Project 2 - Building a shell in C

This is an individual programming project to help you practice and develop your understanding of the Process API in C. You will be building a simple toy shell that can execute commands, similar to the one you use every day.

Project Administration and Policies

Important

Due date: Tuesday, September 30th 2025, 11:59 PM

Follow all CS537 project policies

Project Workflow

Your solution code will be tested in the CS537 Docker container. If you do not already have your container setup, follow the instructions in that repository to create the proper environment. After following those instructions you should have a cs537-projects/ directory where you can clone all of the projects for this course.

Follow the suggested workflow for development of your solution.

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Project Instructions

Warning

⚠️This project will be checked for memory leaks!⚠️

• Your shell must properly free all allocated memory before exiting.
• Use tools like valgrind or gcc --sanitize=address to check for > memory leaks.

Example: valgrind --leak-check=full --show-leak-kinds=all --track-origins=yes ./wsh

Background: What is a Shell?

A shell is a command-line tool that acts as an interface between the users and the operating system. When you type commands in a terminal, you are actually interacting with a shell program (e.g., bash, zsh, fish, etc.).

The shell reads your input, interprets(parses) your commands, executes them and displays the output back to you.

Learning Objectives for this Project

In this project, you will build a simple Unix shell called wsh. The shell is the heart of the command-line interface, and thus is central to the Unix/C programming environment. Knowing how the shell itself is built is the focus of this project. Specifically, you will learn about:

• Process creation and management using system calls like fork(), exec(), and wait().
• Communication between processes using pipes.
• The shell and its internals.

Features of wsh

Your shell, wsh should support the following features. Treat these as checkpoints for your implementation as well as the requirements for your final submission. We strongly recommend doing a second pass to check if all the components are working together correctly.

Important

The grader will do a direct diff between your output/error/rc and the expected output/error/rc. To help you with this, you can use the provided preprocessor directives in wsh.h for the error messages. We have also created a function wsh_warn to help you print to stderr easily.

Examples

// Lets say you have a string variable `cmd`
char* cmd = "NonExistentCommand";
// You can do something like this to print the error message:
wsh_warn(CMD_NOT_FOUND, cmd);

// This works because we have defined CMD_NOT_FOUND in wsh.h as
#define CMD_NOT_FOUND "Command not found or not an executable: %s\n"
// We have setup the wsh_warn as a wrapper around vfprintf to print to 
// let you use string formatting with it.

Sidenote: I call it wsh_warn because even if something goes wrong with a command, the shell doesn't actually "crash" or "fail", it just prints a warning and continues with the next command.

1. Modes of Execution
2. Executing External Commands
3. Builtin Commands
4. Supporting Pipelines

At this point, your shell should be able to handle a variety of commands. We would recommend testing your shell with a variety of commands to ensure it works as expected. Try combining different features and see if your shell is handling them correctly.