Ways to Set Environment Variables With Docker Compose

2024-02-02 1355 words 7 minutes

Contents

Docker Compose is a tool for defining and running multi-container Docker applications. It defines the services that make up an application, the networks they connect to, and the environment variables they need to run.

With Docker Compose, there are multiple ways to set environment variables in containers: the Compose file, or the CLI. Each method is subject to environment variable precedence.

Tip

Don’t use environment variables to pass sensitive information, such as passwords, in to your containers. Use secrets instead.

Compose file

Substitute with an .env file

An .env file in Docker Compose is a text file used to define environment variables that should be made available to Docker containers when running docker compose up. This file typically contains key-value pairs of environment variables, and it centralizes configuration in one place. The .env file is useful when multiple environment variables need storing.

The .env file is the default method for setting environment variables in your containers. The .env file should be placed at the root of the project directory next to your compose.yaml file. For more information on formatting an environment file, see Syntax for environment files.

Below is a simple example:

        
$ cat .env
TAG=v1.5

$ cat compose.yml
services:
  web:
    image: "webapp:${TAG}"

When docker compose up runs, the web service defined in the Compose file interpolates in the image webapp:v1.5 which was set in the .env file. Verify this with the config command, which prints the resolved application config to the terminal:

        
$ docker compose config

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

Additional information

As of Docker Compose version 2.24.0, the .env file can be optional via the env_file attribute. When required is set to false and the .env file is missing, Compose silently ignores the entry.
```
env_file:
  - path: ./default.env
    required: true # default
  - path: ./override.env
    required: false
```
An environment variable defined in the .env file can be referenced directly in compose.yml with the environment attribute. For example, if .env contains DEBUG=1 and compose.yml looks like this:
```
services:
  webapp:
    image: my-webapp-image
    environment:
      - DEBUG=${DEBUG}
```
Docker Compose replaces ${DEBUG} with the value from the .env file
Multiple .env files can be listed in compose.yml with the env_file attribute. Docker Compose reads them in the order specified. If the same variable is defined in multiple files, the last definition takes precedence:
```
services:
  webapp:
    image: my-webapp-image
    env_file:
      - .env
      - .env.override
```
The .env file can live outside the project root. Use one of the following methods so Compose can navigate to it:
- The --env-file option in the CLI
- Using the env_file attribute in the Compose file
Values in your .env file can be overridden from the command line by using docker-compose up -e.
Your .env file can be overridden by another .env if it is substituted with --env-file.

Important

Substitution from .env files is a Docker Compose CLI feature.

It is not supported by Swarm when running docker stack deploy.

Use the environment attribute

Environment variables can be set directly in the Compose file without an .env file, with the environment attribute in compose.yml. It works in the same way as docker run -e VARIABLE=VALUE ...

        
web:
  environment:
    - DEBUG=1

See environment attribute for more examples on how to use it.

Additional information

Omit the value to pass environment variables from the shell straight through to the containers. It works in the same way as 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. Note that in this case no warning is issued if the DEBUG variable in the shell environment is not set.
Interpolation is also available.
```
web:
  environment:
    - DEBUG=${DEBUG}
```
The result is similar to the one above but Compose warns if the DEBUG variable is not set in the shell environment.

Use the env_file attribute

The env_file attribute attaches multiple .env files in a Compose application. It keeps environment variables separate from the main configuration file, so the .env file does not have to sit in the project root.

It works in the same way as docker run --env-file=FILE ....

        
web:
  env_file:
    - web-variables.env

Additional information

If multiple files are specified, they are evaluated in order and can override values set in previous files.
Environment variables declared in the .env file cannot then be referenced again separately in the Compose file.
If both env_file and environment are set, variables from environment take precedence.
The paths to your .env file, specified in the env_file attribute, are relative to the location of your compose.yml file.
Values in your .env files can be overridden from the command line by using docker compose run -e.
Your .env files can be overriden by another .env if it is substituted with --env-file.
As of Docker Compose version 2.24.0, the .env file can be optional via the required field. When required is set to false and the .env file is missing, Compose silently ignores the entry.
```
env_file:
  - path: ./default.env
    required: true # default
  - path: ./override.env
    required: false
```

Substitute from the shell

Existing environment variables can come from the host or from the shell that runs docker compose. Values inject into the Compose configuration at runtime.

For example, suppose the shell contains POSTGRES_VERSION=9.3 and the following configuration is supplied:

        
db:
  image: "postgres:${POSTGRES_VERSION}"

When docker compose up runs with this configuration, Compose looks for POSTGRES_VERSION in the shell and substitutes its value. For this example, Compose resolves the image to postgres:9.3 before running the configuration.

If an environment variable is not set, Compose substitutes with an empty string. In the example above, if POSTGRES_VERSION is not set, the value for the image option is postgres:.

Note

postgres: is not a valid image reference. Docker expects either a reference without a tag, like postgres which defaults to the latest image, or with a tag such as postgres:15.

Important

Values set in the shell environment override those set in the .env file, the environment attribute, and the env_file attribute. For more information, see Environment variable precedence.

CLI

Substitute with –env-file

Default values for multiple environment variables can live in an environment file and be passed as a CLI argument.

The file can live anywhere and be named appropriately, for example,

This file path is relative to the current working directory where the Docker Compose command is executed. Passing the file path is done using the --env-file option:

docker compose --env-file ./config/.env.dev up

Additional information

Useful to temporarily override an .env file already referenced in compose.yml. For example production (.env.prod) vs testing (.env.test). In the following example, .env and .env.dev set different TAG values.
```
$ cat .env
TAG=v1.5

$ cat ./config/.env.dev
TAG=v1.6

$ cat compose.yml
services:
  web:    image: "webapp:${TAG}"
```
If the --env-file is not used in the command line, the .env file is loaded by default:
```
$ docker compose config
services:
  web:    image: 'webapp:v1.5'
```
Passing the --env-file argument overrides the default file path:
```
$ docker compose --env-file ./config/.env.dev config
services:
  web:    image: 'webapp:v1.6'
```
When an invalid file path is being passed as an --env-file argument, Compose returns an error:
```
$ docker compose --env-file ./doesnotexist/.env.dev  config
ERROR: Couldn't find env file: /home/user/./doesnotexist/.env.dev
```
Multiple --env-file options specify multiple environment files. Docker Compose reads them in order. Later files override earlier ones.
```
$ docker compose --env-file .env --env-file .env.override up
```

Specific environment variables can be overridden from the command line when starting containers.

        
$ docker compose --env-file .env.dev up -e DATABASE_URL=mysql://new_user:new_password@new_db:3306/new_database

Set environment variables with docker compose run –env

Similar to docker run --env, environment variables can be set temporarily with docker compose run --env or docker compose run -e:

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

Additional information

A variable can also be passed from the shell by omitting the 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.

Further resources

References

Ways to set environment variables with Compose