This is how the bundle command would look...

bundle.ts

import {main, options} from '@asyncapi/bundler/cli';
import bundle from '@asyncapi/bundler';
import Command from './base';
import {Flags} from '@oclif/core';

export default class Bundle extends Command {
  static strict = false;
  static flags = options.getFlags(Flags);

  async run(){
    const {argv, flags} = this.parse(Bundle);
    await main(argv, flags);
  }
}

cli.js

const bundle = require('./lib');
const fs = require('fs');
const path = require('path');

const main = async (args, flags) => {
  let basepath;
  if (flags['base']){
    basepath = fs.readFile(path.resolve(process.cwd(), flags['base']), 'utf-8')
  }
  
  const document = await bundle(
    args.map(filepath => fs.readFileSync(path.resolve(process.cwd(), filepath), 'utf-8')),
    base: basepath
  );
  
  fs.writeFileSync(path.resolve(process.cwd(), flags['output']), document.yml());
}

const options = {
 getFlags: (Flags) => ({
  base: Flags.string({char: 'b'}),
  output: Flags.string({char: 'o'})
 })
}

module.exports = {
 main,
 options
}