Linux – How to unlink (remove) the special hardlink “.” created for a folder

filesystemshard linklinuxrm

On Linux, when you a create folder, it automatically creates two hard links to the corresponding inode.
One which is the folder you asked to create, the other being the . special folder this folder.

Example:

$ mkdir folder
$ ls -li
total 0
124596048 drwxr-xr-x    2 fantattitude  staff    68 18 oct 16:52 folder
$ ls -lai folder
total 0
124596048 drwxr-xr-x  2 fantattitude  staff   68 18 oct 16:52 .
124593716 drwxr-xr-x  3 fantattitude  staff  102 18 oct 16:52 ..

As you can see, both folder and .'s inside folder have the same inode number (shown with -i option).

Is there anyway to delete this special . hardlink?

It's only for experimentation and curiosity.
Also I guess the answer could apply to .. special file as well.

I tried to look into rm man but couldn't find any way to do it. When I try to remove . all I get is:

rm: "." and ".." may not be removed

I'm really curious about the whole way these things work so don't refrain from being very verbose on the subject.

EDIT: Maybe I wasn't clear with my post, but I want to understand the underlying mechanism which is responsible for . files and the reasons why they can't be deleted.

I know the POSIX standard disallows a folder with less than 2 hardlinks, but don't really get why. I want to know if it could be possible to do it anyway.

Best Answer

It is technically possible to delete ., at least on EXT4 filesystems. If you create a filesystem image in test.img, mount it and create a test folder, then unmount it again, you can edit it using debugfs:

debugfs -w test.img
cd test
unlink .

debugfs doesn't complain and dutifully deletes the . directory entry in the filesystem. The test directory is still usable, with one surprise:

sudo mount test.img /mnt/temp
cd /mnt/temp/test
ls

shows only

..

so . really is gone. Yet cd ., ls ., pwd still behave as usual!

I'd previously done this test using rmdir ., but that deletes the directory's inode (huge thanks to BowlOfRed for pointing this out), which leaves test a dangling directory entry and is the real reason for the problems encountered. In this scenario, the test folder then becomes unusable; after mounting the image, running ls produces

ls: cannot access '/mnt/test': Structure needs cleaning

and the kernel log shows

EXT4-fs error (device loop2): ext4_lookup:1606: inode #2: comm ls: deleted inode referenced: 38913

Running e2fsck in this situation on the image deletes the test directory entirely (the directory inode is gone so there's nothing to restore).

All this shows that . exists as a specific entity in the EXT4 filesystem. I got the impression from the filesystem code in the kernel that it expects . and .. to exist, and warns if they don't (see namei.c), but with the unlink .-based test I didn't see that warning. e2fsck doesn't like the missing . directory entry, and offers to fix it:

$ /sbin/e2fsck -f test.img
e2fsck 1.43.3 (04-Sep-2016)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Missing '.' in directory inode 30721.
Fix<y>?

This re-creates the . directory entry.

Related Solutions

How to safely remove hard-link to home folder

Many shells expand a leading tilde character (~) before a slash at the start of a pathname into the absolute path of your HOME directory. You can prevent this shell expansion by quoting the tilde character or by making sure the tilde doesn't start the pathname:

$ echo "$HOME"
/home/idallen
$ echo ~
/home/idallen
$ echo "~"
~
$ echo \~
~
$ echo ./~
./~

There is nothing special about a tilde character to Unix itself, so you can create a directory named tilde, provided you hide the tilde from expansion by the shell using any of the above methods:

$ mkdir "~"
$ ls -ld '~'
drwxr-xr-x 2 idallen idallen 40 Oct 22 08:45 ~
$ ls -ld \~
drwxr-xr-x 2 idallen idallen 40 Oct 22 08:45 ~
$ ls -ld ./~
drwxr-xr-x 2 idallen idallen 40 Oct 22 08:45 ~
$ ls -ld /tmp/idallen/~
drwxr-xr-x 2 idallen idallen 40 Oct 22 08:45 /tmp/idallen/~

If you don't hide the leading tilde character from the shell, the shell expands it to be your HOME directory. Adding an echo command in front of a command line can show you what the shell does to your command line before it is executed:

$ echo cd ~
cd /home/idallen                 # would go to $HOME directory
$ echo cd ./~
cd ./~                           # would go into directory named ~

You have discovered this need for quoting the tilde, because the commands below would have very different effects, depending on your shell expanding an unquoted leading tilde:

$ echo rm -r ~       
rm -r /home/idallen              # would remove the $HOME directory
$ echo rm -r "~"
rm -r ~                          # would remove the directory named ~
$ echo rm -r ./~
rm -r ./~                        # would remove the directory named ~
$ echo rm -r /tmp/idallen/~
rm -r /tmp/idallen/~             # would remove the directory named ~

Only unquoted tildes at the start of pathnames expand to be your HOME directory.

How to tell which file is original if hard link is created

You can't, because they are literally the same file, only reached by different paths. The first one has no special status.

Best Answer

Related Solutions

How to safely remove hard-link to home folder

How to tell which file is original if hard link is created

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