The Linux boot process is a sequence of steps that occur when a Linux-based operating system is started on a computer.Understanding this process is essential for troubleshooting, system administration, and overall comprehension of how the Linux operating system operates.The boot process can be divided into several key phases:
1. BIOS/UEFI Initialization
When a computer is powered on, the first piece of firmware that runs is the Basic Input/Output System (BIOS) or Unified Extensible Firmware Interface (UEFI). This firmware is responsible for:
Performing a Power-On Self Test (POST) to check hardware components.
Initializing hardware components such as the CPU, memory, and storage devices.
Locating and loading the bootloader from a designated boot device (usually a hard drive, SSD, or USB drive).
2. Boot Loader
Once the BIOS/UEFI has completed its checks, it hands over control to the bootloader. Common bootloaders in Linux include GRUB (GRand Unified Bootloader) and LILO (Linux Loader). The bootloader’s primary functions are:
Presenting a menu to the user to select which operating system or kernel version to boot (in multi-boot systems).
Loading the selected Linux kernel into memory.
Passing control to the kernel, along with necessary parameters.
3. Kernel Initialization
After the bootloader loads the kernel into memory, the kernel begins its initialization process. Key tasks during this phase include:
Setting up the systemβs memory management.
Detecting and configuring hardware components (like disks, network interfaces, etc.).
Mounting the root filesystem.
Starting the initial process, typically referred to as init (or systemd in many modern distributions).
4. Init/Systemd
The init system (or systemd in many distributions) is the first user-space program that the kernel runs. Its responsibilities include:
Setting up the user space environment.
Starting system services and daemons based on configuration files.
Managing the system run-levels or targets (which determine which services should be running).
5. User Space Initialization
In this phase, various system services and applications are started. This can include:
Starting network services (like DHCP).
Initiating system logging services.
Launching graphical user interfaces (if applicable).
6. Login Prompt
Once the system services are up and running, users are presented with a login prompt (for console-based systems) or a graphical login screen (for desktop environments). Users can then log in to start their session.
Summary of the Boot Process π
BIOS/UEFI: Initializes hardware and locates the bootloader.
Boot Loader: Loads the kernel into memory.
Kernel Initialization: Initializes the system and mounts the root filesystem.
Init/Systemd: Starts user-space processes and services.
User Space Initialization: Services and applications are launched.
Login Prompt: Users log in to start their session.
