I-node
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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. The POSIX standard requires at least the following attributes:
- The length of the file in bytes
- Device ID (this identifies the device containing the file)
- The User ID of the file's owner
- The Group ID of the file
- An inode number that identifies the file within the filesystem
- 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
- 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.
The inode number is an integer unique to the device upon which it is stored. 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:
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 became gradually dropped).de:Inode