☯️ Dotenv::Merge
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if ci_badges.map(&:color).all? { it == "green"} 👇️ send money so I can do more of this. FLOSS maintenance is now my full-time job.
👣 How will this project approach the September 2025 hostile takeover of RubyGems? 🚑️
I've summarized my thoughts in this blog post.
🌻 Synopsis 
Dotenv::Merge intelligently merges two versions of a dotenv (.env) file. It is built on ast-merge, plain-merge, and tree_haver. Its parser enters through TreeHaver.parser_for(:dotenv, backend_type: :line) and builds dotenv-specific ownership rules on the shared plain line substrate.
Key Features
- Dotenv-Aware: Understands dotenv file format (KEY=value, comments, exports)
- Intelligent: Matches environment variables by key name
- Comment-Preserving: Comments and blank lines are preserved in their context
- Freeze Block Support: Respects freeze markers (default:
dotenv-merge:freeze/dotenv-merge:unfreeze) for merge control - customizable to match your project's conventions - Full Provenance: Tracks origin of every line
- StructuredMerge Native: Depends on
ast-mergeandtree_haver, matching the rest of the Ruby merge family - Line Backend: Registers
:dotenvand:envparser paths throughTreeHaver.parser_forand carries the underlying plain line analysis. -
Customizable:
- `signature_generator` - callable custom signature generators - `preference` - setting of `:template`, `:destination`, or a Hash for per-node-type preferences - `node_splitter` - Hash mapping node types to callables for per-node-type merge customization (see [ast-merge][ast-merge] docs) - `add_template_only_nodes` - setting to retain variables that do not exist in destination - `freeze_token` - customize freeze block markers (default: `"dotenv-merge"`)
Supported Line Types
| Line Type | Format | Matching Behavior |
|---|---|---|
| Assignment | KEY=value |
Variables match by key name |
| Export | export KEY=value |
Treated as assignment with export flag |
| Comment | # comment text |
Preserved in context |
| Blank | (empty line) | Preserved for readability |
| Double-quoted | KEY="value with\nescapes" |
Escape sequences processed |
| Single-quoted | KEY='literal value' |
No escape processing |
| Inline comment | KEY=value # comment |
Comment stripped from value |
Example
require "dotenv/merge"
template = File.read("template.env")
destination = File.read("destination.env")
merger = Dotenv::Merge::SmartMerger.new(template, destination)
result = merger.merge
File.write("merged.env", result.to_s)
💡 Info you can shake a stick at
| Tokens to Remember |   |
|---|---|
| Works with MRI Ruby 4 |  |
| Support & Community |      |
| Source |     |
| Documentation |      |
| Compliance |      |
| Style |     |
| Maintainer 🎖️ |      |
... 💖 |
    🧊 🐙 🛖 🧪 |
Compatibility
Compatible with MRI Ruby 4.0.0+, and concordant releases of JRuby, and TruffleRuby.
CI workflows and Appraisals are generated for MRI Ruby 4.0.0+.
This test floor is configured by ruby.test_minimum in .kettle-jem.yml and
may be higher than the gem's runtime compatibility floor when legacy Rubies are
not practical for the current toolchain.
The amazing test matrix is powered by the kettle-dev stack.
How kettle-dev manages complexity in tests
| Gem | Source | Role | Total downloads |
|---|---|---|---|
| appraisal2 | GitHub | multi-dependency Appraisal matrix generation |  |
| appraisal2-rubocop | GitHub | RuboCop Appraisal generator integration |  |
| kettle-dev | GitHub | development, release, and CI workflow tooling |  |
| kettle-jem | GitHub | Appraisals & CI workflow templates |  |
| kettle-soup-cover | GitHub | SimpleCov coverage policy and reporting |  |
| kettle-test | GitHub | standard test runner and coverage harness |  |
| rubocop-lts | GitHub | Ruby-version-aware linting |  |
| turbo_tests2 | GitHub | parallel test execution |  |
✨ Installation
Install the gem and add to the application's Gemfile by executing:
bundle add dotenv-merge
If bundler is not being used to manage dependencies, install the gem by executing:
gem install dotenv-merge
⚙️ Configuration
merger = Dotenv::Merge::SmartMerger.new(
template,
destination,
# When conflicts occur, prefer template or destination values
preference: :template, # or :destination (default)
# Add entries that only exist in template
add_template_only_nodes: true, # default: false
)
Signature Match Preference
Control which source wins when both files have the same key:
-
:template- Template values replace destination values- Version files (`VERSION=2.0.0` should replace `VERSION=1.0.0`) - API endpoint updates (`API_URL=https://new-api.example.com`) -
:destination(default) - Destination values are preserved- Local development settings - Project-specific customizations
Template-Only Nodes
Control whether to add entries that only exist in the template:
-
true- Add new entries from template- New required environment variables - New configuration options -
false(default) - Skip template-only entries- Template has placeholder values - Destination is authoritative
🔧 Basic Usage
Simple Merge
require "dotenv/merge"
template = File.read("template.env")
destination = File.read("destination.env")
merger = Dotenv::Merge::SmartMerger.new(template, destination)
result = merger.merge
File.write("merged.env", result)
Working with Freeze Blocks
Freeze blocks protect sections of your .env file from being modified during merges:
# << FREEZE: project_secrets
DATABASE_URL=postgresql://localhost/myapp_dev
SECRET_KEY_BASE=my_local_secret_key_value
# >> FREEZE: project_secrets
# These entries can be updated by template
API_VERSION=v2
Adding Template-Only Entries
# Template introduces a new required variable
template = <<~ENV
DATABASE_URL=postgresql://localhost/template_db
NEW_FEATURE_FLAG=enabled
ENV
destination = <<~ENV
DATABASE_URL=postgresql://localhost/myapp_dev
ENV
merger = Dotenv::Merge::SmartMerger.new(
template,
destination,
add_template_only_nodes: true,
)
result = merger.merge
# Result includes DATABASE_URL from destination + NEW_FEATURE_FLAG from template
🔐 Security
See SECURITY.md.
🤝 Contributing
If you need some ideas of where to help, you could work on adding more code coverage, or if it is already 💯 (see below) check issues or PRs, or use the gem and think about how it could be better.
We so if you make changes, remember to update it.
See CONTRIBUTING.md for more detailed instructions.
Code Coverage
Coverage service badges
📌 Versioning
This library follows for its public API where practical.
For most applications, prefer the Pessimistic Version Constraint with two digits of precision.
For example:
spec.add_dependency("dotenv-merge", "~> 7.0")
📌 Is "Platform Support" part of the public API? More details inside.
Dropping support for a platform can be a breaking change for affected users. If a release changes supported platforms, it should be called out clearly in the changelog and versioned with that impact in mind.
To get a better understanding of how SemVer is intended to work over a project's lifetime, read this article from the creator of SemVer:
See CHANGELOG.md for a list of releases.
📄 License
The gem is available under the following licenses: AGPL-3.0-only, PolyForm-Small-Business-1.0.0. See LICENSE.md for details.
If none of the available licenses suit your use case, please contact us to discuss a custom commercial license.