How could you implement something like this from a shortcode? I don’t think there’s a download function available from the templates (except for JSON/CSV), and we don’t have an exec function to execute arbitrary scripts. See here, here, and here for more discussions on the matter.
If you have a workaround though, I would love to hear about it 
In the meantime, I’ve had some fun developing some sort of content plugin system for my build script. You can find it here. build.sh is the build script in question, but what’s more interesting to our use-case is pre-build.sh.
pre-build.sh will look for all scripts in hooks/pre-build/*.sh and run them a first time, expecting them to reply with a regex that interests them. If the content changes match the regex, the changes will be passed to the script (say, a youtube plugin) to do its stuff.
So here is the youtube.sh hook that fetches the image from Youtube directly when content with the youtube shortcode has been added/modified/deleted. The image is minified with imagemagick, and both versions are kept in static/external/youtube folder.
youtube.sh should really be called from pre-build.sh, because it supports different sources of content changes out-of-the-box:
pre-build.sh initwill look for matches inside the wholecontentfolder (good to fetch previews for youtube videos added with the internal youtube shortcode that are long since pushed)pre-build.sh uncommittedwill look for matches inside your edited but uncommitted content filespre-build.sh committedwill look for matches inside your committed but unpushed stuff, and auto-commit additional files (will not try to hide by amending your commit)
Personally i’m using the last two in my build file. Here’s my new youtube shortcode (way shorter and still responsive as fuck 
).
TODO: Collect the garbage when a video is deleted from the whole content folder. Write more plugins like this? For webmention.io maybe? Twitter?