Unix soft link and hard link

To simply put it, a Unix link is a convenient way to access data of a file. It is similar to shortcuts in windows. But in UNIX there are two types of links, soft link and hard link. To understand the difference between these two, we should first know some basics of Unix file structure.

In UNIX file system, a file contains two parts, data part and filename part. Filename part contains the file name and indoe. Data part contains the actual data stored in the file. Inode is a data structure that contains the information about the file like permissions, group, owner, pointer to the data part etc….Each inode (and hence the data it points to) is identified by an inode number. Data part contains the actual data stored in the file. A user accesses a file using its name while the underlying file system access it with its inode number.

Different files can point to the same inode number. These files are called hard links. They will have different names and may be located in different paths, but under the same file system. Hard links can’t be created across different file systems, this is because the inode number is specific to a file system not the OS as a whole. Each file system maintains a table of inodes within that file system.

A soft link is a special file that contains the file system path to another file as its data. But as soft link (symlink) is a special file, any read/write/modify operation on this file will be redirected to the target file. Since symlinks refer to their target using absolute path and file name, it can cross file system boundaries and can be located anywhere. Soft links can point to both files and directories.

Difference b/w soft and hard links

Soft link	Hard Link
Points to file name of original file	Points to inode of original file
Can link between file systems	Can link only within same file system
Both for file & directory	Only for files
Default permission 777	Default permission 644
Can’t access data after original file is deleted	Can access data even after original file is deleted
Can’t change owner/group/permission	Can change owner/group/permission
Inode number not same as original file	Inode number same as original file

Now create a text file

[root@penguin007 links]# touch link-test.orig
[root@penguin007 links]# echo “Original file created” > link-test.orig
[root@penguin007 links]# cat link-test.orig
Original file created

The command to create link is "ln". Let us create a hard link to the file link-test.orig.

[root@penguin007 links]# ln link-test.orig link-test.hard

Now let us create a soft link to the file link-test.orig.
[root@penguin007 links]# ln -s link-test.orig link-test.soft
[root@penguin007 links]# ls -li
total 8
163774475 -rw-r–r– 2 root root 22 Jul 19 13:38 link-test.hard
163774475 -rw-r–r– 2 root root 22 Jul 19 13:38 link-test.orig
163774476 lrwxrwxrwx 1 root root 14 Jul 19 13:47 link-test.soft -> link-test.orig

Now you can make changes to the original file/soft link/hardlink and the change will be reflected on all others. This is because all of these are pointing to the same data location

Edit from soft link

[root@penguin007 links]# echo “Change from soft link” > link-test.soft
[root@penguin007 links]# cat link-test.orig
Change from soft link
[root@penguin007 links]# cat link-test.hard

Change from soft link

Edit from hard link

[root@penguin007 links]# echo “Change from hard link” > link-test.hard
[root@penguin007 links]# cat link-test.orig
Change from hard link
[root@penguin007 links]# cat link-test.soft

Change from hard link

Edit from original file

[root@penguin007 links]# echo “Change from original” > link-test.orig
[root@penguin007 links]# cat link-test.soft
Change from original
[root@penguin007 links]# cat link-test.hard

Change from original

From the output of "ls -li" you can observe following points.

Both the original file and hard link are having same inode (163774475 - column 1 ). In fact both these files are referred to as hard links and you can't distinguish which file is original and which one is link - there is no need to do that either.

Note the permission on the files, hard links are having standard 644 permissions while soft link has 777 permission. Hard links are treated as regular files but soft links are treated as links- note the first character in the 2^nd column of "ls" output above ("-" for regular file/hard link and "l" for soft link)

When you change permission/ownership of a hard link the change is reflected on all hard links. But when you change permission/ownership of a softlink the change is applied to the original file (and its hard links if any) while soft link retains the permission and ownership.

[root@penguin007 links]# chmod 755 link-test.hard

[root@penguin007 links]# ls -li
total 8
163774475 -rwxr-xr-x 2 root root 22 Jul 19 13:38 link-test.hard
163774475 -rwxr-xr-x 2 root root 22 Jul 19 13:38 link-test.orig
163774476 lrwxrwxrwx 1 root root 14 Jul 19 13:47 link-test.soft -> link-test.orig
[root@penguin007 links]# chmod 644 link-test.soft
[root@penguin007 links]# ls -li
total 8
163774475 -rw-r–r– 2 root root 22 Jul 19 13:38 link-test.hard
163774475 -rw-r–r– 2 root root 22 Jul 19 13:38 link-test.orig
163774476 lrwxrwxrwx 1 root root 14 Jul 19 13:47 link-test.soft -> link-test.orig
[root@penguin007 links]# chown safeer link-test.orig
[root@penguin007 links]# ls -li
total 8
163774475 -rw-r–r– 2 safeer root 22 Jul 19 13:38 link-test.hard
163774475 -rw-r–r– 2 safeer root 22 Jul 19 13:38 link-test.orig
163774476 lrwxrwxrwx 1 root root 14 Jul 19 13:47 link-test.soft -> link-test.orig
[root@penguin007 links]# chown root link-test.soft
[root@penguin007 links]# ls -li
total 8
163774475 -rw-r–r– 2 root root 22 Jul 19 13:38 link-test.hard
163774475 -rw-r–r– 2 root root 22 Jul 19 13:38 link-test.orig
163774476 lrwxrwxrwx 1 root root 14 Jul 19 13:47 link-test.soft -> link-test.orig

The third column in the output shows the hard links count, ie; how many hard links point to that file (2 in our case).

Links do not cause any additional hard disk usage. We can check it now:

[root@penguin007 links]# du -hs ./*
4.0K ./link-test.hard

0 ./link-test.soft

You may note that only one file (that comes alphabetically first) of the hard links is displayed in disk usage result (this happens if both hard links are in the same directory)

Now let us delete the original file.
[root@penguin007 links]# rm -rf link-test.orig
[root@penguin007 links]# cat link-test.hard
Change from hard link
[root@penguin007 links]# cat link-test.soft

cat: link-test.soft: No such file or directory

As you can see, soft link will not work if the original file is deleted, because it points to the name of the original file. But hard links are pointing directly to the inode, so it can continue accessing the data. When a file has multiple hard links, the data part of the file will not be deleted until all the hard links to the file are deleted. The count of hard links is stored in the inode which will be decremented every time a hard link to that data is deleted.