Platform independent whereis function
A couple of small things:


If you're specifically looking for executable files, you might use which instead of whereis command. From this answer, you can see the difference between the two using whatis:


$  whatis which
which                (1)  - shows the full path of (shell) commands

$  whatis whereis
whereis              (1)  - locate the binary, source, and manual page files for a command


Be consistent. You've used two different formatting styles:

"where {}".format(app)


and

"Application ,",app,", not forund.",err  # you also have a spelling mistake here (forund -&gt; found) 


I'd recommend you use the former one
You should have at least two spaces before an inline comment
You should put a space after the comma in almost every situation. (e.g: print("Application,",app,", not found.",err) -> print("Application,", app, ", not found.", err))