What Is an Inode? Understanding File System Metadata and Links

What Is an Inode

An inode (index node) is a data structure that stores a file's metadata such as size, owner, timestamps, permissions, link count, and the locations of its data blocks.

Contents of an Inode

File size in bytes

Owner User ID

Group ID

Read, write, execute permissions

Three timestamps: ctime (inode change), mtime (content change), atime (last access)

Link count (number of directory entries pointing to the inode)

Locations of the file's data blocks

You can view an inode's information with the stat example.txt command.

Inode Size

Inodes also consume disk space. During formatting, the disk is divided into a data area and an inode table. Each inode is typically 128 or 256 bytes, and the total number of inodes is set at format time (e.g., one inode per 1 KB or 2 KB).

On a 1 GB disk with 128‑byte inodes and one inode per 1 KB, the inode table would occupy about 128 MB (12.8% of the disk).

To see inode usage, use df -i. To check the inode size, run sudo dumpe2fs -h /dev/hda | grep "Inode size".

Inode Number

Each inode has a unique number used by the operating system to identify files. Unix/Linux systems use inode numbers internally rather than file names; the name is merely a human‑readable alias.

When a file is opened, the system resolves the name to an inode number, reads the inode, then accesses the data blocks.

Use ls -i to display a file's inode number.

Directory Files

In Unix/Linux, a directory is also a file containing a list of directory entries (dirents). Each entry stores a file name and its corresponding inode number.

The ls /etc command lists file names in the directory, while ls -i /etc shows both names and inode numbers. Detailed file information can be obtained with ls -l /etc, which reads the inode data.

Hard Links

Normally, each inode number maps to a single file name, but Unix/Linux allows multiple names to point to the same inode, creating a hard link.

Hard links share the same inode and data; modifying the file through any name affects all. Deleting one name decrements the link count; when it reaches zero, the inode and its blocks are reclaimed.

Create a hard link with ln source_file target_file.

Directories have a default link count of two ("." and "..") plus one for each subdirectory.

Soft (Symbolic) Links

A soft link (symbolic link) is a special file whose contents are the path to another file. Accessing the soft link redirects the system to the target file.

If the target is removed, the soft link becomes dangling and yields a "No such file or directory" error. Unlike hard links, soft links do not affect the target's link count.

Create a soft link with ln -s source_path target_path.

Special Uses of Inodes

Files with problematic names can be removed by deleting their inode directly.

Renaming or moving a file only changes its directory entry; the inode number remains unchanged.

Running processes identify open files by inode number, allowing updates to replace a file's inode without restarting the software.