Shell Interpreter

A custom Unix shell implementation written in C++ with advanced features including command parsing, I/O redirection, pipes, background processes, and built-in commands.

Features

Core Shell Functionality

• Command Execution: Execute system commands with full argument support
• Pipes: Chain commands using pipes (|)
• I/O Redirection:
  • Output redirection (>, >>)
  • Input redirection (<)
  • Error redirection (2>, 2>>)
  • Combined output and error redirection (>&, >>&)
• Background Processes: Run commands in background using &
• Process Substitution: Advanced process substitution support

Built-in Commands

• cd [directory] - Change directory with support for relative/absolute paths and ~ expansion
• setenv VARIABLE VALUE - Set environment variables
• unsetenv VARIABLE - Remove environment variables
• printenv [VARIABLE] - Display environment variables
• source FILE - Execute commands from a file
• exit - Exit the shell gracefully

Advanced Features

• Wildcard Expansion: Support for * and ? wildcards in filenames
• Environment Variable Expansion:
  • $VARIABLE - Standard variable expansion
  • $_ - Last argument of previous command
  • $? - Exit status of last command
  • $$ - Process ID of shell
  • $! - Process ID of last background command
  • $SHELL - Path to current shell
• Tilde Expansion: ~ expands to home directory, ~user expands to user's home
• Command History: Interactive line editing with history
• Signal Handling: Proper handling of Ctrl-C (SIGINT) and child process cleanup (SIGCHLD)
• Shell Configuration: Automatic execution of .shellrc on startup

Interactive Features

• Custom Prompt: Configurable via PROMPT environment variable
• Line Editing: Interactive command line editing with terminal control
• TTY Detection: Behaves differently when input is from terminal vs file/pipe

Building

Prerequisites

• GCC/G++ compiler with C++17 support
• Flex (lexical analyzer generator)
• Yacc/Bison (parser generator)
• Make build system

Build Instructions

# Clone the repository
git clone <repository-url>
cd Shell

# Build the shell
make

# Optional: Enable debug printing
make PRINTING=1

# Clean build files
make clean

Usage

Running the Shell

# Start interactive shell
./shell

# Run shell script
./shell < script.sh

# Execute commands from file
echo "ls -la" | ./shell

Configuration

Create a .shellrc file in your working directory for automatic startup commands:

# Example .shellrc
setenv PATH /usr/bin:/bin:/usr/local/bin
setenv PROMPT "myshell> "
cd /home/user

Example Commands

# Basic command execution
myshell> ls -la

# Pipes and redirection
myshell> cat file.txt | grep "pattern" > output.txt

# Background processes
myshell> long_running_command &

# Environment variables
myshell> setenv MY_VAR "hello world"
myshell> echo $MY_VAR

# Wildcards
myshell> ls *.cpp

# Change directory
myshell> cd ~/Documents
myshell> cd ..

# Source script
myshell> source setup.sh

Architecture

Core Components

1. Lexical Analyzer (lexical_analyzer.l)

   • Tokenizes input using Flex
   • Handles quotes, escapes, and special characters
   • Implements environment variable and wildcard expansion

2. Syntax Parser (syntax_parser.y)

   • Parses command syntax using Yacc/Bison
   • Builds command structures for execution
   • Handles operator precedence and grammar rules

3. Command Processor (command_processor.cpp/.hh)

   • Executes parsed commands
   • Manages process creation and communication
   • Handles I/O redirection and pipes

4. Simple Command (simple_command.cpp/.hh)

   • Represents individual commands with arguments
   • Manages argument lists and command structure

5. Shell Interpreter (shell_interpreter.cpp/.hh)

   • Main shell loop and initialization
   • Signal handling and process management
   • Prompt display and user interaction

6. Terminal Control (terminal_control.cpp)

   • Low-level terminal operations
   • Enables advanced line editing features

7. Line Editor (line_editor.cpp)

   • Interactive command line editing
   • Command history and completion

Signal Handling

• SIGINT (Ctrl-C): Interrupts current command, returns to prompt
• SIGCHLD: Cleans up zombie processes from background commands

Memory Management

• Proper cleanup of dynamic allocations
• RAII principles for resource management
• Exception-safe command execution

Development

Project Structure

Shell/
├── lexical_analyzer.l      # Flex lexical analyzer
├── syntax_parser.y         # Yacc/Bison parser
├── shell_interpreter.*     # Main shell implementation
├── command_processor.*     # Command execution engine
├── simple_command.*        # Command data structures
├── terminal_control.cpp    # Terminal I/O operations
├── line_editor.cpp         # Interactive editing
└── Makefile               # Build configuration

Build System

The Makefile supports:

• Conditional compilation flags (PRINTING=1 for debug output)
• Automatic dependency tracking
• Clean builds and intermediate file management
• Optional line editing mode compilation

Testing

Run the shell with various command combinations to test functionality:

• Complex pipe chains
• Multiple redirections
• Background process management
• Environment variable operations
• Wildcard expansions
• Error handling scenarios