Fastest way to access host file in Linux client using VirtualBox

First, set up a share folder/directory using the wizard of VirtualBox.
Assuming you have create a shared folder in the VirtualBox and its name is “to-be-shared“.

Then in the Linux client (e.g. Ubuntu), open a new terminal. Make a new directory and mount it using following commands

mkdir share
sudo mount -t vboxsf to-be-shared share

Now it works.
To unmount it just do:

sudo umount share
rmdir share

Then using wizard in the VirtualBox to umount the share folder in the system (you may do this when the client machine is powered off).