Mini Shell

A lightweight, feature-rich Unix shell implementation written in C. This project demonstrates core operating system concepts including process management, I/O redirection, signal handling, and command parsing.

Features

Core Functionality

• Command Execution: Execute any system command available in your PATH
• Built-in Commands: Essential shell commands implemented natively
• Command History: Keep track of previously executed commands
• I/O Redirection: Support for input/output redirection operators
• Background Processes: Run commands in the background
• Signal Handling: Proper handling of Ctrl+C, Ctrl+Z, and other signals
• Colorful UI: Enhanced user experience with colored output

Built-in Commands

Command	Description	Usage
cd	Change directory	cd [directory]
pwd	Print working directory	pwd
echo	Display a line of text	echo [args...]
export	Set environment variable	export VAR=value
history	Show command history	history
clear	Clear the screen	clear
help	Display help information	help
exit	Exit the shell	exit [code]

Redirection Operators

# Output redirection
command > file.txt          # Write output to file (overwrite)
command >> file.txt         # Append output to file

# Input redirection
command < input.txt         # Read input from file

# Background execution
command &                   # Run command in background

Project Structure

mini-shell/
├── src/
│   ├── main.c          # Main shell loop and initialization
│   ├── parser.c        # Command parsing and tokenization
│   ├── executor.c      # Command execution and process management
│   ├── builtins.c      # Built-in command implementations
│   ├── history.c       # Command history management
│   └── utils.c         # Utility functions and signal handlers
├── include/
│   └── shell.h         # Header file with structures and prototypes
├── tests/              # Test files (for future development)
├── docs/               # Documentation (for future development)
├── examples/           # Example scripts
├── Makefile            # Build configuration
└── README.md           # This file

Building the Project

Prerequisites

• GCC compiler (or any C11 compatible compiler)
• Make build system
• POSIX-compliant operating system (Linux, macOS, WSL)

Compilation

# Build the project
make

# Build with debug symbols
make debug

# Build optimized release version
make release

# Clean build files
make clean

# Rebuild from scratch
make rebuild

Running

# Build and run
make run

# Or run directly
./bin/mini-shell

Usage Examples

Basic Commands

# Change directory
mini-shell$ cd /home/user/documents

# Print current directory
mini-shell$ pwd
/home/user/documents

# List files
mini-shell$ ls -la

# Echo text
mini-shell$ echo Hello, World!
Hello, World!

Redirection

# Redirect output to file
mini-shell$ ls -la > files.txt

# Append to file
mini-shell$ echo "New line" >> files.txt

# Read from file
mini-shell$ wc -l < files.txt

Background Processes

# Run long-running command in background
mini-shell$ sleep 60 &
[Process 12345 running in background]

Environment Variables

# Set environment variable
mini-shell$ export MY_VAR=hello

# Use environment variable
mini-shell$ echo $MY_VAR
hello

History

# View command history
mini-shell$ history
  1  ls -la
  2  cd /home/user
  3  pwd
  4  echo Hello

Development

Code Style

• Follow standard C coding conventions
• Use meaningful variable and function names
• Add comments for complex logic
• Keep functions focused and modular

Memory Management

• All allocated memory is properly freed
• No memory leaks (verified with valgrind)
• Proper error handling for allocation failures

Adding New Features

To add a new built-in command:

1. Add function prototype in include/shell.h
2. Implement the function in src/builtins.c
3. Add command name to is_builtin() function
4. Add case in execute_builtin() function
5. Update help text in builtin_help()

Testing

# Run with valgrind for memory leak detection
make valgrind

# Run static analysis
make check

# Format code
make format

Installation

# Install to /usr/local/bin (requires sudo)
make install

# Now you can run from anywhere
mini-shell

# Uninstall
make uninstall

Technical Details

Process Management

• Uses fork() to create child processes
• execvp() for command execution
• waitpid() for process synchronization
• Proper handling of zombie processes

Signal Handling

• SIGINT (Ctrl+C): Interrupt current foreground process
• SIGCHLD: Reap zombie processes automatically
• SIGTSTP (Ctrl+Z): Ignored (can be implemented for job control)
• SIGQUIT (Ctrl+\): Ignored

Input/Output Redirection

• Uses file descriptors and dup2() for redirection
• Supports both input (<) and output (>, >>) redirection
• Proper error handling for file operations

Command Parsing

• Tokenization using strtok_r()
• Support for multiple arguments
• Handles special characters (>, <, &, |)
• Whitespace trimming and empty line detection

Future Enhancements

Planned features for future versions:

• Pipe support (command1 | command2)
• Job control (fg, bg, jobs commands)
• Command-line editing with arrow keys
• Tab completion
• Command aliases
• Shell scripting support
• Globbing (wildcards: *, ?)
• Conditional execution (&&, ||)
• Command substitution
• Configuration file (~/.minishellrc)

Learning Objectives

This project helps understand:

1. Operating System Concepts

   • Process creation and management
   • Inter-process communication
   • Signal handling
   • File descriptors and I/O

2. C Programming

   • Memory management
   • String manipulation
   • Structures and pointers
   • Modular code organization

3. System Programming

   • POSIX system calls
   • Error handling
   • Signal safety
   • Resource management

Common Issues and Solutions

Issue: Permission Denied

# Make sure the binary is executable
chmod +x bin/mini-shell

Issue: Command Not Found

# Verify the command exists in your PATH
which <command>

# Or use absolute path
/bin/ls

Issue: Segmentation Fault

# Run with valgrind to debug
make valgrind

Contributing

Contributions are welcome! Please follow these guidelines:

1. Fork the repository
2. Create a feature branch
3. Write clear commit messages
4. Test your changes thoroughly
5. Submit a pull request

License

This project is licensed under the MIT License - see the LICENSE file for details.

Author

Created as an educational project to demonstrate C programming and operating system concepts.

Acknowledgments

• Inspired by traditional Unix shells (bash, sh, zsh)
• Built with knowledge from operating systems textbooks and courses
• Thanks to the open-source community for resources and inspiration

Resources

• Advanced Programming in the UNIX Environment
• The Linux Programming Interface
• GNU C Library Documentation
• POSIX Standards

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Happy Coding! 🚀