How Do I Pass Environment Variables to Docker Containers?

Environment variables let you configure your Docker containers without hardcoding values. Whether you're passing secrets, app settings, or deployment-specific flags, env vars make it clean and portable.

In this guide, you'll learn several ways to pass environment variables to containers and when to use each method.

Prerequisites

You'll need:

• Docker installed (Docker Engine 24+)
• A basic Docker image and container setup

Option 1: Use -e or --env on the Command Line

You can set environment variables directly with the docker run command:

docker run -e APP_ENV=production -e DEBUG=false my-app-image

This passes APP_ENV and DEBUG into the container at runtime.

Use this for quick tests or when setting a few variables.

Option 2: Use an .env File

For more variables or shared configs, you can use a .env file:

# .env file
APP_ENV=production
DEBUG=false
API_KEY=123abc

Then run:

docker run --env-file .env my-app-image

Docker will load every variable from that file into the container.

This is great for local development or keeping secrets out of your terminal history.

Option 3: Set Env Vars in a Dockerfile

You can bake variables directly into the image using ENV in your Dockerfile:

# Dockerfile
FROM node:18
ENV APP_ENV=production
ENV DEBUG=false

These variables become part of the image and are always available in containers built from it.

Use this for default values that shouldn't change per deployment.

Option 4: Use Compose Files (Recommended for Dev)

If you're using Docker Compose, you can define environment variables in docker-compose.yml:

version: '3.8'
services:
  app:
    image: my-app-image
    environment:
      - APP_ENV=production
      - DEBUG=false

Or pull from a .env file automatically:

# .env file (in same dir as docker-compose.yml)
APP_ENV=production
DEBUG=false

Docker Compose loads this automatically, no extra flags needed.

Bonus: Accessing Env Vars Inside the Container

To see the values inside the container, you can run:

docker exec my-container printenv

Or check a specific variable:

docker exec my-container printenv APP_ENV

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Environment variables give you a clean, flexible way to configure containers. For local work, .env files and Compose are easy to manage. For CI/CD, prefer --env flags or secrets managers.

Keep it clean, keep it configurable.

Happy shipping!