What is a hard link in Linux?

What is a Hard Link in Linux?

In the Linux file system, a hard link is a type of file system object that allows multiple directory entries to point to the same underlying file data. In other words, a hard link is an additional directory entry that references the same inode (i.e., the file's metadata) as the original file.

Understanding Inodes

To understand hard links, it's essential to first understand the concept of inodes in the Linux file system. An inode is a data structure that stores information about a file, such as its permissions, ownership, timestamps, and the physical location of the file's data blocks on the disk. Each file in the Linux file system has a unique inode number, which is used to identify the file.

In the diagram above, both File1 and File2 have directory entries that point to the same inode, which represents the underlying file data.

Creating Hard Links

You can create a hard link to an existing file using the ln command in the Linux terminal. The syntax is as follows:

ln <source_file> <target_file>

For example, let's say you have a file named file.txt in your current directory. To create a hard link to this file, you can run the following command:

ln file.txt file_link.txt

This will create a new directory entry named file_link.txt that points to the same inode as file.txt.

Characteristics of Hard Links

1. Same Inode: Hard links share the same inode, which means they reference the same underlying file data. Modifying the file through any of the hard links will affect the original file.
2. No Nested Links: You cannot create a hard link to another hard link, as this would create a nested directory structure, which is not allowed in the Linux file system.
3. No Cross-Device Links: You can only create hard links within the same file system. You cannot create a hard link to a file on a different file system or device.
4. No Links to Directories: You cannot create hard links to directories, as this could lead to circular references and other file system issues.
5. Increased File Count: Each hard link increases the link count of the file, which is visible in the file's properties.

Benefits of Hard Links

1. Space Efficiency: Hard links allow you to save disk space by avoiding the need to store multiple copies of the same file data.
2. Simplified File Management: If you have multiple references to the same file, you can modify the file through any of the hard links, and the changes will be reflected across all the links.
3. Backup and Restore: Hard links can be useful in backup and restore scenarios, as they preserve the file's metadata and relationships.

Conclusion

In summary, a hard link in Linux is an additional directory entry that points to the same underlying file data as the original file. Hard links share the same inode, which means they reference the same file data. Understanding hard links is essential for efficient file management and storage optimization in the Linux file system.