MyShell - A Simple Unix Shell Implementation

A lightweight, modular Unix shell implementation written in C that supports basic command execution, built-in commands, and optional readline support for enhanced user experience.

Features

• Command Execution: Execute external programs and system commands
• Built-in Commands: Support for cd, exit, and other internal commands
• Input Parsing: Parse command-line input with argument handling
• Modular Design: Clean separation of concerns across multiple modules
• Readline Support: Enhanced input with history and line editing (optional)
• History Management: Command history functionality when readline is available

Project Structure

my_shell/
├── Makefile              # Build configuration
├── main.c                # Entry point and main loop
├── parser.c / parser.h   # Input parsing, redirection, pipes
├── builtins.c / builtins.h # Built-in commands (cd, exit, etc.)
├── executor.c / executor.h # Process execution and pipeline handling
├── prompt.c / prompt.h   # User prompt and readline integration
├── history.c / history.h # Command history management
└── README.md             # This file

Prerequisites

• GCC compiler
• Make utility
• libreadline-dev (optional, for enhanced input features)

Installing Dependencies

Ubuntu/Debian:

sudo apt-get update
sudo apt-get install build-essential libreadline-dev

macOS:

# Install Xcode command line tools
xcode-select --install
# Install readline via Homebrew
brew install readline

Building

1. Clone or download the project files
2. Navigate to the project directory
3. Compile the shell:

make

To clean build artifacts:

make clean

Usage

Starting the Shell

./myshell

Supported Commands

Built-in Commands

• exit: Quit the shell

  exit

• cd: Change directory

  cd /path/to/directory
  cd ..
  cd ~

External Commands

Execute any system command available in your PATH:

ls -la
ps aux
grep "pattern" file.txt
cat file.txt

Example Session

$ ./myshell
myshell> pwd
/home/user/project/myshell
myshell> ls
Makefile  README.md  builtins.c  builtins.h  executor.c  executor.h  history.c  history.h  main.c  myshell  parser.c  parser.h  prompt.c  prompt.h
myshell> cd ..
myshell> pwd
/home/user/project
myshell> exit
$

Development

Adding New Built-in Commands

1. Add the command logic to builtins.c
2. Add the function declaration to builtins.h
3. Update the main loop in main.c to handle the new command

Extending Functionality

The modular design makes it easy to extend:

• Parser: Modify parser.c to support new syntax features
• Executor: Update executor.c to handle new execution modes
• Built-ins: Add new commands in builtins.c

Compilation Notes

• The project automatically detects readline availability
• If readline is not available, basic functionality continues to work
• Use make install-readline for installation instructions

Contributing

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

License

This project is for educational purposes. Feel free to modify and distribute.

Troubleshooting

Common Issues

Readline errors during compilation:

• Install libreadline-dev package
• Or modify Makefile to disable readline support

Permission denied errors:

• Ensure the executable has proper permissions: chmod +x myshell

Command not found:

• Check that external commands are in your PATH
• Use absolute paths for custom executables

Future Enhancements

• Pipe support (|)
• Input/Output redirection (>, <, >>)
• Background process execution (&)
• Environment variable support
• Tab completion
• Signal handling (Ctrl+C, Ctrl+Z)
• Job control
• Scripting support

resources

Shell Program Explained