Custom C Shell

A custom Unix-like shell implementation written in C++ that provides basic shell functionality with command execution, piping, I/O redirection, and process management.

Features

Built-in Commands

• cd [directory] - Change directory

  • cd or cd ~ - Go to home directory
  • cd - - Go to previous directory
  • cd <path> - Change to specified directory

• pwd - Print working directory

• echo [args] - Display text to stdout

  • Supports quoted strings with proper spacing

• ls [options] [directories] - List directory contents

  • -a - Show hidden files
  • -l - Long listing format
  • -la or -al - Combination of both flags

• search <filename> - Search for a file recursively

  • Returns True if found, False otherwise
  • Uses breadth-first search

• history [n] - Display command history

  • history - Shows last 10 commands
  • history <n> - Shows last n commands
  • Maintains up to 20 commands
  • Stores history in history.txt

• pinfo [pid] - Display process information

  • pinfo - Show current process info
  • pinfo <pid> - Show specific process info
  • Displays: PID, status, memory usage, and executable path

• fg <pid> - Bring background process to foreground

  • Sends SIGCONT signal to the specified process

• exit - Exit the shell

Advanced Features

Command Chaining

• Semicolon (;) - Execute multiple commands sequentially

  cd /tmp; ls; pwd

Piping

• Pipe (|) - Chain commands together

  ls -l | grep txt | wc -l

Background Processes

• Ampersand (&) - Run commands in background

  sleep 100 &

I/O Redirection

• Input (<) - Redirect input from file

  sort < input.txt

• Output (>) - Redirect output to file

  ls > output.txt

System Commands

Any command not recognized as a built-in will be executed as a system command using execvp().

Technical Details

Architecture

The shell consists of three main components:

1. main.cpp - Main shell loop and command dispatcher
2. utils.cpp - Implementation of all built-in commands and utilities
3. headers.h - Header file with includes and function declarations

Key Components

Command Processing

• processCommand() - Handles individual command execution
• parseCommandTree() - Parses piped commands into linked list structure
• executeCommandTree() - Executes piped commands with proper file descriptor management

Process Management

• Uses fork() and execvp() for command execution
• Implements proper process groups with setsid()
• Handles foreground and background processes
• Tracks process status (Running, Sleeping, Zombie, Stopped)

History Management

• appendToHistory() - Adds commands to history (max 20)
• printHistory() - Displays command history
• Avoids duplicate consecutive entries
• Persistent storage in history.txt

File Operations

• searchFile() - BFS-based file search
• listDirectory() - Directory listing with detailed info
• Supports I/O redirection with file descriptors

Building and Running

Prerequisites

• C++ compiler with C++17 support (g++)
• macOS (uses macOS-specific libraries like libproc.h)
• Make build system

Compilation

make

This will compile the source files and create the executable myprogram.

Running

./myprogram

Cleaning Build Files

make clean

Project Structure

custom-C-shell/
├── headers.h       # Header file with declarations and includes
├── main.cpp        # Main shell loop and command parsing
├── utils.cpp       # Implementation of built-in commands
├── Makefile        # Build configuration
├── history.txt     # Command history storage
└── README.md       # This file

Example Usage

# Basic navigation
priyanshu@MacBook:~> pwd
/Users/priyanshusharma/Documents/AOS_Assignment2

# List files with details
priyanshu@MacBook:~> ls -la

# Chain commands
priyanshu@MacBook:~> cd /tmp; ls; pwd

# Pipe commands
priyanshu@MacBook:~> ls -l | grep cpp

# Background process
priyanshu@MacBook:~> sleep 30 &
Started background process with PID: 12345

# Process information
priyanshu@MacBook:~> pinfo 12345

# Command history
priyanshu@MacBook:~> history 5

# File search
priyanshu@MacBook:~> search main.cpp
True

# I/O redirection
priyanshu@MacBook:~> ls > files.txt
priyanshu@MacBook:~> grep cpp < files.txt

Limitations

• Platform-specific: Currently designed for macOS (uses libproc.h, proc_pidinfo, etc.)
• Hardcoded paths: Some paths are hardcoded (e.g., home directory path in getRelativePath())
• Limited error handling: Some edge cases may not be fully handled
• No job control: Limited support for complex job control features

Future Enhancements

• Cross-platform support (Linux, Windows)
• Tab completion
• Command history navigation with arrow keys
• More robust signal handling
• Configuration file support
• Aliases and shell functions
• Improved job control (bg, jobs commands)
• Scripting support

Author

Priyanshu Sharma

License

This project is open source and available for educational purposes.