What is inode number in Linux ?

A Linux considers everything a file including hardware devices, printers, directories, and processes. Any text, music, video, image, or multimedia file is considered a regular file. All the regular file contains metadata about the file which describe everything about the file like type, Inode, etc.

In this tutorial, we will discuss the inode number in Linux. How can we find the inode number of a file? What happened to the inode number when we copy, move or delete a file.

What is inode number in Linux

An inode is a unique identification number assigned to each file in Linux. Which is stored in a table called Inode table, which contains the metadata of a file.

Inode number also called index number, it consists following attributes of any file:

• File types ( executable, block special etc )
• Permissions ( read, write etc )
• UID ( Owner )
• GID ( Group )
• FileSize
• Time stamps including last access, last modification and last inode number change.
• File deletion time
• Number of links ( soft/hard )
• Location of ile on harddisk.
• Some other metadata about file.

Check Inodes on a Filesystem

You can find a total number of inodes on disk by using ‘-i‘ option with df command.

df -i /dev/sda1 

Output:
Filesystem      Inodes   IUsed   IFree     IUse% Mounted on
/dev/sda1       1536000  138846  1397154   10%   /

The above command shows the total number of Inodes on /dev/sda1 file system. Also provides the details about used and free inodes.

Each filesystem must-have free inodes available to create a new file. If the inodes are full, the system will not allow you to create a new file.

Find Inode number of File

You can find the inode number of any file using -i command line parameter with the ls command.

ls -il  myfile.txt 

The first field in output is an inode number of the file.

Output:
1150561 -rw-r--r-- 1 root root 0 Mar 10 01:06 myfile.txt

You can also search file with an inode number using find command. For example:

find /home/rahul -inum 1150561 

/home/rahul/myfile.txt

Inode Changes with Copy, Move and Delete

What happens with the inode number when you copy, move or delete a file on the filesystem.

• Copy file: cp allocates a free inode number and create a new entry into the inode table.

  ### Check inode of existing file 
  
  ls -il  myfile.txt 
  1150561 -rw-r--r-- 1 root root 0 Mar 10 01:06 myfile.txt
  
  ### Copy file with new name 
  
  cp myfile.txt myfile_new.txt 
  
  ### Check inode number of new file. Its changed 
  
  ls -il myfile_new.txt 
  1150562 -rw-r--r-- 1 root root 0 Mar 10 01:09 myfile_new.txt
  

• Move or Rename a file: if destination is same filesystem as the source, Has no impact on inode number, it only changes the time stamps in inode table.

  ### Check inode of existing file 
  ls -il  myfile.txt 
  1150561 -rw-r--r-- 1 root root 0 Mar 10 01:06 myfile.txt
  
  ### Moved file to another directory 
  mv myfile.txt /opt/ 
  
  ### Check inode number of moved file. No change in inode 
  ls -il /opt/myfile.txt 
  1150561 -rw-r--r-- 1 root root 0 Mar 10 01:06 /opt/myfile.txt
  

• Delete a file: Once you delete a file in Linux decrements the link count and free the inode number to be reused.

How to Free Inodes in Linux

In the case of inodes are full. You need to remove unused files from the filesystem to make Inode free. There are no options to increase/decrease inodes on disk. It can only be created during the file system creation on any disk.