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Understanding the Linux Boot Process from Power-On to Login.

Have you ever wondered what happens when you click the power button on your Linux system? When you press the power button on your Linux system, a complex sequence of events occurs in milliseconds, bringing your system to life. Whether you're a Linux System Administrator, DevOps engineer, Site Reliability engineer, or a Linux enthusiast, understanding the Linux boot process helps you to troubleshoot issues, optimize performance, and appreciate the inner workings of your system.

Here is a Simplified Overview of What Happens behind the scenes:

1. BIOS/UEFI Initialization (Firmware Stage)

What it does:
When you press the power button, the motherboard's system firmware (BIOS for older systems and UEFI for newer ones) kicks in.

Key Tasks:

  • Performs a diagnostic test known as POST(Power-On Self Test) to check hardware components such as RAM, CPU, storage devices, and peripherals.
  • Detects and initializes boot devices(SSD, HDD, USB, etc)
  • Once the hardware is verified, the firmware hands over control to the bootloader stored on the primary boot device.

Important fact: UEFI is replacing BIOS because it's faster, supports larger drivers (over 2TB), and has a graphical user interface.

2. BOOTLOADER Execution

What it does:
The firmware searches for the bootloader on the storage device and loads it into memory.
The Bootloader is a small program that loads the Linux kernel into memory. The most common Bootloader is GRUB(Grand Unified Bootloader).
Key Tasks:

  • GRUB presents a boot menu to select the OS/Kernel options or automatically load the default Linux kernel.
  • Loads the selected Linux kernel and initramfs/initrd into memory.
  • Passes control to the kernel.

Initramfs/initrd: A temporary filesystem used by the kernel to access necessary drivers before the real root filesystem is mounted.

3. KERNEL Loading

What it does:
The Linux Kernel is now in control of the system
Key Tasks:

  • Hardware Detection: The Kernel detects and initializes the hardware via built-in or loaded drivers(CPU, memory, I/O).
  • Mounts the root file/system.
  • Starts the first user space program, usually /sbin/init or systems

The Kernel also:

  • Manages memory
  • Sets up process scheduling
  • Initializes networking and system calls.

4. Init/Systemd Starts

This is the first user-space process with process ID 1.

What it does:
The Kernel starts the init process (traditionally, but in modern Linux distros, use systemd). It orchestrates the rest of the boot process.

Key Tasks:

  • Reads its initial configurations from /etc/systemd and /etc/initab in older systems.
  • Mounts the remaining file system. (eg,/home, /boot)
  • Runs startup scripts/services to prepare the system, such as starting network services and setting up the graphical environment.

5. User-space Login(TTY or GUI)

Once all services are up, the system presents you with a login interface.

  • A TTY login prompt(text based) for servers or minimal systems.
  • Graphical Login manager (like GDM, lightDM, SDDM) for desktop environment.

After a successful authentication, the login session starts.

  • For TTY login a shell (eg, bash) is launched allowing a command-line interractions.
  • For GUI a desktop environment or windows manager is started, providing a graphical interface for user-interactions. This process signifies the transition of kernel system services to user-level interactions.

Conclusion

In essence, pressing the power button triggers a chain of hardware checks, loads the kernel, sets up the system service, and finally presents you with a usable desktop environment. Despite its complexity, this seamless process ensures your Linux system is ready almost instantly after you power it on.