Inode

The inode is a data structure in a Unix-style file system that describes a filesystem object such as a file or a directory. Each inode stores the attributes and disk block location(s) of the object's data. Filesystem object attributes may include metadata (times of last change, access, modification), as well as owner and permission data.

Directories are lists of names assigned to inodes. A directory contains an entry for itself, its parent, and each of its children.

There has been uncertainty on the Linux kernel mailing list as to the reason for designating these as "i" nodes. The question was brought to Unix pioneer Dennis Ritchie, who replied:

In truth, I don't know either. It was just a term that we started to use. "Index" is my best guess, because of the slightly unusual file system structure that stored the access information of files as a flat array on the disk, with all the hierarchical directory information living aside from this. Thus the i-number is an index in this array, the i-node is the selected element of the array. (The "i-" notation was used in the 1st edition manual; its hyphen was gradually dropped.)

A paper published in 1978 by Ritchie and Ken Thompson strongly suggests the same etymology:

As mentioned in Section 3.2 above, a directory entry contains only a name for the associated file and a pointer to the file itself. This pointer is an integer called the i-number (for index number) of the file. When the file is accessed, its i-number is used as an index into a system table (the i-list) stored in a known part of the device on which the directory resides. The entry found thereby (the file's i-node) contains the description of the file:...

Also, Maurice J. Bach writes:

The term inode is a contraction of the term index node and is commonly used in literature on the UNIX system.

A file system relies on data structures about the files, beside the file content. The former are called metadata—data that describe data. Each file is associated with an inode, which is identified by an integer number, often referred to as an i-number or inode number.

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