ef44f19601
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 <zero-length-file
If `echo` is executed, you will see a blank line. If it is not
executed (i.e. `-r` is specified or the behavior is implied) then
you will see no blank line.
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.