Hard Links Cannot Span Partitions

 

1. Inodes and File System Structure

• Every file in Linux has an inode, which stores metadata (permissions, timestamps, location on disk, etc.)
  
  

• A filename is just a reference (directory entry) pointing to an inode.
  
  

• Multiple filenames can point to the same inode—this is what a hard link is.
  
  

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🔹 2. Hard Links vs Symbolic Links

Feature	Hard Link	Symbolic (Soft) Link
Points to	Inode (actual file)	Path of the target file
Cross-filesystem	❌ Not allowed	✅ Allowed
If target deleted	File still accessible	Broken (dangling) link
Use inode sharing	✅ Yes	❌ No

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🔹 3. Partitions and Mount Points

• A partition is a separate logical division of a disk.
  
  

• Each partition has its own filesystem with a separate inode table.
  
  

• Hard links require pointing to the same inode, so they must be on the same filesystem (i.e., same partition).
  
  

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🔹 4. System Calls: link() vs symlink()

• link() is used for hard links
  
  

• Fails across filesystems because the kernel can’t reference an inode from another partition.
  
  

• symlink() is used for symbolic links
  
  

• Works by storing the path, so cross-partition is OK.
  
  

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🔹 5. Filesystem Types and Link Support

• Not all filesystems support hard links. For example:
  
  

• ext4, xfs: ✅ Support
  
  

• FAT32, NTFS (on Linux): ❌ Partial or no support
  
  

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❌ Why Hard Links Cannot Span Partitions

• Hard links must reference the same inode number, and inodes are local to each filesystem.
  
  

• Since different partitions have independent inode tables, it's technically impossible for a hard link to reference an inode outside its partition.
  
  

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🔧 Example:

# Assume /home and /mnt are on different partitions

ln /home/user/file.txt /mnt/file_hardlink.txt

# Output:

# ln: failed to create hard link '/mnt/file_hardlink.txt': Invalid cross-device link

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✅ What You Can Do Instead

Use a symbolic link if you need cross-partition linking:

ln -s /home/user/file.txt /mnt/file_symlink.txt

This creates a shortcut pointing to the path (not the inode).

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🧠 FAANG-Level Related Concepts and Interview Questions

Here are key related questions you may get:

🔸 Conceptual

1. What is an inode? How does Linux manage file metadata?
   
   

2. What is the difference between a hard link and a soft link?
   
   

3. Why are hard links restricted to a single filesystem?
   
   

4. How do link() and symlink() system calls behave?
   
   

5. What happens to hard/symbolic links when the original file is deleted?
   
   

6. What is a dangling symlink?
   
   

🔸 Practical / Debugging

7. You tried to create a hard link across partitions and it failed. Why?
   
   

8. How do you find how many hard links a file has?
   → Use: ls -l (second column) or stat filename
   
   

How to locate all hard links pointing to a file?
→ Find by inode:

find / -inum <inode_number>

9. 
   

🔸 Edge Cases

10. What are the limitations of symbolic links in terms of permissions?
    
    

11. How can symlinks introduce security vulnerabilities (e.g., symlink race)?
    
    

12. How does cp -a handle hard links?
    
    

13. Can you create a hard link to a directory? Why not?
    
    

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🧪 Bonus: Hands-On Commands

🔹 Check Filesystem Mount Points

df -h

🔹 View Inode Number

ls -li filename

🔹 Count Hard Links

stat filename

🔹 Find Files by Inode

find . -inum $(stat -c %i filename)

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Summary

• ❌ Hard links cannot cross partitions due to inode isolation per filesystem.
  
  

• ✅ Use symbolic links for cross-partition references.
  
  

• 🧠 Understanding inodes, filesystems, and link types is crucial for system-level interviews.