Can we use echo or something available out of the box so that running this would always do something visible and make the sample useful out of the box?

I'm afraid that won't work for the runnable.script.path version. It has to be a path to a file, and the file needs to be present on the running machine. This is useful only when we'd use a custom machine image I think, that has some file where we need it.

Does /usr/bin/echo not work? It should be there on nearly any Linux system.

I don't think I understand. It does run echo Hello world! This is task ${BATCH_TASK_INDEX}. This job has a total of ${BATCH_TASK_COUNT} tasks..

The commented out part about script path is an alternate version, exclusive with running a script by providing it as a text.

As a user, I'm still confused why this is even an option and how it differs from the regular invocation. If all it does is read the test and execute it in the default shell, I might as well write /bin/sh /tmp/test.sh. So why does this exist and under what circumstances may I want to use it?

I agree that it is redundant, however it was not my idea to create it that way :D I just want to present to the end user that it's an option, it's up to them if they want to use it or not.

As a user, I'm still confused why this is even an option and how it differs from the regular invocation. If all it does is read the test and execute it in the default shell, I might as well write /bin/sh /tmp/test.sh. So why does this exist and under what circumstances may I want to use it?