Developing a simple Linux Distro from scratch using Busybox

Greetings, and welcome to my blog. Today, I will discuss how to create a simple Linux distribution using BusyBox. This can be done on any system, whether it is Windows, macOS, or Linux. For Windows, you need WSL, a Docker (Ubuntu) container, or a VM with a Linux distribution installed (Ubuntu or its derivatives are recommended). First, install the prerequisites:

Note: If you are using a container, ensure you run it in privileged mode.

Bash

sudo apt install bc cpio bison libssl-dev libncurses-dev libelf-dev bzip2 make 

sudo apt install automake autoconf git syslinux dosfstools xz-utils build-essential gcc wget

Once you have these dependencies installed, start by creating a directory named distro:

Bash

sudo mkdir /distro
cd /distro

After creating and changing the directory, obtain the Linux Kernel, either from git or wget:

Note: If you use git, you might clone the beta or release candidate version of the kernel.

Bash

sudo git clone --depth 1 https://github.com/torvalds/linux

OR

Bash

sudo wget https://cdn.kernel.org/pub/linux/kernel/vx.x/linux-x.x.x.tar.xz

After obtaining the kernel, extract it (if used wget to obtain archive):

Bash

sudo tar -xvf linux-x.x.x.tar.xz

Now, cd to the extracted or cloned folder:

Bash

cd linux-x.x.x   # If used wget

OR

Bash

cd linux         # If used git

After entering the Linux directory, make the menu configuration:

Bash

sudo make menuconfig

It's not necessary to change the configuration in the TUI menu, however, if you want, you may change it. Then save the configuration and start compiling:

Bash

sudo make -j <number_of_jobs_to_run_in_parallel>

If you're unsure of how many threads your CPU has, you may instead type:

Bash

sudo make -j $(nproc)

This fetches the number of threads in CPU through the nproc variable set by your host system. Now, you will get a success message saying, bzImage successfully generated or created. After you get this message, copy the bzImage as vmlinuz file to your /distro folder:

Bash

sudo cp arch/x86/boot/bzImage /distro/vmlinuz

Now, exit the Linux directory and clone the BusyBox repository:

Bash

cd ..
sudo git clone --depth 1 https://git.busybox.net/busybox

After cloning, cd to the BusyBox directory:

Bash

cd busybox

Now make the menu configuration:

Bash

sudo make menuconfig

There's a problem with the default configuration, so we need to edit the .config file:

Bash

sudo vim .config

In the .config file, change the line with CONFIG_TC = y to CONFIG_TC = n.

After changing, you may start the compilation:

Bash

sudo make -j$(nproc)

After compilation, make a directory initramfs within the /distro directory, and make install the components to the initramfs folder:

Bash

sudo mkdir /distro/initramfs
sudo make CONFIG_PREFIX=/distro/initramfs install

This will install all the components to the initramfs directory. Then, cd to the initramfs directory and delete the linuxrc file:

Bash

sudo rm linuxrc

After removing linuxrc, we need to create an init script that will initialize our system, on what to start after booting up:

Bash

sudo vim init

Vim > Bash

#!/bin/sh
/bin/sh

This script starts the shell, which we can use to type in our commands and execute stuff. Then, staying in the initramfs folder, create a cpio image of the initramfs folder:

Bash

sudo find . | cpio -o -H newc > ../initramfs.cpio

This creates a cpio initramfs image. Now, cd to the root of the distro directory, and type the following commands:

Bash

cd ..
sudo dd if=/dev/zero of=OS bs=1M count=50

The above dd command creates a file named OS, filled with zeros, written via /dev/zero, of size 50 * 1 = 50 MB (approx). Now format the created OS file with FAT filesystem using:

Bash

sudo mkfs -t fat OS

After formatting, mount the OS file to a temporary directory in /distro folder:

Bash

sudo mkdir tmp
sudo mount OS tmp

This mounts the OS file to the tmp directory. Now we can start copying the vmlinuz (bzImage) and initramfs.cpio files to the OS file:

Bash

sudo cp /distro/vmlinuz /distro/tmp/vmlinuz
sudo cp /distro/initramfs.cpio /distro/tmp/initramfs.cpio

After this, finally we install the SYSLINUX bootloader to this OS file and make it bootable. Before starting, you need to create a syslinux.cfg file to automatically load vmlinuz and initramfs.cpio, or else you need to manually type the command in the boot prompt of SYSLINUX:

Bash

sudo vim /distro/tmp/syslinux.cfg

Vim > CFG

PROMPT 0
TIMEOUT 50
DEFAULT MyOS
LABEL MyOS
MENU LABEL MyOS
KERNEL /vmlinuz
INITRD /initramfs.cpio

This is the syslinux.cfg file. After saving it, you may now install the SYSLINUX bootloader to it:

Bash

cd /distro
sudo umount tmp
sudo syslinux OS

You've created a bootable image of your own OS. Boot it using QEMU:

Bash

qemu-system-x86_64 OS

Your system will now boot; however, if you decided not to create the cfg file, you have to manually load the vmlinuz and initrd by typing in the boot prompt:

Bash

/vmlinuz -initrd=/initramfs.cpio

Congratulations! You've booted into your OS now!