Virtualbox: How to use shared folders (aka gemeinsame Ordner)

If you want to use a directory inside a virtualbox vm without copying all files, you can use the shared folders.
Just choose a folder to share:

Then inside the vm type:

[root@localhost oracle]# mount -t vboxsf 12c /mnt

and that's it:

[root@localhost oracle]# df -h |grep 12c
12c                  180G    129G   52G   72% /mnt

or

[root@localhost oracle]# mount | grep 12c
12c on /mnt type vboxsf (rw,nodev,relatime)

When you are done just unmount the folder with

umount /mnt