Very basic todo script for use in an "Intro to Ruby" project I'm developing.

todo.rb

#!/usr/bin/env ruby

require "date"

class Todo
  attr_accessor :complete
  attr_reader :name, :due_on

  def initialize(name, due_on=nil, complete=false)
    @name = name
    @due_on = due_on
    @complete = complete
  end

  def self.from_string(string)
    todo = string.split("::")
    complete = todo[0] == "x"
    name = todo[1]
    due_on = todo[2] ? Date.parse(todo[2]) : nil

    self.new(name, due_on, complete)
  end

  def complete!
    self.complete = true
  end

  def complete?
    self.complete == true
  end

  def to_s
    complete = self.complete? ? "x" : "-"

    [complete, self.name, self.due_on].join("::")
  end
end

class TodoList
  attr_reader :todos

  TODOPATH = File.join(ENV["HOME"], "todos.txt")

  def initialize
    @todos = []

    load_todos
  end

  def all
    self.todos
  end

  def add(name, due_on)
    self.todos << Todo.new(name, due_on)
    save_todos
  end

  def complete(index)
    self.todos[index].complete!
    save_todos
  end

  def move(start_index, end_index)
    todo = self.todos.delete_at(start_index)
    self.todos.insert(end_index, todo)
    save_todos
  end

  def delete(index)
    self.todos.delete_at(index)
    save_todos
  end

  private

  def load_todos
    File.open(TODOPATH, "a+") do |file|
      file.each do |line|
        self.todos << Todo.from_string(line.chomp)
      end
    end
  end

  def save_todos
    File.open(TODOPATH, "w") do |file|
      self.todos.each do |todo|
        file.puts todo.to_s
      end
    end
  end
end

todo_list = TodoList.new

def list_todos(todo_list)
  todo_list.all.each_with_index do |todo, index|
    complete = todo.complete? ? "x" : " "

    line = "#{index + 1}. [#{complete}] #{todo.name}"
    line += " - #{todo.due_on}" if todo.due_on

    puts line
  end
end

def puts_title(title)
  puts title
  puts "-" * title.length
end

puts 

case ARGV[0]
when "list"
  puts_title "Listing todos"
  list_todos(todo_list)

when "add"
  puts_title "Adding a todo"
  todo_list.add(ARGV[1], ARGV[2])
  list_todos(todo_list)

when "complete"
  puts_title "Completing todo #{ARGV[1]}"
  todo_list.complete(ARGV[1].to_i - 1)
  list_todos(todo_list)

when "move"
  puts_title "Moving todo #{ARGV[1]} to #{ARGV[2]}"
  todo_list.move(ARGV[1].to_i - 1, ARGV[2].to_i - 1)
  list_todos(todo_list)

when "delete"
  puts_title "Deleting todo #{ARGV[1]}"
  todo_list.delete(ARGV[1].to_i - 1)
  list_todos(todo_list)

else
  puts <<-EOL
Usage: todo.rb <option> [option argument(s)]
    list                  # List the todo items in your list
    add <name> [due date] # Add new todo to your list with optional due date
    complete <position>   # Mark a todo complete
    move <pos> <new pos>  # Move a todo to a different position
    delete <position>     # Remove a todo at a position
    help
  EOL
end

puts 

Quick annotation, only paying attention to details of the code rather than the whole of it. I can go back over it later with that in mind:

#!/usr/bin/env ruby

# Is that even necessary to require?
require "date"

class Todo
  attr_accessor :complete
  attr_reader :name, :due_on

  # Avoid nil like the plague. Even if you did use it, these will
  # still be nil if you leave off the default there
  def initialize(name, due_on=nil, complete=nil)
    @name = name

    # Short circuiting can come in handy here:
    # @due_on = due_on && Date.parse(due_on)
    @due_on = due_on ? Date.parse(due_on.to_s) : nil

    # Just do a boolean check on it, no reason to || false.
    @complete = complete || false
  end

  def self.from_string(string)
    # I'd agree with avoiding regex unless absolutely necessary.
    # It tends to get hairy fast.
    string.match(/\A(x|-) (.+?)\s*(\d{4}-\d\d-\d\d)?\z/)

