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I just noticed that the link between .. and the parent dir is hardlink, they have the same inode number:

starpinker@host /home/starpinker/unix $ ls -ail
8200794 drwxrwxr-x 2 starpinker starpinker 4096 Aug 23 11:22 .
1409238 drwxrwxr-x 2 starpinker starpinker 4096 Aug 23 11:22 ..

where the inode number of /home/starpinker/unix is 8200794, and the inode number of /home/starpinker is exactly 1409238.

I have a question on this:

The hardlink cannot be create across different file systems. Then what if the parent directory and child directory are in different file systems? For example in my case, if the /home/starpinker and /home/starpinker/unix are in different file system, the unix is just a mount point, then will the link between /home/starpinker/unix/.. and /home/starpinker still be hardlink?

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Apparently so, I just checked with my Windows mount point and it has a normal hard link to the Linux / which is .. for it. Don't know how that works though. If you don't get an answer here after a while, try flagging for migration to U&L. The question is on topic on both sites and is more than welcome to stay here, there are just more hard core *nix experts on U&L. – terdon Aug 31 '13 at 13:28
Thanks, @terdon, Can you revise your link? I follow the U&L link, but it seems incorrect. – StarPinkER Aug 31 '13 at 13:49 But please don't post the same question there, either flag for migration, or delete this one. Also, as I said before, you can leave it here, it is on topic and there may well be someone who can answer for you. – terdon Aug 31 '13 at 14:01
ok, thanks you @terdon. – StarPinkER Aug 31 '13 at 14:04

Why is it a hardlink? Because it points to the parent directory. What else could it be, if not a link?

For the filesystem root directory, .. is still a hardlink – but it points to the directory itself, i.e. it's equivalent to .. The kernel's VFS layer (which manages mountpoints and filesystems) handles this special case and magically resolves /home/starpinker/unix/.. to /home/starpinker without actually looking at the filesystem.

(Also, I haven't researched this, but I think some filesystems don't really keep the . and .. links on disk, but present emulated ones instead. It wouldn't surprise me if the VFS layer actually did that for all .. links, not just the special case...)

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