[OSX-Users] The Personal Data Issue

[Chris Andrews]

I'm not regretting bringing it up in the slightest! :P

One of your points I can expand on - certainly in the old days of Mac OS X
Leopard it *was* possible to create a hard link to a directory, I remember
using it to enable an Apache index.html page to provide a hyperlink to a
directory on a different drive. You had to be very clear about what was
going on and where your link went from and to to avoid accidentally linking
the wrong way around. I think you also had to touch the link first.

I never actually tried to make a loop, presumably because some innate sense
told me this would be A Bad ThingTM, but it might well have been possible
in that variant of HFS+. I wonder whether the upcoming APFS will allow hard
links to directories as well?

Chris

On 6 October 2016 at 11:50, Jules Field <Jules at ecs.soton.ac.uk> wrote:

> There are aliases, soft links and hard links.
>
> Aliases are the Mac's equivalent of Windows Shortcuts. They are just a
> normal file, but one that tells macOS where to find the file it points to.
> Nothing apart from macOS will understand them, everything else will just
> see a normal (but pretty useless) file.
>
> Soft links are at filesystem level, and are a standard feature of Unix
> filesystems. I don't know if Dropbox follows them or not. They are a
> filesystem object that gives a (relative or absolute) path to the file they
> point to. Created with "ln -s filename new-linkname". At file level (e.g.
> calling open() or using most Unix commands that handle files) they will be
> automatically followed. But at directory level (e.g. "ls", "rm", "mv") they
> won't be automatically followed. Many commands such as "tar" have a "-h"
> option to force them to follow soft links, as otherwise they won't.
>
> Hard links are at filesystem level, but lower-level than soft links, and
> are a standard feature of Unix filesystems and NTFS (though Windows doesn't
> encourage users to use them). Dropbox will have to follow them, as does
> everything else, as it can't tell the difference between that and a normal
> file. Created with "ln filename new-linkname". If you imagine that the
> "file contents object" and the directory entry are totally separate things,
> a hard link is merely another directory entry that points to the same file
> contents object. All the ".." directory entries that point to the parent
> directory are just all hard links to the same place. The "." directory
> entry is just a pointer to the directory itself. There are protections in
> place to make sure you can't do silly things like delete "." or ".." (fsck
> will try to put them back if you manage it!). You also can't[1] make a hard
> link to a directory, but only to a file. If you could make a hard link to a
> directory, you could make a loop. That would be "A Bad Thing(tm)".
>
> [1] The exception is hfs on Macs, where this feature is actively used to
> implement non-networked Time Machine backups, and was added by Apple
> specifically for Time Machine. I *believe* the "ln" command on macOS won't
> let you create them, as it's a very bad idea unless you are very careful
> (which Time Machine is). If you wander round the internal structure of a
> non-networked Time Machine backup and start link counting, you will fairly
> quickly see how it is implemented. The result is quite elegant and a very
> neat trick, enabling trivially simple restores and pruning of the oldest
> backups while guaranteeing you still have at least 1 copy of every file.
>
> To see how many directory entries point to the same file, do an "ls -al"
> and look at the number just to the left of the owner's username. The number
> given for "." should be the number of files and sub-directories in that
> directory (excluding .. of course) + 1 for "." itself + 1 for the entry in
> the parent directory that points to this one.
>
> Hopefully by now you're regretting ever bringing up the topic in the first
> place. ;-)
>
> Cheers,
> Jules.
>
>
> On 06/10/2016 09:48, Hugh Davis wrote:
>
>> The same had occurred to me, and this is exactly why I used aliases
>> rather than making links.  I do have links all over the place in my folders
>> and Dropbox definitely treats them differently to aliases.
>>
>> Dropbox does not follow the Aliases and if you check this from the
>> Dropbox web interface you get a deadend at the Alias.
>>
>>
>> Hugh Davis
>>
>>
>>
>>
>>
>> On 6 Oct 2016, at 09:43, Chris Andrews <w at lfie.org<mailto:w at lfie.org>>
>> wrote:
>>
>> Have you checked how the folders appear in the Dropbox web interface? I
>> seem to remember that Dropbox at one point followed certain pointers -
>> think it was hard links rather than aliases, but might be worth a look
>> regardless.
>>
>> Cheers,
>>
>> Chris
>>
>> On Thursday, October 6, 2016, Hugh Davis <hcd at soton.ac.uk<mailto:hcd at so
>> ton.ac.uk>> wrote:
>> Of course, I have *never* committed the crime of keeping other peoples’
>> personal data in Dropbox, but the recent conversation on this list led me
>> to thinking about how I could continue to use Dropbox as my main backup and
>> sync mechanism for my entire Documents repository EXCEPT personal data,
>> while at the same time keeping any legitimately retained personal data
>> backed up and synched “within" the University (OneDrive).
>>
>> So, I don’t know when people last looked at MS OneDrive?  Until recently
>> the “OneDrive for Business” version constantly crashed on OS/X and when it
>> occasionally didn't crash it didn’t seem to sync reliably. The latest
>> version of OneDrive, available from the MAC AppStore (and the IOS AppStore)
>> seems to work perfectly (I’m on Sierra). When you install it asks if you
>> want to use your “School” account, and then redirects you to an iSolutions
>> login. It asks you where you want the OneDrive folder installed, and I
>> choose a place that is not backed up by Dropbox. It is all then set up just
>> like Dropbox, with a folder that you can put in your Finder.  I can confirm
>> that on my systems it works and syncs between systems (MacOS and IOS) just
>> fine.
>>
>> So now what I have done is moved all my sensitive folders into the
>> Onedrive directory, and in my Dropbox directory, where I would like these
>> folders to appear I have put Aliases, so the whole directory navigation is
>> seamless, and your personal data is “safe” in that it is backed up within
>> the approved university systems (Microsoft!).
>>
>>
>> Hope this might be helpful.
>>
>> /h.
>>
>> Hugh Davis
>> Professor of Learning Technologies
>> Web and Internet Science Group
>> Electronics and Computer Science
>> Building 32 Rm 3033
>> Highfield Campus, University of Southampton, SO17 1BJ, UK
>> Tel:  +44 (0)2380 593669
>> Co-ordinates at http://users.ecs.soton.ac.uk/hcd/
>> My calendar is at http://users.ecs.soton.ac.uk/hcd/hcdcal.html
>>
>>
>>
>>
>>
>>
>>
>>
> Jules
>
> --
> Jules Field MEng MBCS CITP CEng
> email+iMessage: Jules at ecs.soton.ac.uk
> Twitter: @JulesFM
>
> Senior Tutor, Electronics and Computer Science
> Teaching Systems Manager, Faculty of Physical Sciences and Engineering
> University of Southampton SO17 1BJ, UK
>
> 'The owls are not what they appear.' - David Lynch (indirectly)
>
>
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