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Minitest / RSpec

By · Reviewed & last updated · Part of Ruby Tutorial

Minitest ships with the Ruby standard library and is the default test framework for new Rails apps. It is small, fast, and offers both classic xUnit-style and spec-style DSLs in one library. Reach for RSpec only if you specifically want its matcher library or shared-example DSL.

Minitest unit, spec, mocks, and parallel runs

EXAMPLE
# test/test_helper.rb
require "minitest/autorun"
require "minitest/pride"          # colourful dots; opt out in CI

# Run tests in parallel processes
Minitest.parallel_executor = Minitest::Parallel::Executor.new(4)

# ---- lib/calc.rb ----
class Calc
  def add(a, b)  = a + b
  def div(a, b)
    raise ArgumentError, "divide by zero" if b.zero?
    a / b
  end
end

# ---- test/calc_test.rb ----
require_relative "test_helper"
require_relative "../lib/calc"

# 1) Classic xUnit style — explicit method names, clearest stack traces
class CalcTest < Minitest::Test
  def setup;    @c = Calc.new end
  def teardown; end

  def test_add_returns_sum
    assert_equal 5, @c.add(2, 3)
  end

  def test_div_by_zero_raises
    err = assert_raises(ArgumentError) { @c.div(1, 0) }
    assert_match(/divide by zero/, err.message)
  end

  def test_skip_on_windows
    skip "wonky on Windows" if RUBY_PLATFORM =~ /mingw|mswin/
    assert_equal 2, @c.div(10, 5)
  end
end

# 2) Spec style — describe/it/expect-style, same engine underneath
describe Calc do
  before { @c = Calc.new }

  it "adds two numbers" do
    _(@c.add(2, 2)).must_equal 4
  end

  it "raises on divide by zero" do
    _ { @c.div(1, 0) }.must_raise ArgumentError
  end
end

# 3) Mocks and stubs — built in
require "minitest/mock"

class MailerTest < Minitest::Test
  def test_sends_welcome
    smtp = Minitest::Mock.new
    smtp.expect(:deliver, true, [String, "alice@example.com"])
    User.new("Alice", "alice@example.com").welcome!(smtp)
    smtp.verify
  end

  def test_stubs_time
    Time.stub :now, Time.new(2026, 6, 11) do
      assert_equal 2026, Time.now.year
    end
  end
end

# Run from CLI
#   ruby -Ilib -Itest test/calc_test.rb            # one file
#   bundle exec rake test                          # via Rake
#   bundle exec rails test                         # in Rails

Why it matters

Spec style is just sugar over the xUnit base — `_(value).must_equal x` desugars to `assert_equal x, value`. Pick one style per file and stick to it; mixing styles in the same suite is the easy way to confuse a future reader of the diff.

Tip: Tweak the snippet with Try it Yourself », then sit the quiz at the bottom of the page.

Example

Example
require 'minitest/autorun'
class UserTest < Minitest::Test
    def test_greet
        assert_equal 'hi, Ada', User.new('Ada').greet
    end
end
Try it Yourself »

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