Fallback for environment variables with docker-compose

Question

Given the following configuration:

mysql:
    environment:
        MY_MYSQL_PORT:
    image: mysql
    ports:
        - "${MY_MYSQL_PORT}:3306"

There's a way to provide a fallback value for MY_MYSQL_PORT without relying on wrapper scripts? I already tested like bash ${MY_MYSQL_PORT-3306} but it doesn't work.

Answer 1

They implemented that feature with compose 1.9 release:

Added support for shell-style inline defaults in variable interpolation.

The supported forms are ${FOO-default} (fall back if FOO is unset) and ${FOO:-default} (fall back if FOO is unset or empty).

Release Notes Docker Compose 1.9

Answer 2

Dockerfile 文档中的解释可以在以下位置找到： https ://docs.docker.com/engine/reference/builder/#environment-replacement