Linux Boot Process Explained

Power On

→ The boot process starts when the computer is powered on.
→ The system hardware initializes and control is handed to BIOS or UEFI.
→ BIOS/UEFI is stored in non-volatile memory and performs POST (Power-On Self Test) to check hardware integrity.

BIOS/UEFI Initialization

→ Detects and initializes hardware devices such as CPU, RAM, disk drives, and peripherals.
→ Determines the bootable devices (like hard drive, USB, or network).
→ Once hardware detection is complete, control moves to the Bootloader.

Boot Device Selection

→ BIOS/UEFI selects the boot device based on configuration order.
→ The boot device contains the bootloader (e.g., GRUB).
→ The system reads the boot sector to locate the bootloader program.

GRUB (Boot Loader)

→ GRUB (GRand Unified Bootloader) is loaded from the selected device.
→ Reads the configuration file /etc/grub2.cfg.
→ Displays a boot menu (if configured) to select the desired kernel or OS version.
→ Loads the chosen Linux kernel into memory.
→ Loads essential kernel modules and libraries required for booting.

Kernel Initialization

→ The Linux kernel decompresses itself and initializes system components.
→ Detects hardware components and mounts the root filesystem (/).
→ Starts the first user-space process — usually systemd.
→ Switches from kernel mode to user mode.

systemd Execution

→ systemd is the first process in user space (PID 1).
→ Responsible for initializing all other services and targets.
→ Reads configuration files from /etc/systemd/system/.
→ Activates target files like:
→ basic.target
→ multi-user.target
→ getty.target
→ ssh.service

Running Startup Scripts

→ Executes system-wide and user-specific startup scripts.
→ Examples:
→ /etc/profile
→ /systemd-logind
→ ~/.bashrc
→ These scripts set up the user environment and background services.

User Login

→ Once all startup scripts and targets are executed, the login screen appears.
→ Users can now log in via terminal, GUI, or SSH.
→ System is now in a fully operational multi-user state.

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