The idea is you have some 3rd-party app, which might accept a parameter and pass it to popen as-is, something like "--data-handler=command"

In this case, current "popen" semantics - a single shell command - works pretty well. You can pass it a custom process:

    --data-handler="scripts/handle_data.py"

or a shell fragment:

    --data-handler="gzip -c > last_data.gz"

or even mini-shell script:

    --data-handler "jq .contents | mail -s 'New data incoming' data-notify"

this is where the "shell command" arguments really shine - and note you cannot simulate all of this functionality with command vector.