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Chapter 2.9 - Boot Process

Learning Objectives​

By the end of this chapter, you will be able to:

  • Understand the Linux boot process
  • Explain the boot steps
  • Identify the components (BIOS/UEFI, bootloader, kernel, init)
  • Troubleshoot boot issues

Boot Overview​

The Linux boot follows a well-defined sequence:

Boot Steps​

1. BIOS/UEFI​

Role: Basic hardware initialization

Actions:

  • Hardware check (POST)
  • Device detection
  • Loading the bootloader from disk

Difference BIOS vs UEFI:

  • BIOS: Old, limitations (2TB disk)
  • UEFI: Modern, supports GPT, Secure Boot

2. Bootloader (GRUB)​

Role: Load the kernel

Actions:

  • Displays the boot menu
  • Loads the kernel and initramfs
  • Passes parameters to the kernel

Files:

  • /boot/grub/grub.cfg (configuration)
  • /boot/vmlinuz-* (kernel)
  • /boot/initrd.img-* (initramfs)

3. Linux Kernel​

Role: System initialization

Actions:

  • Hardware detection and initialization
  • Mounting the root filesystem
  • Starting the init process

Kernel parameters:

  • root= : Root partition
  • quiet : Fewer messages
  • splash : Boot screen

4. Init/systemd (PID 1)​

Role: First user process

Actions:

  • Mounting filesystems
  • Starting services
  • Transitioning to target runlevel

systemd sequence:

  1. default.target activated
  2. Required services started
  3. System ready

Initramfs​

initramfs (Initial RAM File System) is a temporary filesystem in memory:

Roles:

  • Load necessary kernel modules
  • Detect hardware
  • Mount the root filesystem
  • Prepare the environment for init

Detailed systemd Sequence​

Troubleshooting Boot​

Common Issues​

1. Corrupted bootloader:

# Repair GRUB
sudo grub-install /dev/sda
sudo update-grub

2. Kernel does not start:

  • Check kernel parameters
  • Try a previous kernel
  • Check logs: dmesg

3. Missing initramfs:

# Regenerate initramfs
sudo update-initramfs -u

4. Corrupted filesystem:

# Rescue mode
fsck /dev/sda1

Useful Commands​

# View boot messages
dmesg
journalctl -b

# View boot time
systemd-analyze
systemd-analyze blame

# View dependency chain
systemd-analyze critical-chain

# View services slowing down boot
systemd-analyze blame | head -10

Summary​

In this chapter, you learned:

Boot Sequence: BIOS → GRUB → Kernel → initramfs → systemd
BIOS/UEFI: Hardware initialization
GRUB: Bootloader, loads the kernel
Kernel: Initializes the system
initramfs: Temporary filesystem in memory
systemd: First process, starts services
Troubleshooting: GRUB repair, logs, fsck

Next Steps​

Chapter 2.10: Inter-Process Communication
Module 3: Filesystem

Chapter created on: December 2024