Linking and Copying

Question

I have a few questions regarding links in UNIX

1. Can I say Soft links in UNIX are analogous to shortcuts in windows?
2. Difference between copying and hard-linking?
3. Can anyone give me a use-case where I should prefer hard-linking over copying?

I'm so messed up right now. Any help is highly appreciated

Answer 1

The basic thing is, that copying makes a copy of the file, and linking (soft or hard) does not.

As an abstraction model, think of your directory as a table with:

filename       where the file is     content of the file
---------------------------------------------------------
a.txt          sector 13456          abcd
b.txt          sector 67679          bcde

When I copy a file, cp a.txt c.txt, I get the following:

filename       where the file is     content of the file
---------------------------------------------------------
a.txt          sector 13456          abcd
b.txt          sector 67679          bcde
c.txt          sector 79774          abcd

When I hard-link a file ln b.txt d.txt, I get the following:

filename       where the file is     content of the file
---------------------------------------------------------
a.txt          sector 13456          abcd
b.txt          sector 67679          bcde
c.txt          sector 79774          abcd
d.txt          sector 67679          bcde

So, now b.txt and d.txt are exactly the same file. If I add a character f to d.txt, it will also appear in b.txt

The problem with hard linking is that you can only do it on the same filesystem. Therefore, most people use soft links, ln -s a.txt e.txt:

filename       where the file is     content of the file
---------------------------------------------------------
a.txt          sector 13456          abcd
b.txt          sector 67679          bcde
c.txt          sector 79774          abcd
d.txt          sector 67679          bcde
e.txt          sector 81233          "Look at where a.txt is located"

As a first order approximation, soft links are a bit like shortcuts in Windows. However, soft links are a part of the filesystem, and will therefore work with every program. Windows shortcuts are just a file that is interpreted by explore.exe (and some other programs). But Windows programs need to do something in interpreting the shortcut, where as in Linux, soft links are handled automatically.

Most uses of links use soft links, because they are more flexible, can point to other filesystems, can be used with NFS et cetera.

The one use-case I have seen for hard links is to make sure that a file is not deleted by a user. The sysadmin created hard-links in a "pointer" directory and when the user inadvertently rm-ed a file (which apparently happened a lot there) he could in no-time restore the file without the use of tape, without double disk space etc.

That works as follows:

filename       where the file is     content of the file
---------------------------------------------------------
a.txt          sector 13456          abcd
b.txt          sector 67679          bcde

When the user types rm a.txt, the table will be:

filename       where the file is     content of the file
---------------------------------------------------------
b.txt          sector 67679          bcde

All reference to a.txt is lost. The disk space may be reclaimed for other files.

However, if the sysadmin keeps a copy of links to important files, the tabel will be:

filename       where the file is     content of the file
---------------------------------------------------------
a.txt          sector 13456          abcd
b.txt          sector 67679          bcde
link.a.txt     sector 13456          abcd
link.b.txt     sector 67679          bcde

When a user now types rm a.txt, the table becomes:

filename       where the file is     content of the file
---------------------------------------------------------
b.txt          sector 67679          bcde
link.a.txt     sector 13456          abcd
link.b.txt     sector 67679          bcde

Because there is still a reference to the file starting at 13456, the disk space of the file will not be marked as free. So the file is still there. When the user now asks if it would be possible somehow restore the a.txt, the sysadmin simply dose ln link.a.txt a.txt and the file a.txt re-appears! And with its latest edits too. (of course, the link.a.txt is in another directory on the same filesystem and this doesn't mean that you can forget about backups, but at that time and place, it was a useful option).

Answer 2

1. No, shortcuts are independent files and have to be first opened by the Windows Explorer/Shell which then opens an application handling them or executes an application. On the other hand Unix applications can open symbolic links directly and work with them as real files.

2. Copying creates a copy of a file, while hard-linking just creates an new inode which is linked to the same data on the disk. Hard link basically occupies next to zero space on the disk.

3. I've never used hard links to any extent, so I cannot give you any example. Here's a good comparison between symbolic and hard links in Unix: https://blog.usejournal.com/what-is-the-difference-between-a-hard-link-and-a-symbolic-link-8c0493041b62 https://www.geeksforgeeks.org/soft-hard-links-unixlinux/