msh: Maini Shell

A modular command line interpreter developed in C. This project implements the core mechanics of the Unix Sh-family architecture, focusing on robust process lifecycle management, strict memory hygiene, and environment-aware execution.

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Technical Features

• Advanced Process Management: Orchestrates command execution via fork(), execvp(), and synchronized waitpid() logic with status boundary checks.
• Multi-Stage Pipeline (Piping): Supports command chaining using pipe() and dup2(), allowing data to flow between multiple processes (e.g., ls | grep msh).
• Environment Variable Expansion: Dynamic resolution of system variables (e.g., $USER, $PATH) performed before the parsing stage for native shell behavior.
• Struct-Mapped Builtins: High-extensibility architecture using a t_builtin dispatch table for efficient command resolution and modularity.
  • cd: Optimized with $HOME defaults and OLDPWD tracking for 'cd -' support.
  • help: Dynamic discovery of available shell internal commands.
  • exit: Clean termination of the REPL session with proper exit status.
• Signal Resilience: Custom signal masking. SIGINT (Ctrl+C) and SIGQUIT (Ctrl+) are ignored by the parent shell but restored to default behavior in child processes.
• Automated Build System: Pro-tier Makefile featuring wildcard discovery, header dependency tracking (-MMD -MP), and debug symbol integration.

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Architecture Overview

The shell operates on a refined Read-Expand-Parse-Execute pipeline:

1. Lexical Analysis: Tokenizes raw input strings into discrete command components and operators.
2. Expansion Engine: A pre-parsing layer in src/env.c that scans for $ tokens and replaces them with system environment values using getenv().
3. Dispatch Engine: A centralized msh_execute layer that differentiates between internal built-ins, piped commands, and external binaries.
4. Execution Layer:
   • Manages the fork-join model for external binaries.
   • Implements pipe-line synchronization for concurrent process execution.
   • Restores signal defaults in the child context to maintain standard terminal interactivity.

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Project Structure

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├── include/           # Modular headers (msh.h, executor.h, builtins.h, parser.h)
├── src/               # Implementation logic
│   ├── main.c         # Entry point & REPL loop
│   ├── env.c          # Environment variable expansion logic ($VAR)
│   ├── executor.c     # Forking, pipe handling & process execution
│   ├── builtins.c     # Internal shell command implementations
│   └── parser.c       # String tokenization & I/O handling
├── obj/               # Generated object files (automatically managed)
└── Makefile           # Automated build system with dependency tracking

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Roadmap

• I/O Redirection: Implement >, >>, and < using open() and dup2().
• Command History: Persistent history tracking across sessions.
• Logical Operators: Support for && and || conditional execution.

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Getting Started

Prerequisites

• GCC or Clang compiler
• GNU Make
• Standard C Library (libc)

Installation

1. Clone the repository: git clone https://github.com/phantekzy/msh.git cd msh

2. Compile the executable: make

3. Initialize the shell: ./msh

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Development & Testing

The project is validated against the following criteria:

• Valgrind: Comprehensive heap analysis to ensure zero memory leaks.
• Signal Stressing: Verification of parent/child signal isolation.
• Header Integrity: Automatic recompilation on header changes via Makefile dependency tracking.

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License

The project is intended for educational purposes within the scope of systems programming and Unix architecture studies.