    # Look into the english regex terms, assume perlisms like $1 are off-limits
    complete = $1 == "x"
    name = $2
    due_on = $3

    self.new(name, due_on, complete)
  end

  def complete!
    self.complete = true
  end

  # You can just use self.complete here
  def complete?
    self.complete == true
  end
end

class TodoList
  attr_reader :todos

  def initialize
    @todos = []

    load_todos
  end

  # Look into Enumerable, it'll allow you to iterate over things by
  # defining an each method, and a comparator (<=>)
  def all
    self.todos
  end

  # Look into after-method hooks so you can abstract the save_todos
  # method into being executed after certain methods.

  def add(name, due_on)
    self.todos << Todo.new(name, due_on)
    save_todos
  end

  def complete(index)
    self.todos[index.to_i - 1].complete!
    save_todos
  end

  # Whole words won't kill anyone. Type them out instead of using odd
  # abbreviations.
  def move(start_idx, end_idx)
    todo = self.todos.delete_at(start_idx.to_i - 1)
    self.todos.insert(end_idx.to_i - 1, todo)
    save_todos
  end

  def delete(index)
    self.todos.delete_at(index.to_i - 1)
    save_todos
  end

  private

  def load_todos
    # Make the file path a constant in the head of this
    File.open(File.join(ENV["HOME"], "todos.txt"), "a+") do |file|
      # Consider the use of map instead:
      #
      # self.todos.concat file.map { |line| Todo.from_string(line.chomp) }
      file.each do |line|
        self.todos << Todo.from_string(line.chomp)
      end
    end
  end

  def save_todos
    File.open(File.join(ENV["HOME"], "todos.txt"), "w") do |file|
      self.todos.each do |todo|
        file.puts "%c %s %s" % [todo.complete? ? "x" : "-", todo.name, todo.due_on]
      end
    end
  end
end

todo_list = TodoList.new

def list_todos(todo_list)
  # If you do that Enumerable bit above, you can get rid of all
  todo_list.all.each_with_index do |todo, index|
    # This ternary is repeated multiple times. Abstract it to a method instead
    line = "%.2d [%c] %s" % [index + 1, todo.complete? ? "x" : " ", todo.name]
    # use <<, as it's faster
    line += " - %s" % todo.due_on.strftime("%b %d, %Y") if todo.due_on

    puts line
  end
end

puts 

# Might want to chomp that, newlines make for a bad time.
case ARGV[0]
when "list"
  # nifty method idea for formatting:
  #
  # def puts_title(string)
  #   puts string
  #   puts '-' * string.length
  # end
  puts "Listing todos"
  puts "-------------"
  list_todos(todo_list)

when "add"
  puts "Adding a todo"
  puts "-------------"
  todo_list.add(ARGV[1], ARGV[2])
  list_todos(todo_list)

when "complete"
  puts "Completing todo #{ARGV[1]}"
  puts "------------------"
  todo_list.complete(ARGV[1])
  list_todos(todo_list)

when "move"
  puts "Moving todo #{ARGV[1]} to #{ARGV[2]}"
  puts "--------------------"
  todo_list.move(ARGV[1], ARGV[2])
  list_todos(todo_list)

when "delete"
  puts "Deleting todo #{ARGV[1]}"
  puts "----------------"
  todo_list.delete(ARGV[1])
  list_todos(todo_list)

else
  puts <<-EOL
Usage: todo.rb <option> [option argument(s)]
    list                  # List the todo items in your list
    add <name> [due date] # Add new todo to your list with optional due date
    complete <position>   # Mark a todo complete
    move <pos> <new pos>  # Move a todo to a different position
    delete <position>     # Remove a todo at a position
    help
  EOL
end

puts 

Great! Thanks everyone for your input. I implemented most of the suggestions, but not all; some was left in for the purpose of illustration, even if not the "Ruby Way"(tm).

I really appreciate the help and constructive feedback.

Looks great. I am a bit surprised that people feel regex is seldom used. It is a good skill to have, but definitely a subject in and of itself :P