command output interpolation in docker-compose.yml

[rino1]

Description

Hello everyone, here is my scenario

+ lsb_release -a
Description:	Ubuntu 24.04.1 LTS

+ bash --version
GNU bash, versione 5.2.21(1)-release (x86_64-pc-linux-gnu)

+ cat docker-compose.yml
services:
    composer:
        image: bash/bash
        environment:
            # INTERPOLATED: NO
            NEW_DATE: $(date)
            # INTERPOLATED: YES
            NEW_PATH: $PATH

Let's do some tests

# echo command output
+ echo $(date)
ven 8 nov 2024, 07:31:58, CET

OK, but...

+ docker compose config | grep NEW
      NEW_DATE: $$(date)
      NEW_PATH: /usr/local/sbin:/usr/local/bin:/usr/sbin...etc...etc...

As you can see, $(date) is not interpolated (evaluated).

Is it a bug?
Is it a feature not implemented?
Or am I doing something wrong?

[ndeloof]

Docker Compose interpolates variables set by environment (or .env file) using a bash-like ${VAR} syntax, but this is definitely not just a full bash implementation and it does not support sub-command execution ($(command)) - and never will, as this would just break portability (how can you guarantee all system will have this command installed, with a consistent version and output?)

see https://docs.docker.com/compose/how-tos/environment-variables/variable-interpolation/

What's your actual use-case ? To access current date your container can just run date on its own, or equivalent syscall from application code.

[rino1]

thanks