3. Links

The best way to understand what links are is to look at an example. Let's create a (regular) file:

$ pwd
/home/queen/example
$ ls
$ touch a
$ ls -il a
32555 -rw-r--r--  1 queen queen 0 Aug  6 19:26 a

The -i option of the ls command prints the inode number, which is the first field on the output. As you can see, before we created file a, there were no files in the directory. The other field of interest is the third one, which is the number of file links (well, inode links, in fact).

The touch a command can be separated into two distinct actions:

But now, if we type:

$ ln a b
$ ls -il a b
32555 -rw-r--r--  2 queen queen 0 Aug  6 19:26 a
32555 -rw-r--r--  2 queen queen 0 Aug  6 19:26 b
$

We create another link to the same inode. As you can see, we didn't create a file named b. Instead, we just added another link to the inode numbered 32555 in the same directory, and attributed the name b to this new link. You can see on the ls -l output that the link counter for the inode is now 2 rather than 1.

Now, if we do:

$ rm a
$ ls -il b
32555 -rw-r--r--  1 queen queen 0 Aug  6 19:26 b
$

We see that even though we deleted the “original file”, the inode still exists. But now, the only link to it is the file named /home/queen/example/b.

Therefore a file in UNIX® has no name; instead, it has one or more link(s) in one or more directories.

Directories themselves are also stored in inodes. Their link count coincides with the number of sub-directories within them. This is due to the fact that there are at least two links per directory: the directory itself (represented by the entry .) and its parent directory (represented by ..). So a directory with two sub-directories will have at least four links: ., .. and links for each sub-directory.

Typical examples of files which are not linked (i.e.: have no name) are network connections. You will never see the file corresponding to your connection to the Mandriva Linux web site in your file tree, no matter which directory you look in. Similarly, when you use a pipe in the shell, the inode corresponding to the pipe exists, but it is not linked. Temporary files are another example of inodes without names. You create a temporary file, open it, and then remove it. The file exists while it's open, but nobody else can open it (as there is no name to open it). This way, if the application crashes, the temporary file is removed automatically.