Buffl
Buffl
LFCA: Linux Fundamentals

Understanding Linux Startup: Overview

as
by abdullah S.

The Big Picture


The Linux startup process is like a relay race with 4 runners, each passing the baton to the next until the system is fully operational.


┌─────────────────────────────────────────────────────────────────┐

│ LINUX BOOT SEQUENCE │

└─────────────────────────────────────────────────────────────────┘

Power On

↓

┌──────────────────┐

│ 1. BIOS/UEFI │ ← Hardware Manager

│ (Firmware) │ • Checks hardware (POST)

└────────┬─────────┘ • Finds bootable device

↓ • Loads bootloader

┌──────────────────┐

│ 2. GRUB │ ← Boot Menu

│ (Bootloader) │ • Shows OS choices

└────────┬─────────┘ • Loads kernel + initramfs

↓ • Passes boot parameters

┌──────────────────┐

│ 3. Kernel │ ← Operating System Core

│ (vmlinuz) │ • Initializes hardware

└────────┬─────────┘ • Mounts root filesystem

↓ • Starts init system

┌──────────────────┐

│ 4. systemd │ ← Service Manager

│ (Init System) │ • Starts all services in parallel

└────────┬─────────┘ • Reaches target state

↓ • System ready!

Login Prompt

or GUI

Stage-by-Stage Breakdown


Stage 1: BIOS/UEFI ⚙️ (The Hardware Checker)


What it is: Firmware chip on the motherboard

What it does:

  1. ✅ POST (Power-On Self-Test) - Verifies CPU, RAM, keyboard, etc.

  2. ✅ Detects devices - Finds hard drives, USB, network cards

  3. ✅ Locates bootloader - Finds GRUB in MBR or ESP partition

  4. ✅ Hands off control - Starts the bootloader

Key files/locations:

  • Stored in motherboard chip

  • Settings: Press F2/F10/Del during boot

Time taken: ~2-5 seconds

Stage 2: GRUB (Bootloader) 🎯 (The Launcher)


What it is: GRand Unified Bootloader

What it does:

  1. ✅ Shows boot menu - Choose OS or kernel version

  2. ✅ Loads kernel - Reads /boot/vmlinuz-*

  3. ✅ Loads initramfs - Reads /boot/initramfs-* or /boot/initrd.img-*

  4. ✅ Passes parameters - Sends boot options to kernel

  5. ✅ Transfers control - Starts the kernel

Key files/locations:

  • /boot/grub/grub.cfg - Main configuration

  • /etc/default/grub - Settings (edit this!)

  • /boot/vmlinuz-* - Kernel image

  • /boot/initramfs-* - Initial RAM filesystem

Key commands:

sudo update-grub # Update GRUB config sudo grub-install /dev/sda # Install GRUB

Time taken: ~3-5 seconds

Stage 3: Kernel 🧠 (The Brain)


What it is: Core of the Linux operating system

What it does:

  1. ✅ Decompresses itself - Unpacks compressed kernel

  2. ✅ Initializes hardware - CPU, memory, devices

  3. ✅ Loads drivers - From initramfs (temporary root)

  4. ✅ Mounts root filesystem - Real root (/) partition

  5. ✅ Starts systemd - First process (PID 1)

Key files/locations:

  • /boot/vmlinuz-* - Compressed kernel

  • /boot/initramfs-* - Temporary filesystem with drivers

  • /proc/cmdline - Boot parameters used

  • /proc/version - Kernel version info

Key commands:

uname -r # Show kernel version dmesg # View kernel messages journalctl -k # View kernel logs

Time taken: ~5-10 seconds

Stage 4: systemd (Init System) 🚀 (The Orchestrator)


What it is: First process (PID 1), service manager

What it does:

  1. ✅ Starts as PID 1 - First user-space process

  2. ✅ Reads configuration - Determines default target

  3. ✅ Starts services in parallel - Network, logging, etc.

  4. ✅ Reaches target state - multi-user or graphical

  5. ✅ System ready! - Login prompt appears

Key files/locations:

  • /etc/systemd/system/ - Custom service files

  • /lib/systemd/system/ - Distribution service files

  • /etc/systemd/system/default.target - Default target (symlink)

Key commands:

systemctl status # System status systemctl get-default # Show default target systemctl list-units # List all units journalctl -b # View boot logs systemd-analyze # Boot time analysis

Time taken: ~10-30 seconds (depends on services)

Visual Timeline


Time Stage What's Happening

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

0s [Power On] Press power button

2s [BIOS/UEFI] 🔍 Checking hardware...

✓ CPU OK

✓ RAM OK

✓ Disk found

5s [GRUB] 📋 GRUB Menu (5 sec timeout)

→ Loading kernel...

→ Loading initramfs...

8s [Kernel] 🧠 Kernel starting...

✓ Hardware initialized

✓ Drivers loaded

✓ Root filesystem mounted

13s [systemd] 🚀 Starting services...

✓ networking.service

✓ sshd.service

✓ NetworkManager.service

✓ gdm.service (if GUI)

25s [Ready] ✅ System ready!

→ Login prompt or GUI

Quick Command Reference


# BIOS/UEFI

[ -d /sys/firmware/efi ] && echo "UEFI" || echo "BIOS"

# Bootloader (GRUB)

cat /proc/cmdline # Boot parameters used

ls /boot/vmlinuz-* # Installed kernels

# Kernel

uname -r # Current kernel version

dmesg | head -20 # Early kernel messages

lsmod # Loaded kernel modules

# systemd

systemctl status # System state

systemctl --failed # Failed services

journalctl -b # This boot's logs

systemd-analyze blame # Service start times

Common Boot Problems & Solutions


Problem

Stage

Solution

"No bootable device"

BIOS/UEFI

Check boot order in BIOS settings

"GRUB error"

GRUB

Reinstall GRUB: grub-install /dev/sda

"Kernel panic"

Kernel

Boot with older kernel from GRUB menu

"Service failed"

systemd

Check logs: journalctl -u service_name

Black screen

Any

Add nomodeset to kernel parameters


Boot to Different Modes

At GRUB Menu (press 'e' to edit):


# Single-user/Rescue mode

Add: systemd.unit=rescue.target

# Multi-user (no GUI)

Add: systemd.unit=multi-user.target

# Emergency mode (minimal)

Add: systemd.unit=emergency.target

# Verbose boot (see all messages)

Remove: quiet splash

The Complete Flow (One-Liner)


Power → BIOS (hardware check) → GRUB (load kernel) →

Kernel (initialize system) → systemd (start services) → Login

Join Course
Preview

Author

abdullah S.

Information

Last changed

Report course
© 2023 Buffl GmbH