What's the difference between a hard links and copied files?

Question

RHEL 6

My understanding is that hard links include a copy of the original file and that I could delete a hard-linked file in one directory, and it would still exist in the other directory.

If that's the case, why would I want to use hard links at all? Why not just have two separate files?

Answer 1

If you copy a file it will duplicate the content. So if you modify the content of a single file, that has no effect on the other one.

If you do a hardlink that will create a file pointing on the same content. So if you change the content of either of the files the change will be be seen on both.

Answer 2

A hard link is basicly a second filename for the same file. So if you hardlink a file, it will only be once on the filesystem, and therefore only take up space once. So you want to use this if you wish to save diskspace

Answer 3

On unix file systems every filename is actually a hard link to the location of the data on the disc, called an inode. If you create a new hard link to an existing file, it will take no extra space on the disk as it is just another pointer to the same data. If you edit the data by one or other link (or edit the in ode directly) both files will be changed.

The system keeps a count of how many hard links each in ode has. When the link count is 0, the file can no longer be reached, and the data is marked as safe to be overwritten. So given a file with 2 hard links, if you delete either link, the data will not be deleted. Only if you delete both will the data be gone.

A soft link, on the other hand, points to another file by its file name. If you move or delete the original file, the link will be broken.

Answer 4

With reference to the part of the question that asked, "why would I want to use hard links at all?":

Hard-linked files (or for that matter soft (symbolic)-linked ones offer a useful way of having a single executable file that can be made to masquerade for different purposes.

That is, the name by which the code is invoked can be examined in order to determine what options are available for execution. This allows the development and packaging of one large piece of code with all the shared functionality needed for slightly different purposes. From the user's perspective, the specification of "what" to run (by name) limits the choices and presentation to a more manageable subset of options.

A classic example is LVM. At one time HP-UX used hard-linked files for the various executables (e.g. vgdisplay, vgcreate, vgextend, etc. Today, like Linux, these commands are actually symbolic (soft) links to the lvm executable.