[NTG-context] check installed program using Lua

Dear list, is there a way if a program is installed on the computer using Lua. I have the following sample: \starttext \startluacode filename = tex.jobname .. ".pdf" os.exec("dir " .. filename) \stopluacode \stoptext Is there a way to wrap os.exec() in the sample so that it only runs if "dir" is available? I know that os.name would be an option here, but not in my real world document. Many thanks for your help, Pablo -- http://www.ousia.tk