If you’ve used the GUI tool to share a folder with your Ubuntu guest machine, you probably are confused why it’s not showing up anywhere.
The problem is that you need to run a command to make it show up. First, create a mount point, and then use the special mount syntax to actually “map” the folder to the shared VirtualBox folder.
sudo mkdir /media/vboxshared
Now you can map the folder using the share name that you setup earlier in VirtualBox
sudo mount -t vboxsf SHARENAME /media/vboxshared