mitchellh/vagrant — pull request #3902 by zigg (2014-05-25 22:48:11+00:00)
Title: drop `-r` flag from `xargs` on OS X

OS X's `xargs` does not support the `-r` flag (which means "do not
execute the command even once if there are no arguments"), but
behaves by default as if it was specified.

You can verify this yourself with this test:

$ touch zero-length-file
$ xargs echo