Unix filesystem

An Example of a Unix-like Root Filesystem

In Unix and operating systems inspired by it, the file system is considered a central component of the operating system.^[1] It was also one of the first parts of the system to be designed and implemented by Ken Thompson in 1969. Like in other operating systems, it provides information storage and interprocess communication, in the sense that the many small programs that traditionally comprise a Unix system can store information in files so that other programs can read these, although pipes complemented it in this role starting with the Third Edition.

The rest of this article uses "Unix" as a generic name to refer to both the original Unix operating system as well as its many workalikes.

Principles

The original Unix file system supported three types of files: ordinary files, directories, and "special files", also known as device files.^[1] The Berkeley Software Distribution added sockets and symbolic links to this, and modern Unix system may support additional types of files.<ref name="43bsd>Leffler, Samuel J.; Marshall Kirk McKusick, Michael J. Karels and John S. Quarterman (October 1989). The Design and Implementation of the 4.3BSD UNIX Operating System. Addison-Wesley. ISBN 0-201-06196-1.  Cite uses deprecated parameters (help)</ref> The filesystem appears as a single rooted tree of directories.^[1] Instead of addressing separate volumes such as disk partitions, removable media, and network shares as separate trees (as done in MS-DOS and Windows: each "drive" has a drive letter that denotes the root of its file system tree), such volumes can be "mounted" on a directory, causing the volume's file system tree to appear as that directory in the larger tree.^[1] The root of the entire tree is denoted /.

In the original Bell Labs Unix, a two-disk setup was customary, where the first disk contained startup programs, while the second contained users' files and programs. This second disk was mounted at the empty directory named usr on the first disk, causing the two disks to appear as one filesystem, with the second's disks contents viewable at /usr.

Unix directories do not "contain" files. Instead, they contain the names of files paired with references to so-called inodes, which in turn contain both the file and its metadata (owner, permissions, time of last access, etc., but no name). Multiple names in the file system may refer to the same file, a feature known as (hard) linking.^[1] (If this feature is taken into account, the file system is a limited type of directed acyclic graph rather than a tree. As originally envisioned ca. 1968-69, the file system would in fact be a more general type of directed graph, but navigating this turned out to be difficult, and the decision was made to disallow multiple links to a single directory, enforcing a tree structure.)

Conventional directory layout

Certain conventions exist for locating particular kinds of files, such as programs, system configuration files and users' home directories. These were first documented in the hier(7) man page since Version 7 Unix;^[2] subsequent versions, derivatives and clones typically have a similar man page.^{[3][4][5][6][7]}

The details of the directory layout have varied over time. Although the file system layout is not part of the Single UNIX Specification, several attempts exist to standardize it, such as the Linux Foundation's Filesystem Hierarchy Standard (FHS).

Here is a generalized overview of common locations of files on a Unix operating system:

References

• This article incorporates material from the Citizendium article "Unix filesystem", which is licensed under the Creative Commons Attribution-ShareAlike 3.0 Unported License but not under the GFDL.