Drush: --yes-if-newer behavior with pm-download? I'm writing an Acquia Cloud Hook where I install Registry Rebuild. In the interest of performance, I'd like to only install it if it's not already at the latest version. In other words, --yes unless it's already the current version.

Is there a simple way to do this in current Drush or shell?
I'm afraid there is no easy way: as the command version isn't stored in any file, there is nothing to compare with. If you really want to do it, you can download the command and run a diff with the version you have, and install it if it differs, but I doubt the efficiency of such a check compared to install it in any case