mtsh — Empty Shell — Minimal Teaching Shell

mtsh (pronounced like em tee shell) is a tiny, educational Unix-like shell written in C.
It’s designed to be simple enough to understand line-by-line, while still being a functional interactive shell you can extend over time.

It starts as a bare-bones REPL that can run external commands, and is designed to grow feature-by-feature as you learn. Every line of code is intended to be readable, hackable, and easily extended — making mtsh a hands-on guide to understanding how real shells work under the hood.

Features

• POSIX.1-2008 compliant — portable across most modern Unix-like systems.
• Minimal command loop:
  • Reads a line of input
  • Parses it into arguments
  • Runs external commands via execvp()
• Built-in commands:
  • cd — change the working directory
  • exit — quit the shell
• Proper signal and exit code handling.

Why mtsh?

There are many full-featured shells. mtsh is not one of these!
Instead, it’s a teaching tool: a clean starting point for learning shell internals without wading through tens of thousands of lines of code.

You can:

• Experiment with process management.
• Add more built-ins.
• Implement piping, redirection, job control, or scripting support.
• Learn by modifying a shell you fully understand.

  ┌─────────┐
  │  Start  │
  └────┬────┘
       │
       ▼
┌─────────────┐
│  Read line  │  ← get command from user
└──────┬──────┘
       │
       ▼
┌─────────────┐
│  Evaluate   │  ← parse + run the command
└──────┬──────┘
       │
       ▼
┌─────────────┐
│   Print     │  ← show output or error
└──────┬──────┘
       │
       ▼
┌─────────────┐
│   Loop?     │  ← exit if user typed "exit"
└──────┬──────┘
       │yes
       ▼
     (stop)

Build

# Clone this repository
git clone https://github.com/simocoder/emptyshell.git
cd emptyshell

# Build
cc -std=c11 -Wall -Wextra -O2 -o mtsh mtsh.c

Or use make:

make

Install (optional)

You can run mtsh directly from the repo, or install it somewhere in your PATH:

sudo cp mtsh /usr/local/bin/

Usage

./mtsh

Example session:

$ ./mtsh
                      _             _          _ _ 
  ___ _ __ ___  _ __ | |_ _   _ ___| |__   ___| | |
 / _ \ '_ ` _ \| '_ \| __| | | / __| '_ \ / _ \ | |
|  __/ | | | | | |_) | |_| |_| \__ \ | | |  __/ | |
 \___|_| |_| |_| .__/ \__|\__, |___/_| |_|\___|_|_|
               |_|        |___/                    
 emptyshell — Minimal Teaching Shell v0.1
 POSIX.1-2008 • Simple • Hackable
-----------------------------------------------------

> pwd
/home/user
> cd /tmp
> ls
example.txt
> exit
$

Roadmap Ideas

• Command history (readline or custom)
• Tab completion
• Pipelining (cmd1 | cmd2)
• I/O redirection
• Background jobs (&)
• Configuration files (.mtshrc)

License

MIT — see LICENSE for details.

Blog

https://www.rebelyaml.com/posts/emptyshell-mtsh-minimal-teaching-shell/