Naval Kishor Upadhyay

Posted on Sep 15

Linux Boot Process: From Power-On to Login

#linux #devops #beginners

Booting a Linux system is a chain of handoffs: firmware initializes hardware, a bootloader loads the kernel, the kernel prepares the system, and systemd brings user space to life.

High-Level Overview

1) Power-On, POST, and Firmware (BIOS/UEFI)

POST (Power-On Self-Test) verifies CPU, RAM, basic chipset, storage controllers, and peripherals.
Hardware initialization configures controllers (SATA/NVMe/USB, GPU, timers) so code can be read from disks/USB/Net.
Boot selection uses a firmware-defined order to choose a bootable device (SSD/HDD/USB/Network).

2) Primary vs Secondary Boot Loader

Primary boot loader (BIOS/MBR): ~446 bytes of code at disk sector 0; too small for menus/filesystems—just finds and loads next stage.
UEFI: loads an EFI executable (e.g., \EFI\GRUB\grubx64.efi, \EFI\systemd\systemd-bootx64.efi) from the EFI System Partition (ESP).
Secondary boot loader (e.g., GRUB, systemd-boot): shows a menu, accepts kernel parameters, loads kernel and initramfs.

Common kernel parameters (set by the bootloader):

root= (e.g., root=/dev/nvme0n1p2 or root=UUID=...) – tells the kernel where the real root filesystem is.
ro/rw – mount root initially read-only or read-write.
quiet/verbose – control console verbosity.
single or systemd.unit=rescue.target – maintenance boot modes.
init= – override PID 1 (rare, for rescue/debug).

3) Kernel Initialization (Early Userspace with initramfs)

Kernel decompression & start: kernel unpacks in RAM and begins execution.
Hardware bring-up: CPU & SMP, memory manager & page tables, device buses (PCI/USB), interrupts & timers.
Driver loading: built-in drivers + modules from initramfs to access storage, crypto, filesystems, etc.
Root filesystem discovery: initramfs scripts discover and mount the real root FS, then pivot_root/switch_root to it.
PID 1 start: kernel executes the first userspace process (normally /sbin/init → systemd).

4) systemd Takes Over (PID 1)

Unit files describe services, sockets, timers, mounts, targets (goal states).
Parallel startup honors dependencies (After=, Wants=, Requires=) to speed boot.
Targets replace SysV runlevels:
- rescue.target (single-user), multi-user.target (server/text), graphical.target (desktop).
Key tools: systemctl (control units), journalctl (logs), systemd-analyze (boot timing).

Example service dependencies:

network-online.target before services needing network (e.g., docker.service).
local-fs.target ensures local filesystems are mounted before dependent services.

5) Login Stage

Server consoles: systemd spawns agetty@ttyX.service for virtual TTYs (Ctrl+Alt+F1..F6).
Desktops: a Display Manager (GDM, SDDM, LightDM) provides GUI login and launches the session (GNOME/KDE/etc.).
After authentication, shell or desktop session starts and userland completes initialization (user services, autostart apps).

6) Common Failure Points & Recovery Hints

Firmware/Boot order: ensure the correct disk/ESP entry is first.
GRUB issues: from live media, chroot and reinstall grub, regenerate configs (grub-mkconfig).
Kernel panic (root FS): verify root=UUID=..., rebuild initramfs (e.g., dracut, update-initramfs), confirm modules for storage controller.
Emergency/Rescue: boot with systemd.unit=rescue.target, inspect logs journalctl -b -p err, disable problematic services with systemctl disable --now ....

Quick Reference: Useful Commands

Firmware/ESP: efibootmgr -v (UEFI entries), lsblk -f (filesystems & UUIDs)
Bootloader: grub-install, grub-mkconfig -o /boot/grub/grub.cfg
Kernel/initramfs: uname -a, lsinitramfs / lsinitrd, update-initramfs -u / dracut -f
systemd: systemd-analyze blame/critical-chain, systemctl list-dependencies, journalctl -b

Final Takeaway

From POST to login, Linux traverses predictable stages. Knowing where you are—firmware, bootloader, kernel, or systemd—turns a mysterious “it won’t boot” into a solvable problem with concrete tools and checkpoints.

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