Change Read to Include Printing the Inode Number of the File Being Opened

Information construction describing a file-system object (e.m. file, directory) that stores the attributes and deejay cake location(south) of the object information

The inode (alphabetize node) is a information structure in a Unix-way file system that describes a file-system object such as a file or a directory. Each inode stores the attributes and disk block locations of the object's data.^[i] File-system object attributes may include metadata (times of last modify,^[ii] access, modification), as well as owner and permission information.^[3]

A directory is a list of inodes with their assigned names. The listing includes an entry for itself, its parent, and each of its children.

Etymology [edit]

There has been uncertainty on the Linux kernel mailing list about the reason for the "i" in "inode". In 2002, the question was brought to Unix pioneer Dennis Ritchie, who replied:^[4]

In truth, I don't know either. It was only a term that we started to use. "Alphabetize" is my all-time judge, because of the slightly unusual file organization construction that stored the admission data of files as a apartment array on the disk, with all the hierarchical directory information living aside from this. Thus the i-number is an alphabetize 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 1978 paper past Ritchie and Ken Thompson bolsters the notion of "index" being the etymological origin of inodes. They wrote:^[5]

[…] 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 every bit an index into a system table (the i-listing) stored in a known part of the device on which the directory resides. The entry establish thereby (the file'southward i-node) contains the clarification of the file.

Additionally, Maurice J. Bach wrote that an inode "is a contraction of the term index node and is usually used in literature on the UNIX system".^[6]

Details [edit]

A file organization relies on data structures about the files, as opposed to the contents of that file. The former are called metadata—data that describes data. Each file is associated with an inode, which is identified by an integer, oft referred to as an i-number or inode number.

Inodes store information well-nigh files and directories (folders), such as file ownership, access way (read, write, execute permissions), and file type. On many older file system implementations, the maximum number of inodes is fixed at file system creation, limiting the maximum number of files the file system can concord. A typical allocation heuristic for inodes in a file system is one inode for every 2K bytes contained in the filesystem.^[8]

The inode number indexes a table of inodes in a known location on the device. From the inode number, the kernel'due south file system driver tin can access the inode contents, including the location of the file, thereby allowing access to the file. A file's inode number tin can be found using the ls -i control. The ls -i command prints the i-node number in the first column of the written report.

Some Unix-manner file systems such equally ZFS, OpenZFS, ReiserFS, btrfs, and APFS omit a fixed-size inode tabular array, but must store equivalent information in order to provide equivalent capabilities. The data may be called stat data, in reference to the stat organisation call that provides the data to programs. Mutual alternatives to the stock-still-size table include B-trees and the derived B+ trees.

File names and directory implications:

• Inodes exercise not contain its hardlink names, only other file metadata.
• Unix directories are lists of association structures, each of which contains one filename and 1 inode number.
• The file system commuter must search a directory looking for a particular filename and and then convert the filename to the right respective inode number.

The operating organization kernel'southward in-retentiveness representation of this data is called struct inode in Linux. Systems derived from BSD utilise the term vnode (the "v" refers to the kernel'south virtual file arrangement layer).

POSIX inode description [edit]

The POSIX standard mandates file-system behavior that is strongly influenced by traditional UNIX file systems. An inode is denoted by the phrase "file serial number", defined equally a per-file system unique identifier for a file.^[9] That file serial number, together with the device ID of the device containing the file, uniquely identify the file within the whole system.^[x]

Inside a POSIX system, a file has the following attributes^[10] which may be retrieved by the stat system call:

• Device ID (this identifies the device containing the file; that is, the telescopic of uniqueness of the serial number).
• File serial numbers.
• The file manner which determines the file type and how the file's owner, its group, and others tin access the file.
• A link count telling how many hard links point to the inode.
• The User ID of the file'southward owner.
• The Group ID of the file.
• The device ID of the file if information technology is a device file.
• The size of the file in bytes.
• Timestamps telling when the inode itself was last modified (ctime, inode alter time), the file content last modified (mtime, modification time), and terminal accessed (atime, access time).
• The preferred I/O block size.
• The number of blocks allocated to this file.

Implications [edit]

Filesystems designed with inodes will have the following administrative characterisics.

