To determine whether a particular utility is available on shinyapps.io servers, you can add something like:
to your app.R. Deploy the application to shinyapps.io, run it, then look in the application log - you should see something like:
2016-06-09T13:32:29.491205+00:00 shinyapps: /bin/tar
if the utility is available. If it is not available, you will see no output at all.