`require` vs. `gem` methods?

Question

What's the difference between the require and gem methods?

For example, what's the difference betweenrequire 'minitest' and gem 'minitest'?

Answer 1

Say you have two versions of the gem foo installed:

$ gem list foo

*** LOCAL GEMS ***

foo (2.0.1, 2.0.0)

---

If you use only require, the newest version will be loaded by default:

require 'foo'       # => true

Foo::VERSION        # => "2.0.1"

---

If you use gem before calling require, you can specify a different version to use:

gem 'foo', '2.0.0'  # => true
require 'foo'       # => true

Foo::VERSION        # => "2.0.0"

---

Note: using gem without subsequently calling require does not load the gem.

gem 'foo'           # => true

Foo::VERSION        # => NameError: uninitialized constant Foo

Answer 2

Kernel#require activates the latest version of a gem.

Kernel#gem (added by RubyGems) allows activation of specific gem versions.