Minishell

A simple shell implementation in C that mimics basic functionality of bash. This project implements a command-line interpreter with support for pipes, redirections, environment variables, and built-in commands.

Features

• State Machine Parser/Lexer: Robust parsing system using finite state machine for tokenization and syntax analysis
• Multiprocess Execution: Parallel execution of pipeline commands for optimal performance
• Built-in Commands: Support for echo, cd, pwd, export, unset, env, exit
• Redirections: Input (<), output (>), and append (>>) redirections
• Pipes: Command chaining with pipe (|) operator
• Environment Variables: Variable expansion and management ($VAR, ${VAR}, $?)
• Signal Handling: Proper handling of Ctrl+C (SIGINT), Ctrl+D (EOF), and Ctrl+\ (SIGQUIT)
• History: Command history using readline library

Prerequisites

Before compiling the project, you need to install the readline library:

Ubuntu/Debian:

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

macOS (with Homebrew):

brew install readline

CentOS/RHEL/Fedora:

# For CentOS/RHEL
sudo yum install readline-devel

# For Fedora
sudo dnf install readline-devel

Arch Linux:

sudo pacman -S readline

Installation

1. Clone the repository:

   git clone https://github.com/muhsinalikulbak/Minishell.git
   cd Minishell
   

2. Compile the project:

   make
   

Makefile Commands

• make: Compiles the project and creates the minishell executable
• make re: Recompiles the entire project from scratch (equivalent to make fclean + make)
• make clean: Removes all object files (.o files) but keeps the executable
• make fclean: Removes all object files and the executable (complete cleanup)

Usage

Run the shell:

./minishell

Once started, you can use it like a regular shell:

minishell$ echo "Hello World"
Hello World
minishell$ ls -la | grep minishell
-rwxr-xr-x 1 user user 45678 Dec 10 15:30 minishell
minishell$ export MY_VAR=42
minishell$ echo $MY_VAR
42
minishell$ echo "Exit status: $?"
Exit status: 0
minishell$ echo "Shell PID: $$"
Shell PID: 12345
minishell$ cat < input.txt > output.txt
minishell$ ls | wc -l | cat -e
15$
minishell$ exit

Supported Commands

Built-in Commands:

• echo [-n] - Display text
• cd [path] - Change directory
• pwd - Print working directory
• export [var=value] - Set environment variable
• unset [var] - Unset environment variable
• env - Display environment variables
• exit [code] - Exit the shell

External Commands:

• Any executable in PATH
• Support for absolute and relative paths

Special Features:

• Pipes: command1 | command2 | command3
• Redirections:
  • command > file (output redirection)
  • command >> file (append redirection)
  • command < file (input redirection)
• Environment Variables:
  • $VAR or ${VAR} - Variable expansion
  • $? - Exit status of last command
  • $$ - Process ID of shell
• Command History: Use arrow keys to navigate through previous commands

Architecture

Parser/Lexer (State Machine)

The parsing system uses a finite state machine approach to tokenize and parse input:

• Lexical Analysis: Converts input string into tokens
• Syntax Analysis: Validates command structure and builds parse tree
• State Management: Handles quotes, escapes, and special characters

Executor (Multiprocess)

The execution system implements parallel processing for pipeline commands:

• Process Creation: Each pipeline segment runs in its own process
• IPC Management: Proper pipe setup and file descriptor handling
• Synchronization: Parent process waits for all child processes
• Resource Cleanup: Automatic cleanup of processes and file descriptors

Variable Expansion

The shell supports comprehensive variable expansion:

• Environment Variables: $HOME, $PATH, $USER etc.
• Custom Variables: Variables set with export
• Special Variables:
  • $? - Exit status of the last executed command
  • $$ - Process ID of the current shell
• Expansion in Quotes:
  • Double quotes: Variables are expanded
  • Single quotes: Variables are treated literally
• Parameter Expansion: ${VAR} syntax for complex cases

Signal Handling

The shell implements proper signal management:

• SIGINT (Ctrl+C):
  • In interactive mode: Displays new prompt on new line
  • During command execution: Terminates current command
  • Exit status set to 130
• SIGQUIT (Ctrl+):
  • Ignored in interactive mode
  • During command execution: Terminates with core dump
  • Exit status set to 131
• EOF (Ctrl+D):
  • Exits the shell gracefully
  • Prints "exit" message
• Signal Inheritance: Child processes receive appropriate signal handling

Error Handling

The shell handles various error conditions gracefully:

• Invalid commands and syntax errors
• File permission issues
• Memory allocation failures
• Signal interruptions
• Pipe and redirection errors

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 part of the 42 School curriculum and follows the school's academic guidelines.