How Linux Is Booted on Ozbem

Bootloader

info@ozbem.com (ebeem) — Tue, 23 Jun 2026 00:00:00 +0000

Introduction

We discussed earlier the objective of the firmware which was to test the hardware and run the bootloader. The bootloader is a small program that has one objective, to load the kernel properly in RAM hand control to it.

GRUB (GRand Unified Bootloader) is one of the most popular bootloaders available for GNU/Linux, other options like systemd-boot also exist which are more lightweight.

Objective

The bootloader will perform multiple steps until it hands controls to the kernel.

Loading Kernel & Initramfs

The Linux kernel vmlinuz is the core of kernel. It's small and contains only essential components. Other modules like drivers and filesystem support are loaded on demand and their extension is .ko.

Initializing RAM

The initramfs (Initial RAM Filesystem) contains the needed modules and drivers required in order to allow the kernel to read your filesystem. This may look a bit confusing as a very valid question will be raised here, why these needed drivers aren't just part of the kernel itself? and the answer to that is, each machine will require different drivers to boot, disk types and encryption applied to them will make these drivers different. In order to support all of these machines, all the drivers will be loaded which will make the kernel very large. The solution is that each machine will actually build the initramfs file during the installation process which will contain exactly the drivers it needs to boot. You will notice this file also being generated every time you update your kernel.

Customizing the Bootloader

GRUB is highly customizable, it allows you to reorder the list of operating system to boot, the timeout before selecting the default system to boot, and the parameters to pass to the kernel like recovery mode.

All of these parameters and more can be found in the file /etc/default/grub. This file has comments explaining each parameter and you can find more in GRUB Manual - Configuration.

It's also important to highlight that making changes to this file will not take effect until you regenerate the grub configuration file which is may different based on your distro.

Debian / Ubuntu:

sudo update-grub

Arch Linux / Manjaro:

sudo grub-mkconfig -o /boot/grub/grub.cfg

This process of updating grub via grub-mkconfig (Debian actually calls this command if you use update-grub) is going to produce a new configuration file in the specified path after the -o parameter which is /boot/grub/grub.cfg in the case of the Arch Linux example. The new configuration file is going to pull your configuration from /etc/default/grub in addition to all files in the directory /etc/grub.d/ in order by name. In addition to pulling your final configuration file, it scans your disks for other operating systems like Linux or Windows and adds them to your boot menu in the final configuration file.

Themes and Visuals

There are many available themes that can make your bootloader look way more interesting. The manual explains in detail how to make your own theme in GRUB Manual - Themes.

You can find a nice collection of GRUB themes available on this Github Repository. Below is a screenshot showcasing one of those creative themes.

Firmware (BIOS and UEFI)

info@ozbem.com (ebeem) — Sat, 20 Jun 2026 00:00:00 +0000

Introduction

In this series, I will try to explain how your system (specifically Linux) is booted from the moment you press that power button until you see your desktop environment. I will also highlight the different software available for each task/step and what customization you can apply.

The firmware is the first software to be loaded once you power on your machine, this software exists in your motherboard itself and it usually is proprietary and hard to replace as the motherboard manufacturers don't share the internals of their chips.

Objective

The firmware has two objectives: to diagnose hardware and to locate and run the bootloader.

Power-On Self Test (POST)

The firmware performs the Power-On Self Test (POST) which tests and initializes hardware components like CPU and RAM, it will report a failure if something goes wrong in this step by displaying an error message, digit codes, or producing beeping sounds.

Running the Bootloader

The firmware will have to locate and run another specialized program called the bootloader. This other program will take care of launching the operating system.

BIOS vs UEFI

You may hear about these terms quite often in firmware. Computers used the Basic Input/Output System (BIOS) in the early days. Today, almost all modern computers use the Unified Unified Extensible Firmware Interface (UEFI). The advantages you will get from using UEFI include:

Unlimited drive sizes, BIOS is limited to 2.2 TB
Modern interface with high resolution and mouse support, BIOS is keyboard-only and has a retro blue text-based interface.
Fast booting as it initializes components in parallel, BIOS does it one component at a time.

Upgrading Firmware

Since the firmware is actually a piece of software, regular updates are released for both BIOS and UEFI. These updates usually are to patch security vulnerabilities, fix bugs, or add hardware support. This process is usually called flashing the BIOS and the instructions to perform it varies based on the motherboard.

Warning: Flashing your firmware carries a small amount of risk and it may "brick" your motherboard if you flash incompatible firmware or power goes down during the process. Always ensure your system has stable power and you have identified the correct firmware supply before updating.

Customizing Firmware

Many UEFI interfaces nowadays offer built-in customization options such as boot order, hardware virtualization, and interface theme. However, to really customize the firmware, you must use an open-source one which is usually not possible.

Some projects like Coreboot and Libreboot try to replace the proprietary, closed-source firmware shipped by motherboard manufacturers. Unfortunately, installing these alternatives requires hardware compatibility and technical expertise. They also may "brick" your motherboard if not done right.

The benefits obtained from installing these custom firmware include: much faster boot times because they strip out hidden unwanted code and also give users full transparent control.