02-environment

02 - Environment Variables

When docker-compose spins-up the service, it is possible to pass some information from the host to the container via the environment variables. These variables can then be accessed by programs running in the container.

The containers.*.variables section supports placeholder interpolation using ${..} syntax. This can be used to access outputs from metadata or resources. The placeholder syntax can be escaped with a double-$ like $${..}.

The environment resource to collect them from the current shell:

apiVersion: score.dev/v1b1

metadata:
  name: hello-world

containers:
  hello:
    image: busybox
    command: ["/bin/sh"]
    args: ["-c", "while true; do echo $${GREETING} $${NAME}!; sleep 5; done"]
    variables:
      GREETING: Hello
      NAME: ${resources.env.NAME}
      WORKLOAD_NAME: ${metadata.name}
      ESCAPED: $$_$${fizzbuzz}

resources:
  env:
    type: environment

Like example 01, we use generate to build a compose file:

$ score-compose init
$ score-compose generate score.yaml

And it returns

name: 02-environment
services:
    hello-world-hello:
        annotations:
            compose.score.dev/workload-name: hello-world
        command:
            - -c
            - while true; do echo $${GREETING} $${NAME}!; sleep 5; done
        entrypoint:
            - /bin/sh
        environment:
            ESCAPED: $$_$${fizzbuzz}
            GREETING: Hello
            NAME: ${NAME}
            WORKLOAD_NAME: hello-world
        hostname: hello-world
        image: busybox

Now we can set this variable using a .env file (see below) or provide it when we run docker compose:

$ NAME=John docker-compose -f ./compose.yaml up hello-world
[+] Running 2/2
 ⠿ Network compose_default          Created
 ⠿ Container compose-hello-world-1  Created
Attaching to compose-hello-world-1
compose-hello-world-1  | Hello John!
compose-hello-world-1  | Hello John!

Using .env files

For workloads relying on many environment variables it is convenient to manage all required settings in one place, the .env file.

docker compose will load .env from the current directory or any other file passed as --env-file:

$ cat .env
NAME=John

$ NAME=John docker-compose -f ./compose.yaml up hello-world
[+] Running 2/2
 ⠿ Network compose_default          Created
 ⠿ Container compose-hello-world-1  Created
Attaching to compose-hello-world-1
compose-hello-world-1  | Hello Bob!
compose-hello-world-1  | Hello Bob!

score-compose can generate the initial env file for you if you're not sure what variables are used. To do this, specify the --env-file flag when running the generate subcommand.

$ score-compose generate --env-file sample.env
$ cat sample.env
NAME=