Minimal UNIX Shell 🐚

A modular, POSIX-compliant shell implemented in C, following the RPEL (Read-Parse-Execute-Loop) architecture. This project serves as a foundation for understanding OS-level process management, memory handling, and terminal interaction.

Architecture: The RPEL Model

The shell is designed with modularity in mind, separating concerns into individual components:

1. Read:
   • read_line.c: Handles interactive input using getline.
   • read_stream.c: Handles piped/scripted input using a character-by-character buffer.
2. Parse:
   • split_line.c: Tokenizes input into commands and arguments using strtok. Supports comments (#).
3. Execute:
   • execute_args.c: A dispatcher that checks for built-in commands before attempting to launch external programs.
   • builtins.c: Internal implementations for cd, env, help, and exit.
   • new_process.c: Creates child processes using fork(), replaces them with execvp(), and monitors status with waitpid().
4. Loop:
   • shell_interactive.c: Manages the prompt and life cycle for terminal usage.
   • shell_no_interactive.c: Manages execution flow for non-terminal input.

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Installation & Setup

This shell relies on POSIX system calls (fork, exec, wait). Follow the instructions below based on your Operating System.

🐧 Linux (Recommended)

1. Install Build Tools:

   sudo apt update
   sudo apt install build-essential

2. Compile and Run:

   make hsh
   ./hsh

🍎 macOS

1. Install Command Line Tools:

   xcode-select --install

2. Compile and Run:

   make hsh
   ./hsh

🪟 Windows

Native Windows (Command Prompt/PowerShell) does not support the fork() system call. To run this shell, you MUST use a POSIX-compatible layer.

Option A: WSL (Windows Subsystem for Linux) - Highly Recommended

1. Open PowerShell as Administrator and run: wsl --install.
2. Restart your PC and open the Ubuntu terminal.
3. Follow the Linux instructions above.

Option B: MSYS2 / Cygwin

1. Install MSYS2.
2. Open the MSYS2 MINGW64 terminal.
3. Install the toolchain: pacman -S mingw-w64-x86_64-toolchain make.
4. Compile using make hsh.

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Usage

Interactive Mode

Run the shell directly to see the prompt:

./hsh
simple_prompt$ ls -l
simple_prompt$ cd ..
simple_prompt$ help

Non-Interactive Mode

Pipe commands into the executable:

echo "ls -l" | ./hsh

Cleanup

To remove compiled object files and the executable:

make clean

Limitations

• Environment: Does not run natively on Windows cmd.exe.
• Features: Does not currently support piping (|), redirection (>, <), or globbing (*).
• Expansion: Environment variable expansion (e.g., $HOME) is handled by the system's execvp but not by the parser itself for argument building.