- Shut down virtual machine.
- Go to settings>Storage, and under “controller: ID” option, add an optical drive with the guest additions CD
- Reboot your virtual machine.
- Once the machine is booted, click Devices>Insert Guest Additions CD image
- Cd to the optical drive /media/username/VBox_GAs_5.2
- Run sudo sh ./VBoxLinuxAdditions.run
You should be able to see /media/sf_shared in Ubuntu.
You may also like to read How to setup serial port in Virtualbox
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