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{{Short description|Directory structure used by a Unix-like operating system}}
[[File:Version 7 UNIX SIMH PDP11 Filesystem Layout.png|thumb|Version 7 Unix filesystem layout: subdirectories of "/" and "/usr"]]▼
{{Redirect|Unix file system|UFS, a specific file system used by many Unix and Unix-like operating systems|Unix File System}}
[[File:Standard-unix-filesystem-hierarchy.svg|thumb|An overview of a Unix filesystem layout]]▼
▲[[File:Version 7 UNIX SIMH PDP11 Filesystem Layout.png|thumb|[[Version 7 Unix]] filesystem layout: subdirectories of "/" and "/usr"]]
▲[[File:Standard-unix-filesystem-hierarchy.svg|thumb|An overview of a [[Unix]] filesystem layout]]
In [[Unix]] and [[operating system]]s inspired by it, the [[file system]] is considered a central component of the operating system.<ref name="
As in other operating systems, the filesystem provides information storage and retrieval, and one of several forms of [[interprocess communication]], in that the many small programs that traditionally form a Unix system can store information in files so that other programs can read them, although [[Pipeline (Unix)|pipes]] complemented it in this role starting with the [[Research Unix|Third Edition]]. Also, the filesystem provides access to other resources through so-called ''[[device
The rest of this article uses ''Unix'' as a [[Generic trademark|generic name]] to refer to both the original Unix operating system and its many [[Unix-like|workalikes]].
==Principles==
The filesystem appears as one [[rooted tree]] of directories.<ref name="Ritchie"/> Instead of addressing separate volumes such as [[Disk partitioning|disk partitions]], removable media, and [[network share]]s as separate trees (as done in [[DOS]] and [[Microsoft Windows|Windows]]: each ''drive'' has a drive letter that denotes the root of its file system tree), such volumes can be ''[[mount (Unix)|mounted]]'' on a directory, causing the volume's file system tree to appear as that directory in the larger tree.<ref name="Ritchie"/> The root of the entire tree is denoted <code>/</code>.
In the original [[Research Unix|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 <code>usr</code> on the first disk, causing the two disks to appear as one filesystem, with the second
Unix directories do not ''contain'' files. Instead, they contain the names of files paired with references to so-called [[inode]]s, 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 termed a ''[[hard link]]''.<ref name="Ritchie"/> The mathematical traits of hard links make the file system a limited type of [[directed acyclic graph]], although the ''directories'' still form a tree, as they typically may not be hard-linked.<!--Mac OS X allows this, according to the article 'Hard link'.--> (As originally envisioned in 1969, the Unix file system would in fact be used as a general graph with hard links to directories providing navigation, instead of path names.<ref name="evolution">{{cite conference |first=Dennis M. |last=Ritchie |year=1979 |title=The Evolution of the Unix Time-sharing System |conference=Language Design and Programming Methodology Conf. |url=http://cm.bell-labs.com/cm/cs/who/dmr/hist.html |access-date=2014-04-20 |archive-date=2015-04-08 |archive-url=https://web.archive.org/web/20150408054606/http://cm.bell-labs.com/cm/cs/who/dmr/hist.html |url-status=dead }}</ref>)
===File types===
{{Main |Unix file types}}
The original Unix file system supported three types of files: ordinary files, [[Directory (computing)|directories]], and "special files", also termed device files.<ref name="Ritchie"/> The [[Berkeley Software Distribution]] (BSD) and [[UNIX System V|System V]] each added a file type to be used for [[interprocess communication]]: BSD added [[Berkeley sockets|sockets]],<ref name="43bsd">{{cite book |last1= Leffler |first1= Samuel J. |
BSD also added [[symbolic link]]s (often termed "symlinks") to the range of file types, which are files that refer to other files, and complement hard links.<ref name="43bsd"/> Symlinks were modeled after a similar feature in [[Multics]],<ref name="FFS">{{cite web|last1=McKusick|display-authors=etal|first1=Marshall Kirk|title=A Fast Filesystem for Unix|url=https://docs.freebsd.org/44doc/smm/05.fastfs/paper.pdf|website=Freebsd.org|publisher=CSRG, UC Berkeley|
==Conventional directory layout==
{{Redirect|Usr||USR (disambiguation)}}
Certain conventions exist for locating some kinds of files, such as programs, system configuration files, and users' [[home directory|home directories]]. These were first documented in the <code>hier(7)</code> [[man page]] since [[Version 7 Unix]];<ref name="v7">{{man|7|hier|v7}}</ref> subsequent versions, derivatives and clones typically have a similar man page.<ref>{{man|7|hier|FreeBSD}}</ref><ref name="openbsd">{{man|7|hier|OpenBSD}}</ref><ref>{{cite web|title=hier(7) man page for 2.9.1 BSD|url=http://www.freebsd.org/cgi/man.cgi?query=hier&manpath=2.9.1+BSD}}</ref><ref>{{cite web|title=hier(7) man page for ULTRIX 4.2|url=http://www.freebsd.org/cgi/man.cgi?query=hier&manpath=ULTRIX+4.2}}</ref><ref>{{cite web|title=hier(7) man page for SunOS 4.1.3|url=http://www.freebsd.org/cgi/man.cgi?query=hier&manpath=SunOS+4.1.3}}</ref><ref>{{man|7|hier|Linux}}</ref>
