Understanding Linux Boot: From BIOS POST to Systemd Targets

Overview

When a Linux PC powers on, it passes through four distinct phases before presenting the login prompt: BIOS integrity check (POST), the GRUB2 bootloader, kernel initialization, and the systemd init system.

1. BIOS Integrity Check (POST)

The BIOS runs a Power‑On Self Test (POST) to verify critical hardware such as HDD/SSD, keyboard, RAM, USB ports, and other components. Errors or missing devices may trigger beep codes or on‑screen messages requiring user intervention. If all checks pass, the boot process proceeds.

2. Bootloader (GRUB2)

After POST, the BIOS locates the Master Boot Record (MBR) on the primary disk (e.g., /dev/sda) and loads the GRUB2 bootloader into RAM. GRUB2 presents a menu that lets users select a kernel version or edit kernel parameters. The configuration file is typically /boot/grub2/grub2.cfg. Modern Linux distributions use GRUB2, while older bootloaders like LILO are now obsolete.

3. Kernel Initialization

The selected Linux kernel is decompressed and begins execution. The first user‑space process, init (PID 1), mounts the root filesystem as defined in /etc/fstab and loads the initial RAM disk ( initrd) as a temporary root until the real root filesystem is mounted. This stage also starts essential daemons.

4. Systemd Startup

Systemd replaces the legacy SysV init system and becomes the parent of all processes. It reads /etc/systemd/system/default.target to determine the default target (runlevel). Common targets include: poweroff.target (runlevel 0): shut down the system. rescue.target (runlevel 1): start a rescue shell. multi-user.target (runlevels 2‑4): non‑graphical multi‑user mode. graphics.target (runlevel 5): graphical desktop. reboot.target (runlevel 6): reboot.

On a desktop the default target is usually graphics.target (runlevel 5); on a server it is multi-user.target (runlevel 3).

Useful commands: systemctl get-default To change the target or runlevel, use: init <runlevel-value> For example, init 3 switches to non‑graphical mode, while init 6 reboots the system. Remember to run these commands with sudo when required.