"Also, if you want to back up, say, your Documents folder to Dropbox in this manner, the backup won't take up any extra physical space in the Dropbox folder on your Mac."

Right, but it won't use up space on your Mac. If you have, for example, a laptop with an SSD, and your Documents folder is 50 GB, adding a symlink to your Dropbox folder won't use another 50 GB on your SSD. (It will, of course, count against your Dropbox quota.)

It wasn't misleading to me. He said "on your Mac", not on the DropBox server.

This is exactly why I haven't been using DropBox. I don't need it for sharing, and have been wanting to use it for backups, but didn't want to have the same files in two places on my Mac.

I use links extensively with dropbox. They are very useful, but it's important to understand how they work.

Symlinks will work exactly like you had copied the folder into your dropbox. If you modify a file, such as edit the exif or id3 data, it will be modified on all computers. This means that if you share a folder via symlink with somebody, and they delete those files, they delete *your* files. Beware of that. This option is best used for synchronization across computers, not sharing.

Hard links, however, do not function this way, and still save disk space. If you hard link a file into dropbox, whenever it is edited in its dropbox location by any computer, that link is broken and the original file stays the same. If the dropbox versions are deleted, the original hard linked copy stays on the disk. This makes hard links much more useful for sharing, but not useful for synchronization across computers. Unfortunately you can't hard-link a folder, so you have to resort to things like gnu cp or rsync --link-dest to make a tree of hard linked files.


And remember the safety net that dropbox provides. Don't be scared to try it out, you can go back within the last 30 days and download your old versions from your dropbox history if something goes wrong. :)

One Dropbox functionality that breaks, at least for me, is that the "Dropbox" contextual menu in Finder doesn't appear when right-clicking on symlinked files, or files in a symlinked directory.

http://en.wikipedia.org/wiki/Hard_link
http://en.wikipedia.org/wiki/Symbolic_link
http://en.wikipedia.org/wiki/Alias_(Mac_OS)

Basically, with a hard link, the same file data exists in two places at once. So ~/Documents/myfile.txt and ~/Dropbox/myfile.txt are both names for the same data; editing ~/Documents/myfile.txt will instantly change the data in ~/Dropbox/myfile.txt too, because it is the same data. Hard links work perfectly 100% of the time for all operations, even if you move or rename files, but can only be used within a single filesystem (you can't make a hard link to a file on another disk), and cannot be used for folders.

A symlink is a small file that redirects to another file or folder. When you try to open/copy/delete/whatever a symlink, your actions simply get sent to the path that the symlink points to and applied to whatever is found there. Symlinks can point to files and folders, can point to items on other disks, and work for almost all operations. But they are pretty dumb; if you move or rename the file that the symlink points to, the symlink will be broken, because it now points to nothing.

An alias is a pointer (like a symlink). It is able to point to files, folders, other computers on network, and more. An alias is usually able to find its target even if the target moves or is renamed, which makes aliases much more robust than symlinks. However, aliases are Mac specific (they were invented for use by the Finder) and don't usually work with low-level operations (e.g. UNIX command line programs).

Dropbox uses various low-level operations to do its magic, and so it can't follow aliases. Putting an alias file into your Dropbox folder will result only in the alias file itself getting uploaded to Dropbox.

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The alias will only work on the Mac you created the alias on. On another computer linked to your Dropbox account, the alias file will copy to your Dropbox, but will be useless.

Symlinks are covered in the Dropbox documentation as the way to achieve what we're talking about here. While this hint is great and very well written, I would recommend that everyone who is interested in working with symlinks and Dropbox to just check out the Dropbox documentation.

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There's a subtle point that, once you understand it, makes symlinks much more useful in Dropbox.

The whole design of symlinks in Unix tries to make them invisible to programs that don't specifically try to manipulate them. So suppose 'sym' is a symlink to 'file'. If a program opens 'sym' for read, it actually gets the data in 'file'. If it appends to 'sym', it actually appends to 'file'. However, if it deletes 'sym', what disappears is the link 'sym', not the file 'file'. Opening 'sym' for writing as a new file - not appending to it - is equivalent to deleting the old file and creating a new one: It leaves 'file' unchanged and creates an entirely new file named 'sym' which no longer has any connection with 'file'.

A link to a directory follows the same rules. Looking a file up using the symlink as the name really searches the linked-to directory. Creating a file through the symlink is like appending: It creates the entry in the linked directory. And so on.

A program that wishes to do something special - like change where a symlink points - has to be aware that it's dealing with a symlink and use special OS calls for that exact purpose.

