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Gemfile

What is a .gemfile file?

The Ruby project's dependency manifest — Bundler reads this and installs every gem listed, pinning resolved versions in Gemfile.lock for reproducibility.

Safe format
Type Code
By André Arko / Yehuda Katz (Bundler)
MIME text/x-ruby

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What is it

Every Ruby project past hello-world has a Gemfile at its root. It's Ruby's equivalent of `package.json` (Node), `requirements.txt` (Python), or `Cargo.toml` (Rust): a single file declaring which gems (Ruby libraries) the project depends on. Bundler reads this file, resolves a compatible set of versions, installs them, and writes the resolved versions to a Gemfile.lock that locks the entire dependency tree.

The syntax is a Ruby DSL — yes, the Gemfile is itself executable Ruby code. `source 'https://rubygems.org'` at the top declares the gem registry. `gem 'rails', '~> 7.1'` adds Rails with a pessimistic version constraint (>= 7.1, < 8.0). Gems can be grouped (`group :development, :test do ... end`) so production deploys skip dev-only dependencies. Versioned operators include `~>` (pessimistic), `>=` (minimum), and `=` (exact).

Gemfile and Gemfile.lock both belong in version control for applications. Gemfile.lock pins exact versions across machines so `bundle install` produces identical environments everywhere. `bundle update` upgrades dependencies within the constraints in Gemfile and rewrites the lock file. The pair is foundational to Ruby — every Ruby tool (Rails, Sinatra, Jekyll, Sidekiq) assumes you're using Bundler. The CLI is `bundle install`, `bundle exec`, `bundle update`.

Technical details
Full Name
Gemfile
MIME Type
text/x-ruby
Developer
André Arko / Yehuda Katz (Bundler)
Magic Bytes
N/A
Safety
.gemfile is a known, safe format.
What opens it
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FAQ
What's the difference between Gemfile and Gemfile.lock?
Gemfile is what you write — declared dependencies with version constraints. Gemfile.lock is what Bundler generates — the resolved, pinned versions of every gem (and every gem's dependencies, recursively). Both belong in git: Gemfile expresses intent, Gemfile.lock guarantees reproducibility.
Should I commit Gemfile.lock for libraries?
For applications: yes, always. For libraries published as gems: traditionally no, because users of your library shouldn't be locked to your specific dependency versions. Bundler 2 made this debatable — recent guidance leans toward committing it for libraries too, but treating it as a development convenience rather than a contract.
Why is the Gemfile written in Ruby?
Because Bundler is a Ruby tool and the simplest way to parse a config file in Ruby is to make it Ruby. The DSL approach lets you do conditional logic (`gem 'pg' if ENV['DATABASE_URL']`) which JSON or TOML can't easily express.
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