C-Shell: A Custom UNIX Shell

A robust, modular, and fully functional command-line interface (CLI) shell implemented entirely in C from scratch. This project mimics the core behaviors of traditional standard shells (like bash or zsh), introducing specialized built-in commands alongside advanced job control, I/O redirection, and inter-process communication via piping.

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Key Features

1. Advanced Process & Job Control

• Background Processes (&): Execute long-running tasks asynchronously in the background.
• Job Management:
  • activities: List all currently running background processes.
  • fg <pid>: Bring a background process to the foreground.
  • bg <pid>: Resume a stopped background process.
  • ping <pid> <signal>: Transmit specific signals to processes directly from the shell.

2. Multi-Stage Piping & Redirection

• Piping (|): Supports chaining multiple commands together, effortlessly passing the standard output of one command as the standard input to the next (e.g., cmd1 | cmd2 | cmd3).
• I/O Redirection (<, >, >>): Route standard input/output to and from files seamlessly, allowing for overwriting (>) or appending (>>) data.

3. Custom Built-in Commands

• hop: A smart directory navigation tool (equivalent to cd).
• reveal: A robust directory listing and inspection tool (equivalent to ls).
• log: A persistent command history tracker that saves your session data to a local .shell_log file across restarts.

4. Robust Signal Handling

• Gracefully intercepts keyboard signals such as SIGINT (Ctrl+C) and SIGTSTP (Ctrl+Z), ensuring that the shell itself doesn't crash while managing child processes correctly.

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System Architecture & Implementation

The shell is designed with modularity in mind, cleanly separating input parsing, command execution, and signal management into distinct C source files (e.g., parser.c, pipes.c, signals.c).

The shell utilizes a raw input parser that tokenizes strings and offloads valid syntaxes to an executor module. It heavily leverages fork(), execvp(), POSIX file descriptors for piping, and waitpid() for process synchronization.

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Quick Start Guide

Prerequisites

• A UNIX-based environment (Linux, macOS, or WSL).
• GCC Compiler and make.

Build & Run

# Navigate to the project directory
cd C-Shell

# Compile the project using the provided Makefile
make

# Launch the custom shell (assuming the output binary is named 'shell' based on your makefile)
./shell

Usage Examples

# Multi-stage piping and redirection
c-shell> reveal -l -a | grep "test" > output.txt

# Background process management
c-shell> sleep 10 &
[1] 12345
c-shell> activities
[1] 12345 : sleep 10 - Running
c-shell> fg 12345

# Persistent history
c-shell> log
c-shell> log execute 3

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