Require 'Rubygems'

require 'rubygems'

It is often superfluous. It will allow you to require specific versions of particular gems though, with the gem command.

https://guides.rubygems.org/patterns/#requiring-rubygems

When do we need to require 'rubygems'?

Use require 'rubygems' when you are using a gem that you installed with Rubygems. The reason is that Ruby uses Rubygems to determine the path of the gem that Rubygems installed. (is unable to locate the gem you want to use)

Alternatively, you can pass the -rubygems flag when you invoke your script, or set export RUBYOPT=rubygems in your profile (~/.bashrc or ~/.bash_profile or ~/.profile) which is basically the same as the flag, except it is implicit.

On 1.9, rubygems is required implicilty, and you shouldn't have to do that.

Here are some docs about it http://docs.rubygems.org/read/chapter/3

Note: Some have built solutions (zozo and faster_rubygems) to avoid Rubygems overhead http://www.ruby-forum.com/topic/212463

When is the 'require' necessary when using a ruby gem?

Usually, an application that is using a gem needs to require the gem:

require "my_awesome_gem"
MyAwesomeGem.do_something_great

However, if an application is using bundler, which defines the application's gem in a file called "Gemfile":

source 'http://rubygems.org'
gem 'my_awesome_gem'

then the application may invoke bundler in a way that automatically requires all of the gems specified in the Gemfile:

require "bundler"
Bundler.require
MyAwesomeGem.do_something_great

Rails projects use Bundler.require, so a Rails application will not need to explicitly require a gem in order to use it: Just add the gem to the Gemfile and go.

For more about Bundler.require, see the bundler documentation

For more about how Rails uses Bundler, see How Does Rails Handle Gems? by Justin Weiss.

Is `require rubygems` returning `false` ok?

It should be fine. Requiring a file the second time results in a false response. With load, it's a different thing, which would load (require) the file each time it is requested.

So it just means that irb is starting with rubygems already required, which is not a surprise.

For example, load is heavily used in Rails' development mode, so your changes can immediately be shown.

Since Ruby 1.9 rubygems are automatically required.

Require a specific version of RubyGems

I don't believe it's possible to select from among multiple rubygems versions in a single Ruby installation. But you can ensure that the required version is being used. Put the following somewhere where it will be run early in your application's startup, such as the top of your Gemfile (which is just a Ruby file), or at the beginning of a script which doesn't use bundler:

required_rubygems_version = '2.6.0'
if Gem.rubygems_version < Gem::Version.create(required_rubygems_version)
  raise "Please upgrade to rubygems #{required_rubygems_version}"
end

Is this the right way to require ruby gems?

Get rid of these lines from your config.ru file:

require 'rubygems'
require 'bundler'

Bundler.require

Just make sure you run

bundle install 

from the terminal to install your gems before starting the application.

How does require rubygems help find rubygem files?

Here's the current version of the relevant source: https://github.com/rubygems/rubygems/blob/02ead548e38ff90923444fa7c0ff9f6a5dbd87b0/lib/rubygems/custom_require.rb. (Edit: here's an earlier version (1.5.2) that more clearly expresses what happens.)

The docs say:

When RubyGems is required, Kernel#require is replaced with our own which
is capable of loading gems on demand.

When you call require 'x', this is what happens:

• If the file can be loaded from the existing Ruby loadpath, it
  is.
• Otherwise, installed gems are searched for a file that matches.
  If it's found in gem 'y', that gem is activated (added to the
  loadpath).

The normal require functionality of returning false if
that file has already been loaded is preserved.

It does this by opening up module Kernel and aliasing the original require with alias gem_original_require require, then redefining require to first call the original version, and look at the gems if that doesn't work.

So the load path is only changed when you require a gem:

ruby-1.8.7-p330 :002 > $:.length
=> 9 
ruby-1.8.7-p330 :003 > require 'rubygems'
=> true 
ruby-1.8.7-p330 :004 > $:.length
=> 9 
ruby-1.8.7-p330 :005 > require 'haml'
=> true 
ruby-1.8.7-p330 :006 > $:.length
=> 10 

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