• Files tin have multiple names. If multiple names hard link to the aforementioned inode so the names are equivalent; i.due east., the starting time to be created has no special condition. This is unlike symbolic links, which depend on the original proper noun, not the inode (number).
• An inode may have no links. An unlinked file is removed from disk, and its resources are freed for reallocation merely deletion must await until all processes that have opened it finish accessing information technology. This includes executable files which are implicitly held open up past the processes executing them.
• It is typically non possible to map from an open up file to the filename that was used to open it. The operating organisation immediately converts the filename to an inode number and then discards the filename. This means that the getcwd() and getwd() library functions search the parent directory to discover a file with an inode matching the working directory, and so search that directory's parent, and and so on until reaching the root directory. SVR4 and Linux systems maintain extra information to brand this possible.
• Historically, information technology was possible to hard link directories. This made the directory construction into an arbitrary directed graph opposite to a directed acyclic graph. It was fifty-fifty possible for a directory to be its own parent. Modern systems more often than not prohibit this disruptive state, except that the parent of root is yet defined as root. The almost notable exception to this prohibition is found in Mac OS X (versions ten.5 and higher) which allows difficult links of directories to be created by the superuser.^[11]
• A file'southward inode number stays the same when information technology is moved to another directory on the same device, or when the disk is defragmented which may change its physical location, allowing it to be moved and renamed fifty-fifty while existence read from and written to without causing interruption. This also implies that completely conforming inode behavior is impossible to implement with many not-Unix file systems, such as FAT and its descendants, which don't have a way of storing this invariance when both a file's directory entry and its data are moved around.
• Installation of new libraries is elementary with inode file systems. A running process can admission a library file while another process replaces that file, creating a new inode, and an all-new mapping will exist for the new file so that subsequent attempts to admission the library get the new version. This facility eliminates the need to reboot to supervene upon currently mapped libraries.
• Information technology is possible for a device to run out of inodes. When this happens, new files cannot be created on the device, even though there may be gratis space available. This is almost common for use cases like mail servers which comprise many small files. File systems (such as JFS or XFS) escape this limitation with extents or dynamic inode allotment, which can "grow" the file system or increase the number of inodes.

Inlining [edit]

It can make sense to store very pocket-sized files in the inode itself to salve both space (no data cake needed) and lookup time (no further disk access needed). This file system feature is called inlining. The strict separation of inode and file information thus tin can no longer be assumed when using modern file systems.

If the data of a file fits in the space allocated for pointers to the data, this infinite tin conveniently exist used. For example, ext2 and its successors store the data of symlinks (typically file names) in this manner if the data is no more than 60 bytes ("fast symbolic links").^[12]

Ext4 has a file organization option called inline_data that allows ext4 to perform inlining if enabled during file system creation. Because an inode's size is limited, this merely works for very pocket-size files.^[13]

In non-Unix systems [edit]

• NTFS has a master file table (MFT) storing files in a B-tree. Each entry has a "fileID", analogous to the inode number, that uniquely refers to this entry.^[fourteen] The iii timestamps, a device ID, attributes, reference count, and file sizes are found in the entry, just dissimilar in POSIX the permissions are expressed through a different API.^[xv] The on-disk layout is more than circuitous.^[16] The earlier Fat file systems did non take such a tabular array and were incapable of making hard links.
  • NTFS besides has a concept of inlining small files into the MFT entry.^[17]
  • The derived ReFS has a homologous MFT. ReFS has a 128-bit file ID; this extension was besides backported to NTFS, which originally had a 64-bit file ID.^[fifteen]
• The same stat-like GetFileInformationByHandle API can be used on Cluster Shared Volumes and SMB 3.0, and then these systems presumably have a similar concept of a file ID.^[xv]

See also [edit]

• inode pointer structure
• inotify

References [edit]

1. ^ Tanenbaum, Andrew S. Modern Operating Systems (3rd ed.). p. 279.
2. ^ JVSANTEN. "Deviation betwixt mtime, ctime and atime - Linux Howtos and FAQs". Linux Howtos and FAQs.
3. ^ "Anatomy of the Linux virtual file system switch". ibm.com.
4. ^ Linux Kernel list archive. Retrieved on 2011-01-12.
5. ^ Ritchie, Dennis One thousand.; Thompson, Ken (1978). "The UNIX Time-Sharing System". The Bong Organisation Technical Journal. 57 (half dozen): 1913–1914. Retrieved xix December 2015.
6. ^ Maurice J. Bach (1986). The Pattern of the UNIX Operating System. Prentice Hall. ISBN978-0132017992.
7. ^ Bach, Maurice J. (1986). The Design of the UNIX Operating System. Prentice Hall. p. 94. Bibcode:1986duos.book.....B.
8. ^ "linfo". The Linux Information Projection . Retrieved 11 March 2020.
9. ^ "Definitions - iii.176 File Serial Number". The Open Group . Retrieved x January 2018.
10. ^ ^{a b} "<sys/stat.h>". The Open Grouping . Retrieved 15 January 2018.
11. ^ "What is the Unix control to create a hardlink to a directory in OS X?". Stack Overflow. 16 Jan 2011. Archived from the original on v January 2020. Retrieved five January 2020.
12. ^ "The Linux kernel: Filesystems". tue.nl.
13. ^ "Ext4 Disk Layout". kernel.org . Retrieved August 18, 2013.
14. ^ "Does Windows have Inode Numbers like Linux?". Stack Overflow.
15. ^ ^{a b c} "GetFileInformationByHandle function (fileapi.h) - Win32 apps". docs.microsoft.com.
16. ^ "[MS-FSCC]: NTFS Attribute Types". docs.microsoft.com.
17. ^ "Windows - Maximum size of file that tin be stored entirely in NTFS Master File Table (MFT)".

External links [edit]

• Anatomy of the Linux File Organisation
• Inode definition
• Caption of Inodes, Symlinks, and Hardlinks

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Source: https://en.wikipedia.org/wiki/Inode

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