In computing, an inode is a data structure on a traditional Unix-style file system such as UFS. An inode stores basic information about a regular file, directory, or other file system object. Memory (Random Access Memory) Look up computing in Wiktionary, the free dictionary. ... A binary tree, a simple type of branching linked data structure. ... Filiation of Unix and Unix-like systems Unix (officially trademarked as UNIX®) is a computer operating system originally developed in the 1960s and 1970s by a group of AT&T employees at Bell Labs including Ken Thompson, Dennis Ritchie and Douglas McIlroy. ... In computing, a file system (often also written as filesystem) is a method for storing and organizing computer files and the data they contain to make it easy to find and access them. ... The UNIX file system (UFS) is a file system used by many Unix and Unix-like operating systems. ... A computer file is a collection of information that is stored in a computer system and can be identified by its full path name. ... In computing, a directory, catalog, or folder, is an entity in a file system which can contain a group of files and/or other directories. ...

Details

When a file system is created, data structures that contain information about files are created. Each file has an inode and is identified by an inode number (often "i-number" or even shorter, "ino") in the file system where it resides. Inodes store information on files such as user and group ownership, access mode (read, write, execute permissions) and type of file. There is a fixed number of inodes, which indicates the maximum number of files each filesystem can hold.

A file's inode number can be found using the ls -i command, while the ls -l command will retrieve inode information.

Non-traditional Unix-style filesystems such as ReiserFS may avoid having a table of inodes, but must store equivalent data in order to provide equivalent function. The data may be called stat data, in reference to the stat system call that provides the data to programs. ReiserFS is a general-purpose, journaled computer file system designed and implemented by a team at Namesys led by Hans Reiser who is referred to as the projects Benevolent Dictator for Life. ... stat() is a Unix system call that returns useful data about an i-node. ... In computing, a system call is the mechanism used by an application program to request service from the operating system. ...

The kernel's in-memory representation of this data is called struct inode in Linux. Systems derived from BSD use the term vnode, with the v of vnode referring to the kernel's virtual file system layer. Linux (IPA pronunciation: ) is a Unix-like computer operating system. ... BSD redirects here; for other uses see BSD (disambiguation). ... A virtual file system (VFS) or virtual filesystem switch is an abstraction layer on top of a more concrete file system. ...

The POSIX standard mandates filesystem behavior that is strongly influenced by traditional UNIX filesystems. Regular files are required to have the following attributes: POSIX or Portable Operating System Interface[1] is the collective name of a family of related standards specified by the IEEE to define the application programming interface (API) for software compatible with variants of the Unix operating system. ... Filiation of Unix and Unix-like systems Unix (officially trademarked as UNIX®) is a computer operating system originally developed in the 1960s and 1970s by a group of AT&T employees at Bell Labs including Ken Thompson, Dennis Ritchie and Douglas McIlroy. ...

• 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.
• The file mode, which determines what users can read, write, and execute the file.
• Timestamps telling when the inode itself was last modified (ctime, change time), the file content last modified (mtime, modification time), and last accessed (atime, access time).
• A reference count telling how many hard links point to the inode.
• Pointers to the disk blocks that store the file's contents.

The term inode usually refers to inodes on block devices that manage regular files, directories, and possibly symbolic links. The concept is particularly important to the recovery of damaged file systems. In computer science a byte is a unit of measurement of information storage, most often consisting of eight bits. ... On Unix-like systems, users are represented by a user identifier, often abbreviated UID. The range of values for a UID varies amongst different systems; at the very least, a UID can be between 0 and 32767, with some restrictions: The Superuser must always have a UID of zero (0). ... In Unix-like systems, multiple users can be combined to form groups. ... In computer science, reference counting is a technique of storing the number of references, pointers, or handles to a resource such as an object or block of memory. ... In computing, a hard link is a reference, or pointer, to physical data on a storage volume. ... In computing (specifically data transmission and data storage), block size indicates a nominal size, usually expressed in bytes or bits, of a block of data. ... In computing, a symbolic link (often shortened to symlink and also known as a soft link) consists of a special type of file that serves as a reference to another file. ...

The inode number indexes a table of inodes in a known location on the device; from the inode number, the kernel can access the contents of the inode, including the data pointers, and so the contents of the file.

Inodes do not contain filenames. Unix directories are lists of "link" structures, each of which contains one filename and one inode number. The kernel can search a directory, looking for a particular filename, and convert the filename to the correct corresponding inode number if the name is found.

