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Shadow OS

Shadow OS is a minimal experimental operating system written in C programming language and x86 Assembly. It demonstrates how a simple operating system boots, loads a kernel, and provides a basic command-line interface.

The project is designed for **learning operating system internals**, including bootloaders, kernel compilation, disk images, and virtualization.

The OS can run inside the QEMU emulator or Oracle VM VirtualBox.

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Project Overview

Shadow OS implements the fundamental parts of an operating system:

• Bootloader initialization • Kernel loading • Command-line interface • File operations simulation • Basic system commands

The system boots from a GRUB-based ISO image and runs inside an emulator.

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Screenshot

Example of Shadow OS running inside QEMU

The OS provides a terminal-like interface with commands such as:

clear
pwd
echo
create
write
read
remove
list
time
date
help
exit

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Features

Bootable Operating System

The OS boots from a bootable ISO image.

Command Line Interface

Users interact with the system through commands.

File System Simulation

The OS simulates file creation, reading, writing, and deletion.

Kernel Execution

The kernel handles command execution and system behavior.

Virtualization Support

Shadow OS can run inside:

• QEMU • VirtualBox

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Project Architecture

The system architecture consists of the following layers:

Hardware / Emulator
        ↓
Bootloader (Assembly)
        ↓
Kernel (C)
        ↓
Command Interpreter
        ↓
User Commands

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Directory Structure

ShadowOS
│
├── boot.asm        # Bootloader code (Assembly)
├── kernel.c        # Kernel implementation
├── linker.ld       # Kernel linker script
├── boot.o          # Bootloader object file
├── kernel.o        # Kernel object file
├── kernel.bin      # Final kernel binary
├── iso/            # Bootable ISO structure
│   └── boot/
│       └── grub/
│           └── grub.cfg
└── myos.iso        # Bootable ISO image

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Command System

Shadow OS provides several built-in commands.

Command	Description
clear	Clears the screen
pwd	Prints working directory
echo	Displays a message
create	Creates a new file
write	Writes content to a file
read	Reads content from a file
remove	Deletes a file
list	Lists files
time	Shows current time
date	Shows system date
help	Displays command list
exit	Aborts the system

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Example Usage

Display help

$ help

Output:

Available commands:
clear   - Clear the screen
pwd     - Print working directory
echo    - Display a message
create  - Create a new file
write   - Write content to a file
read    - Read content from a file
remove  - Delete a file
list    - List all files
time    - Get time
date    - Get date
help    - Show this help
exit    - System Abort

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Development Environment

Shadow OS is developed on Ubuntu using common low-level programming tools.

Required tools include:

• NASM assembler • GCC cross compiler • GRUB bootloader tools • QEMU emulator

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Installing Required Tools

Install the required packages:

sudo apt install build-essential nasm xorriso grub-pc-bin
sudo apt install qemu-kvm -y
sudo apt install gcc-i686-linux-gnu binutils-i686-linux-gnu
sudo apt install grub-pc-bin xorriso
sudo apt install mtools

Verify the cross compiler:

i686-linux-gnu-gcc --version
i686-linux-gnu-ld --version

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Create Workspace

mkdir -p myos/{boot,iso}
cd myos

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Assemble Bootloader

Compile the assembly bootloader.

nasm -f elf32 boot.asm -o boot.o

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Compile Kernel

Compile the kernel using the cross compiler.

i686-linux-gnu-gcc -ffreestanding -m32 -c kernel.c -o kernel.o

Explanation:

• -ffreestanding – compile without standard libraries • -m32 – target 32-bit architecture

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Link the Kernel

Link bootloader and kernel together.

i686-linux-gnu-ld -T linker.ld -o kernel.bin kernel.o boot.o

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Create Bootable ISO

Create the directory structure:

mkdir -p iso/boot/grub

Copy kernel:

cp kernel.bin iso/boot/kernel.bin

Create GRUB configuration:

nano iso/boot/grub/grub.cfg

Add:

set timeout=0
set default=0

menuentry "MyOS" {
    multiboot /boot/kernel.bin
    boot
}

Create ISO:

grub-mkrescue -o myos.iso iso

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Create Persistent Disk Image

Create a virtual disk for storage.

qemu-img create -f raw persistent.img 50M

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Run in QEMU

Run the OS using QEMU.

qemu-system-i386 -cdrom myos.iso -drive file=persistent.img,format=raw -m 512M

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Running in VirtualBox

You can also boot Shadow OS using Oracle VM VirtualBox.

Step 1 Create VM

Name: MyOS Type: Other Version: Other/Unknown

Step 2 Configure Hardware

Memory: 128MB minimum

Hard Disk: Do not add disk

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Step 3 Attach ISO

Settings → Storage → IDE Controller

Attach:

myos.iso

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Step 4 Boot

Start the virtual machine.

If everything is correct, Shadow OS will boot.

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VirtualBox Storage Controllers

Use Case	Recommended Controller
Booting ISO	IDE
Modern OS installation	SATA
High performance disks	NVMe

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Educational Concepts

This project demonstrates several operating system concepts:

• Bootloaders • Kernel development • Cross compilation • Assembly programming • GRUB boot process • OS virtualization

It is a great starting point for learning operating system development.

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Future Improvements

Possible future improvements:

• Real file system support • Keyboard drivers • Memory management • Process scheduling • Interrupt handling • Graphical interface • Multitasking

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Author

Abinesh N

GitHub https://github.com/Abineshabee

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LICENSE

This project is licensed under the GNU General Public License v3.0 - see the LICENSE file for details.