What are soft and hard links in Unix/Linux?

Unix/Linux operating systems provide users with powerful tools for managing files and directories. Among these tools, soft and hard links are essential concepts that contribute to efficient file organization and resource management. In this Answer, we’ll delve into the differences between soft and hard links, exploring their uses and how they enhance file system functionality.

Soft links

Soft links, also known as symbolic links or symlinks, are references to other files or directories by name. Instead of directly pointing to the data blocks of the target file like hard links do, soft links store the path to the target file or directory. This means that when you access a soft link, the system looks up the path stored in the link and then follows it to reach the actual data.

To create a soft link, you use the ln command with the -s option, followed by the source file and the name of the link:

Symbolic links are commonly used for creating shortcuts to files or directories. For example, imagine a scenario where we have a file named document.txt located in the /home/user/documents/ directory. Now, we want to create a soft link to this file on our desktop for quick access. Using the ln -s command, we can create a symbolic link on our desktop that points to document.txt. When we click the symbolic link, the system reads the stored path and redirects us to the original file in the /home/user/documents/ directory.

Soft links, therefore, provide a way to reference files or directories indirectly. They act as aliases, allowing users to create convenient shortcuts without physically duplicating the data.

Hard links

Hard links, on the other hand, are different from soft links in that they directly point to the inode of a file. An inode contains metadata about a file, such as its size, permissions, and disk block location. When we create a hard link, we essentially create an additional directory entry that points to the same inode as the original file.

Hard links can be created using the ln command without any options:

Hard links are beneficial for efficient disk space utilization since they allow multiple directory entries to reference the same data blocks. They are often used for version control systems and backups where changes in one file are reflected in all hard links. For example, imagine we have a critical configuration file named settings.conf located in the /etc/ directory, which is a common location for system configuration files on Unix/Linux systems. This configuration file is crucial for the proper functioning of various system processes. To ensure the integrity and accessibility of this configuration file, we decide to create a hard link in another directory, /backup/. Now, if the original settings.conf file is accidentally deleted, the hard link in /backup/ still holds a reference to the same inode. This means that your critical configuration information is not lost, as it is still accessible through the hard link in the /backup/ directory.

Soft links vs. hard links

Let's see how the soft links behave differently than the hard links.

The illustration below will help in better understanding the comparison:

Example

In this example, we will follow certain steps to visually demonstrate the difference between the soft links and the hard links:

1. Click “Click to Connect....” to start the terminal.

2. Create a new text file, original.txt, using the command:

3. To confirm the creation of this newly created file, read its content using the command:

4. Create a soft link, soft_link_to_original.txt, using the command:

5. Create a hard link, hard_link_to_original.txt, using the command:

6. Use the command ls -l to see the details of both the soft and hard links.

7. Add some more content to original.txt via the soft link using the command:

8. Add some more content to original.txt via the hard link using the command:

9. Read the updated content of original.txt again using cat original.txt. Similarly, read the content of soft_link_to_original.txt and hard_link_to_original.txt as well. All of them must have the same content by now.

10. Delete the original text file using the command:

11. Use ls to see how the color of soft_link_to_original.txt has been changed to red, demonstrating its dangling state. Then, try reading the soft_link_to_original.txt file again. We’ll get an error this time around.

12. Now, try to read the hard_link_to_original.txt file. The read would be successful, as expected. There will be no impact of deleting the original.txt file on this hard link.

Understanding the differences between soft and hard links in Unix/Linux is crucial for effective file management. Soft links provide flexibility in referencing files across file systems, while hard links offer efficient disk space usage and maintain consistency across multiple directory entries. By incorporating these concepts into the file organization strategies, one can enhance the efficiency and organization of their Unix/Linux file system.