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Boot

graph TD
  ON["POWER ON"]
  B["BIOS / UEFI"]
  P["POST
  (runs inside BIOS/UEFI)"]
  D["BOOT DEVICE"]
  G["BOOT LOADER"]
  K["Kernel"]
  DR["Kernel Initialization
  (Drivers, Modules, Initramfs)"]
  SYS["Systemd (PID 1)"]
  TARGET["Run .target files"]
  STARTUP["User space startup
  (login, desktop, services)"]

  ON --> B --> P
  B ---> D --> G --> K --> DR --> SYS --> TARGET --> STARTUP

BIOS / UEFI

  • Firmware that initializes hardware and starts the boot process

POST (Power On Self Test)

  • Tests all important hardware parts

BOOT DEVICE

  • SSD, HDD, CD
  • System partition

BOOT LOADER

  • GRUB2
  • Is a part of the distribution (not Linux Kernel)

Kernel

  • Core software that manages hardware, memory, and system resources

Kernel Initialization

  • Load:
  • Device Drivers
  • Kernel Modules
  • Initramfs
  • Mount the root filesystem
  • Start the first user-space process (/lib/systemd/systemd)

Systemd is responsible for booting the user space and managing services:

  • Acts as parent of all processes on Linux
  • Mounts file systems and disks
  • Launches background processes (networking, sound, & power management)
  • Handles user logins
  • Starts the desktop environment (if installed)

.target files 

basic.target (Mounts filesystems)
  ↓
multi-user.target (Starts networking, login, SSH, etc.)
  ↓
graphical.target (if a desktop environment is installed)