Environment variables in Compose

There are multiple parts of Compose that deal with environment variables in one sense or another. This page should help you find the information you need.

Substitute environment variables in Compose files

It’s possible to use environment variables in your shell to populate values inside a Compose file:

web:
  image: "webapp:${TAG}"

For more information, see the Variable substitution section in the Compose file reference.

Set environment variables in containers

You can set environment variables in a service’s containers with the ‘environment’ key, just like with docker run -e VARIABLE=VALUE ...:

web:
  environment:
    - DEBUG=1

Pass environment variables to containers

You can pass environment variables from your shell straight through to a service’s containers with the ‘environment’ key by not giving them a value, just like with docker run -e VARIABLE ...:

web:
  environment:
    - DEBUG

The value of the DEBUG variable in the container is taken from the value for the same variable in the shell in which Compose is run.

The “env_file” configuration option

You can pass multiple environment variables from an external file through to a service’s containers with the ‘env_file’ option, just like with docker run --env-file=FILE ...:

web:
  env_file:
    - web-variables.env

Set environment variables with ‘docker-compose run’

Just like with docker run -e, you can set environment variables on a one-off container with docker-compose run -e:

docker-compose run -e DEBUG=1 web python console.py

You can also pass a variable through from the shell by not giving it a value:

docker-compose run -e DEBUG web python console.py

The value of the DEBUG variable in the container is taken from the value for the same variable in the shell in which Compose is run.

The “.env” file

You can set default values for any environment variables referenced in the Compose file, or used to configure Compose, in an environment file named .env:

$ cat .env
TAG=v1.5

$ cat docker-compose.yml
version: '3'
services:
  web:
    image: "webapp:${TAG}"

When you run docker-compose up, the web service defined above uses the image webapp:v1.5. You can verify this with the config command, which prints your resolved application config to the terminal:

$ docker-compose config

version: '3'
services:
  web:
    image: 'webapp:v1.5'

Values in the shell take precedence over those specified in the .env file. If you set TAG to a different value in your shell, the substitution in image uses that instead:

$ export TAG=v2.0
$ docker-compose config

version: '3'
services:
  web:
    image: 'webapp:v2.0'

When you set the same environment variable in multiple files, here’s the priority used by Compose to choose which value to use:

1. Compose file
2. Shell environment variables
3. Environment file
4. Dockerfile
5. Variable is not defined

In the example below, we set the same environment variable on an Environment file, and the Compose file:

$ cat ./Docker/api/api.env
NODE_ENV=test

$ cat docker-compose.yml
version: '3'
services:
  api:
    image: 'node:6-alpine'
    env_file:
     - ./Docker/api/api.env
    environment:
     - NODE_ENV=production

When you run the container, the environment variable defined in the Compose file takes precedence.

$ docker-compose exec api node

> process.env.NODE_ENV
'production'

Having any ARG or ENV setting in a Dockerfile evaluates only if there is no Docker Compose entry for environment or env_file.

Specifics for NodeJS containers

If you have a package.json entry for script:start like NODE_ENV=test node server.js, then this overrules any setting in your docker-compose.yml file.

Configure Compose using environment variables

Several environment variables are available for you to configure the Docker Compose command-line behavior. They begin with COMPOSE_ or DOCKER_, and are documented in CLI Environment Variables.

Environment variables created by links

When using the ‘links’ option in a v1 Compose file, environment variables are created for each link. They are documented in the Link environment variables reference.

However, these variables are deprecated. Use the link alias as a hostname instead.