If you want to get the path of a file relative to another, you can use the expand_path method with either the constant __FILE__ or the method __dir__. Read this card for more information about __FILE__ and __dir__.

Example

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Structure:

.
├── bin
│   ├── format_changelog
├── CHANGELOG.md

bin/format_changelog:

#!/usr/bin/env ruby

changelog_path = ? # How to get the path to ../CHANGELOG.md independent of the working dir of the caller
changelog = File.read(changelog_path)

# ... further actions here

Ruby >= 2.0

changelog_path = File.expand_path('../CHANGELOG.md', __dir__)

Ruby < 2.0

changelog_path = File.expand_path('../../CHANGELOG.md', __FILE__)

Rails

changelog_path = Rails.root.join('CHANGELOG.md')