A Detailed Look at the Linux Boot Process

When you power on your Linux computer, it undergoes a systematic sequence of phases before presenting a login screen that prompts you for your username and password.

Every Linux distribution goes through four distinct stages during the boot-up process:

• BIOS or UEFI Power-On Self-Test (POST)

• Loading of the Boot Loader (GRUB2)

• Kernel Initialization

• Starting systemd (Parent of All Processes)

Let’s dive into each of these stages in greater detail.

1. BIOS and UEFI Integrity check (POST)

Upon system boot, the Basic Input/Output System (BIOS) or the Unified Extensible Firmware Interface (UEFI) program initiates and performs a Power-On Self-Test (POST). This diagnostic routine validates the integrity of various hardware components and peripherals.

The POST process checks and tests essential components like the hard disk drive (HDD) or solid-state drive (SSD), keyboard, random-access memory (RAM), USB ports, and other connected hardware. It ensures that the computer's hardware is functioning correctly.

If the test detects any errors or issues, it typically displays an error message on the screen, prompting user intervention. For instance, if the RAM is not detected correctly, the POST might produce a series of beep codes.

However, if all the expected hardware components are present and functioning properly, the boot process proceeds to the next stage.

It's important to note that BIOS and UEFI are firmware interfaces used by computers to initiate the operating system (OS) boot process. However, they differ in their approach to storing metadata about the drive:

• BIOS uses the Master Boot Record (MBR) partitioning scheme.

• UEFI uses the GUID Partition Table (GPT) partitioning scheme.

2. Loading of the Boot loader (GRUB2)

After the BIOS or UEFI has completed the POST to verify the machine's status, it searches for information about the boot loader and disk partitioning scheme.