dollarsign_handling

Research into, how docker compose treats dollar signs.

Templating

The .env-file states the environment variables that you can use, when the docker-compose file is evaluated.

The contents of the .env file are not available inside the running container.

If you want to use variables defined in your .env-file inside the container. You can refer them in the environments part of the the docker compose file.

Example

Refer to the following docker compose file and environment file.

When running with docker up:

> docker compose up
[+] Running 1/1r Desktop   o View Config   w Enable Watch
 ✔ Container myapp-foo-1  Recreated                                                                                                                                                                                                   0.1s 
Attaching to foo-1


foo-1  | TO DAY NAME_WITH_ and NAME_FROM_ENV

foo-1  | ENV_NAME=ENV_NAMEX
foo-1  | NAME_WITH_DOLLAR_SIGN_FROM_ENV=NAME_WITH_$_SIGN_FROM_ENV
                                        
foo-1 exited with code 0

Only the environment variables mentioned in the environments part of the docker file are included (show running printenv). The variable NAME_FROM_ENV was only available when the docker compose file was evaluated.

Also, notice how the dollar signs are 'eaten'.

Docker compose config shows how the docker compose file is evaluated.

> docker compose config
name: myapp
services:
  foo:
    command:
      - sh
      - -c
      - echo TO DAY NAME_WITH_$$_SIGN_FROM_ENV and NAME_FROM_ENV && printenv
    environment:
      ENV_NAME: ENV_NAMEX
      NAME_WITH_DOLLAR_SIGN_FROM_ENV: NAME_WITH_$$_SIGN_FROM_ENV
    image: busybox
    networks:
      default: null
networks:
  default:
    name: myapp_default

Try it out

If you want to try it out. The .env.sample file should be copied into the file .env.