Gems & Bundler

Ruby is a dynamic, beginner-friendly programming language with a famous ecosystem of gems — packaged libraries you can install and require — managed reliably with Bundler.

Learn Gems & Bundler in our free Ruby course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick reference.

Part of the free Ruby course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

By the end of this lesson you'll install and require gems, write a Gemfile, and understand how Bundler locks versions for reproducible projects.

What You'll Learn in This Lesson

1️⃣ Installing and Using Gems

Install a gem from your terminal with gem install name , then load it in code with require . Ruby's standard library already includes many useful libraries — like json — that you can require without installing anything.

2️⃣ Bundler: A Gemfile for Reproducible Projects

Real projects declare their gems in a Gemfile . Running bundle install resolves versions and writes a Gemfile.lock , so everyone installs the same versions. Run your app with bundle exec to use exactly those gems.

Your turn. Use the json gem to serialise and parse settings. Confirm the two TODO lines, then run it.

Author a real Gemfile with a source, a pinned gem, and a test-group gem — then note the commands you'd run. This is the file at the heart of every Ruby project.

📋 Quick Reference — Gems & Bundler

Practice quiz

What is a gem?

  • A Ruby keyword
  • A packaged, shareable Ruby library
  • A type of variable
  • A built-in data structure

Answer: A packaged, shareable Ruby library. A gem is a packaged Ruby library with code plus metadata, published to RubyGems.org.

Which command installs a gem globally from the terminal?

  • bundle install
  • gem install name
  • require name
  • gem load name

Answer: gem install name. gem install name fetches and installs a single gem globally on your system.

How do you load an installed gem in your code?

  • import gem
  • include gem
  • require "name"
  • load gem

Answer: require "name". You load a gem (or standard-library module) in code with require "name".

What file declares a project's gems for Bundler?

  • Gemfile
  • package.json
  • gems.txt
  • Rakefile

Answer: Gemfile. A file literally named Gemfile lists the gems a project needs.

What does write to lock exact versions?

  • Gemfile
  • Gemfile.lock
  • versions.rb
  • lockfile.json

Answer: Gemfile.lock. bundle install resolves versions and records the exact ones in Gemfile.lock.

What does the version constraint mean?

  • Exactly this version
  • Any version
  • Pessimistic: safe minor/patch upgrades, no breaking major bump
  • Less than this version

Answer: Pessimistic: safe minor/patch upgrades, no breaking major bump. ~> is the pessimistic operator; ~> 0.21 means at least 0.21 but less than 1.0.

Why run your app with ?

  • It is faster
  • It ensures the exact gems from the Gemfile/lock are used, not some global version
  • It installs gems
  • It compiles the app

Answer: It ensures the exact gems from the Gemfile/lock are used, not some global version. bundle exec runs your app with exactly the gems Bundler resolved, avoiding wrong global versions.

Which library ships with Ruby and needs no install before ?

  • httparty
  • rails
  • json
  • rspec

Answer: json. json is part of Ruby's standard library, so you can require it without installing anything.

What error appears when a required gem isn't installed?

  • SyntaxError
  • LoadError (cannot load such file)
  • ArgumentError
  • NameError

Answer: LoadError (cannot load such file). Requiring a missing gem raises LoadError: 'cannot load such file'.

What is the difference between gem install and bundle install?

  • No difference
  • gem install adds one gem globally; bundle install installs the whole Gemfile set with locked versions
  • gem install needs a Gemfile
  • bundle install installs only one gem

Answer: gem install adds one gem globally; bundle install installs the whole Gemfile set with locked versions. gem install is for single global tools; bundle install installs a project's whole declared, version-locked set.