Do docker-compose's two `command` forms behave differently? - docker

Question

In Dockerfile, we can specify the CMD to be in one of three different forms:
...
CMD ["executable","param1","param2"] (exec form, this is the preferred form)
CMD ["param1","param2"] (as default parameters to ENTRYPOINT)
CMD command param1 param2 (shell form)
...
If you use the shell form of the CMD, then the <command> will execute in /bin/sh -c:
FROM ubuntu
CMD echo "This is a test." | wc -
If you want to run your <command> without a shell then you must express the command as a JSON array and give the full path to the executable. This array form is the preferred format of CMD. Any additional parameters must be individually expressed as strings in the array:
FROM ubuntu
CMD ["/usr/bin/wc","--help"]
Source
In docker-compose, we can also use two different forms for its command:
...
Override the default command.
command: bundle exec thin -p 3000
The command can also be a list, in a manner similar to dockerfile:
command: ["bundle", "exec", "thin", "-p", "3000"]
Source
Do these two different forms in docker-compose behave the same way as the exec and shell forms in Dockerfile do?

Answer 1

No they do not. docker-compose's command always uses the equivalent of Dockerfile's exec form. This can be easily seen with a quick demo:
Dockerfile with shell form processes shell symbols:
FROM alpine:3.11.5
CMD echo foo && echo bar
$ docker run example
foo
bar
Dockerfile with exec form doesn't do any shell processing:
FROM alpine:3.11.5
CMD ["echo", "foo", "&&", "echo", "bar"]
$ docker run example
foo && echo bar
docker-compose's first command form doesn't do any shell processing:
version: "3.7"
services:
example:
image: alpine:3.11.5
command: echo foo && echo bar
$ docker-compose up
Starting example_example_1 ... done
Attaching to example_example_1
example_1 | foo && echo bar
example_example_1 exited with code 0
And neither does docker-compose's second command form:
version: "3.7"
services:
example:
image: alpine:3.11.5
command: ["echo", "foo", "&&", "echo", "bar"]
$ docker-compose up
Starting example_example_1 ... done
Attaching to example_example_1
example_1 | foo && echo bar
example_example_1 exited with code 0
 
So, if one does want shell processing in docker-compose's command, it has to be explicitly enabled by passing the command to sh -c:
version: "3.7"
services:
example:
image: alpine:3.11.5
command: sh -c "echo foo && echo bar"
$ docker-compose up
Starting example_example_1 ... done
Attaching to example_example_1
example_1 | foo
example_1 | bar
example_example_1 exited with code 0