The booting process of a Linux server begins with the system’s firmware, typically the BIOS or UEFI, which initializes the hardware components and performs a Power-On Self Test (POST) to ensure that all necessary hardware is functioning correctly. Once the POST is completed, the firmware locates the bootable device, which could be a hard drive, SSD, or other storage media containing the Linux operating system. The firmware then loads the bootloader from the designated device into memory. Common bootloaders for Linux include GRUB (GRand Unified Bootloader) and LILO (Linux Loader), which are responsible for loading the Linux kernel.
After the bootloader is loaded, it presents a menu to the user, allowing them to select which operating system or kernel version to boot if multiple options are available. Once a selection is made, the bootloader loads the selected Linux kernel into memory and passes control to it. The kernel then initializes the system by setting up the necessary hardware drivers, mounting the root filesystem, and starting essential system processes. This stage is crucial as it prepares the environment for user-space applications and services.
Following the kernel initialization, the init process, which is the first user-space program started by the kernel, takes over. Traditionally, this process is represented by the PID (Process ID) of 1. The init process reads its configuration files to determine which services and processes to start, often utilizing system initialization systems like Systemd or SysVinit. As these services are launched, the server becomes fully operational, allowing users to log in and interact with the system. This entire sequence from firmware initialization to the start of user services constitutes the boot process of a Linux server, ensuring that all components are correctly configured and ready for use.
