img-5.4.0-42-generic

# Boot

boot

# View environment variables

set

# Search for file

search --file /vmlinuz

# Display help

help

1. Kernel (vmlinuz-*)

• vmlinuz = Compressed Linux kernel

• Location: /boot/vmlinuz-[version]

• Purpose: Core of the operating system

• Size: Typically 5-10 MB

Example:

/boot/vmlinuz-5.15.0-56-generic

2. Initial RAM Disk/Filesystem

Two types:

initrd (Initial RAM Disk) - Older format

• File: /boot/initrd.img-[version]

• Block device in RAM

• Used by: Older systems

initramfs (Initial RAM Filesystem) - Modern format

• File: /boot/initramfs-[version]

• Compressed cpio archive

• Used by: Modern systems (RHEL, Fedora, CentOS)

Purpose:

• Contains essential drivers and modules

• Provides temporary root filesystem

• Needed before the real root filesystem can be mounted

• Contains tools for mounting encrypted/LVM/RAID volumes

Example:

/boot/initrd.img-5.15.0-56-generic # Debian/Ubuntu

/boot/initramfs-5.15.0-56.el8.x86_64 # RHEL/CentOS

Symptoms:

GRUB loading...

error: no such partition

grub rescue>

Solution:

1. Boot from live USB/CD

2. Mount your root partition:

sudo mount /dev/sda1 /mnt

sudo mount --bind /dev /mnt/dev

sudo mount --bind /proc /mnt/proc

sudo mount --bind /sys /mnt/sys

1. Chroot into system:

sudo chroot /mnt

1. Reinstall GRUB:

grub-install /dev/sda

update-grub

1. Exit and reboot:

exit

sudo umount /mnt/dev /mnt/proc /mnt/sys /mnt

sudo reboot

Solution: Boot with previous kernel from GRUB menu

1. Select "Advanced options"

2. Choose older kernel version

3. Once booted, remove problematic kernel:

sudo apt remove linux-image-[bad-version] # Debian/Ubuntu

sudo dnf remove kernel-[bad-version] # Fedora

Solution: Boot into single-user mode

1. Press 'e' at GRUB menu

2. Find line starting with linux or linux16

3. Add single or init=/bin/bash at end

4. Press Ctrl+X to boot

5. Reset password:

passwd root

1. Reboot

GRUB 2 can boot multiple operating systems:

Detecting Other OS:

# Install os-prober

sudo apt install os-prober # Debian/Ubuntu

sudo dnf install os-prober # Fedora

# Detect other operating systems

sudo os-prober

# Update GRUB to include detected OS

sudo update-grub

Edit /etc/grub.d/40_custom:

menuentry "My Custom Linux" {

set root=(hd0,1)

linux /vmlinuz-5.15.0-56-generic root=/dev/sda1 ro quiet

initrd /initrd.img-5.15.0-56-generic

}

Then update GRUB:

sudo update-grub

Edit /etc/grub.d/40_custom:

# Generate password hash

grub-mkpasswd-pbkdf2

# Enter password when prompted

# Add to 40_custom:

set superusers="admin"

password_pbkdf2 [REDACTED:PASSWORD] [hash-from-above]

Update GRUB:

sudo update-grub

Now users need password to edit boot entries or access GRUB command line.

GRand Unified Bootloader

To load the Linux kernel and initrd/initramfs into memory and pass control to the kernel

/boot/grub/grub.cfg (or /boot/grub2/grub.cfg on some systems)

No, it's auto-generated and changes will be overwritten. Edit /etc/default/grub instead and regenerate the config.

/etc/default/grub

udo update-grub or sudo grub-mkconfig -o /boot/grub/grub.cfg

sudo grub2-mkconfig -o /boot/grub2/grub.cfg

sudo grub-install /dev/sda

/etc/grub.d/

vmlinuz- (e.g., /boot/vmlinuz-5.15.0-56-generic)

initrd (initrd.img-) and initramfs (initramfs-)

/boot/ directory

e (edit)

Ctrl + X or F10

c

GRUB_DEFAULT=0 (where 0 is the first entry, 1 is second, etc.)

GRUB_TIMEOUT=5 (in seconds)

single or systemd.unit=rescue.target

Suppresses most boot messages, showing only critical errors

Displays a graphical splash screen during boot instead of text messages

Remove "quiet" from the kernel command line, or remove both "quiet splash"

Provides a temporary root filesystem with essential drivers and tools needed to mount the real root filesystem

dpkg --list | grep linux-image or ls /boot/vmlinuz-*

rpm -qa | grep kernel or ls /boot/vmlinuz-*

In the Master Boot Record (MBR) - first 512 bytes of the disk

In the EFI System Partition (ESP), typically at /boot/efi/EFI/[distro]/

ls (from GRUB command line)

(hd0,1) or (hd0,msdos1) for MBR, (hd0,gpt1) for GPT

sudo os-prober

Run sudo update-grub or sudo grub2-mkconfig

/etc/grub.d/40_custom or /etc/grub.d/41_custom

root=/dev/sda1 (or appropriate partition)

init=/bin/bash

nomodeset

BIOS/UEFI → Bootloader (GRUB) → Kernel → initrd/initramfs → Init System (systemd)

GRUB cannot find its configuration files or the boot partition, usually due to partition changes or corruption

Specifies kernel boot parameters that apply to all Linux boot entries (normal and recovery)

Specifies kernel boot parameters for normal boot only (not recovery mode)

Add systemd.unit=multi-user.target or 3 to kernel parameters

Add systemd.unit=graphical.target or 5 to kernel parameters

grub-install --version or grub2-install --version

GRUB 2 is the modern, more powerful replacement for GRUB Legacy, with better features, flexibility, and support for UEFI

It passes boot parameters to the kernel and transfers control to it

GRUB_SAVEDEFAULT=true (also requires GRUB_DEFAULT=saved)

Contains kernel symbol table for debugging (maps memory addresses to function names)

100-550 MB, formatted as FAT32

grub-mkpasswd-pbkdf2

Your changes won't take effect because grub.cfg hasn't been regenerated

It detects and adds Linux kernels installed in /boot to the GRUB menu

It detects other operating systems on the system and adds them to the GRUB menu