How can I tell if a utility I need is available on the shinyapps.io servers?

To determine whether a particular utility is available on shinyapps.io servers, you can add something like:

base::system("which tar")

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[88391]: /bin/tar

if the utility is available.  If it is not available, you will see no output at all.