The command line tool to create a link is ln, which is short for link, and by default, the ln command will create hard links. Let's look at a simple example. First, let's create a test file, f.

#> touch f
#> ls -l
-rw-r--r--  1 aviv  staff  0 Mar 23 12:51 f

In this format of ls -l output we have the following information:

   links--.                  .--file size
           \                / 
-rw-r--r--  1 aviv  staff  0 Mar 23 12:51 f
\________/    \_________/    \__________/  \_
     |             |              |          |
permissions    user/group       mod time     file name

All the fields except for links should be familiar to you, and link field describe how many hard links except to the underlying i-node that this file references. Let's see what happens when we create a hard link to f:

#> ln f hl   #Create a hard link to f named hl
#> ls -l
total 0
-rw-r--r--  2 aviv  staff  0 Mar 23 12:51 f
-rw-r--r--  2 aviv  staff  0 Mar 23 12:51 hl

You see the number of links in both f and hl incremented by 1 to 2, because there are now two files that link to the same i-node. We can further explore this by using the -i option on the ls output, which shows us the i-node number:

#> ls -li
34996364 -rw-r----- 2 aviv scs 0 Mar 23 18:14 f
34996364 -rw-r----- 2 aviv scs 0 Mar 23 18:14 hl

And, there it is, the same i-node for each hard link.

You've actually seen hard links before. Let's rerun ls -li with the -a option so we can see all files and directories:

#> ls -lia
34996362 drwxr-x--- 2 aviv scs 4096 Mar 23 18:14 .
34996236 drwxr-x--- 3 aviv scs 4096 Mar 23 18:14 ..
34996364 -rw-r----- 2 aviv scs    0 Mar 23 18:14 f
34996364 -rw-r----- 2 aviv scs    0 Mar 23 18:14 hl

Notice that the . or current directory has 2 hard links. Where do those come from? That's because every . and .. reference is actually a hard-link you get for free. The current directory is referenced by . to itself, and then directly to itself by name in the upper directory, that's 2. The .. directory have 3 references: the current directly link ..; the upper level directory . link; and the name of the directory itself.

Another thing to note is that a directory and a file in Unix is actually the same thing with respect to v-nodes/i-nodes. The only difference is that a file stores data and a directory stores a list of files within the directory.

In the same way we can add links, we can also remove links. The command to do that is unlink. Let's see the result:

#> ls -li
total 0
34996364 -rw-r----- 2 aviv scs 0 Mar 23 18:14 f
34996364 -rw-r----- 2 aviv scs 0 Mar 23 18:14 hl
#> unlink hl
#>ls -l
total 0
-rw-r----- 1 aviv scs 0 Mar 23 18:14 f

Notice that all that happened when we un-linked hl is that we removed it, and that is exactly what remove does —it un-links a v-node from a i-node. In other terms, the unlink command removes an entry from the file system by removing a link. The result of the unlinking can be the remove of a file if no links exist further. In the above example, f still links to the i-node, so the i-node is not removed. An i-node without any links can be reclaimed for further use, like freeing memory.

Two files that are hard-linked to the same file block or i-node is specific to the file system that is mounted. That means you cannot create a hard-link across mounted file systems because the link may span different kind of devices. If you think about it, this make sense because the i-node also stores device specific information for how to read and write from that device, it is not possible for a link to span different mounted file systems.

We can see this for ourselves. Recall the mount table from running mount on a lab machine:

#> mount
/dev/sda1 on / type ext4 (rw,errors=remount-ro)
proc on /proc type proc (rw,noexec,nosuid,nodev)
sysfs on /sys type sysfs (rw,noexec,nosuid,nodev)
none on /sys/fs/fuse/connections type fusectl (rw)
none on /sys/kernel/debug type debugfs (rw)
none on /sys/kernel/security type securityfs (rw)
udev on /dev type devtmpfs (rw,mode=0755)
devpts on /dev/pts type devpts (rw,noexec,nosuid,gid=5,mode=0620)
tmpfs on /run type tmpfs (rw,noexec,nosuid,size=10%,mode=0755)
none on /run/lock type tmpfs (rw,noexec,nosuid,nodev,size=5242880)
none on /run/shm type tmpfs (rw,nosuid,nodev)
binfmt_misc on /proc/sys/fs/binfmt_misc type binfmt_misc (rw,noexec,nosuid,nodev)
rpc_pipefs on /run/rpc_pipefs type rpc_pipefs (rw)
gvfs-fuse-daemon on /var/lib/lightdm/.gvfs type fuse.gvfs-fuse-daemon (rw,nosuid,nodev,user=lightdm)
nuzee.academy.usna.edu:/home/mids on /home/mids type nfs (rw,nfsvers=3,bg,addr=10.1.83.28)
zee.cs.usna.edu:/courses on /courses type nfs (rw,bg,addr=10.1.83.18)
zee.cs.usna.edu:/home/scs on /home/scs type nfs (rw,nfsvers=3,bg,addr=10.1.83.18)

