Linux Boot Process

The below image outlines the typical flow of a Linux system boot.

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BIOS (Basic Input/Output System) & UEFI (Unified Extensible Firmware Interface)

BIOS (Basic Input/Output System) and UEFI (Unified Extensible Firmware Interface) are both firmware interfaces that provide a critical layer between your hardware and operating system during the startup process. However, they differ significantly in their functionality and capabilities.

Similarities:

Both BIOS and UEFI are pre-installed on your motherboard and activate when you turn on your computer.
They perform essential tasks like initializing hardware components, performing power-on self-tests (POST), and loading the operating system from a storage device.

MBR - master boot record & GPT - GUID partition table

MBR (Master Boot Record) and GPT (GUID Partition Table) are both partition table formats used on hard disk drives (HDDs) and solid-state drives (SSDs) to define how the storage space is organized. However, they have some key differences:

Similarities:

Both MBR and GPT are located at the beginning of a storage device and define how the storage space is divided into partitions.
They both store information about each partition, such as its size, file system type, and location on the disk.
Both allow you to create multiple partitions on a single storage device, enabling you to manage different operating systems or data types more efficiently.
Identification of boot loader (typically at beginning of disk) - Once identified, the boot process is handled by the boot loader - most commonly GRUB2.

Note; MBR is an older standard of partitioning, most systems will now use GUID.

GRUB2 (Grand Unified Boot Loader)

GRUB2 is a software program, specifically a bootloader. Its primary function is to take control during the computer startup process after the firmware (BIOS or UEFI) has initialized the hardware. Here's a breakdown of its role:

Loading the Operating System Kernel: GRUB2 locates the kernel (the core program) of your operating system on the storage device. It then loads this kernel into memory.
starts kernel and systemd.

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Initrd & Kernel

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InitrdWhat &is Kernelinitrd?

initrd

(initialisation

~~Which~~RAM ~~file~~disk) is essentially a set of instructions used to load the kernel. initrd is stored temporarily in ~~/boot~~system ismemory ~~the kernel?~~

~~Typically,~~whilst the kernel ~~file itself (located within /boot)~~ is ~~prefaced with 'vmlinuz', for example:~~

vmlinuz-5.15.0-106-generic

loaded.

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What is a Linux Kernel?

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Which file in /boot is the kernel?

Typically, the kernel file itself (located within /boot) is prefaced with 'vmlinuz', for example:

vmlinuz-5.15.0-106-generic

Working on Linux systems, you may see systems with various kernels installed.

Which file in /boot is initrd?

The initrd file will be prefaced with just that - initrd:

initrd.img-5.15.0-106-generic

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To check which kernel is currently being treated as the primary one (which will be loaded on boot), you can check the symlinks (in /boot), as below:

vmlinuz -> vmlinuz-5.15.0-107-generic

vmlinuz.old -> vmlinuz-5.15.0-106-generic

The same applies to the initrd files:

initrd.img -> initrd.img-5.15.0-107-generic

initrd.img.old -> initrd.img-5.15.0-106-generic

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Systemd

Systemd is the first process that starts on a booting OS.

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