Ubuntu Image

Introduction

Linux is often treated as a black box: you press the power button, and somehow you end up with a login prompt or a desktop environment.
In reality, Linux follows a strict, well-defined sequence of steps that transform raw hardware into a multi-user operating system.

This post walks through the Linux boot process and internal architecture, focusing on what actually happens, not marketing abstractions.

1. Power-on and Firmware Stage

When you power on a system, Linux is not involved yet.

The CPU begins execution at a predefined memory address provided by the firmware:

BIOS (legacy systems)
UEFI (modern systems)

Responsibilities at this stage:

Hardware initialization
Memory testing
Locating a bootable device

Once complete, the firmware hands control to a bootloader.

2. Bootloader (GRUB)

Most Linux systems use GRUB.

GRUB’s responsibilities:

Load the Linux kernel into memory
Load the initial RAM filesystem (initramfs)
Pass kernel parameters

At this point, control is transferred to the kernel.

3. Kernel Initialization

Once the kernel starts executing, it performs several critical tasks:

3.1 Hardware Abstraction

Detects CPUs
Initializes memory management
Sets up device drivers (built-in ones)

3.2 Process Management

Initializes the scheduler
Creates the very first process

This process always has PID 1.

4. `init` / `systemd` (PID 1)

The kernel does not manage user services.

Instead, it launches a userspace process defined by:

On modern systems, this is usually systemd.

systemd responsibilities:

Mount filesystems
Start system services
Manage service dependencies
Handle shutdown and reboot

5. User Space Initialization

After systemd completes its startup sequence:

Login services start
Networking becomes available
Desktop environment or TTY is launched

At this point, the system is fully operational.

From here on:

Applications run in user space
Kernel interactions happen via system calls

6. Kernel Space vs User Space

A critical Linux design principle:

| Kernel Space | User Space | |-------------|------------| | Full hardware access | Restricted access | | Runs kernel code | Runs applications | | Can crash the system | Can be killed safely |

Applications never access hardware directly.
They go through the kernel.

7. Why This Matters

Understanding Linux internals helps you:

Debug boot failures
Optimize system startup
Secure systems
Write better low-level software

This knowledge is foundational for:

Systems programming
Cybersecurity
DevOps
Kernel development

Conclusion

Linux is not magic.
It is a carefully layered system where each stage hands control to the next.

Once you understand this flow, many “mysterious” Linux behaviors become predictable and logical.