Cannot use shell form for command specified in Compose file

[ijessen-mitll]

Description

A command override in a Compose service definition will never execute in shell form, only ever exec form. This is contrary to the documentation statement that "the value can also be a list, in a manner similar to Dockerfile". A Dockerfile CMD specified as a list triggers exec form, while a Dockerfile CMD specified as a plain string triggers shell form (see here).

Steps To Reproduce

Example compose.yaml:

services:

  exec-form:
    image: alpine:latest
    command: ["/bin/sh", "-c", "echo $$HOSTNAME"]

  shell-form-compose:
    image: alpine:latest
    command: echo $$HOSTNAME

  shell-form-dockerfile:
    build:
      dockerfile_inline: |
        FROM alpine:latest
        CMD echo $$HOSTNAME
    image: alpine:latest

Results (notice the difference between the compose shell form output from the other two):

$ docker compose -f compose.yaml up
[+] Running 3/0
 ✔ Container config-exec-form-1              Created         0.0s 
 ✔ Container config-shell-form-compose-1     Created         0.0s 
 ✔ Container config-shell-form-dockerfile-1  Created         0.0s 


exec-form-1               | 03eae6e24cbf
shell-form-compose-1      | $HOSTNAME
shell-form-dockerfile-1   | 2d7438fcb6a1

Compose Version

Docker Compose version v2.31.0

Docker Environment

Client: Docker Engine - Community
 Version:    27.4.0
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.19.2
    Path:     /usr/libexec/docker/cli-plugins/docker-buildx
  compose: Docker Compose (Docker Inc.)
    Version:  v2.31.0
    Path:     /usr/libexec/docker/cli-plugins/docker-compose

Anything else?

If the behavior is as-intended, the Compose reference documentation should be updated to clarify that a string command will be executed in exec mode (contrary to the Dockerfile behavior).

[ndeloof]

This is the expected behavior
"in a manner similar to Dockerfile" indeed is weaving hands without a clear definition in docs, need to be clarified.

[ijessen-mitll]

For completeness - why is this intended behavior?

[ndeloof]

Basically: preserve backward compatibility