To follow-up on yesterday’s post, the inevitable question is: how do files get like this in the first place?

As far as I can figure out, this floating owner “feature” is part of OSX’s OS9 compatibility, and also has to do with the “Ignore ownership on this volume” checkbox you’ll find in the Get Info window for non-boot volumes.

It’s important to remember that OS9 has no concept of ownership whatsoever. So, when a file is created by an OS9 application, running in OS9 itself, it has to choose a user that’s going to own the file.

But what user should it choose? As I said, there’s no real concept of ownership in OS9. And if you randomly just pick a user—say, the first one—there’s no guarantee that you only have one user on your machine… and no way to ensure that, once you’re in OSX, the correct user owns the file.

There’s a bit more complexity to this whole “user” thing. While a user has a name (the “short user name” you select when you first create the account), the system doesn’t really assign the “name” as the owner of the file. Instead, it uses a User ID, which is a number.

On OSX, the first user account created is given the User ID 501. The next account created is 502, etc. These numbers are what are associated with the files on your disk, not the short user name, which is more for your convenience than anything else.

This can create some confusing situations, though, when you bring a disk to another computer, and turn “Ownership” on. If it’s another user’s computer, and their account was the first created (most are), that means their User ID is the same as yours. So, your files will look like they’re owned by them!

Similarly, if you have more than one computer in your office (or family), and the accounts weren’t created in the same order, your account may be “501” on one machine, and “505” on another! (Here, to avoid this problem, I make a habit of creating all my accounts in the same order on all machines.)

No doubt this is why Apple allows you to turn ownership off when you attach a FireWire drive to your machine… this causes all the file owners to float!

So, Apple compromised. Since they couldn’t randomly pick a user, and they didn’t want to force permissions on OS9—which would cause serious compatibility problems—they did the next best thing. They made these OS9 files owned by “everyone”, all at once. In other words, they made the owner float.

When the owner’s floating, it looks like it’s owned by the user looking at it. So, if the file is saved in OS9 (or stored on a FireWire disk with “Ignore ownership on this volume” checked), it’ll be owned by whatever user looks at it.

Even if that user is a Backup program running as root, like SuperDuper!. Which brings us full circle.