Soft Links & Hard Links

What are links in Linux?


A Link in Linux is a pointer to a file. Like Pointers in any programming languages, links in Linux are pointers pointing to a file or a directory. Creating Links is a Kind of shortcuts to access a file. The two different type of links in UNIX are.


• Soft Links (or) Symbolic Links
• Hard Links


Difference Between Soft links and hard links:



Soft Links	Hard Links
Soft Links Can be created across file systems	Had Links can be created only within the File System
Soft Link has a different inode number than the original file	Had links have the same inode number as the original File
On deleting the original file, soft links cannot be accessed	On deleting the original file, had linked file can still be accessed
Soft links need extra memory to store the original file name as its data	Had links do not need any extra data memory to save since  it uses links
Source file need not exist for soft link file creation	Source file should exist
Can be created on a file or on a directory	Can be created only on files, not on directories
Access to the file is slower due to the overhead to access  file	Access to the file is faster compared to soft link



How do we create Soft Links and Hard Links? And How do we access them?


We have a file named "file1" with the following contents:


$ cat file1
Welcome


To create a hard link of file1:


$ ln file1 file2


To create  a soft link of file1:


$ ln -s file1 file3


Once the links are created, the linked files contain the same content as of the original file. See below


$ cat file2
welcome
$ cat file3
Welcome


Note: Soft links can be created on non-existent files as well.


•  "file2" and "file3" being the linked files, can I say which is soft link & which is hard link?


Yes. When we do the listing of the files with "ls -li" option:


$ ls -li
total 20
9962464 -rw-r--r-- 2 guru users 8 Mar  9  file1
9962464 -rw-r--r-- 2 guru users 8 Mar  9  file2
9962471 lrwxrwxrwx 1 guru users 5 Mar  9  file3 -> file1


If you notice "file3" shows "->" towards "file1". This indicates file3 is a soft-link to file1. In case of file2, if we notice the inode numbers of file1 and file2, they are the same. Same indoe number indicates file1 and file2 are hard links. Also note, the link count of these files is 2.


• File1 and file2 are hard links? Can we say which is the original file and which is the hard linked file?:


No. We can not say which is the original file and which one is hard-linked file. Once a hard-linked is created, it is like 2 files pointing to the same location. In fact, once a hard link is created on a file, using the term "original file" is actually incorrect.




• How is it possible that while deleting the original file, still the hard linked file is accessible and the soft linked file is not?


Let us look into the inode structure of the hard and soft links to understand in details. 













Fig1: Hard link representation ( Files pointing to indoe, in turn pointing to data location)


File1 and File2 are hard links, As we know, they both have the same inode number ( 9962464) and hence they both point to the same indoe structure. One of the parameters in the inode tells the location of the file contents, and hence the contents are accessed by both the files.


Now, say we try to delete the file "File2". When the file is deleted, only the link connecting of File2 to the indoe structure  gets disconnected. However, the inode and the file contents and the link of FIle1  are still in tact, and hence the file is still accessible.


However, the same is not the case with the soft link. In case of Soft Link.

















Fg2: Soft Link Representation (Files pointing to inodes, in turn pointing to data location)


File1 and File3 both have different inode numbers. These different inode numbers point to different data locations. File1 location points to the contents of the file. However, if you see the content location of File3, it contains "File1" which is nothing but the original file from which the soft link is formed. Now, when "File3" is accessed, through its inode metadata, it gets the name "File1" and searches for "File1" and reaches to the content of "File1" Hence once the "File1" is deleted, "File3" becomes dangling meaning it points to something which does not exist. And so, the soft link becomes inaccessible.


For these same reasons, we can create a soft link on an non-existent file, however the same is not possible for hard links.


• Soft links can span across File systems whereas hard links can be created only within a file system. Why so?


Hard links refers to its file using the inode number. Inode numbers are specific within a file system. This means a file "f1" in file system "X" can have the same inode numbers as of a file  "f2" in file system "Y". This is very much possible. For this reason, had links cannot be shared across file systems.


However, the same is not true for soft links, Soft links refer to the files using the file name as seen in the earlier case. Hence a soft links can be accessed across the file systems.


• Does soft links and hard links work only at the file level? Does it work at the directory level as well?


Hard links works only at the file level. However, soft links works at the directory level as well. This particular use of soft links, to create links at the directory level, serves us a shortcut for many frequently used directories.