☯️ Bash::Merge

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👣 How will this project approach the September 2025 hostile takeover of RubyGems? 🚑️

I've summarized my thoughts in this blog post.

🌻 Synopsis

Bash::Merge intelligently merges two versions of a Bash script using the StructuredMerge Ruby stack. It is built on ast-merge and tree_haver, with Bash parsing supplied through the current tree-sitter language-pack backend.

Key Features

Tree-Sitter Powered: Uses tree-sitter-bash for accurate AST parsing
Script-Aware: Understands Bash syntax including functions, variables, and commands
Intelligent: Matches functions and variable assignments by name
Comment-Preserving: Comments are preserved in their context
Shebang Handling: Properly handles #!/bin/bash and similar shebangs
Freeze Block Support: Respects freeze markers (default: bash-merge:freeze / bash-merge:unfreeze) for merge control - customizable to match your project's conventions
Full Provenance: Tracks origin of every node
StructuredMerge Native: Depends on ast-merge and tree_haver; parser availability comes from the registered backend providers

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 nodes that do not exist in destination
- `freeze_token` - customize freeze block markers (default: `"bash-merge"`)

Supported Node Types

Node Type	Signature Format	Matching Behavior
Function Definition	`[:function, name]`	Functions match by name
Variable Assignment	`[:assignment, name]`	Variables match by name
Command	`[:command, name, args...]`	Commands match by name and arguments
Comment	`[:comment, text]`	Comments preserved in context
Pipeline	`[:pipeline, commands...]`	Pipelines match by command sequence
If Statement	`[:if, condition_sig]`	Conditionals match by condition signature
For Loop	`[:for, variable]`	For loops match by loop variable
While Loop	`[:while, condition_sig]`	While loops match by condition signature

Example

require "bash/merge"

template = File.read("template.sh")
destination = File.read("destination.sh")

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

File.write("merged.sh", result.to_bash)

💡 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
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 bash-merge

If bundler is not being used to manage dependencies, install the gem by executing:

gem install bash-merge

⚙️ Configuration

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

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

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

  # Custom signature generator (optional)
  # Receives a node, returns a signature array or nil
  signature_generator: ->(node) { [:function, node.name] if node.type == :function_definition },
)

🔧 Basic Usage

Simple Merge

require "bash/merge"

# Template defines the structure
template = <<~BASH
  #!/bin/bash

  # Configuration
  APP_NAME="myapp"
  DEBUG=false

  # Main function
  main() {
    echo "Starting $APP_NAME"
    setup
    run
  }

  setup() {
    echo "Setting up..."
  }

  run() {
    echo "Running..."
  }

  main "$@"
BASH

# Destination has customizations
destination = <<~BASH
  #!/bin/bash

  # Configuration
  APP_NAME="myapp-custom"
  DEBUG=true
  LOG_FILE="/var/log/myapp.log"

  # Main function with custom logging
  main() {
    echo "Starting $APP_NAME" | tee -a "$LOG_FILE"
    setup
    run
  }

  setup() {
    echo "Custom setup..."
  }

  main "$@"
BASH

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

Using Freeze Blocks

Freeze blocks protect sections from being overwritten during merge:

#!/bin/bash

# Configuration
APP_NAME="myapp"

# bash-merge:freeze Custom credentials
DB_USER="production_user"
DB_PASS="super_secret_password"
API_KEY="my_production_api_key"
# bash-merge:unfreeze

# Standard functions
main() {
  echo "Starting $APP_NAME"
}

main "$@"

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

Adding Template-Only Nodes

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

🔐 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("bash-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:

"Major Version Numbers are Not Sacred"

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.