Creating directory hard links in Mac OS X

How can I create a hard link to a directory in Mac OS X?

This feature has been added to their file system in Mac OS X v10.5 (Leopard) (for time machine), but I could not find any information on actually using it from the command line.

I have bundled up the suggested answer in a Git repository if anybody is interested: https://github.com/selkhateeb/hardlink

Once installed, create a hard link with:

hln source destination

I also noticed that unlink command does not work on Mac OS X v10.6 (Snow Leopard), so I added an option to unlink:

hln -u destination

---

To install Hardlink, use Homebrew and run:

brew install hardlink-osx

Unfortunately Apple has crippled the ln command. You can use the following program to create a hard link to a directory:

#include <unistd.h>
#include <stdio.h>

int main(int argc, char* argv[]) {
 if (argc != 3) {
  fprintf(stderr,"Use: hlink <src_dir> <target_dir>\n");
  return 1;
 }
 int ret = link(argv[1],argv[2]);
 if (ret != 0)
  perror("link");
 return ret;
}

Take into account that the hard linked directories may not be in the same parent directory, so you can do this:

$ gcc hlink.c -o hlink
$ mkdir child1
$ mkdir parent
$ ./hlink child1 parent/clone2

In the answer to the question by the_undefined about how to remove a hardlink to a directory without removing the contents of other directories to which it is linked: As far as I can tell, it can't be done from the command line using builtin commands. However, this little program (inspired by Freeman's post) will do it:

#include <unistd.h>
#include <stdio.h>

int main(int argc, char* argv[]) {
    if (argc != 2) {
        fprintf(stderr,"Use: hunlink <dir>\n");
        return 1;
    }
    int ret = unlink(argv[1]);
    if (ret != 0)
        perror("unlink");
    return ret;
}

To follow on with Freeman's example,

$ gcc hunlink.c -o hunlink
$ echo "foo bar" > child1/baz.txt
$ ./hunlink parent/clone2

will remove the hardlink at parent/clone2, but leave the directory child1 and the file child1/baz.txt alone.

Another solution is to use bindfs https://code.google.com/p/bindfs/ which is installable via port:

sudo port install bindfs
sudo bindfs ~/source_dir ~/target_dir