Hard and Soft Links?

[davejjj]

So if there are several links in the system pointing to a particular file and the file is then moved what happens?

[yancek]

Nothing, I expect. You will probably get an error message like file not found or couldn't open file.
Never actually tried this myself.

[rcgreen]

It depends on what you move. If foo1 foo2 foo3 and foo4 are links to the same data,
if you move foo1, the data has not moved, only the link has moved. You assume that
when you move a file to another place you are moving the data, but that is not necessarily
true.

[Cabhan]

So, it depends on what sort of link you have.

The way that UNIX filesystems work is that the filesystem contains a link to an inode. The inode contains information like the permissions of the file, where it is actually stored on disk, etc. When I say link here, I refer to what is sometimes called a "hard link". So there can be multiple "hard links" to a single inode.

The "mv" command is basically a wrapper around "create a new link, remove the old link". So when you say "mv foo bar", it actually runs:

ln foo bar # Create a new link "bar" that points to the inode pointed at by "foo"
rm foo # Remove the link "foo"

So if we have this situation:

link1            link2
 |                |
 |                |
 |--> inode <------

And we run the command "mv link2 link3", we have this:
So if we have this situation:

link1            link3
 |                |
 |                |
 |--> inode <------

As you see, link1 is entirely unaffected.


A symbolic link (aka soft link) is a totally different story. A symbolic link points to a specific filepath. So, we might have a situation like this:

symlink ---> /home/user/file
/home/user/file ---> inode

Now, if we move /home/user/file with "mv /home/user/file /home/user/different_file", we have:

symlink ---> /home/user/file
/home/user/different_file ---> inode

Because symlink now points to a file that doesn't exist, it doesn't point to anything unless /home/user/file is recreated.