Your home directories are mounted at /home/scs and /home/mids, which comes from the network file server zee, but the root file system is a disc from device /dev/sda1. Let's see what happens when we try and create a hard link from our home directory to the /tmp directory on the root file system:

#> ln f /tmp/f
ln: failed to create hard link `/tmp/f' => `f': Invalid cross-device link

You also create symbolic links with ln, but with an -s option:

#> ln -s f sl
#> ls -l
-rw-r----- 1 aviv scs 0 Mar 23 18:14 f
lrwxrwxrwx 1 aviv scs 1 Mar 23 18:44 sl -> f

You can see sl links to f, and this is indicated in the ls output. The permission of sl are not clear, since this is also part of the linking. If f has a particularly permission, so does sl.

Since symbolic links do not need to reference an i-node, it's ok if the linked too file does not exist. We can even remove the file that is being linked and sl just sticks around as before:

#> rm f
rm: remove regular empty file `f'? y
#> ls -l
lrwxrwxrwx 1 aviv scs 1 Mar 23 18:44 sl -> f
#> cat sl
cat: sl: No such file or directory

But if we try and use sl and follow the link, we'll get an error since f does not exist. A symbolic link that links to a nonexistent file or directory is described as a dangling link or broken link.

Unlike a hard link where the i-node must exist, a symbolic link can dangle. This means we can create symbolic links across file systems mounts.

#> ln -s f /tmp/sl
#> ls -l /tmp/sl
lrwxrwxrwx 1 aviv scs 1 Mar 23 18:51 /tmp/sl -> f

Think about why we can create symbolic links across file systems while we cannot create hard links. A symbolic link only references a file, it has nothing to do with the underlying device and file storage, so symbolic links can cross device boundaries. Further, since a symbolic link can be bad, not link anything, then even if that file system is not mounted, the symbolic link can exist irrespective.

One thing to be careful about with symbolic links is if the links are absolute or relative. An absolute symbolic link refers to a file by its absolute path, while a relative link uses an relative path from the location of the file. Let's look at a quick example. The setup is as follows:

#> echo "Hello World" > a
#> cat a
Hello World
#> mkdir dir1
#> ls -l
total 8
-rw-r----- 1 aviv scs   12 Mar 25 08:59 a
drwxr-x--- 2 aviv scs 4096 Mar 25 09:00 dir1

There is a file a with "Hello World" stored in it and a directory dir1. We want to create a symbolic link from within dir1 to a. Let's start by doing the obvious thing:

#> ln -s a dir1/a
#> ls -l dir1
total 0
lrwxrwxrwx 1 aviv scs 1 Mar 25 09:03 a -> a

This seemed to work. There is a symbolic link in dir1 called a which refers to a, but which a is that? Is it the one in the upper level directory, the one with "Hello World" in it, or is it the one in its own directory? We can see by trying to follow the link:

#> cat dir1/a 
cat: dir1/a: Too many levels of symbolic links

It is a self-referencing symbolic link and that's because the link is relative. When we see a -> a the link is relative to the location of the symbolically linking file; that is, dir/a refers to itself. What we need is for dir/a to refer to ../a, the a in the upper level directory with "Hello World."

To create a proper relative link we have to specify the right path in relation to the location of the link.

#> ln -s ../a dir1/a
#> ls -l dir1
total 0
lrwxrwxrwx 1 aviv scs 4 Mar 25 09:07 a -> ../a

Now, we can use that link properly:

#> cat dir1/a
Hello World

An easy trick to get your absolute paths right when creating symbolic links is to always change into the working directory where you wish the link to exist.

#> cd dir1/
#> ln -s ../a b
#> ls -l
total 0
lrwxrwxrwx 1 aviv scs 4 Mar 25 09:07 a -> ../a
lrwxrwxrwx 1 aviv scs 4 Mar 25 09:12 b -> ../a

Now its a bit more direct that you are linking ../a to b rather then having to envision the relative path from within file system.