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What is Bundler in Ruby on Rails?

Bundler is a crucial tool for any Ruby on Rails application. If you are learning programming and have come across Ruby on Rails, you might have heard of Bundler. But what exactly is Bundler, and why is it so essential for your Rails applications? In this blog post, we will dive deep into the world of Bundler, explaining its purpose, how it works, and how to use it effectively in your Rails applications.

What is Bundler?

Bundler is a package manager for the Ruby programming language. It helps you manage the dependencies of your Ruby applications, ensuring that all the required libraries (called "gems" in Ruby) are installed and available for your application to run smoothly. It also makes it easy to keep your application's dependencies up to date.

Gems in Ruby

In Ruby, libraries are called gems. Gems are self-contained packages that include code, documentation, and other files needed to provide functionality to your application. They can be easily installed, updated, and managed using several tools, including Bundler.

Why do we need Bundler?

When you start building a Ruby on Rails application, you quickly realize that you need to use several external libraries to add functionality to your application. These libraries are called gem dependencies.

Managing these dependencies can become complicated, especially for large applications with many gem dependencies. For example, you may have two different gems that depend on different versions of a third gem. How do you ensure that both gems can work together in your application?

That's where Bundler comes in. Bundler helps you manage your application's dependencies, ensuring that your application uses the correct versions of each gem and installing the necessary gems when needed. It also helps you avoid conflicts between different gem versions.

How does Bundler work?

Bundler relies on a file called Gemfile that lists all the gems your application depends on, along with their version requirements. When you run Bundler, it reads the Gemfile and generates a Gemfile.lock file, which contains the exact versions of the gems that your application should use. Bundler then installs the specified gems if they are not already installed on your system.

The Gemfile

The Gemfile is a plain text file that lives in the root directory of your Rails application. It lists all the gems your application depends on, along with any version requirements or other options.

Here's an example of a simple Gemfile:

source ''

gem 'rails', '6.1.4'
gem 'pg', '>= 0.18', '< 2.0'
gem 'puma', '~> 5.0'
gem 'sass-rails', '>= 6'
gem 'webpacker', '~> 5.0'
gem 'turbolinks', '~> 5'
gem 'jbuilder', '~> 2.7'
gem 'redis', '~> 4.0'

In this example, the Gemfile specifies that the application depends on Rails version 6.1.4, PostgreSQL gem with a version between 0.18 and 2.0, and Puma web server with a version around 5.0.

The Gemfile.lock

The Gemfile.lock is a file generated by Bundler that contains the exact versions of all gems specified in the Gemfile, as well as any dependencies those gems have. This file should be committed to your version control system (such as Git) to ensure that all developers working on the project use the same gem versions.

Here's an example of a Gemfile.lock:

    actioncable (6.1.4)
      actionpack (= 6.1.4)
      activesupport (= 6.1.4)
      nio4r (~> 2.0)
      websocket-driver (>= 0.6.1)
    actionmailbox (6.1.4)
      actionpack (= 6.1.4)
      activejob (= 6.1.4)
      activerecord (= 6.1.4)
      activestorage (= 6.1.4)

In this example, the Gemfile.lock specifies the exact gem versions needed to run the application, including dependencies of the gems listed in the Gemfile.

Installing and updating gems with Bundler

To install the gems specified in your Gemfile, you simply need to run the following command in your terminal:

bundle install

This command reads the Gemfile, installs the required gems, and generates the Gemfile.lock file.

To update the gems in your application, you can use the following command:

bundle update

This command updates your gems to the latest versions allowed by the Gemfile and updates the Gemfile.lock.

Using Bundler effectively in your Rails applications

Now that you understand what Bundler is and how it works, here are some tips to help you use Bundler effectively in your Rails applications:

1. Keep your Gemfile organized

It's essential to maintain an organized and clean Gemfile. Group your gems by functionality or environment, and add comments to explain the purpose of each gem. This will make it easier for other developers to understand your application's dependencies.

For example:

# Web framework
gem 'rails', '6.1.4'

# Database
gem 'pg', '>= 0.18', '< 2.0'

# Web server
gem 'puma', '~> 5.0'

# CSS preprocessor
gem 'sass-rails', '>= 6'

# Development and test gems
group :development, :test do
  gem 'byebug', platforms: [:mri, :mingw, :x64_mingw]
  gem 'factory_bot_rails'
  gem 'rspec-rails', '~> 5.0'

2. Be mindful of gem version requirements

When specifying gem version requirements in your Gemfile, it's important to strike a balance between allowing for updates and ensuring stability. You can use different operators to specify the desired gem version range, such as:

  • gem 'rails', '6.1.4': Use exactly version 6.1.4 of Rails
  • gem 'pg', '>= 0.18', '< 2.0': Use a version of PostgreSQL gem between 0.18 and 2.0
  • gem 'puma', '~> 5.0': Use a version of Puma around 5.0 (e.g., 5.0.x, 5.1.x, but not 6.x)

It's generally a good idea to use the tilde-greater-than operator (~>) to allow for updates within a specific major or minor version, depending on the gem's stability and update frequency.

3. Use Bundler to manage your application's environments

Bundler can help you manage different environments for your Rails application, such as development, test, and production. You can use the group keyword in your Gemfile to specify gems that should only be installed for specific environments.

For example:

group :development, :test do
  gem 'rspec-rails', '~> 5.0'

group :production do
  gem 'unicorn'

In this example, the rspec-rails gem will only be installed for development and test environments, while the unicorn gem will only be installed for production environments. This helps keep your application lean and avoids installing unnecessary gems in production.

4. Use bundle exec to run commands

When you run a command that uses a gem installed by Bundler, it's a good idea to prefix the command with bundle exec. This ensures that the command will use the correct gem versions specified in your Gemfile.lock.

For example:

bundle exec rails server

This command will start the Rails server using the gem versions specified in your Gemfile.lock.


Bundler is an essential tool for managing the dependencies of your Ruby on Rails applications. It helps you manage your application's gem dependencies, ensuring that your application runs smoothly and avoids conflicts between different gem versions. By understanding how Bundler works and following best practices, you can make your Rails application more robust, maintainable, and enjoyable to work with.