Dropbox works with symlinks *but it doesn't do anything special with them*. So suppose you put that 'sym' linked to 'file' in your Dropbox directory. Dropbox comes along, finds a new file, and sends it to its servers. What does it send? Well, first the name 'sym', and then the "contents" - i.e., what it gets from reading 'sym' which is exactly the contents of 'file'. On the server, and then later on other clients, what you will find is a normal file named 'sym' with the contents of 'file'. *There is no connection with a file named 'file'.* If you change 'file' on the system where 'sym' links to it, the changes propagate. If you change it anywhere else, the changes propagate back - but Dropbox doesn't modify files in place, it writes entire new ones. So the effect back on the original system is to break the link and write a new file named 'sym' with the latest contents - but no connection to 'file'.

I know of no way to keep a link to a *file* as a symlink across updates. But the story is different for *directories*. Unlike ordinary files, directories are normally updated in place (unless you explicit delete and recreate them). So you can do the following:

1. Create directory 'dir' anywhere you like.
2. Create symlink 'dirlink' pointing to 'dir' in your Dropbox folder.
3. Wait for 'dirlink' to appear on all other clients. It will appear as an ordinary directory, not as a symlink. If the original 'dir' had files in it, those will now appear as files on the clients, too.
4. On each client, rename 'dirlink' to 'dir' *in the place you want it to appear in your directory tree*. (Renaming only works if you are staying not the same device. Otherwise, you need to create 'dir' and move all the files.) This need not be the same on all clients, though it's easier to keep track of if it is.
5. On each client, create symlink 'dirlink' pointing to 'dir'.

Now you have a 'dirlink' on each client, which will to Dropbox look like a subdirectory - and it will sync all the files in that "subdirectory". Changes made on any client to any file in 'dir' aka 'dirlink' will be synced to all the other clients as well. Files created or deleted in 'dir' will be created/deleted on every other client as well.

It's probably easiest to do all this while there are no files in 'dir'. Otherwise, Dropbox sometimes repeatedly syncs the same files until everything eventually settles down.

The limitations here:
- Some platforms (e.g., iOS) don't support symlinks. To them, 'dirlink' will just be an ordinary subdirectory.
- Any time you add a new client, you have to go through the process for that client. Certain reset operations in Dropbox - anything that requires re-syncing every file in the Dropbox folder on a client - will require the same, because Dropbox doesn't know how to *create* symlinks - it'll just create an ordinary subdirectories.

I've used this configuration for a couple of years. You have to watch out for the reset situations and such, but generally once you have it set up, it "just works".

I have been using symlinks in Dropbox for several months, and the Dropbox contextual Finder menu works - but occasionally (twice I think) it has disappeared. It returns with a re-installation of the Dropbox app.

For those that are Teminal impaired: SymbolicLinker (http://www.macupdate.com/app/mac/10433/symboliclinker) is a Contextual Menu/Service to create symbolic links.
I have been using symlinks to group folders in a SL server without any problems (I am aware that the DropBox backup does not preserve permissions, but it's not a problem in my case),

It's worth noting that when you add a sym link inside the Dropbox folder, Dropbox follows the sym link and syncs the data it points to, meaning that on the Dropbox servers, and on any other linked computers, that data exists as actual files, not as a sym link.

For this reason, it's actually generally better to put the actual data in the Dropbox folder in the first place, and put sym links in the original locations outside the Dropbox folder.

Whoa! So if I follow the original hint, I end up with duplicate data? Then that is no good for syncing data between computers, right?

Whoa! So if I follow the original hint, I end up with duplicate data? Then that is no good for syncing data between computers, right?

...it's actually generally better to put the actual data in the Dropbox folder in the first place, and put sym links in the original locations outside the Dropbox folder.

I tried this, and it worked. Both computers running 10.6.8. On one computer, the OS wouldn't let me move the original Desktop folder to the trash or out of the home folder, which I needed to do to replace it with the symlink, so I logged in as root to get that done. Upon logging back in as a the regular user, Viola! I have the same desktop on both computers. I am stunned.

The Finder window sidebar shows the name of the folder in Dropbox ("Dropbox Desktop",) not the name of the symlink ("Desktop"). That is probably a good thing, to remind me of the setup. But if I changed the Dropbox folder name to desktop, I could have a "transparent" set up.

The Desktop still works as expected of Dropbox isn't running, but of course sync doesn't take place.

Aliases placed on one desktop appear on the other desktop, but don't work.

I'd recommend a solid backup of everything before attempting this.

I have senior clients with multiple homes who may like this setup. They cannot really deal with any complexity at all: this gives them the illusion of having single desktop appear on multiple computers.

For easier creation of symlinks, you can add this script http://cl.ly/ImZ5 to "User/Library/Services" and right click on finder items to create these.

Using the symbolic link trick works for me if my source folder is on my HD. If the source folder is on my NAS all it does is that just create the subdirectories but not actually copying any files.
Any idea what's going on? Anyone got it working correctly with network shares?
thanks
OsamaS