How to shutdown VirtualBox

For a scriptable control of Virtual Box machines we can make use of the VBoxManage commands:

• List running machines (returns name and UUID):
  

  VBoxManage list runningvms
  

• Stop running VMs by "hibernating" them (reommended to avoid data loss)
  

  VBoxManage controlvm  savestate
  

• Poweroff running VMs (not recommended because we may lose data in the guest)
  

  VBoxManage controlvm  poweroff
  

• Use ACPI in an ACPI-aware guest OS (preferable to poweroff for graceful shutdown of guests)
  

  VBoxManage controlvm  acpipowerbutton
  

Also see: How to safely shutdown Guest OS in VirtualBox using command line

Update from OP

Based on this selected correct answer below, I've added this bash script "$HOME/bin/stop-vagrant.sh". So now I have something that can safely begin a stop of all vagrant VMs that I might have turned on yet forgotten about in a session.

vboxmanage list runningvms | sed -r 's/.*\{(.*)\}/\1/' | xargs -L1 -I {} VBoxManage controlvm {} savestate

Command Explained:

vboxmanage list runningvms | -- gets a list of all running vms under VirtualBox
sed -r 's/.*\{(.*)\}/\1/' | -- strips the string down to id number
xargs -L1 -I {} VBoxManage controlvm {} savestate -- runs the save state command on each box that's open.
On xargs

• -L1 - take one line at a time
• -I {} - uses {} as a place holder for the next command