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About this tool
Generate a Bundler Gemfile with source, ruby version, groups, git and path sources, and pessimistic version constraints.
A Gemfile generator produces the Bundler manifest that lists a Ruby project's gem source, Ruby version and dependencies, including grouped and git-sourced gems. It also expands every pessimistic constraint so you can see the real range: ~> 1.2 means >= 1.2 and < 2.0, while ~> 1.2.3 means >= 1.2.3 and < 1.3.0. Useful when starting a Ruby or Rails project, or when reviewing whether a constraint is as tight as the author intended.
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Turns every ~> requirement into the explicit >= and < range RubyGems resolves it to.
Rejects duplicate gems, branch without a git source, and github: values that are not owner/repo.
Gem names, forks and internal paths never leave your browser.
It is the pessimistic version constraint: it allows the last digit you wrote to increase but nothing above it. ~> 6.4 permits 6.4, 6.5 and 6.9 but not 7.0, while ~> 6.4.1 permits 6.4.1 and 6.4.9 but not 6.5.0. It is the usual way to accept bug fixes without accepting breaking changes.
The Gemfile states the ranges you accept; Gemfile.lock records the exact version of every gem, including transitive ones, that Bundler resolved. Commit the lock file for applications so deploys and CI install identical versions — bundle install reads it, bundle update recalculates it.
When Bundler should install the gem but not load it automatically on Bundler.require. Command-line tools such as rubocop, and gems like bootsnap that you require yourself at a specific point in boot, are the common cases; it shortens boot time and avoids load-order surprises.
It is optional but recommended for applications, because Bundler then refuses to install under a mismatched interpreter and hosting platforms read it to pick a runtime. Use ruby file: ".ruby-version" to keep the value in one place rather than duplicating it.