The stat system call retrieves a file's inode number and some of the information in the inode. stat() is a Unix system call that returns useful data about an i-node. ...

The exact reasoning for designating these as "i" nodes is unsure. When asked, Unix pioneer Dennis Ritchie replied: Dennis Ritchie Dennis MacAlistair Ritchie (born September 9, 1941) is a computer scientist notable for his influence on ALTRAN, B, BCPL, C, Multics, and Unix. ...

'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).'

Example of structure:

Image File history File links Ext2-inode. ...

Implications

The properties of a file system that makes use of the concept of inodes surprise many users who are not used to it at first:

• If multiple names link to the same inode (they are all hard links to it) then all of the names are equivalent. The first one to have been created has no special status. This is unlike sometimes more familiar symbolic links where all of the links depend on the original name.
• An inode can even have no links at all. Normally such a file would be removed from the disk and its resources freed for reallocation (the normal process of deleting a file) but if any processes are holding the file open, they may continue to access it, and the file will only be finally deleted when the last reference to it is closed. This includes executable images which are implicitly held open by the processes executing them. For this reason, when programs are updated, it is recommended to delete the old executable first and create a new inode for the updated version, so that any instances of the old version currently executing may continue to do so unbothered.
• Typically, it is not possible to map from an open file to the filename that was used to open it. The operating system would convert the filename to an inode number at the first possible chance, then forget the filename. This means that the getcwd() and getwd() library functions would need to search the parent directory to find a file with an inode matching the "." directory, then search the grandparent directory for that directory, and so on until reaching the "/" directory. SVR4 and Linux systems retain extra information to avoid this awkwardness.
• Historically, it was possible to hard link directories. This made the directory structure be an arbitrary directed graph instead of a tree. It was possible for a directory to be its own parent. Modern systems generally prohibit this confusing state.
• A file's inode number will stay the same when it is moved to another directory on the same device, or when the disk is defragmented. Therefore, moving either a file's directory entry or its data (or both) is not enough to prevent a running process from accessing it, if the process ever had a chance of finding out the inode number. This also implies that completely conforming behavior of inodes is impossible to implement with many non-Unix file systems, such as FAT and its descendants, which don't have a way of storing this lasting "sameness" when both a file's directory entry and its data are moved around.

In computing, a symbolic link (often shortened to symlink and also known as a soft link) consists of a special type of file that serves as a reference to another file. ... A parent directory of a given directory A is such a directory B in which the directory A is located. ... It has been suggested that Traditional Unix be merged into this article or section. ... Linux (IPA pronunciation: ) is a Unix-like computer operating system. ... This article just presents the basic definitions. ... In the context of administering computer systems, defragmentation (or defragging) is a process that reduces the amount of fragmentation in file systems. ... File Allocation Table (FAT) is a partially patented file system developed by Microsoft for MS-DOS and was the primary file system for consumer versions of Microsoft Windows up to and including Windows Me. ...

Practical considerations

Many computer programs used by system administrators in UNIX operating systems often give i-node numbers to designate a file. Popular disk integrity checking utility fsck or pfiles command may serve here as examples. Thus need arises to translate i-node numbers to file pathnames and vice versa. This can be accomplished using file-finding utility find with option -inum or ls command with proper option which on many platforms is -i. A computer program is a collection of instructions that describe a task, or set of tasks, to be carried out by a computer. ... A system administrator, or sysadmin, is a person employed to maintain, and operate a computer system or network. ... Filiation of Unix and Unix-like systems Unix (officially trademarked as UNIX®) is a computer operating system originally developed in the 1960s and 1970s by a group of AT&T employees at Bell Labs including Ken Thompson, Dennis Ritchie and Douglas McIlroy. ... An operating system (OS) is a set of computer programs that manage the hardware and software resources of a computer. ... Typical hard drives of the mid-1990s. ... The system utility fsck (for file system check or file system consistency check) is a tool for checking the consistency of a file system in the Unix system and clones thereof. ... A computer file is a collection of information that is stored in a computer system and can be identified by its full path name. ... A path is the general form of a file or directory name, giving a files name and its unique location in a file system. ... For the EP by Hidden in Plain View, see Find (EP) The find program is a search utility, mostly found on Unix-like platforms. ... LS may refer to: .ls, the Internet top-level domain for Lesotho Jet2. ...

External links

• The Linux Virtual File-system Layer: Inodes and Operations - I-nodes in Linux