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Env Interpolation

Environment variable interpolation replaces ${VAR} or $VAR in configuration files and templates with the environment variable value, supporting defaults and nested expansion.

Purpose

To inject runtime environment variables into configuration so the same configuration fits multiple deployment environments.

Mechanism

At parse time ${VAR} and $VAR are replaced with process.env[VAR]; ${VAR:-default} supplies a default; ${VAR:+alt} uses alt when VAR is set. Recursive expansion is supported (a VAR referencing VAR2).

Use cases

  • Docker Compose variable interpolation
  • CI/CD configuration templates
  • Multi-environment configuration

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References