Requiring Files and Libraries

As programs grow, you split them into several files, and you use code that other people have written. Two methods load code from elsewhere.

require_relative — your own files

require_relative loads a file by its path relative to the current file. Given two files in the same folder:

# greetings.rb
def greet(name)
  "Hello, #{name}!"
end

# main.rb
require_relative "greetings"

puts greet("Ada")

$ ruby main.rb
Hello, Ada!

The .rb extension is optional. A file is only loaded once, even if it's required several times.

require — libraries

require loads a library by name. Ruby ships with a standard library of useful ones that just need requiring:

require "date"
require "securerandom"

puts Date.new(2025, 1, 15).strftime("%d %B %Y")
puts SecureRandom.hex(4).length

15 January 2025
8

Some everyday standard libraries: json, date, time, set, fileutils, net/http, securerandom, csv, yaml.

Libraries written by the community are packaged as gems and installed with the gem or bundle commands. Gems and Bundler are a Level 2 topic.

Common mistakes

• Using require "greetings" for your own file. require searches Ruby's library paths, not the current folder, and fails with cannot load such file -- greetings (LoadError). Use require_relative.
• Defining methods at the top level of many files. As programs grow, put related methods inside a module or class (Level 2) so names don't clash.

What's next

You now know enough Ruby to build real programs. Try the Beginner Projects, then continue to Level 2 — Intermediate, which starts with classes and objects.