Ruby Cheatsheet

Updated September 2026 · Quick reference

Ruby is an expressive, object-oriented language known for developer happiness and the Rails web framework.

Everything is an object. Learn blocks and Enumerable early—they power most everyday Ruby code.

Basics

Hello

Print with puts. Run a file with ruby app.rb.

puts "Hello"

Variables

Snake_case locals. Constants start with an uppercase letter.

name = "Ada"
MAX = 100

Symbols

Lightweight immutable labels, often used as hash keys.

status = :ok
user = { name: "Ada", id: 1 }

String interpolation

Double-quoted strings interpolate with #{...}.

puts "Hi, #{name}"

irb / pry

Interactive shells for trying snippets.

irb

Arrays, hashes, Enumerable

Arrays

Ordered lists with rich methods.

nums = [1, 2, 3]
nums << 4

Hashes

Key/value maps. Prefer symbol keys in new code.

book = { title: "Practical Ruby", year: 2024 }
book[:title]

map / select

Transform and filter with Enumerable.

nums.map { |n| n * 2 }
nums.select { |n| n.even? }

each

Iterate with a block.

nums.each do |n|
  puts n
end

Methods & classes

Method

Define with def. Last expression is the return value.

def add(a, b)
  a + b
end

Keyword args

Name parameters for clarity.

def greet(name: "friend")
  "Hi, #{name}"
end

Class

Instance variables use @. Expose with attr_reader / attr_accessor.

class User
  attr_reader :name
  def initialize(name)
    @name = name
  end
end

Modules

Namespaces and mixins shared across classes.

module Greetable
  def greet
    "Hi"
  end
end

Errors & gems

begin / rescue

Handle exceptions; rescue specific classes when possible.

begin
  File.read("missing.txt")
rescue Errno::ENOENT => e
  warn e.message
end

raise

Raise an error with a message or exception class.

raise ArgumentError, "name required"

Gemfile

Bundler manages gem dependencies.

bundle init
bundle add sinatra
bundle install

require

Load a gem or file.

require "json"
JSON.parse('{"a":1}')

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