> ## Documentation Index
> Fetch the complete documentation index at: https://tally.wharflab.com/llms.txt
> Use this file to discover all available pages before exploring further.

# tally/ruby/redundant-bundler-install

> `gem install bundler` on official `ruby:*` images is redundant and re-installs an already-present tool.

`gem install bundler` on official `ruby:*` images is redundant — Bundler 2.x already ships with the image.

| Property | Value                 |
| -------- | --------------------- |
| Severity | Warning               |
| Category | Performance           |
| Default  | Enabled               |
| Auto-fix | Yes (`FixSuggestion`) |

## Description

The official [`docker-library/ruby`](https://github.com/docker-library/ruby/blob/master/Dockerfile-debian.template)
image installs and pre-resolves Bundler before publishing each tag. Bundler is on `$PATH` immediately, ready
to use. Running `gem install bundler` after `FROM ruby:...` re-downloads, recompiles, and re-installs the
same tool — wasting build time and introducing version drift between local development and CI.

The corpus shows this pattern in **55 of 196 Dockerfiles**, including some from large projects
(`basecamp/kamal`, `basecamp/fizzy`, `chatwoot/chatwoot`).

This rule fires when:

* The stage's effective base image is an official `ruby:*` image (or a known devcontainer Ruby image).
* A `RUN` instruction invokes `gem install bundler` (in any of its parsed forms).

The rule treats stage refs (`FROM <stage>`) as inheriting the original external base, so a stage that's
based on an earlier `FROM ruby:3.3-slim AS builder` is also flagged.

Stages explicitly named `dev`, `development`, `test`, `testing`, `ci`, or `debug` are skipped.

### Suppressions

The rule does NOT fire when the base image is **not** an official `ruby:*` image — for example
`FROM debian:bookworm-slim` followed by manual Ruby installation, or `FROM alpine` plus `apk add ruby`. In
those cases `gem install bundler` may be the right thing to do because the base image doesn't ship Bundler.

It also does NOT fire on Ruby derivatives like `jruby:*`, `truffleruby:*`, or `phusion/passenger-ruby:*`.
These images don't carry the same Bundler-pre-install guarantee, so a manual install may be intentional.

### Context-aware refinement

When tally is invoked with `--context` and `Gemfile.lock` is observable, the violation detail mentions the
exact Bundler version recorded in the lockfile's `BUNDLED WITH` block. If a project genuinely needs that
specific Bundler version, the recommended replacement is Bundler's own version-aware shim:

```dockerfile theme={null}
RUN bundle _2.5.6_ install
```

— which uses the BUNDLED WITH version from `Gemfile.lock` directly, without a fresh `gem install`.

## Examples

### Before

```dockerfile theme={null}
FROM ruby:3.3-slim
RUN gem install bundler
COPY Gemfile Gemfile.lock ./
RUN bundle install
```

### After

```dockerfile theme={null}
FROM ruby:3.3-slim
COPY Gemfile Gemfile.lock ./
RUN bundle install
```

If a specific Bundler version is required:

```dockerfile theme={null}
FROM ruby:3.3-slim
COPY Gemfile Gemfile.lock ./
RUN bundle _2.5.6_ install
```

## Auto-fix

The rule offers a `FixSuggestion`:

* When the offending `RUN` contains **only** `gem install bundler`, the fix deletes the entire `RUN` line.
* When the install is part of a chained command (`RUN gem install bundler && bundle install`), the fix is a
  non-edit suggestion — chained command rewrites are too easy to get wrong, so the rule explains what to do
  but doesn't auto-rewrite. Edit the chain manually.

## References

* [docker-library/ruby Dockerfile.template](https://github.com/docker-library/ruby/blob/master/Dockerfile-debian.template) —
  Bundler is installed and pre-resolved before publishing.
* [Bundler `bundle _<version>_` version-shim documentation](https://bundler.io/v2.5/man/bundle.1.html) —
  the right way to pin a specific Bundler version.


## Related topics

- [tally/ruby/jemalloc-installed-but-not-preloaded](/rules/tally/ruby/jemalloc-installed-but-not-preloaded.md)
- [tally/gpu/no-redundant-cuda-install](/rules/tally/gpu/no-redundant-cuda-install.md)
- [IDE integration](/guides/ide-integration.md)
- [Installation](/installation.md)
- [CI/CD integration](/guides/ci-cd.md)