subshell not propagating error, bash 4.4

Question

I have this in a github workflow

      - run: bash --version
      - name: run postgres
        run: |
          set -e
          shopt -s inherit_errexit
          export ID=$(docker run \
            --publish 5432:5432 \
            --network skynet \
            --network-alias db.host \
            --env-file .github/env/postgres \
            --detach ${{ steps.postgres-tag.outputs.data }} )
        shell: bash -euET -o pipefail -O inherit_errexit {0}

this is its run output

GNU bash, version 4.4.20(1)-release (x86_64-pc-linux-gnu)

  set -e
  shopt -s inherit_errexit
  export ID=$(docker run \
    --publish 5432:5432 \
    --network skynet \
    --network-alias db.host \
    --env-file .github/env/postgres \
    --detach  )
  shell: /bin/bash -euET -o pipefail -O inherit_errexit {0}
  env:
    AWS_DEFAULT_REGION: us-east-1
    AWS_REGION: us-east-1
    AWS_ACCESS_KEY_ID: ***
    AWS_SECRET_ACCESS_KEY: ***
"docker run" requires at least 1 argument.
See 'docker run --help'.

docker run [OPTIONS] IMAGE [COMMAND] [ARG...]

minimal reproduction on OS X with bash 5

bash-5.0$ export ID=$(docker run)
"docker run" requires at least 1 argument.
See 'docker run --help'.

Usage:  docker run [OPTIONS] IMAGE [COMMAND] [ARG...]

Run a command in a new container
bash-5.0$ echo $?
0
bash-5.0$ docker run
"docker run" requires at least 1 argument.
See 'docker run --help'.

Usage:  docker run [OPTIONS] IMAGE [COMMAND] [ARG...]

Run a command in a new container
> echo $?                                                                            # services -> feature/RS2-1228-optimize-build $ ! RC=1
1

However, the shell is not exiting with an error. I understand that normally a subshell with $() will not cause an error to happen... but I thought that setting shopt -s inherit_errexit fixed that. Am I missing something in my code?

Answer 1

Contrast:

$ bash -e -c 'export a=$(false); echo >&2 "If you see this, the previous command succeeded"'
If you see this, the previous command succeeded
$ bash -e -c 'a=$(false); echo >&2 "If you see this, the previous command succeeded"' 

In your attempt, the command inside the command substitution failed, but the command containing the command substitution succeeded. The status of a command substitution does not influence the status of the containing command if it's an ordinary command, including a call to the export builtin. The status of a command substitution only matters if it's in an assignment: the status of an assignment command is the status of the last command substitution in the command, or 0 if there is no command substitution (assuming there is no error in the assignment itself, such as attempting to assign to a read-only variable).

In order for your script to halt on the first error, you need to enable inherit_errexit and put each command substitution in a separate assignment. It has to be a plain assignment: exporting the variable must be a separate command.

$ bash -e -O inherit_errexit -c 'export a=$(false; echo >&2 "If you see this, errexit is off inside command substitution"); echo >&2 "If you see this, the previous command succeeded"'
If you see this, the previous command succeeded
$ bash -e -O inherit_errexit -c 'a=$(false; echo >&2 "If you see this, errexit is off inside command substitution"); echo >&2 "If you see this, the previous command succeeded"'