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ENV

An ENV file defines environment variables in KEY=VALUE format for application configuration; it is the recommended approach of 12-factor apps.

Purpose

To provide the standard way of injecting configuration through environment variables, separating configuration from code.

Mechanism

Each line is KEY=VALUE; # marks comments and blank lines are ignored. Values usually carry no quotes (quote them when they contain spaces). A shell or a dotenv library loads them into process.env. No types (all strings) and no nesting.

Use cases

  • Node.js .env configuration
  • Docker container environment variables
  • CI/CD secret injection
  • Local development configuration

Examples

  • Input 
    PORT=3000
DB_URL=postgres://...
    Output 
    process.env.PORT="3000"

    Everything is a string

Common errors

Committing .env to version control leaks secrets
Add it to .gitignore and provide a .env.example template

Related terms

dotenv
dotenv is the library and convention that loads .env files into environment variables; it is the most popular in the Node.js ecosystem and externalizes configuration.
GLOSSARY
INI
INI is a traditional configuration file format that groups settings with [section] headers and key=value pairs; it is simple but unstandardized, and implementations differ slightly.
GLOSSARY
Properties
Java Properties files use KEY=VALUE or KEY:VALUE format; they are the traditional configuration format of the Java ecosystem and support Unicode escapes.
GLOSSARY
YAML
YAML (YAML Ain't Markup Language) is a configuration-oriented data serialization format using indentation for hierarchy, supporting comments and references, and better suited to hand editing than JSON.
GLOSSARY

References