creating links: hard vs. symbolic

Recently, I wrote about some less than intuitive directory placement with respect to iCloud.

While an alias is one way to solve this issue, another is to use a symbolic link (commonly referred to as a symlink).

As I investigated creating links, I noticed there was a distinction between a hard link and a soft (or symbolic) link.

Pros And Cons Of Hard And Soft Links

Before digging into the differences, it’s worth noting that in most situations I’ve come across, the soft link is the preferred approach. Unlike hard links, soft links can be directories.

Example

The following example demonstrating some of the differences between a hard link and symbolic link is derived from this answer on askubuntu.com, itself a derivative of these two answers on StackOverflow here and here.

Starting with the basics. Hard links and soft links are really only comparable at the file level because hard links can only be to files.

Let’s envision a file myfile.txt:

Notice that the file also has a pointer to its location in the file system in the form of an inode.

With this understanding, the diagram makes it clear that a hard link is a pointer to the inode itself and not the file. This is in contrast to the soft link which points to the filename.

To see the implications of this, an example is useful:

touch fileOne; echo "Cat" > fileOne;
touch fileTwo; echo "Dog" > fileTwo;

Confirm the data was written successfully:

$ cat fileOne; cat fileTwo;
Cat
Dog

At this point, we have two equivalents to the myfile.txt in the diagram above. Let’s examine how hard and soft links differ by creating one for each and then moving the underlying file.

To create the links, we’ll use the ln utility.

First, the hard link:

ln fileOne fileOne-hard

(The first argument is the target, the second is the name of the new link we’re creating)

Now the soft link:

ln -s fileTwo fileTwo-soft

If we look at our directory now:

$ ls -la
total 24
drwxr-xr-x   6 stephen  staff   192B Mar  6 11:11 .
drwxr-xr-x+ 81 stephen  staff   2.5K Mar  6 11:11 ..
-rw-r--r--   2 stephen  staff     4B Mar  6 10:50 fileOne
-rw-r--r--   2 stephen  staff     4B Mar  6 10:50 fileOne-hard
-rw-r--r--   1 stephen  staff     4B Mar  6 10:51 fileTwo
lrwxr-xr-x   1 stephen  staff     7B Mar  6 11:11 fileTwo-soft -> fileTwo

We can see the links have been created. Now let’s start changing the original files by moving them and see how this affects our links:

$ mv fileOne newFileOne
$ mv fileTwo newFileTwo
$ ls -la
total 24
drwxr-xr-x   6 stephen  staff   192 Mar  6 11:25 .
drwxr-xr-x+ 81 stephen  staff  2592 Mar  6 11:25 ..
-rw-r--r--   2 stephen  staff     4 Mar  6 10:50 fileOne-hard
lrwxr-xr-x   1 stephen  staff     7 Mar  6 11:11 fileTwo-soft -> fileTwo
-rw-r--r--   2 stephen  staff     4 Mar  6 10:50 newFileOne
-rw-r--r--   1 stephen  staff     4 Mar  6 10:51 newFileTwo

We can already start to suspect a problem at this point: fileTwo-soft is pointing to fileTwo which no longer exists! But let’s see it in practice:

$ cat fileOne-hard
Cat
$ cat fileTwo-soft
cat: fileTwo-soft: No such file or directory

And what about if we delete the original files altogether?

$ rm newFileOne; rm newFileTwo; ls -la
total 8
drwxr-xr-x   4 stephen  staff   128 Mar  6 11:27 .
drwxr-xr-x+ 81 stephen  staff  2592 Mar  6 11:27 ..
-rw-r--r--   1 stephen  staff     4 Mar  6 10:50 fileOne-hard
lrwxr-xr-x   1 stephen  staff     7 Mar  6 11:11 fileTwo-soft -> fileTwo

Our links are still there, but… while fileOne-hard still points to the data:

$ cat fileOne-hard
Cat

The soft link is linked to a non-existant file and so will error if we try to access it.

This demonstrates the biggest risk (in my opinion) with soft links: moving the underlying data breaks the link. Still, in that case, it’s a matter of recreating the link and then carrying on.