I-node Guide, Meaning , Facts, Information and Description

In Unix computing, an i-node (or inode) is a data structure on a file system that stores basic information about a file, directory, or other file system object. This information includes:

• The size of the file and its physical location (if the file resides on a block device, such as a hard disk);
• The file's owner and group;
• The file mode, which determines what users can read, write, and execute the file;
• Timestamps telling when the inode was created, last modified, and last accessed; and
• A reference count telling how many hard links point to the inode.

Less formally, the term inode can refer to this data structure and the device blocks that it manages (for a regular file, the blocks constituting the body of the file).

The term inode usually refers to inodes on block devices that manage regular files, directories, and symbolic links. The concept is particularly important to the recovery of damaged file systems.

Each inode has an inode number that is unique within its device. All files are hard links to inodes. Whenever a program refers to a file by name, the system uses the filename to look up the corresponding inode, which gives the system the information it needs about the file to perform further operations.

The stat system call retrieves a file's inode number and some of the information in the inode.

The exact reasoning for designating these as "i" nodes is unsure. When asked, UNIX pioneer Dennis Ritchie replied

 Subject: Re: What does the "i" in inode stand for?  Nobody seems to know...
 Date: Sat, 20 Jul 2002 00:52:39 -0400
 From: Dennis Ritchie 
     
 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 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 became gradually
 dropped).
     
  Dennis

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