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Standard library and a short note on Rails

The libraries that ship with Ruby and cover most scripting needs, plus what Rails adds on top.

Standard library you will actually use

require "json"
require "csv"
require "set"
require "fileutils"
require "securerandom"

data = { name: "Ada", roles: ["admin", "editor"] }
json = JSON.generate(data)
parsed = JSON.parse(json, symbolize_names: true)

File.write("out.json", JSON.pretty_generate(parsed))
FileUtils.mkdir_p("tmp/reports")

CSV.foreach("rows.csv", headers: true) do |row|
  puts row["email"]
end

ids = Set.new([1, 2, 2])            # => #<Set: {1, 2}>
token = SecureRandom.urlsafe_base64(32)

text = "the quick brown fox jumps"
puts text.scan(/\w+/).tally.sort_by { |_, n| -n }.first(3).inspect
RequirementLibrary or method
Parse or emit JSONjson: JSON.parse, JSON.generate
CSV filescsv: CSV.foreach, CSV.read
Unique collections and fast membershipset, which Ruby 3 makes available without require
Files and directoriesFileUtils, Pathname, Dir.glob
Random tokens and UUIDssecurerandom
Time zonesTime for basics, the tzinfo gem for real zone handling
HTTP requestsnet/http in the standard library, faraday or httparty in practice

A short note on Rails

Rails is a full framework built on Ruby: convention over configuration, MVC, and generators that write the boilerplate for you. Ruby is the language; Rails is a library you happen to load.

gem install rails
rails new blog
cd blog
bin/rails server

bin/rails generate model Post title:string body:text published_at:datetime
bin/rails generate controller Posts index show
bin/rails db:migrate
# config/routes.rb
Rails.application.routes.draw do
  resources :posts
  root "posts#index"
end

# app/models/post.rb
class Post < ApplicationRecord
  belongs_to :user

  validates :title, presence: true, length: { maximum: 200 }
  scope :published, -> { where.not(published_at: nil) }
end
  • Active Record maps a table to a class, so queries read as Ruby: Post.published.order(created_at: :desc).
  • Convention replaces configuration: Post maps to the posts table and PostsController to app/controllers/posts_controller.rb.
  • Gems add authentication, background jobs and admin panels without wiring a stack from scratch, which is why Rails ships products quickly.
  • Learn blocks, modules and the Enumerable methods first: most Rails magic is those three ideas used heavily.
💡
Sinatra is a good middle step: the same Ruby on the same Rack interface with a fraction of the framework. If Rails feels like more structure than a small service needs, use Sinatra or plain Rack first and grow into Rails later.

FAQ

Do I need Rails to write Ruby?
No. Ruby is widely used for scripts, CLIs, data pipelines and background workers, and many HTTP services use Sinatra or plain Rack instead of a full framework.
Why does my require fail for a gem in the Gemfile?
Run bundle install first, then use bundle exec so the versions in Gemfile.lock are the ones loaded. Running from the project directory also matters.

Objects, modules and exceptions Syntax and blocks

Last refreshed 2026-09-18.