Understanding Inodes in Linux

An inode (short for index node) is a data structure used to store metadata about a file or directory in a Linux file system. Every file and directory has an associated inode that contains essential information about the file.

What is an Inode?

An inode is a fundamental concept in Unix-like file systems.
It helps maintain the file system structure by tracking metadata about files.
Every file, directory, character file, and block device has an inode.

What Metadata Does an Inode Store?

An inode does not store the filename itself but contains critical file-related details such as:

🔹 File Size
🔹 Permissions (read, write, execute)
🔹 Owner & Group
🔹 File Location on Disk
🔹 Timestamps (creation, modification, access)
🔹 Other File Attributes

Note: Each used inode refers to one file.

Where is Inode Information Stored?

Inodes are stored in a table-like structure in each partition.
Typically, inode tables are located near the beginning of the partition for fast access.

Commands:

To find the inode number of a file.
```
 ls -li <file-name>
```
To Search A File Using Inode Number
```
 find . -inum <inode-number>
```
To Check Inode Utilization On The Filesystem
```
 df -i
```
How To Reduce The Inode Usage

Remove the empty files
```
 find . -empty -delete
```

How does inode work?

When a new file is created, it is assigned an inode number and a file name.
Both name and inode number are stored as entries in a directory.
Inodes are unique within a partition, meaning:
- Same inode numbers can exist across different partitions.
- Two files on the same partition cannot have the same inode.

Scenario 1

When you use the cp (copy) command to duplicate a file, the inode of the source file remains the same, and a new inode is created for the copied file.
This means that the copied file is treated as a separate and distinct file from the original, and any changes made to one do not affect the other.

Scenario 2

When you use the mv (move) command to rename or relocate a file within the same file system, the file retains the same inode.
The mv command essentially updates the file's metadata (name and location) without creating a new file.
The content of the file remains the same, and there is no duplication of data.

Hard Link

A Hard Link is a copy of the original file that serves as a pointer to the same file, allowing it to be accessed even if the original file is deleted or relocated.
Inode is same for original and hard link
Deleting, renaming or removing the original file will not affect the link
Unlike soft links, modifications to hard-linked files affect other files, and the hard link remains active even if the source file is deleted from the system.

Create a Hard Link:

ln original.txt hardlink.txt

Now, hardlink.txt and original.txt are identical and share the same inode.

Why is there no data loss after deleting the original file, but the hard link still has the data?

This happens because hard links do not point to the file name; they point directly to the inode (which stores the actual data).
When you create a hard link, it creates another filename pointing to the same inode. Both the original file and the hard link are equal references to the same data.

What Happens When You Delete the "Original" File?

Deleting a file (using rm) only removes one reference to the inode.
If other hard links exist, the inode and data blocks remain untouched.
The data is only deleted when the link count (number of references to the inode) reaches zero.

Use Case:

Backup Without Duplicating Data: Instead of making full copies of large files, you can create hard links to save space.

2. Faster File Access in Multiple Locations: Instead of copying large files across different directories, you can create a hard link to save space and improve efficiency.

Soft Link

A soft link is a short pointer file that links a filename to a pathname.
Inode for soft link is different.
It's nothing more than a shortcut to the original file, much like the Windows OS's shortcut option.
The soft link serves as a pointer to another file without the file's actual contents.
It allows the users to delete the soft links without affecting the original file's contents.
You can make changes in the symbolic link that will also be reflected in the original file.
If the original file is deleted, the symbolic link still exists but becomes "dangling" and points to nothing.

Command to create Soft Link
```
  ln -s original_file soft_link
```
Deleting the soft link

Difference Between Hard Link and Soft Link in Linux

Feature	Hard Link	Soft Link (Symbolic Link)
Definition	A direct reference to the same inode (file data).	A shortcut pointing to the original file’s path.
Inode Sharing	Shares the same inode number as the original file.	Has a different inode from the original file.
File Deletion	Original file deletion does not affect the hard link (file remains).	If the original file is deleted, the soft link breaks (dangling link).
Works Across Filesystems	❌ No (must be on the same filesystem).	✅ Yes (can link files across different filesystems).
Works for Directories	❌ No (cannot create hard links for directories).	✅ Yes (can create soft links for directories).
Storage	Does not take additional space (same inode).	Stores the file path separately, so it takes a few extra bytes.
Speed	Faster (direct access to the same data).	Slightly slower (needs to resolve the path).
Modification Effect	Changing the hard link modifies the original file.	Changing the soft link modifies the original file unless it’s broken.
Command to Create	`ln original.txt hardlink.txt`	`ln -s original.txt softlink.txt`

Working with links