Linux

Modern Linux Boot Process ?

Rohit Kumar

Oct 8, 2024 — 2 min read

The modern Linux boot process typically involves multiple stages that transition from system power-on to a fully running operating system. Here's an overview of the key steps in the Linux boot process:

BIOS/UEFI Initialization:

BIOS (Basic Input/Output System) or UEFI (Unified Extensible Firmware Interface) is the first code that runs when the computer is powered on.
POST - Power On Self Test. It initializes and tests hardware components
It looks for a bootable device (such as hard drives, SSDs, or USBs), and transfers control to the bootloader located in the Master Boot Record (MBR) or GUID Partition Table (GPT).

Bootloader

The bootloader (GRUB) is responsible for loading the Linux kernel into memory and passing control to it.
It may provide a boot menu to select different operating systems or kernel versions.

Initial RAM Disk (initramfs)

The kernel initializes system hardware, including CPU, memory, and device drivers.
It mounts the initial RAM disk (initramfs), a temporary root file system that allows access to necessary drivers and modules required to mount the real root file system.
The kernel detects and configures all necessary hardware for the system to operate.

Systemd

Once the kernel has loaded and set up basic hardware, it hands over control to the systemd (first process of the kernel).
systemd reads configuration files from /etc/systemd/, and it manages services, mounts file systems, sets up network interfaces, and starts daemons.

Target

systemd switches to the target specified in its configuration. (typically multi-user.target for a full multi-user, non-GUI system, or graphical.target for GUI-based systems).
At this point, all necessary services and daemons like networking, login, display managers (e.g., GDM, LightDM) are started.

Once systemd completes its work, the system prompts the user with either a command-line login or a graphical login screen (depending on whether the system is in text or graphical mode).
After login, the user's shell environment or desktop environment is launched, and the system is fully operational.

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