Linux Boot Process

The Linux boot process describes the sequence of events that occur from powering on the computer to the system being fully operational. This involves hardware initialization, loading the kernel, and setting up user-space services to make the system ready for use. Below is an overview of each stage of the boot process.

1. BIOS/UEFI Initialization

BIOS (Basic Input/Output System) or UEFI (Unified Extensible Firmware Interface) is the first software that runs when you power on your computer.

It performs a POST (Power-On Self Test) to check the hardware components such as the CPU, memory, and storage devices.

After completing hardware checks, BIOS/UEFI looks for a bootable device (such as a hard drive, SSD, or USB stick) and loads the bootloader.

2. Bootloader Stage

The bootloader is responsible for loading the Linux kernel into memory. Two common bootloaders used in Linux are:

a. GRUB (Grand Unified Bootloader)

GRUB is the most widely used bootloader in Linux. It allows users to select the operating system or kernel version to boot in case of a dual-boot or multiple kernel configurations.

It loads the kernel and the initrd (initial RAM disk) into memory and hands over control to the kernel.

b. LILO (Linux Loader)

LILO is an older bootloader, less commonly used now. It directly loads the Linux kernel and transfers control to it without offering a menu for kernel selection.

Once the bootloader is loaded, it proceeds to load the Linux kernel.

3. Kernel Loading

The Linux kernel is loaded into memory by the bootloader. The kernel is responsible for managing hardware resources, handling system calls, and providing essential services for the operating system.

Along with the kernel, the initrd (or initramfs) is also loaded. This temporary root filesystem contains essential tools and drivers needed to mount the real root filesystem.

Kernel Initialization

The kernel initializes the system’s hardware, such as CPU, memory, storage devices, and network interfaces.

It loads drivers and necessary modules, and it sets up the kernel environment.

The kernel also mounts the root filesystem and prepares the system to enter the user-space environment.

4. Init Process (System Initialization)

The init process is the first user-space process that is started by the kernel. It is assigned PID 1 (process ID 1).

init (or its replacement, systemd, in most modern Linux distributions) is responsible for initializing the system and starting all the necessary system services.

The init system can vary between distributions:

systemd: A modern and widely used init system that offers parallel service startup for faster boot times.

sysvinit: A traditional init system that starts services sequentially based on runlevels.

Upstart: An older init system, now mostly replaced by systemd in many distributions.

5. Systemd or Init Services

If systemd is used:

Systemd handles parallel startup of services, improving boot time and efficiency. It also provides tools for managing system services, logs, and other tasks.

Systemd manages services via unit files and targets.

If sysvinit is used:

sysvinit starts services in a sequential manner according to predefined runlevels (e.g., single-user, multi-user, graphical modes).

Services are initialized one after the other using rc scripts.

6. Mounting the Root Filesystem

The root filesystem (/) is mounted once the kernel has initialized the hardware.

If the root filesystem is encrypted, the system might prompt for a password at this point.

The system might also mount additional filesystems (like /home, /var, etc.) during this stage, depending on the configuration.

7. Starting User-Space Services

At this stage, the system continues to initialize background services required for the system to operate properly. Some common services include:

Networking: Setting up network interfaces and configuring network settings.

Display Manager: If the system uses a graphical interface, the display manager (e.g., GDM, LightDM, or SDDM) is started to manage user logins.

System Services: Services such as logging (e.g., rsyslog), process scheduling (e.g., cron), and hardware monitoring are also started.

8. User Login

Once the necessary services are running, the system presents a login prompt to the user.

In a graphical environment, the user is prompted by a display manager to log in using a GUI.

In a text-based environment, users log in via the command line (TTY).

After a successful login, the user gains access to the system and can begin interacting with it.

Summary of the Linux Boot Process

BIOS/UEFI performs hardware checks and locates the boot device.

The bootloader (like GRUB) loads the kernel and initrd.

The kernel initializes the hardware, loads drivers, and mounts the root filesystem.

The init process (or systemd) initializes the system, launching required services.

Background system services are started, including networking, display, and other services.

The system presents a login prompt, allowing the user to authenticate and start interacting with the system.