Understanding Linux Startup: Overview

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

│ 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

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

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

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

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)

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

# 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

# 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

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

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