What is inode number in Linux ?

An inode is an entry in inode table, containing information ( the metadata ) about a regular file and directory. An inode is a data structure on a traditional Unix-style file system such as ext3 or ext4.

Inode number also called as index number , it consists following attributes.

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.

To check inode number of file use following command, first field in ouput is an inode number.

# ls -il  tecadmin.txt
1150561 -rw-r--r-- 1 root root 0 Mar 10 01:06 tecadmin.txt

Search file on basis of inode number.

# find /root -inum 1150561
/root/tecadmin.txt

What happen with inode when copy, move or delete a file:

Copy file: cp allocates a free inode number and placing a new entry in inode table.

# ls -il  tecadmin.txt
1150561 -rw-r--r-- 1 root root 0 Mar 10 01:06 tecadmin.txt
# cp tecadmin.txt tecadmin.net.txt
# ls -il tecadmin.net.txt
1150562 -rw-r--r-- 1 root root 0 Mar 10 01:09 tecadmin.net.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.

# ls -il  tecadmin.txt
1150561 -rw-r--r-- 1 root root 0 Mar 10 01:06 tecadmin.txt
# mv tecadmin.txt /opt/
# ls -il /opt/tecadmin.txt
1150561 -rw-r--r-- 1 root root 0 Mar 10 01:06 /opt/tecadmin.txt

Delete a file: Deleting a file in linux decrements the link count and freeing the inode bumber to be reused.