README.md

Linux Mini Shell

📌 Project Overview

A lightweight, POSIX-compliant UNIX shell written in C from scratch. This project demonstrates core operating system concepts such as process creation, inter-process communication (IPC), file descriptor manipulation, and robust signal handling. It closely mirrors the behavior of standard shells like bash or sh while remaining modular and heavily documented.

🎥 Demo

✨ Features

• Execution: Runs standard foreground and background (&) processes.
• Pipelining: Supports unlimited chained pipelines (e.g., ls -l | grep src | wc -l).
• I/O Redirection: Handles input (<), output (>), and append (>>) file redirection.
• Built-in Commands: cd, exit, and history.
• Signal Handling: Gracefully handles SIGINT (Ctrl+C) without crashing, and automatically reaps background zombie processes via SIGCHLD.

🏗️ Architecture Overview

The shell is built with a clean, modular architecture separating concerns across dedicated C modules:

• shell.c: The main REPL (Read-Eval-Print Loop).
• parser.c: Parses raw string input into structured Command data objects.
• execute.c: Handles single-command fork() and execvp() logic.
• pipe.c: Orchestrates N-1 pipes for N commands, chaining standard I/O streams.
• redirect.c: Manages file opens and dup2() descriptor mapping.
• signals.c: Installs isolated signal handlers and manages state restoration for child processes.
• builtin.c & history.c: Implements internal commands and memory-managed command history.

📂 Folder Structure

linux-mini-shell/
├── Makefile
├── README.md
└── src/
    ├── builtin.c / builtin.h
    ├── command.c / command.h
    ├── execute.c / execute.h
    ├── history.c / history.h
    ├── main.c
    ├── parser.c / parser.h
    ├── pipe.c / pipe.h
    ├── redirect.c / redirect.h
    ├── shell.c / shell.h
    └── signals.c / signals.h

🛠️ System Calls Used

This project extensively utilizes UNIX system calls to interface directly with the kernel:

• Process Management: fork(), execvp(), waitpid()
• IPC & Pipes: pipe(), dup2()
• File I/O: open(), close(), read(), write()
• Signals: signal(), sigaction(), kill()
• Environment: chdir()

🚀 Build Instructions

To build and run the project locally, ensure you have gcc and make installed.

# Clone the repository
git clone https://github.com/HarshEvolves/linux-mini-shell.git
cd linux-mini-shell

# Build the project
make

# Run the shell
make run
# OR manually run: ./build/minishell

💡 Usage Examples

Standard Execution & Backgrounding

myshell$ ls -la
myshell$ sleep 10 &
[12345]

Unlimited Pipelining

myshell$ cat README.md | grep "C" | wc -l

I/O Redirection

myshell$ echo "Hello, World!" > output.txt
myshell$ cat < output.txt >> history.log

Signal Resilience

myshell$ sleep 10
^C  # Kills the sleeping child, but returns cleanly to the shell prompt
myshell$ 

🧠 Key OS Concepts Demonstrated

1. Process Forking & Execution: Replacing a child process image with a new executable while preserving parent state.
2. Zombie Process Reaping: Using non-blocking waitpid(WNOHANG) inside a SIGCHLD handler to prevent resource leaks.
3. File Descriptor Chaining: Routing stdout of one process into the stdin of another using kernel pipes and dup2().
4. Interruption Safety: Safely handling EINTR errors from blocking calls like getline() and waitpid() when signals are caught.

🚀 Future Improvements

• Logical Operators: Support for && and ||.
• Job Control: Implementation of fg, bg, and jobs commands using SIGTSTP.
• Quoting & Escaping: Support for parsing strings containing spaces inside double/single quotes.
• Environment Variables: Support for expanding $VAR variables and export.

🎓 Learning Outcomes

Building this shell provided deep, hands-on experience with systems programming in C, kernel-level process management, memory safety (avoiding leaks across forks), and designing robust architectures that can safely handle asynchronous OS signals.