Building Your First CLI Tool

Hello developers!! You must have use multiple CLI tools in your everyday development like echo, ls, cd etc.

Let’s try to understand it in a better way by creating your own simple CLI tool.

In this article, we will create a simple Calculator CLI tool.

🎯 What is CLI Tool?

A CLI (Command Line Interface) tool is just a program you run in the terminal by typing commands with your keyboard, instead of clicking buttons in a GUI (Graphical User Interface).

🎯 Project Setup

1. Setup the Project with initial command

npm init

2. Create a folder named src in the root directory of your project.

3. Inside src create a file called index.js This is going to be the entry point of our CLI.

4. In the package.json file, change the “main” part to src/index.js.

5. Now manually add another entry into the package.json file called bin and set its key to calc and it's value to ./src/index.js.

Your resultant package.json file would look like this:

{
  "name": "calculator-cli-tool",
  "version": "1.0.0",
  "description": "A CLI tool used for calculations",
  "main": "src/index.js",
  "bin": {
    "calc": "./src/index.js"
  },
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC"
}

The key calc, is the keyword for calling the CLI. This is the keyword that user will type in the terminal for using your CLI.

🎯 Commander

• Commander is a popular Node.js library that makes it easy to build CLI tools.

• What commander gives you?

  1. Commands → Define sub-commands like add, multiply.

  2. Arguments → Easily get values like 5 7.

  3. Options (flags) → Support things like --verbose or --format json.

  4. Automatic help → Generates --help output for free.

  5. Versioning → Add --version easily.

Let’s install commander:

npm install commander

In src/index.js file:

#!/usr/bin/env node
const { Command } = require('commander');
const program = new Command();

program
  .name('calc')
  .description('A simple calculator CLI')
  .version('1.0.0');

// Command: add
program
  .command('add <a> <b>')
  .description('Add two numbers')
  .action((a, b) => {
    console.log(Number(a) + Number(b));
  });

// Command: multiply
program
  .command('multiply <a> <b>')
  .description('Multiply two numbers')
  .option('-v, --verbose', 'Show detailed steps')
  .action((a, b, options) => {
    const result = Number(a) * Number(b);
    if (options.verbose) {
      console.log(`${a} * ${b} = ${result}`);
    } else {
      console.log(result);
    }
  });

program.parse();

🎯 Link it globally

Now if you run the below command in your project folder, this creates a global symlink so you can use calc anywhere.

npm link

Now your CLI tool is ready to use anywhere in your system. Let’s try it now:

🎯 CLI Building Blocks

As you noticed in the above examples, there are multiple parts in the single commands, let’s break it down.

• calc is the main command (the tool itself).

• add, multiply represents sub-command (an action inside calc).

• 5, 7 represents the arguments (values you pass)

• —verbose is a option which acts like a flag & modifies behavior

🎯 Wrap Up!!

That's all for this article. Thank you for your time!! Let's connect to learn and grow together. LinkedIn Twitter Instagram