This might help me hunt down some system configuration bug.
Suppose I make a local directory (e.g., “/foo”) in one disk partition, and copy some files to it. But later on I forget about it, and configure the same directory as mounting point for another disk partition in /etc/fstab.
Now if I reboot the system, what will happen, will /foo still hold local files or point to the other disk partition?
When you mount some device to /foo, then the files on the new device appear, and the ones on the original device are hidden.
If you unmount /foo, then the original files will reappear.
Source : Link , Question Author : user180574 , Answer Author : Michael Hampton