The Linux Kernel

At first, compile the Linux kernel, https://kernel.org/ and grab the latest Linux kernel.

wget https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-6.7.1.tar.xz
tar xf linux-6.7.1.tar.xz
cd linux-6.7.1.tar.xz

General preparation environment to compile the kernel...

sudo apt install build-essential libncurses-dev bison flex libssl-dev libelf-dev

To configure the kernel:

make defconfig

This will create the default config file for compiling Linux. Then run

make menuconfig

to enter a TUI to edit the config file. One thing for older devices is unchecking 64 bit kernel. Use make -j $(nproc) to compile using all the CPU cores.

# Example: Using 4 parallel jobs
make -j4

After compiling, you should get the arch/x86/boot/bzImage . If not, check the make file log.

BusyBox

Next use BusyBox to create a minimal file system. Get the source code and extract it.

wget https://busybox.net/downloads/busybox-1.36.1.tar.bz2
tar xf busybox-1.36.1.tar.bz2
cd busybox-1.36.1

Run...

make defconfig

to make a default configuration for the BusyBox. Then...

make menuconfig

to bring the TUI and edit the configs. The option which you must edit is located in Settings, and then Build static binary (no shared libs). The reason to enable this option is we do not want to compile Glibc into the Linux distro.

Use..

make -j $(nproc)

to compile BusyBox. To check if the compiled file is fine, use command...

file busybox

The file must be statically linked.

Creating The File System

Create the file system which contains BusyBox. Run make install . This will create a folder called _install and change directory to _install and you will see a hierarchy like Linux file system.

In this directory, run the following command to create the folders needed for kernel.

mkdir dev proc sys

Now create a file called init and open it with a text editor. Copy and paste the following data to it:

#!/bin/sh
mount -t devtmpfs none /dev
mount -t proc none /proc
mount -t sysfs none /sys
echo "Welcome to my Linux!"
exec /bin/sh

Then make the script executable with...

chmod +x init

Done. BusyBox made one executable which is capable of providing us a lot of Linux utilities such as sh , echo , vi and so on. With make install we made a filesystem hierarchy which contains these programs as links to BusyBox executable. Next, we make a shell script called init. This script will be ran after kernel loads. At first, it mounts dev, proc and sys directories. It then prints a welcome message and runs sh to open a shell.

You can put ANY executable as init file as long as it is statically linked. You might also want to mount dev, sys and proc directories when the program starts using mount.h header. Last thing we need to do is to create the filesystem itself... To do so, run these commands inside _install directory.

find . -print0 | cpio --null -ov --format=newc | gzip -9 >
../initramfs.cpio.gz

This will make the file initramfs.cpio.gz in upper directory.