在 docker-compose.yml 的 env_files 中使用系统环境变量

Question

I have an env file called myenvfile.env which is wanted to be defined from a system environment variable.

.env

SYSTEM_ENV_VAR=prefix

myenvfile.env

MY_COMPOSED_ENV_VAR=${SYSTEM_ENV_VAR}_suffix

docker-compose.yml

version: '3.4'

services:
  myservice:
    env_file: 
      - env_files/myenvfile.env

But when I create container with docker-compose up -d , I look into the container environment variables, and MY_COMPOSED_ENV_VAR = "${SYSTEM_ENV_VAR}_suffix" instead of desired "prefix_suffix" .

How can I achieve that? Thanks

Edit: More information

If I define SYSTEM_ENV_VAR in Jenkins job, for example, it works in Linux and Windows. But if I try to do it in a shell, it doesn't work in Windows.

Answer 1

Try declaring SYSTEM_ENV_VAR before the docker-compose up command, something like:

$ export SYSTEM_ENV_VAR=test
$ docker-compose up

You'll see that it'll expand correctly.

This behaviour happens because the SYSTEM_ENV_VAR is expected to be declared previous to the docker-compose up command, as it'll not expand with environment variables that are declared in the container creation instant (the env_file key section, for example), only the ones that are already available when the docker-compose runs.

That's why the Jenkins job works in this case.

Answer 2

.env file is used only at docker compose start time by replacing $VARIABLE with values given in .env file. So you cannot expect it to update contents of your myenvfile.env file. (check this )

Instead you should declare new environment variable inside docker-compose.yaml like:

environment:
    - MY_COMPOSED_ENV_VAR=${SYSTEM_ENV_VAR}_suffix