How Linux Hardlinks Work and Link to Other Files

Introduction

This tutorial will provide a comprehensive understanding of how Linux hardlinks work and how they can be used to link to other files on your system. You will learn the basics of creating and managing hardlinks, as well as explore their practical applications in various scenarios.

Skills Graph

What is a Hardlink?

In the Linux file system, a hardlink is a reference to a file that points directly to the file's data on the disk. When you create a hardlink, you're essentially creating an additional name for the same file. This means that multiple filenames can all refer to the same underlying file data.

Hardlinks are different from symbolic links (also known as symlinks) in that they don't create a new file, but rather a new directory entry that points to the same inode (a data structure that stores information about a file, such as its location, permissions, and ownership).

The key characteristics of hardlinks are:

• They don't create a new file, but rather a new directory entry that points to the same file data.
• They have the same inode number as the original file.
• They share the same file data, so changes to the file content are reflected across all hardlinks.
• They can only be created for files, not directories.
• They can only be created within the same file system, as they rely on the inode number which is unique within a file system.

Hardlinks are useful in various scenarios, such as:

• Backup and Archiving: When creating backups or archives, hardlinks can be used to save disk space by avoiding duplicate file data.
• File Deduplication: Hardlinks can be leveraged in file deduplication systems to eliminate redundant file data.
• Version Control: Hardlinks can be used to maintain multiple versions of a file, where each version is a hardlink to the same underlying data.

To create a hardlink in the Linux terminal, you can use the ln command followed by the source file and the new hardlink name:

ln source_file new_hardlink

For example, to create a hardlink named "file2.txt" for the file "file1.txt", you would run:

ln file1.txt file2.txt

This will create a new hardlink named "file2.txt" that points to the same file data as "file1.txt".

Creating and Managing Hardlinks

Creating Hardlinks

To create a hardlink in the Linux terminal, you can use the ln command followed by the source file and the new hardlink name:

ln source_file new_hardlink

For example, to create a hardlink named "file2.txt" for the file "file1.txt", you would run:

ln file1.txt file2.txt

This will create a new hardlink named "file2.txt" that points to the same file data as "file1.txt".

Verifying Hardlinks

You can use the ls -l command to view the hardlink count for a file. The hardlink count is displayed in the second column of the output.

$ ls -l
-rw-r--r-- 2 user group 12 Apr 12 12:34 file1.txt
-rw-r--r-- 2 user group 12 Apr 12 12:34 file2.txt

In the example above, both "file1.txt" and "file2.txt" have a hardlink count of 2, indicating that they are hardlinks to the same file data.

Managing Hardlinks

Hardlinks can be managed just like regular files. You can perform various operations on them, such as:

• Renaming a Hardlink: You can use the mv command to rename a hardlink, which will update the directory entry without affecting the underlying file data.
• Deleting a Hardlink: You can use the rm command to delete a hardlink. This will not delete the file data until the last hardlink is removed.
• Determining the Number of Hardlinks: You can use the ls -l command to view the hardlink count for a file, as mentioned earlier.

It's important to note that deleting a hardlink does not delete the file data, as long as there are other hardlinks pointing to the same inode. The file data will only be deleted when the last hardlink is removed.

Hardlinks in Practice

Backup and Archiving

Hardlinks can be useful when creating backups or archives, as they can help save disk space by avoiding duplicate file data. When you create a backup or archive, any files that have hardlinks will be stored only once, with the hardlinks pointing to the same underlying data.

For example, let's say you have a directory with two files, "file1.txt" and "file2.txt", and you create a hardlink for "file2.txt" named "file3.txt". If you create a backup of this directory, the backup will only store the file data once, with all three filenames (file1.txt, file2.txt, and file3.txt) pointing to the same underlying data.

$ ls -l
-rw-r--r-- 2 user group 12 Apr 12 12:34 file1.txt
-rw-r--r-- 2 user group 12 Apr 12 12:34 file2.txt
-rw-r--r-- 2 user group 12 Apr 12 12:34 file3.txt

File Deduplication

Hardlinks can also be used in file deduplication systems to eliminate redundant file data. By identifying files with the same content and creating hardlinks between them, these systems can significantly reduce the amount of storage required.

This is particularly useful in scenarios where you have multiple copies of the same file, such as in a shared network storage environment or a backup system.

Version Control

Hardlinks can be used to maintain multiple versions of a file, where each version is a hardlink to the same underlying data. This can be useful in version control systems or file versioning workflows.

For example, you could create a new hardlink for a file every time you make a change, allowing you to easily revert to a previous version by simply deleting the latest hardlink.

$ ls -l
-rw-r--r-- 3 user group 12 Apr 12 12:34 file1.txt
-rw-r--r-- 3 user group 12 Apr 12 12:34 file1.txt.v2
-rw-r--r-- 3 user group 12 Apr 12 12:34 file1.txt.v3

In this example, "file1.txt", "file1.txt.v2", and "file1.txt.v3" are all hardlinks to the same underlying file data, allowing you to maintain multiple versions of the file.

Summary

By the end of this tutorial, you will have a solid grasp of how Linux hardlinks work and how they can be used to link to other files on your system. You will be able to create and manage hardlinks effectively, and understand their practical applications in real-world scenarios.