env.active_machines can potentiall return 'invalid' machines: - Ignore machines that are not declared in current Vagrantfile - Warn when machines are missing (it usually occurs when the VM is removed without `vagrant destroy` and some orphan metadata remains in .vagrant/machines/...)
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.