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Soft Links and Hard Links in Shell

In Unix-like operating systems, both soft (symbolic) and hard links are used to link files together. However, they behave differently and serve different purposes.

1. Hard Link

A hard link is a reference (pointer) to the physical data on a disk. Multiple hard links can refer to the same data on the disk, and deleting one hard link does not affect the others.

Characteristics of Hard Links:

  • A hard link points directly to the inode of the file.
  • Multiple hard links share the same inode.
  • If the original file is deleted, the data is still accessible through any remaining hard links.
  • Hard links cannot span across different file systems (partitions).

Syntax for Creating a Hard Link:

ln source_file hard_link_name

Example:

# Create a file
echo "This is the original file." > original.txt

# Create a hard link to the original file
ln original.txt hard_link.txt

# List the inodes of the files
ls -i original.txt hard_link.txt

In this example, both original.txt and hard_link.txt will have the same inode number, meaning they point to the same data on disk.

Even if you delete original.txt, the data can still be accessed through hard_link.txt because they share the same inode.

rm original.txt
cat hard_link.txt  # The content is still accessible

2. Soft Link (Symbolic Link)

A soft link (also called a symbolic link or symlink) is a pointer to another file or directory. Unlike hard links, a soft link does not point directly to the inode but rather references the file name.

Characteristics of Soft Links:

  • A soft link points to the file name, not the inode.
  • Deleting the original file breaks the symbolic link.
  • Soft links can span across different file systems and partitions.
  • Soft links can point to directories, unlike hard links.

Syntax for Creating a Soft Link:

ln -s source_file soft_link_name

Example:

# Create a soft link to the original file
ln -s original.txt soft_link.txt

# List the files to see the symbolic link
ls -l original.txt soft_link.txt

In this case, soft_link.txt is a symbolic reference to original.txt. If you delete original.txt, the soft link will be broken, and accessing soft_link.txt will result in an error:

rm original.txt
cat soft_link.txt  # This will return an error since the link is broken

Key Differences Between Hard and Soft Links

Feature Hard Link Soft Link
Points to The inode (actual data) The file name (path)
Inode number Same as the original file Different from the original file
Effect when original is deleted The link remains valid, and data is still accessible The link is broken, and the file becomes inaccessible
Cross-filesystem support No Yes
Can link to directories No Yes

Example of Listing Inodes

To visualize the difference between hard and soft links, you can list the inode numbers of files:

# Create a hard link
ln original.txt hard_link.txt

# Create a soft link
ln -s original.txt soft_link.txt

# List the inode numbers of all three files
ls -i original.txt hard_link.txt soft_link.txt

Output:

123456 original.txt
123456 hard_link.txt
789012 soft_link.txt

Notice that original.txt and hard_link.txt share the same inode number, while soft_link.txt has a different one.

Conclusion

  • Hard links directly reference the same data (inode), making them resilient to file deletions but restricted to the same filesystem.
  • Soft links reference the file name, allowing more flexibility (like linking to directories or spanning filesystems), but they break if the original file is deleted.

Understanding the use cases for both types of links can help you manage files and storage effectively in Unix-like systems.