System Boot process
Here is an Illustrated Linux Boot Process
Power On: When the power is turned on, the BIOS (Basic Input/Output System) or UEFI (Unified Extensible Firmware Interface) firmware is loaded from non-volatile memory and performs the POST (Power On Self Test).
Device Detection: The BIOS/UEFI detects and initializes connected devices such as the CPU, RAM, and storage.
Boot Device Selection: The system selects a boot device from which to load the operating system. This could be the hard drive, a network server, or a CD-ROM.
Boot Loader Execution: The BIOS/UEFI loads the boot loader (such as GRUB), which presents a menu to select the operating system or kernel functions.
Kernel Initialization: Once the kernel is loaded, control transfers to user space. The kernel initiates systemd as the first user-space process, which manages processes and services, detects remaining hardware, mounts filesystems, and starts the desktop environment.
Default Target Activation: systemd activates the default target unit during boot, along with other necessary units.
Startup Scripts: The system executes a series of startup scripts to configure the environment.
User Login: Finally, the system presents a login window, and the system is ready for use.
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