Docker ENV File Parser

A .env file stores environment variables as KEY=VALUE pairs, one per line. It's used by Docker, Node.js (via dotenv), Python (via python-dotenv), and many frameworks to inject configuration without hardcoding values in source code. Lines starting with # are comments.

ARG is a build-time variable only available during docker build. ENV sets a runtime environment variable that persists in the container image and is available to running processes. Use ARG for build secrets or version pins; use ENV for app configuration that the container needs at runtime.

The env_file directive in docker-compose.yml reads a file and injects all KEY=VALUE pairs as environment variables into the container. It supports the same .env format: comments (#), blank lines, and optional quotes around values. Variables defined this way do not expand other variables.

Use --env-file <path> to pass all variables from a file, or -e KEY=VALUE for individual variables. Example: docker run --env-file .env myimage. The --env-file flag does not support variable expansion or multi-line values.

No. Never commit .env files containing real secrets. Add .env to your .gitignore. Instead, commit a .env.example file with placeholder values so other developers know what variables are required. Use a secrets manager (AWS Secrets Manager, Vault, Doppler) for production credentials.