Install VBox additions!

Port forward from the CLI (to allow to ssh into the virtual machine)

VBoxManage modifyvm "ubuntu20.04" --natpf1 "SSH,tcp,127.0.0.1,2522,10.0.2.15,22"

After that, just ssh into the machine with:

ssh 127.0.0.1 -p 2522

Start headless machine CLI

VBoxManage startvm ubuservloc --type headless

Get ip(v4) address of a vbox machine (named ubuntu)

Put Bridged Adapter in the Network Settings

VBoxManage guestproperty enumerate ubuntu | grep "Net.*V4.*IP"

Add shared folder from ubuntu guest

sudo mount -t vboxsf -o uid=$UID,gid=$(id -g) NAME_OF_SHARED_FOLDER_IN_VBOX FOLDER_IN_WHICH_TO_MOUNT

Share folder between ubuntu host and windows guest

• Add folder to shared folders in VBox settings
• In windows cli: net use <letra>: \\vboxsvr\shared_folder_name

Resize emulated hard drive

user@pc :~$ VBoxManage modifyhd filename.vdi --resize 46080

Problems

SSH doesn't work

Try bridged mode

Premature end of data in tag VirtualBox line 2

• Looks like it was caused (in my case) because of the host machine running out of space
• Found this and some luck.
• backup the vbox file, and substitute the /path/to/vm/win7/win7.vbox file with the contents of /path/to/vm/win7/win7.vbox. Lost some progress, though.