Note:Some confusion may have arisen. Working together on a lab is ok. But you are to write the labs yourselves. So the labs should look substantially different as one would expect when two people write up ideas they worked on together.

4.3.3: Shared files (links)

• ``Shared'' files is Tanenbaum's terminology.
• More descriptive would be ``multinamed files''.
• If a file exists, one can create another name for it (quite possibly in another directory).
• This is often called creating a (or another) link to the file.
• Unix has two flavor of links, hard links and symbolic links or symlinks.
• Dos/windows has symlinks, but I don't believe it has hard links.
• These links often cause confusion, but I really believe that the diagrams I created make it all clear.

Hard Links

• Symmetric multinamed files.
• When a hard like is created another name is created for the same file.
• The two names have equal status.
• It is not, I repeat NOT true that one name is the ``real name'' and the other one is ``just a link''.

Start with an empty file system (i.e., just the root directory) and then execute:

cd /
mkdir /A; mkdir /B
touch /A/X; touch /B/Y

We have the situation shown on the right.

Note that names are on edges not nodes. When there are no multinamed files, it doesn't much matter.

Now execute

ln /B/Y /A/New

At this point there are two equally valid name for the right hand yellow file, /B/Y and /A/New. The fact that /B/Y was created first is NOT detectable.

• Both point to the same inode.
• Only one owner (the one who created the file initially).
• One date, one set of permissions, one ... .

Assume Bob created /B and /B/Y and Alice created /A, /A/X, and /A/New. Later Bob tires of /B/Y and removes it by executing

rm /B/Y

The file /A/New is still fine (see third diagram on the right). But it is owned by Bob, who can't find it! If the system enforces quotas bob will likely be charged (as the owner), but he can neither find nor delete the file (since bob cannot unlink, i.e. remove, files from /A)

Since hard links are only permitted to files (not directories) the resulting file system is a dag (directed acyclic graph). That is, there are no directed cycles. We will now proceed to give away this useful property by studying symlinks, which can point to directories.

Symlinks

• Asymmetric multinamed files.
• When a symlink is created another file is created, one that points to the name of original file.
• A hard link in contrast points to the original file. The examples will make this clear.

Again start with an empty file system and this time execute

cd /
mkdir /A; mkdir /B
touch /A/X; touch /B/Y
ln -s /B/Y /A/New

We now have an additional file /A/New, which is a symlink to /B/Y.

• The file named /A/New has the name /B/Y as its data (not metadata).
• The system notices that A/New is a diamond (symlink) so reading /A/New will return the contents of /B/Y (assuming the reader has read permission for /B/Y).
• If /B/Y is removed /A/New becomes invalid.
• If a new /B/Y is created, A/New is once again valid.
• Removing /A/New has no effect of /B/Y.
• If a user has write permission for /B/Y, then writing /A/New is possible and writes /B/Y.

The bottom line is that, with a hard link, a new name is created that has equal status to the original name. This can cause some surprises (e.g., you create a link but I own the file). With a symbolic link a new file is created (owned by the creator naturally) that points to the original file.

Question: Consider the hard link setup above. If Bob removes /B/Y and then creates another /B/Y, what happens to /A/New?
Answer: Nothing. /A/New is still a file with the same contents as the original /B/Y.

Question: What about with a symlink?
Answer: /A/New becomes invalid and then valid again, this time pointing to the new /B/Y. (It can't point to the old /B/Y as that is completely gone.)