Bare_minimum_EmbeddedLinux.md

// Booting a bare minimum linux using QEMU

Command to install Qemu

$ sudo apt-get install qemu-system-x86

Utilities

Command to install build essentials

$ sudo apt-get install build-essential

$ sudo apt-get install lzop flex bison libncurses-dev

Download Linux Kernel and BusyBox

Create workspace directory and cd to workspace directory

$ mkdir ~/workspace_kernel_mini && cd ~/workspace_kernel_mini ( && or ;; for running both the commands)

Downloading latest Linux Kernel

The official Linux Kernel Archives lists the current stable release as 6.15.5, published on July 6, 2025. You can download the tarball from: linux-6.15.5.tar.gz (stable release) $ wget https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-6.15.5.tar.gz

Downloading latest Busybox -- the latest official BusyBox release is version 1.37.0, which was released on September 27, 2024

$ wget https://busybox.net/downloads/busybox-1.37.0.tar.bz2

Creating Bare minimum Linux

$ cd ~/workspace_kernel_mini

extracting the kernel source

$ tar -xvf /linux-6.15.5.tar.gz

change directory to extracted linux source directory

$ cd linux-6.15.5/

Configure smallest possible kernel

$ make allnoconfig

"This will create .config file setting values to 'n' as much as possible. For more refer Documentation/admin-guide/README.rst at kernel.org"

Customization

$ make menuconfig

This will open configuration options. Now set following options … Option 1: Host name General setup > Default hostname “bare-mini-linux-kernel"

Option 2: Enable 64 bit support 64-bit Kernel

Option 3: Enable support for RAM disk General Setup > Initial RAM filesystem and RAM disk (initramfs/initrd) support

Option 4: Enable printk support General Setup > Configure standard kernel features (expert users) > Enable support for printk

Option 5: Ensure Gzip Kernel compression General Setup >kernel compression mode (Gzip)

Option 6: ELF binary and script Executable file formats > Kernel support for ELF binaries Executable file formats > Kernel support for scripts starting with #!

Option 7: Enable devtmpfs Device Driver > Generic Driver Options > Maintain a devtmpfs filesystem to mount at /dev Device Driver > Generic Driver Options > Automount devtmpfs at /dev, after the kernel mounted the rootfs

Option 8: Enable TTY Device Driver > Character devices > Enable TTY

Option 9: Enable Serial Drivers Device Driver > Character devices > Serial Drivers > 8250/16550 and compatible serial support Device Driver > Character devices > Serial Drivers > Console on 8250/16550 and compatible serial port

Option 10: Pseudo filesystems File systems > Pseudo filesystems > /proc file system support File systems > Pseudo filesystems > /sysfs file system support

Now, save and close configuration window. This will save .config file.

We can review .config file …

$ gedit bare-mini-linux-kernel.config

Building Linux Kernel

$ make -j4 ( -j4 runs 4 threads in 4 cores of the processor to speed up the process of compilation)

To check cores in your PC $ nproc

Enter to boot directory in linux kernel

$ /home/lokesh/workspace_kernel_mini/linux-6.15.5/arch/x86/boot (will take a while)

Depending on the internet speed and PC configuration, this will take some time. Once build is complete, you can verify presence of kernel.

Path "$pwd"

example : ~/workspace_kernel_mini/linux-6.15.5/arch/x86/boot $ ls -sh bzImage 1.6M bzImage

Now we have Linux kernel available. Next is creating initram disk.

Creating Initramfs

change directory to workspace directory

$ cd ~/workspace_kernel_mini

extracting the Busybox source tree

$ tar -xvf busybox-1.37.0.tar.bz2

change directory to busybox

$ cd busybox-1.37.0/

Customize busybox

$ make menuconfig

This will start configuration menu for BusyBox. We need only one setting. Settings > Build static binary (no shared libs)

Now, exit and save. It is time to build busybox.

Build

$ make -j4

Install

$ make install This will install binaries in “./_install” directory

change directory to workspace directory

$ cd ~/workspace_kernel_mini

creating mini_linux directory and cd to mini_linux

$ mkdir mini_linux && cd mini_linux

its time to create required directory. We need etc, proc, sys and dev directory.

$ mkdir -p etc proc sys dev

Coping all busybox installed files to ~/workspace_kernel_mini/mini_linux

$ cp -a ~/workspace_kernel_mini/busybox-1.37.0/_install/* .

Create init script in “mini_linux” directory. This is the content of init script. /** #!/bin/sh mount -t proc none /proc mount -t sysfs none /sys cat <<! boot took $(cut -d' ' -f1 /proc/uptime) seconds exec /bin/sh **/

check the init file

$ cat init

It is time to make init file executable. Give executable permission to init file

$ chmod +x init

Creating initramfs as cpio archieve

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

Note : Description of “find . -print0 | cpio --null -ov --format=newc | gzip -9 > initramfs.cpio.gz”

find . -print 0 : this will create a list of all files. print the full file name on the standard output, followed by a null character cpio --null -ov --format=newc: copy file to archive, use newc (SVR4) archive format , -o = create , -v verbosly list all files processed gzip -9 > initramfs.cpio.gz : creating gzip file, with compression speed 9. initramfs.cpio.gz should be created.

Booting Mini Linux in QEMU

Now we have, Linux kernel ~/workspace_kernel_mini/linux-6.15.5/arch/x86/boot/bzImage

And ramfs .. ~/workspace_kernel_mini/mini_linux/initramfs.cpio.gz

it is time to start QEMU and booting our mini Linux.

$ qemu-system-x86_64
-kernel ~/workspace_kernel_mini/linux-6.15.5/arch/x86/boot/bzImage
-initrd ~/workspace_kernel_mini/mini_linux/initramfs.cpio.gz
-append "init=/bin/sh console=ttyS0" -nographic

Command

Description

qemu-system-x86_64

Command to start QEMU for system.

-kernel ~/workspace_kernel_mini/Linux -6.15.5/arch/x8664/boot/bzImage \

“-kernel” option to provide path of bzImage.

-initrd ~/workspace_kernel_mini/mini_linux/initramfs.cpio.gz \

“-initrd” option to provide initramfs file.

-append "init=/bin/sh console=ttyS0"

“-append” provide command line argument for Linux kernel.

-nographic

We don’t want to use graphics.

Review of bare minimum Linux Kernel running in QEMU

Now we can try multiple commands to review our bare minimum Linux. $ uname -a

$ top

top command display all the process running in Linux. You can see we have /bin/sh which is bourne shell . Process ID (PID) of bourne shell is 1. and second process is kthreadd, process Id is 2. kthreadd is the kernel thread started after init and all kernel threads are descendant of kthreadd.

$ cat /proc/cmdline

This command shows command line argument give to the kernel.

$ cat /proc/cpuinfo

you can review parameter related to CPU. Model name is QEMU Virtual CPU

$ dmesg

This command shows all kernel messages which were routed to ttyS0. You should able to find following information in kernel messages.