Peeking at the contents of a mount point

[Patrick Asselman]

On 2012-09-12 16:53, Kevin O'Gorman wrote:
> On Wed, Sep 12, 2012 at 12:57 AM, Karl Auer <kauer at biplane.com.au> 
> wrote:
>> On Wed, 2012-09-12 at 15:40 +1000, Basil Chupin wrote:
>>> On 12/09/12 12:25, Kevin O'Gorman wrote:
>>> > I recall some time ago seeing a command -- a variant or argument 
>>> to
>>> > 'mount' I think -- which allowed you to see the real contents of 
>>> a
>>> > mount point while something else was still mounted on it.
>>> > I've looked back through 2 years of postings and can't find it.
>>> > Anybody have a clue?
>>>
>>> The "real mount point" is what you have mounted on it already. And 
>>> I
>>> don't know of any "fake" mount points :-) .
>>
>> A mount point is a directory - like "/". A mount point can contain
>> files, just like any other directory. If a filesystem is mounted on 
>> a
>> directory that contains files, any files below the mount point are
>> hidden by the mounted filesystem. If you unmount the filesystem, the
>> files below the mount point become visible again.
>>
>> Try it - put a file in (say) /mnt, then mount something on /mnt. The
>> file you created will "disappear" - to reappear unharmed when you
>> unmount the filesystem.
>>
>> You can see the contents of the mountpoint under a mounted 
>> filesystem by
>> binding the mount point to another name:
>>
>>    mount --bind /mnt /mnt_view
>>
>> ... then "ls /mnt_view" should show you the "hidden" files.
>>
>> This is from memory, so do please test carefully.
>>
>> Regards, K.
>
>
> That's what I thought too, but it does not work.  The --bind option
> lets me place part of the heirarchy in multiple places, but does not
> allow me to expose any underlying mount points.
>
> For example, my home directory is mounted separately so that any
> runaway experiments don't use up the disk space of other things, but
> if I have root do
>    mount --bind /home/kevin /mnt
>    ls /mnt
> I see the same thing as if I just did
>     ls /home/kevin
>

If you had files in /mnt and they are hidden due to something mounted 
on /mnt then doing
mount --bind / /all
should show the original files under /all/mnt/

Best regards,
Patrick Asselman