Bug 89429

I want a mode for systemctl to "dry run" or "preflight" a start/stop/isolate/etc operation.

This would be useful for debugging unit files ("what does it actually try to do when I start this?") or isolate commands to custom targets.
It would also enable some other niceties, such as not needing to ask for authentication to do nothing.


As an example, here's a sample shell session I would want to have on my dev system:

    $ systemctl start -n postgresql
    To start: postgresql.service postgresql@9.4-main.service
    Already started: basic.target
    $ systemctl start -n basic.target
    Nothing to do.
    $ systemctl start -P postgresql

(Auth, it starts...)

    $ systemctl start -n postgresql
    Nothing to do.
    $ systemctl start -P postgresql

(No PolicyKit dialog shows up, as the preflight check returns with 'nothing to do', so no need to auth!)

I'm aware that the -n option is already taken, but it doesn't apply to the (sub)commands that would be affected by this.