VBoxHeadless

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Running Virtual Machines With VirtualBox 4.1 On A Headless Ubuntu 11.10 Server
This guide explains how you can run virtual machines with VirtualBox 4.1 on a headless Ubuntu 11.10 server. Normally you use the VirtualBox GUI to manage your virtual machines, but a server does not have a desktop environment. Fortunately, VirtualBox comes with a tool called VBoxHeadless that allows you to connect to the virtual machines over a remote desktop connection, so there's no need for the VirtualBox GUI.
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Now we must add the user that will run VirtualBox (administrator in this example) to the vboxusers group:

sudo adduser administrator vboxusers

VirtualBox is now installed and ready to be used.
3 Using VirtualBox On The Command Line
3.1 Creating A VM To create a VM on the command line, we can use the VBoxManage command. See

VBoxManage --help

for a list of available switches and (highly recommended!) take a look at http://www.virtualbox.org/manual/ch08.html.
I will now create an Ubuntu 11.10 Server VM with 512MB memory and a 10GB hard drive from the Ubuntu 11.10 Server iso image (which I have stored in /home/ubuntu-11.10-server-amd64.iso):

VBoxManage createvm --name "Ubuntu 11.10 Server" --register
VBoxManage modifyvm "Ubuntu 11.10 Server" --memory 512 --acpi on --boot1 dvd --nic1 bridged --bridgeadapter1 eth0
VBoxManage createhd --filename Ubuntu_11_10_Server.vdi --size 10000
VBoxManage storagectl "Ubuntu 11.10 Server" --name "IDE Controller" --add ide
VBoxManage storageattach "Ubuntu 11.10 Server" --storagectl "IDE Controller" --port 0 --device 0 --type hdd --medium Ubuntu_11_10_Server.vdi
VBoxManage storageattach "Ubuntu 11.10 Server" --storagectl "IDE Controller" --port 1 --device 0 --type dvddrive --medium /home/ubuntu-11.10-server-amd64.iso

3.2 Importing An Existing VM
Let's assume you have a VM called examplevm that you want to reuse on this host. On the old host, you should have a directory Machines/examplevm in the VirtualBox directory; Machines/examplevm should contain the examplevm.xml file. Copy the examplevm directory (including the examplevm.xml file) to your new Machines directory (if your user name is administrator, this is /home/administrator/.VirtualBox/Machines - the result should be /home/administrator/.VirtualBox/Machines/examplevm/examplevm.xml). In addition to that copy the examplevm.vdi file from the old VDI directory to the new one (e.g. /home/administrator/.VirtualBox/VDI/examplevm.vdi).
Afterwards, you must register the imported VM:

VBoxManage registervm Machines/examplevm/examplevm.xml

3.3 Starting A VM With VBoxHeadlessRegardless of if you create a new VM or import an old one, you can start it with the command:

VBoxHeadless --startvm "Ubuntu 11.10 Server"

(Replace Ubuntu 11.10 Server with the name of your VM.)
VBoxHeadless will start the VM and a VRDP (VirtualBox Remote Desktop Protocol) server which allows you to see the VM's output remotely on another machine.
To stop a VM, run

VBoxManage controlvm "Ubuntu 11.10 Server" poweroff

To pause a VM, run

VBoxManage controlvm "Ubuntu 11.10 Server" pause

To reset a VM, run

VBoxManage controlvm "Ubuntu 11.10 Server" reset

To learn more about VBoxHeadless, take a look at

VBoxHeadless --help

and at http://www.virtualbox.org/manual/ch07.html#vboxheadless