This is just a bad idea, as there is no way to tell the difference between a hard link and an original name.
Allowing hard links to directories would break the directed acyclic graph structure of the filesystem, possibly creating directory loops and dangling directory subtrees, which would make
fsck and any other file tree walkers error prone.
First, to understand this, let’s talk about inodes. The data in the filesystem is held in blocks on the disk, and those blocks are collected together by an inode. You can think of the inode as THE file.
Inodes lack filenames, though. That’s where links come in.
A link is just a pointer to an inode. A directory is an inode that holds links. Each filename in a directory is just a link to an inode. Opening a file in Unix also creates a link, but it’s a different type of link (it’s not a named link).
A hard link is just an extra directory entry pointing to that inode. When you
ls -l, the number after the permissions is the named link count. Most regular files will have one link. Creating a new hard link to a file will make both filenames point to the same inode. Note:
% ls -l test ls: test: No such file or directory % touch test % ls -l test -rw-r--r-- 1 danny staff 0 Oct 13 17:58 test % ln test test2 % ls -l test* -rw-r--r-- 2 danny staff 0 Oct 13 17:58 test -rw-r--r-- 2 danny staff 0 Oct 13 17:58 test2 % touch test3 % ls -l test* -rw-r--r-- 2 danny staff 0 Oct 13 17:58 test -rw-r--r-- 2 danny staff 0 Oct 13 17:58 test2 -rw-r--r-- 1 danny staff 0 Oct 13 17:59 test3 ^ ^ this is the link count
Now, you can clearly see that there is no such thing as a hard link. A hard link is the same as a regular name. In the above example,
test2, which is the original file and which is the hard link? By the end, you can’t really tell (even by timestamps) because both names point to the same contents, the same inode:
% ls -li test* 14445750 -rw-r--r-- 2 danny staff 0 Oct 13 17:58 test 14445750 -rw-r--r-- 2 danny staff 0 Oct 13 17:58 test2 14445892 -rw-r--r-- 1 danny staff 0 Oct 13 17:59 test3
-i flag to
ls shows you inode numbers in the beginning of the line. Note how
test2 have the same inode number,
test3 has a different one.
Now, if you were allowed to do this for directories, two different directories in different points in the filesystem could point to the same thing. In fact, a subdir could point back to its grandparent, creating a loop.
Why is this loop a concern? Because when you are traversing, there is no way to detect you are looping (without keeping track of inode numbers as you traverse). Imagine you are writing the
du command, which needs to recurse through subdirs to find out about disk usage. How would
du know when it hit a loop? It is error prone and a lot of bookkeeping that
du would have to do, just to pull off this simple task.
Symlinks are a whole different beast, in that they are a special type of “file” that many file filesystem APIs tend to automatically follow. Note, a symlink can point to a nonexistent destination, because they point by name, and not directly to an inode. That concept doesn’t make sense with hard links, because the mere existence of a “hard link” means the file exists.
So why can
du deal with symlinks easily and not hard links? We were able to see above that hard links are indistinguishable from normal directory entries. Symlinks, however, are special, detectable, and skippable!
du notices that the symlink is a symlink, and skips it completely!
% ls -l total 4 drwxr-xr-x 3 danny staff 102 Oct 13 18:14 test1/ lrwxr-xr-x 1 danny staff 5 Oct 13 18:13 test2@ -> test1 % du -ah 242M ./test1/bigfile 242M ./test1 4.0K ./test2 242M .
You can use bind mount to simulate hard linking directories
sudo mount --bind /some/existing_real_contents /else/dummy_but_existing_directory sudo umount /else/dummy_but_existing_directory
With the exception of mount points, each directory has one and only parent:
One way to do
pwd is to check the device:inode for ‘.’ and ‘..’. If they are the same, you have reached the root of the file system. Otherwise, find the name of the current directory in the parent, push that on a stack, and start comparing ‘../.’ with ‘../..’, then ‘../../.’ with ‘../../..’, etc. Once you’ve hit the root, start popping and printing the names from the stack. This algorithm relies on the fact that each directory has one and only one parent.
If hard links to directories were allowed, which one of the multiple parents should
.. point to? That is one compelling reason why hardlinks to directories are not allowed.
Symlinks to directories don’t cause that problem. If a program wants to, it could do an
lstat() on each part of the pathname and detect when a symlink is encountered. The
pwd algorithm will return the true absolute pathname for a target directory. The fact that there is a piece of text somewhere (the symlink) that points to the target directory is pretty much irrelevant. The existence of such a symlink does not create a loop in the graph.