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The Linux boot process is one of those topics that almost every DevOps engineer encounters early in their career and keeps encountering throughout interviews. It looks simple on the surface, but once you start peeling the layers, you realize how many components quietly work together just to bring a server to life.

Interviewers love this question not because it’s obscure, but because it exposes how well you understand system fundamentals. A server that fails to boot is not just a technical issue; it’s an availability issue, a reliability issue, and sometimes a business outage. That’s why the boot process still matters, even in a cloud-native, auto-scaling world.

Let’s walk through the Linux boot process slowly, logically, and in plain language, exactly how it happens in the real world.

Before diving into servers, it helps to think about something familiar: your phone or laptop.

When you press the power button on your phone, you don’t immediately see the home screen. First, the hardware is checked. Then the operating system loads. Then background services start. Only after all that do you get a usable device.

A Linux server follows the same philosophy, just with stricter rules and more transparency. Everything happens in a strict sequence, and if any step fails, the system stops right there.

Power on Self test

The moment you power on a machine, the CPU begins executing instructions stored in firmware. The very first job of the system is not to load Linux, it is to make sure the machine itself is healthy enough to proceed. This is where Power-On Self Test (POST) comes in.

POST checks whether essential hardware components such as memory, CPU, and storage controllers are working correctly. If RAM is faulty or the CPU is unreachable, there is no point continuing. The system halts immediately. This is why some failures never even show a Linux error message, they happen before Linux exists . At this stage, the operating system is completely irrelevant. We are still in the hardware world.

Firmware: BIOS vs UEFI 

POST is executed by firmware, which lives on the motherboard. Historically, this firmware was called BIOS (Basic Input/Output System). Modern machines use UEFI (Unified Extensible Firmware Interface). BIOS was designed in the 1980s, when disks were small and hardware was simple. Over time, hardware evolved dramatically larger disks, more memory, faster CPUs. BIOS could not scale with these changes.

One famous limitation of BIOS is that it can only address disks up to 2.2 TB, due to 32-bit sector addressing. That limitation alone made BIOS unsuitable for modern systems. UEFI solved this by introducing 64-bit addressing, graphical interfaces, faster boot times, and better extensibility.

From the boot process perspective, firmware has one primary job: Find a bootable device and transfer control to it, Everything before this point is still independent of Linux .

Locating the boot Information on Disk

Once firmware finishes its work, it looks for boot information on a storage device.

On older BIOS systems, this information lives in the Master Boot Record (MBR) the very first 512 bytes of the disk. Those bytes contain just enough code to locate and start a bootloader.

On modern UEFI systems, this role is handled by the GUID Partition Table (GPT), which removes size and partition count limitations and works seamlessly with UEFI firmware.

At this stage, the system is still not loading Linux itself. It is preparing to load the bootloader, whose only job is to load the operating system.

GRUB

In most Linux systems, the bootloader is GRUB (Grand Unified Bootloader).

GRUB sits between firmware and the Linux kernel. Its responsibility is simple but critical:

• Show available operating systems
• Allow kernel selection
• Load the selected kernel into memory

GRUB does not start services.GRUB does not mount filesystems. GRUB only loads the kernel and hands over control.

This is why a broken GRUB configuration can make a system unbootable even though Linux itself is perfectly fine .

Kernel Loading

Once GRUB loads the kernel into memory, Linux finally takes control. But the kernel has a problem.

At this point, the kernel does not yet know how to access disks, network cards, or filesystems reliably. It needs drivers, but loading all drivers permanently into the kernel would make it massive and inefficient.

This is where initramfs becomes essential.

initramfs

initramfs (Initial RAM Filesystem) is a small, temporary filesystem loaded into memory alongside the kernel. It contains only the minimal drivers and tools required to:

• Detect storage devices
• Mount the real root filesystem
• Prepare the system for full operation

The idea is elegance through minimalism. The kernel stays lean, and initramfs provides just enough functionality to bridge the gap between firmware and a fully running system .

If initramfs is missing or corrupted, the kernel cannot mount /, and the boot process fails with a kernel panic.

Switching from temporary to Real Root

Once initramfs successfully mounts the real root filesystem, the system performs a switch_root operation.

This step discards the temporary initramfs environment and transitions the system to the actual root filesystem on disk. At this point, the system finally has full access to its files, libraries, and configurations. This transition is subtle, but absolutely critical. Without it, the system cannot move forward .

systemd

The kernel starts the very first userspace process: systemd, which always runs as PID 1.

systemd becomes the parent of every other process on the system. It manages:

• Service startup
• Dependencies
• System state
• Targets (formerly runlevels)

If systemd fails to start, nothing else will .Older Linux systems used init, but modern distributions have standardized on systemd due to its powerful dependency and parallel startup model.

Targets

systemd does not start everything blindly. Instead, it brings the system to a specific target, which defines what services should be running.

For example:

• A server might boot into multi-user.target
• A desktop might boot into graphical.target

Each target represents a meaningful system state. systemd ensures all required services for that target are started in the correct order.

The Login Prompt

Once the target is reached and all services are running, the system presents a login prompt or GUI.

At this point:

• Hardware is verified
• Kernel is running
• Filesystems are mounted
• Services are active
• Networking is available

The Linux boot process is complete.

Why this question never goes away in Interviews

The Linux boot process is a goldmine for interviewers because they can pause at any step and ask:

• What happens if this fails?
• How would you debug it?
• Where would you look for logs?
• What tools would you use?

Understanding the boot process is not about memorization; it’s about thinking like a systems engineer.