dolittle.rb

#!/usr/bin/env ruby
require 'thor'

# Simple Todo list CLI
class Dolittle < Thor
  def initialize(*args)
    super
    @file = File.open('todolist.dat', 'a+')
  end

  desc 'list', 'list all tasks'
  def list
    @file.readlines.each_with_index do |line, index|
      puts "#{index + 1} => #{line}"
    end
  end

  desc 'add TASK', 'adds a new task'
  def add(task)
    @file.puts task
  end

  desc 'done TASK', 'delete a task'
  def done(task_index)
    `sed -i '#{task_index}d' #{File.basename(@file)}`
  end
end

Dolittle.start(ARGV)

Gemfile

# frozen_string_literal: true
source "https://rubygems.org"

# gem "rails"
gem 'thor'