Bash-Inspired Shell

Project Overview

A Unix shell implementation inspired by Bash, built from scratch in C as a learning exercise. This project demonstrates a comprehensive understanding of Unix system programming, process management, file descriptors, and inter-process communication.

Tech Stack:

• C Programming Language
• Unix/Linux System Calls
• POSIX Standards
• Process Management APIs

Core Features:

• Command parsing and execution
• Multi-process piping
• Input/output redirection
• Built-in shell commands
• Signal handling
• Interactive REPL

Architecture

Command Processing Pipeline

The shell operates on a classic REPL (Read-Eval-Print Loop) model:

1. Read: Parse user input into tokens
2. Evaluate: Determine command type and prepare execution
3. Execute: Fork processes and manage I/O
4. Print: Display results and wait for next command

Key Components

Command Parser

• Tokenizes input strings into command arguments
• Identifies special operators (pipes, redirections)
• Handles quoted strings and escape sequences

Process Manager

• Creates child processes using fork-exec pattern
• Manages parent-child synchronization
• Handles background process execution

I/O Handler

• Manages file descriptor redirection
• Creates and maintains pipes between processes
• Handles standard streams (stdin, stdout, stderr)

Signal Controller

• Intercepts keyboard interrupts (Ctrl+C, Ctrl+Z)
• Manages process termination
• Prevents shell from exiting on signals

Feature Implementation

Command Execution

Uses the classic Unix fork-exec pattern:

• Fork creates a child process
• Exec replaces child process with target program
• Wait synchronizes parent with child completion

Piping

Implements multi-process pipelines by:

• Creating pipe file descriptors between processes
• Redirecting stdout of one process to stdin of next
• Coordinating multiple child processes simultaneously

I/O Redirection

Supports standard redirection operators:

• > - output redirection (overwrite)
• < - input redirection
• >> - output redirection (append)

Built-in Commands

Implements essential shell built-ins:

• cd - change directory
• pwd - print working directory
• echo - print arguments
• exit - terminate shell
• history - show command history

Background Processes

Executes commands asynchronously:

• Appending & runs command in background
• Parent process continues without waiting
• Displays process ID for tracking

Technical Challenges

File Descriptor Management

Managing file descriptors across multiple processes requires careful attention:

• Duplicating descriptors with dup2()
• Closing unused descriptors to prevent leaks
• Ensuring proper inheritance in child processes

Process Synchronization

Coordinating multiple processes involves:

• Using waitpid() for specific child processes
• Handling zombie processes
• Managing background process completion

Memory Management

C requires explicit memory management:

• Dynamic allocation for variable-length input
• Reallocation for growing token arrays
• Proper cleanup to prevent memory leaks

Signal Handling

Implementing robust signal handling:

• Using sigaction() for reliable signal handling
• Preventing shell termination on Ctrl+C
• Distinguishing between shell and child signals

Systems Programming Concepts

Process Control

The project demonstrates core Unix process concepts:

• Process creation and termination
• Process states and transitions
• Parent-child relationships
• Zombie and orphan processes

Inter-Process Communication

Multiple IPC mechanisms are utilized:

• Pipes for streaming data between processes
• File descriptors as communication channels
• Process exit codes for status reporting

File I/O

Low-level file operations:

• Opening and closing file descriptors
• Reading and writing with system calls
• File descriptor duplication and redirection

Testing & Validation

The shell was tested with various scenarios:

Basic Operations:

• Single command execution
• Commands with multiple arguments
• Built-in command functionality

Advanced Features:

• Multi-stage pipelines (cmd1 | cmd2 | cmd3)
• Combined redirections (cmd < in.txt > out.txt)
• Background process execution

Edge Cases:

• Empty input handling
• Invalid command errors
• File permission issues
• Malformed syntax handling

Learning Outcomes

This project provided hands-on experience with:

Low-Level Programming:

• Direct interaction with system calls
• Manual memory management
• Pointer manipulation and safety

Operating System Internals:

• Process lifecycle and management
• File descriptor mechanics
• Signal handling and propagation

Software Design:

• Modular architecture
• Error handling strategies
• Resource cleanup patterns

Problem-Solving:

• Debugging complex multi-process interactions
• Understanding race conditions
• Managing system resources efficiently

Conclusion

Building a shell from scratch illuminated the inner workings of Unix systems. The project required understanding not just how to use system calls, but why they exist and how they interact.

Working at this low level, without frameworks or abstractions, reinforced that complex systems are built from simple primitives. Understanding these fundamentals enables building sophisticated software even with limited tools.