Koa Shell

Koa Shell is a simple, humble command-line interpreter (shell) written entirely in C. I created this project as a personal endeavor and as my final project for my Operating Systems course during my second year of studying Computer Science.

About the Project

The goal of this project was to understand the fundamental concepts behind how an operating system interacts with the user and manages processes. By building a shell from scratch, I was able to peek under the hood of programs I use every day as a developer.

This shell implements a classic REPL (Read-Evaluate-Print Loop) structure:

1. Prompts the user for input with -> .
2. Reads the line of input from standard input.
3. Parses and splits the line into appropriate commands and arguments.
4. Executes the commands either natively (if they are built-in) or by creating new processes.

Features

• Built-in commands:
  • cd: Change the current working directory.
  • help: Display a simple help snippet about the shell.
  • exit: Terminate the shell loop and exit the program.
• External commands: Executes standard system programs (like ls, pwd, or echo) by relying on the OS's environment path.
• Dynamic parsing: The shell allocates memory dynamically for parsing input strings and argument arrays, expanding the buffer sizes as necessary while the user types.

What I Learned

Throughout the development of Koa Shell, I gained practical experience with several core C programming and operating system concepts:

• Process Management: I learned how to deal with process creation and termination using system calls. Specifically, using fork() to create a new child process, execvp() to replace the child process's image with a new executable, and waitpid() to make the parent process wait for the child's execution to complete.
• Memory Management: Parsing the command line input required dynamic memory allocation using malloc, realloc, and getline. This reinforced my understanding of pointers, arrays of strings, and importantly, preventing memory leaks by properly using free().
• System Calls vs Library Functions: I gained a better understanding of the boundary between user space and kernel space, specifically differentiating when to use standard C library functions versus making direct system calls.
• String Manipulation in C: Working with strtok() for parsing and tokenizing user input based on specific delimiters taught me how C handles strings and char arrays at a low level.
• Modular Code Design: I structured the project around separating the underlying concerns, keeping reading logic, command execution, and built-ins in their respective areas while using a unity build approach to combine them.

Compilation and Usage

This project uses a unity build system; all dependencies are #included directly in main.c for simplicity.

1. Ensure you have a C compiler like gcc installed on your system.
2. Compile the shell by running:

   gcc src/main.c -o myshell

3. Run the compiled executable:

   ./myshell

Building Koa Shell started as an academic requirement but quickly became a deeply satisfying puzzle that helped solidify my understanding of operating systems theory. I'm very proud of having my own custom shell running natively.