[ILUG] odd linking

[Niall]

On Sat, May 13, 2000 at 09:33:57PM +0100, David Meagher wrote:

> this evening i was trying to link two directories on two different
> harddrives
> 
> i tried
> ll -D /dir1 /mnt/DOS-hdb1/dir2
> and
> ll -F /dir1 /mnt/DOS-hdb1/dir2
> 
> it either gave me a
> you can't hard link these two directories
> or
> won't connect directories on different devices

Hmm - I don't know what command ll is but I'll proceed on the assumption
that it's some kind of alias for ln or that you mis-typed it :-)

It's not possible to make a hard link between two directories (or files, for
that matter) which are on different drives. To make a hard link to a file
(or directory, which is only a special kind of file) means to make another
directory entry pointing to the same inode as the file to which you are
linking. Recall that an inode is a structure which is part of a filesystem,
and it should be obvious that a directory entry on filesystem A can't
physically point to an inode on filesystem B, whether these filesystems be
on different drives or not. From the info page for ln

   A "hard link" is another name for an existing file; the link and the
original are indistinguishable.  Technically speaking, they share the same
inode, and the inode contains all the information about a file--indeed, it
is not incorrect to say that the inode _is_ the file.

You can do what you want with a symbolic link (ln -s) which is a text based
re-direction. It's fractionally slower in use than a hard link (not that
you'll notice), but works across disks.


Interesting aside - I do remember reading that ln on Linux won't make hard
links to directories except for the super-user, because there is serious
potential for filesystem confusion and the man page seems to concur where it
says 

-d, -F, --directory
        hard link directories (super-user only)
	

However, the info page says 

On all existing implementations, you cannot make a hard link to a directory,

and this seems to be true, at least on the two boxes I tried (SuSE 6.3 /
2.2.13, Mandrake 5.3 / 2.2.12)

Did any version of ln on Linux ever support hard linking directories ? (I
know I've done this on Solaris - can't remember why, but I'm sure I had a
good reason :-) )


Regards,


Niall