If you’re creating a gem from scratch, you can use bundler’s built in gem skeleton to create a base gem for you to edit.
$ bundle gem my_gem
This will create a new directory named my_gem with your new gem skeleton.
If you already have a gem, you can create a Gemfile and use Bundler to manage your development dependencies. Here’s an example.
source "https://rubygems.org"
gemspec
gem "rspec", "~> 3.9"
gem "rubocop", "0.79.0"
In this Gemfile, the gemspec method imports gems listed with add_runtime_dependency in the my_gem.gemspec file, and it also installs rspec and rubocop to test and develop the gem.
All dependencies from the gemspec and Gemfile will be installed by bundle install, but rspec and rubocop will not be included by gem install mygem or bundle add mygem.
Runtime dependencies in your gemspec are treated as if they are listed in your Gemfile, and development dependencies are added by default to the group, :development.
You can change that group with the :development_group option:
gemspec :development_group => :dev
You can also point to a specific gemspec directory using the :path option. If your gemspec is in /gemspec/path, use:
gemspec :path => '/gemspec/path'
If you omit :path, Bundler will look for gemspecs in the same directory as the Gemfile (usually the project root).
If you have multiple gemspecs in the same directory, specify which one you’d like to reference using :name. This refers to the gem name declared inside the gemspec, not the gemspec filename:
gemspec :name => 'my_awesome_gem'
This will match the gemspec where spec.name = "my_awesome_gem", regardless of whether it’s defined in my_awesome_gem.gemspec or another file.
That’s it! Use bundler when developing your gem, and otherwise, use gemspecs normally!
$ gem build my_gem.gemspec