☯️ Dotenv::Merge

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Dotenv::Merge is a standalone Ruby module that intelligently merges two versions of a dotenv (.env) file. It's like a smart "git merge" specifically designed for environment configuration files. Built on top of ast-merge, it shares the same architecture as prism-merge for Ruby source files.

• 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
• Standalone: Minimal dependencies - just ast-merge
• 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 docs)
  • add_template_only_nodes - setting to retain variables that do not exist in destination
  • freeze_token - customize freeze block markers (default: "dotenv-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.to_s)

Example implementations for the gem templating use case:

merger = Dotenv::Merge::SmartMerger.new(
  template_content,
  dest_content,
  # Which version to prefer when variables match
  # :destination (default) - keep destination values
  # :template - use template values
  preference: :destination,

  # Whether to add template-only variables to the result
  # false (default) - only include variables that exist in destination
  # true - include all template variables
  add_template_only_nodes: false,

  # Token for freeze block markers
  # Default: "dotenv-merge"
  # Looks for: # dotenv-merge:freeze / # dotenv-merge:unfreeze
  freeze_token: "dotenv-merge",

  # Custom signature generator (optional)
  # Receives an EnvLine, returns a signature array or nil
  signature_generator: ->(line) { [:env, line.key] if line.assignment? },
)

require "dotenv/merge"

# Template defines the structure
template = <<~ENV
  # Database configuration
  DATABASE_URL=postgres://localhost/myapp_dev
  DATABASE_POOL=5

  # API keys
  API_KEY=your_api_key_here
  API_SECRET=your_secret_here
ENV

# Destination has customizations
destination = <<~ENV
  # Database configuration
  DATABASE_URL=postgres://production.example.com/myapp
  DATABASE_POOL=25

  # Custom setting not in template
  CUSTOM_SETTING=my_value
ENV

merger = Dotenv::Merge::SmartMerger.new(template, destination)
result = merger.merge
puts result

Freeze blocks protect sections from being overwritten during merge:

# Database configuration
DATABASE_URL=postgres://localhost/myapp_dev

# dotenv-merge:freeze Custom API credentials
API_KEY=my_secret_production_key
API_SECRET=super_secret_value
# dotenv-merge:unfreeze

# Other settings
DEBUG=false

Content between # dotenv-merge:freeze and # dotenv-merge:unfreeze markers is preserved from the destination file, regardless of what the template contains.

merger = Dotenv::Merge::SmartMerger.new(
  template,
  destination,
  add_template_only_nodes: true,
)
result = merger.merge
# Result includes variables from template that don't exist in destination

This gem is part of a family of gems that provide intelligent merging for various file formats:

Tokens to Remember
Works with JRuby
Works with Truffle Ruby
Works with MRI Ruby 3
Support & Community
Source
Documentation
Compliance
Style
Maintainer 🎖️
... 💖	🧊 🐙 🛖 🧪

Compatible with MRI Ruby 3.2.0+, and concordant releases of JRuby, and TruffleRuby.

Federated DVCS Repository	Status	Issues	PRs	Wiki	CI	Discussions
🧪 kettle-rb/dotenv-merge on GitLab	The Truth	💚	💚	💚	🐭 Tiny Matrix	➖
🧊 kettle-rb/dotenv-merge on CodeBerg	An Ethical Mirror (Donate)	💚	💚	➖	⭕️ No Matrix	➖
🐙 kettle-rb/dotenv-merge on GitHub	Another Mirror	💚	💚	💚	💯 Full Matrix	💚
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Alternatively:

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

This gem is cryptographically signed, and has verifiable SHA-256 and SHA-512 checksums by stone_checksums. Be sure the gem you install hasn’t been tampered with by following the instructions below.

Add my public key (if you haven’t already, expires 2045-04-29) as a trusted certificate:

gem cert --add <(curl -Ls https://raw.github.com/galtzo-floss/certs/main/pboling.pem)

You only need to do that once. Then proceed to install with:

gem install dotenv-merge -P HighSecurity

The HighSecurity trust profile will verify signed gems, and not allow the installation of unsigned dependencies.

If you want to up your security game full-time:

bundle config set --global trust-policy MediumSecurity

MediumSecurity instead of HighSecurity is necessary if not all the gems you use are signed.

NOTE: Be prepared to track down certs for signed gems and add them the same way you added mine.

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
)

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

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

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)

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

# 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

While kettle-rb tools are free software and will always be, the project would benefit immensely from some funding. Raising a monthly budget of... "dollars" would make the project more sustainable.

We welcome both individual and corporate sponsors! We also offer a wide array of funding channels to account for your preferences (although currently Open Collective is our preferred funding platform).

If you're working in a company that's making significant use of kettle-rb tools we'd appreciate it if you suggest to your company to become a kettle-rb sponsor.

You can support the development of kettle-rb tools via GitHub Sponsors, Liberapay, PayPal, Open Collective and Tidelift.

Support us with a monthly donation and help us continue our activities. [Become a backer]

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I’m driven by a passion to foster a thriving open-source community – a space where people can tackle complex problems, no matter how small. Revitalizing libraries that have fallen into disrepair, and building new libraries focused on solving real-world challenges, are my passions. I was recently affected by layoffs, and the tech jobs market is unwelcoming. I’m reaching out here because your support would significantly aid my efforts to provide for my family, and my farm (11 🐔 chickens, 2 🐶 dogs, 3 🐰 rabbits, 8 🐈‍ cats).

If you work at a company that uses my work, please encourage them to support me as a corporate sponsor. My work on gems you use might show up in bundle fund.

I’m developing a new library, floss_funding, designed to empower open-source developers like myself to get paid for the work we do, in a sustainable way. Please give it a look.

Floss-Funding.dev: 👉️ No network calls. 👉️ No tracking. 👉️ No oversight. 👉️ Minimal crypto hashing. 💡 Easily disabled nags

See SECURITY.md.

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 reek, 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.

See CONTRIBUTING.md.

Everyone interacting with this project's codebases, issue trackers, chat rooms and mailing lists agrees to follow the .

Made with contributors-img.

Also see GitLab Contributors: https://gitlab.com/kettle-rb/dotenv-merge/-/graphs/main

This Library adheres to . Violations of this scheme should be reported as bugs. Specifically, if a minor or patch version is released that breaks backward compatibility, a new version should be immediately released that restores compatibility. Breaking changes to the public API will only be introduced with new major versions.

dropping support for a platform is both obviously and objectively a breaking change
—Jordan Harband (@ljharb, maintainer of SemVer) in SemVer issue 716

I understand that policy doesn't work universally ("exceptions to every rule!"), but it is the policy here. As such, in many cases it is good to specify a dependency on this library using the Pessimistic Version Constraint with two digits of precision.

For example:

spec.add_dependency("dotenv-merge", "~> 1.0")

SemVer should, IMO, but doesn't explicitly, say that dropping support for specific Platforms is a breaking change to an API, and for that reason the bike shedding is endless.

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:

• "Major Version Numbers are Not Sacred"

See CHANGELOG.md for a list of releases.

The gem is available as open source under the terms of the MIT License . See LICENSE.txt for the official Copyright Notice.

• Copyright (c) 2025 Peter H. Boling, of Galtzo.com , and dotenv-merge contributors.

Maintainers have teeth and need to pay their dentists. After getting laid off in an RIF in March, and encountering difficulty finding a new one, I began spending most of my time building open source tools. I'm hoping to be able to pay for my kids' health insurance this month, so if you value the work I am doing, I need your support. Please consider sponsoring me or the project.

To join the community or get help 👇️ Join the Discord.

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Thanks for RTFM. ☺️