What happens inside when you turn on a computer? A detailed breakdown of the 6 stages of Linux booting. Covering Dual Booting, Secure Boot, Kernel Modules, Systemd Unit Files, and troubleshooting with Journalctl.
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We press the power button, grab a coffee, and the login screen appears. It seems simple. But in those few seconds, the computer performs a miraculous transition from a hunk of dead metal to a sentient machine capable of running applications.
This process is called Booting, short for "Bootstrapping". It comes from the phrase "Pull yourself up by your bootstraps." Since software needs an OS to run, but the OS itself is software, who runs the OS? The computer has to lift itself up, stage by stage.
Let's dissect the 6 stages of the Linux boot process. Understanding this is crucial for troubleshooting system failures (like the dreaded "Kernel Panic" or "Boot Loop").
When electricity flows into the motherboard, the CPU looks at a specific memory address mapped to the ROM (Read-Only Memory). There resides the BIOS (Basic Input/Output System) or its modern successor, UEFI.
POST (Power-On Self-Test) First, it checks its own body.
Secure Boot UEFI has a feature called "Secure Boot". It checks the digital signature of the bootloader to ensure it hasn't been tampered with by malware (Rootkits). If the signature doesn't match Microsoft's or the OEM's key, booting stops.
Once the Boot Device is found, the BIOS reads the absolute first sector of that storage. This sector (512 bytes) is called the MBR (Master Boot Record).
It's tiny but critical. It contains:
In modern UEFI systems, MBR is replaced by GPT (GUID Partition Table) and a special EFI System Partition (ESP). UEFI is smarter; it can read .efi files directly from the partition instead of relying on a magic raw sector. However, the concept is the same: Handover control to the Bootloader.
The MBR code loads the full Bootloader. The standard for Linux is GRUB2 (GRand Unified Bootloader).
This is the screen where you see a menu:
Dual Booting works here. GRUB scans the disks for other Operating Systems (like Windows) and adds them to this menu. If you choose Windows, GRUB hands over control to the Windows Boot Manager. If you don't press anything, GRUB loads the default kernel image into memory.
Sometimes you lose your password.
In the GRUB menu, you can edit the boot parameters by pressing e.
If you add init=/bin/bash to the end of the line starting with linux, you bypass the normal boot process and drop directly into a Root Shell without a password.
This is called Single User Mode. It's a powerful tool for fixing systems, but also a security risk if anyone has physical access to your machine.
The Kernel is the core of the operating system.
It is usually a compressed file (like vmlinuz) located in /boot.
Once loaded into RAM, it decompresses itself and takes full control of the hardware.
dmesg output) and initializes drivers.initrd.img) is loaded into RAM./) is mounted, initramfs is discarded.The kernel is smart. It doesn't load every single driver for every possible device. It uses Modules.
Modules are pieces of code (.ko files) that can be loaded and unloaded on demand.
lsmod: List loaded modules.modprobe: Load a module.
This keeps the core kernel small and efficient.Now that the kernel is running and the disk is mounted, it needs to start "User Space" programs.
The kernel executes the first program: /sbin/init (or linked to /lib/systemd/systemd).
This process gets PID (Process ID) 1.
Historically we used System V Init (SysVinit), but now Systemd is the de-facto standard. Systemd checks its configuration (default.target) to decide what to do next.
Systemd starts launching background services (Daemons) in parallel.
It follows a target dependency tree defined in Unit Files (.service).
sysinit.target: Basic system initialization (Swap, Filesystems).multi-user.target (Runlevel 3): Network, SSH, Cron, Databases. (Text mode only).graphical.target (Runlevel 5): Starts the Display Manager (GDM, LightDM) for the GUI login screen.Unit Files
You can control these services using systemctl.
sudo systemctl start nginxsudo systemctl enable nginx (Start on boot)
The unit files live in /etc/systemd/system/ or /lib/systemd/system/.Once the Display Manager asks for your username/password and you login, a Shell session (like Bash or Zsh) is started. Congratulations! The boot is complete.
When boot fails after the kernel loads, Systemd often captures the reason.
The journalctl command is your best friend here.
journalctl -b: Show logs from the current boot.journalctl -b -1: Show logs from the previous boot (useful if it crashed).journalctl -p 3 -xb: Show only errors (priority 3) and explanations.systemd-analyze blame: Shows what took so long during boot.Common boot failures:
The boot process is a relay race of software layers, each handing the baton to the next, building up abstraction until you can move your mouse.