CShell — A Unix Shell Written in C

A lightweight, POSIX-compliant command-line interpreter built from scratch in C. This project demonstrates core operating-system principles — process management, inter-process communication, file descriptor manipulation, and manual memory management — implemented without relying on any existing shell.

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Contents

• Features Implemented
• Architecture & System Calls
• Getting Started
• Manual Testing Examples
• Future Work / Roadmap

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Features Implemented

Feature	Details
Process execution	Executes standard system commands (ls, grep, cat, etc.) by searching the PATH environment variable
Built-in: cd	Changes the shell's current working directory
Built-in: pwd	Prints the current working directory
Built-in: history	Displays a ring buffer of recently executed commands
Built-in: exit	Safely terminates the shell and frees all allocated memory
Output redirection	Truncating (>) and appending (>>) to a file
Input redirection	Reading command input from a file (<)

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Architecture & System Calls

The shell runs an infinite REPL (Read–Eval–Print Loop), split into distinct modules:

1. Parser (parser.c) Reads raw input with getline() (avoids fixed-size buffer overflows) and tokenizes it, separating redirection symbols (>, >>, <) from the command's own arguments.

2. Executor (executor.c) Handles process creation:

   • fork() clones the parent process.
   • execvp() replaces the child's memory image with the target executable.
   • waitpid() blocks the parent until the child finishes, preventing zombie processes.

3. File descriptor redirection When redirection is detected, the child opens the target file with open() and uses dup2() to overwrite STDIN_FILENO (0) or STDOUT_FILENO (1) before calling execvp() — rerouting I/O transparently, with no changes needed in the executed program itself.

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Getting Started

Prerequisites

• GCC
• GNU Make
• A Unix/Linux environment (or WSL on Windows) — this project relies on POSIX system calls

Build and run

# Clone the repository
git clone https://github.com/RahulBiswas224/CShell.git
cd CShell

# Compile using the included Makefile
make

# Run the shell
./myshell

Clean build artifacts

make clean

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Manual Testing Examples

myshell> ls -la
myshell> echo "Hello World" > output.txt
myshell> cat < output.txt
myshell> echo "Appending text" >> output.txt
myshell> history
myshell> exit

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Future Work / Roadmap

The architecture was designed to be extensible. Planned next:

• Pipes (|) — wire one process's stdout directly into another's stdin using pipe(), e.g. ls -la | grep .c
• Background jobs (&) — run processes asynchronously, with a zombie-reaping loop using waitpid() and the WNOHANG flag so the shell isn't blocked waiting on background children
• Signal handling — catch SIGINT (Ctrl+C) and SIGTSTP (Ctrl+Z) so they interrupt the running child process without killing the shell itself

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Author

Built by Rahul Biswas as a systems-programming project ahead of technical interviews focused on C, operating systems, and process management.