In UNIX-like operating systems, every file is represented internally by a data structure called an inode (index node).
An inode stores metadata about a file, but not the filename and not the file’s data.

1. What is an Inode?

An inode is a data structure on disk that contains all the information about a file except its name and actual content.

Every file and directory has exactly one inode, identified by an inode number.

2. What Information Does an Inode Store?

An inode contains:

• File type (regular file, directory, symlink, device, pipe, etc.)

• File size

• Owner (UID)

• Group (GID)

• Permissions (rwx)

• Timestamps:

  • atime (last access)

  • mtime (last modification)

  • ctime (last metadata change)

• Number of hard links

• Pointers to data blocks on disk:

  • Direct blocks

  • Indirect blocks

  • Double-indirect blocks

  • Triple-indirect blocks

• Flags (immutable, append-only, etc.)

3. What Inodes Do Not Contain

An inode does NOT store:

• The filename

• The file’s directory path

• File contents

Filenames are stored in directory entries, which map:

filename → inode number

This is why:

• Multiple filenames (hard links) can point to the same inode.

• Renaming a file does not change its inode.

4. Inode Number

Each inode has a unique number within a filesystem.
You can view it with:

ls -i

Example:

123456 file.txt

Here, 123456 is the inode number.

5. How Inodes Store File Data

The inode contains pointers to data blocks:

• Direct blocks: point directly to data (fast for small files)

• Single indirect: points to a block that contains more pointers

• Double indirect: pointer to a block of indirect blocks

• Triple indirect: for very large files

This hierarchical structure allows files to grow to very large sizes.

6. Why Inodes Matter

Disk full but still space left

If the filesystem runs out of inodes, you cannot create new files even if free disk space exists.

Hard links

Hard links point multiple filenames to the same inode.

File deletion behavior

If a file is deleted while a process is using it:

• The directory entry is removed,

• But the inode and data remain until no process references it.

Backups and recoveries

Tools like fsck use inode structures to repair filesystems.

7. How to View Inode Information

stat file.txt

This shows:

• inode number

• permissions

• ownership

• timestamps

• size

List inode usage on filesystem:

df -i

8. Summary