Hard link

Hard Link Definition

A hard link is a reference to a file on a file system, connecting the file to another location without creating a separate copy. It allows multiple directory entries to refer to the same file, providing a way to access the same data from different locations.

How Hard Links Work

When a hard link is created, it points directly to the file's inode. An inode is a data structure in a Unix-style file system that stores information about a file or directory, including its location, size, and permissions.

One important characteristic of hard links is that they do not create separate copies of the file. Instead, they create additional references to the same underlying data. This means that changes made to the file through one hard link are reflected in all the other hard links that point to the same inode. For example, if you modify the content of a file through one hard link, those modifications will be visible when accessing the file through any other hard links.

It is worth noting that hard links are limited to files within the same file system. This is because the file system keeps track of inodes within a particular file system, so hard links can only reference files within that file system.

Advantages of Hard Links

• Space-efficient: Hard links do not consume additional storage space since they point to the same inode and share the same data.
• Efficient file management: Hard links allow for multiple directory entries to refer to the same file, making it easy to organize and access files from different locations.
• Quick access: Since hard links directly point to the file's inode, accessing the file through a hard link is just as fast as accessing it through its original name.

Considerations and Limitations

• Renaming: Renaming a hard link will not affect the file it points to or other hard links to the same file. Each hard link functions independently, and their names can be changed without affecting the others.
• Deletion: Deleting the original file will not affect the hard links, as they point directly to the data rather than the file name. However, if all hard links to a file are deleted, the data will only be permanently removed once the original file is deleted as well.
• Permissions: Modifying the permissions on a hard link or the original file will affect both. Since hard links are just references to the same data, any changes to permissions or access controls will be reflected in all the hard links.
• Different file systems: Hard links can only exist within the same file system. If you try to create a hard link between files in different file systems, it will result in an error.

Practical Examples

Example 1:

In a Unix-based operating system, a user may have a directory named "/home/user/documents" that contains important files. If there is a need to access a specific file frequently from another directory, a hard link can be created using the "ln" command, like this: ln /home/user/documents/file.txt /usr/local/bin/file-link This will create a hard link named "file-link" in the "/usr/local/bin" directory, which points to the same inode as the original file.

Example 2:

For version control purposes, a software development team may want multiple developers to work on the same source code file simultaneously. By creating hard links to the file, each developer can access and modify the file independently, while all changes are reflected in real-time.

Related Terms

• Symbolic Link: A symbolic link, also known as a soft link, is a file that serves as a reference or shortcut to another file or directory. Unlike a hard link, a symbolic link points to the file name rather than directly to the underlying data.
• Inode: An inode is a data structure in a Unix-style file system that stores metadata about a file or directory. It includes information such as the file's location, size, permissions, and timestamps.

Sources

• Wikipedia - Hard link
• Linuxize - Understanding Hard and Soft (Symbolic) Links in Linux
• nixCraft - Unix / Linux: Create Symbolic Links Only ( Without Actual File / Directory )