Linux Boot Process

An overview of the Linux boot process, explaining what happens from the moment you press the power button to when you reach the login screen.

Interactive version: https://www.hackers-game.com/2020/08/23/linux-boot-process/

This video covers the various stages of Linux system initialization, providing fundamental knowledge for Linux system administrators and anyone wanting to troubleshoot boot-related issues.

Stage 1: BIOS/UEFI

When you press the power button:

1. Power-On Self Test (POST): Hardware initialization and verification
2. Device enumeration: Identifying connected hardware
3. Boot device selection: Finding the bootloader

BIOS vs UEFI

• BIOS: Legacy firmware, MBR partition tables, limited to 2TB drives


• UEFI: Modern firmware, GPT partition tables, secure boot support

Stage 2: Bootloader (GRUB)

GRUB (Grand Unified Bootloader) is the most common Linux bootloader:

1. Stage 1: Loaded from MBR or EFI partition
2. Stage 1.5: Filesystem drivers (for BIOS systems)
3. Stage 2: Full GRUB environment with menu

GRUB Configuration



• Located at /boot/grub/grub.cfg


• Generated by grub-mkconfig or update-grub


• Allows kernel parameter modifications

Stage 3: Kernel Loading

The Linux kernel initialization:

1. Kernel decompression: The compressed kernel extracts itself
2. Hardware initialization: Setting up CPU, memory, devices
3. Mounting initramfs: Temporary root filesystem
4. Loading drivers: Required for accessing the real root filesystem

Stage 4: Init System (systemd)

Modern Linux distributions use systemd:

1. PID 1: systemd becomes the first user process
2. Target units: Defining system states (multi-user, graphical)
3. Service startup: Parallel service initialization
4. Dependency management: Ensuring correct startup order

Key systemd Commands




Check boot time

systemd-analyze
View boot blame

systemd-analyze blame
Check service status

systemctl status
View boot log

journalctl -b

Stage 5: Login

Finally, the login prompt appears:

• Console login: getty on virtual terminals


• Graphical login: Display manager (GDM, SDDM, LightDM)


• Session initialization: User environment setup

Troubleshooting Boot Issues

Understanding the boot process helps diagnose problems:


• BIOS/UEFI issues: Hardware or firmware configuration


• GRUB issues: Bootloader corruption or misconfiguration


• Kernel issues: Driver problems or kernel panics


• systemd issues: Service failures or dependency problems

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