This is mainly for my own reference, but here it goes.

1. Share a folder on the host OS

• In VirtualBox, click your OS on the left and click on Settings.
• Click on the Shared Folders tab.
• Click on the folder with the plus on the right.
• Browse to a folder of your choice in the folder path.
• Enter a folder name with no spaces e.g. “Share”.
• Check Auto-mount and Make Permanent, if available.
• Click on OK.

2. Mount the folder in the guest OS

• Create a folder in your guest OS that you want to share.
• Open up Terminal.
• Type in id and press ENTER— remember that ID.
• Switch to the root user using sudo su and enter your password.
• Browse to the etc folder using cd /etc.
• Edit the rc.local file using vi rc.local.
• Move your cursor right above exit 0 and press the letter “i” on your keyboard to insert text.
• Type in the following: sudo mount -t vboxsf -o uid=1000,gid=1000 Share /home/username/Documents/Share
  • 1000 should match the ID you noted down earlier.
  • Share should match the folder name from step 1.
  • username should match your Linux username.
  • /Documents/Share should be the absolute path of the new folder you created.
• Now hit “ESC”, type :wq and hit ENTER to save and quit the file editing.

After you restart the guest OS, your shared folder will be automatically mounted.