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 (parts of) it, such as the [[UNIX System V|System V]] [[Application Binary Interface]], the [[Intel Binary Compatibility Standard]], the Common Operating System Environment, and [[Linux Foundation]]'s [[Filesystem Hierarchy Standard]] (FHS).<ref>{{cite web |title=Where to Install My Products on Linux? |url=http://www.linuxjournal.com/article/4121 |website=[[Linux Journal]] |date=1 November 2000 |author=George Kraft IV |
Here is a generalized overview of common locations of files on a Unix operating system:
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|Stands for ''[[computer program|binaries]]'' and contains certain fundamental utilities, such as <code>ls</code> or <code>cp</code>, that are needed to mount <code>/usr</code>, when that is a separate filesystem, or to run in one-user (administrative) mode when <code>/usr</code> cannot be mounted. In System V.4, this is a symlink to <code>/usr/bin</code>. Otherwise, it needs to be on the root filesystem itself.
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|Contains all the files needed for successful booting process. In [[Research Unix]], this was one file rather than a directory.<ref name="upe"/> Nowadays usually on the root filesystem itself, unless the system, bootloader etc. require otherwise.
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|Stands for ''devices''. Contains [[device node|file representations]] of peripheral devices and [[Device file#Pseudo-devices|pseudo-devices]]. See also: [[Linux Assigned Names and Numbers Authority]]. Needs to be on the root filesystem itself.
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|Contains system-wide configuration files and system databases; the name stands for ''[[et cetera]]''
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|Contains user home directories on Linux and some other systems. In the original version of Unix, home directories were in <code>/usr</code> instead.<ref name="notes72">{{cite web|last=Ritchie|first=Dennis|title=Unix Notes from 1972|url=https://www.bell-labs.com/usr/dmr/www/notes.html|
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|Originally ''essential libraries'': [[C (programming language)|C]] libraries, but not [[Fortran]] ones.<ref name="upe"/> On modern systems, it contains the shared libraries needed by programs in <code>/bin</code>, and possibly [[loadable kernel module]]
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|Default mount point for removable devices, such as USB sticks, media players, etc. By common sense, the directory itself, whose subdirectories are mountpoints, is on the root partition itself.
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|Stands for ''mount''. Empty directory commonly used by system administrators as a temporary mount point. By common sense, the directory itself, whose subdirectories are mountpoints, is on the root partition itself.
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|Contains locally installed software. Originated in [[UNIX System V|System V]], which has a [[package manager]] that installs software to this directory (one subdirectory per package).<ref>''[http://www.sco.com/developers/devspecs/gabi41.pdf System V Application Binary Interface]'' Edition 4.1 (1997-03-18)</ref>
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|[[procfs]] virtual [[File system|filesystem]] showing information about [[process (computing)|processes]] as files.
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|The home directory for the [[superuser]] ''root'' - that is, the system administrator. This account's home directory is usually on the initial filesystem, and hence not in /home (which may be a mount point for another filesystem) in case specific maintenance needs to be performed, during which other filesystems are not available. Such a case could occur, for example, if a hard disk drive suffers
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|Stands for "[[computer program|system (or superuser) binaries]]" and contains fundamental utilities, such as <code>init</code>, usually needed to start, maintain and recover the system. Needs to be on the root partition itself.
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|Server data (data for services provided by system).
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|In some [[Linux distribution]]s, contains a [[sysfs]] virtual [[File system|filesystem]], containing information related to hardware and the operating system. On BSD systems, commonly a symlink to the kernel sources in <code>/usr/src/sys</code>.
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|A place for temporary files not expected to survive a reboot. Many systems clear this directory upon startup or use [[tmpfs]] to implement it.
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|The Unix [[kernel (
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|The "user file system": originally the directory holding user home directories,<ref name="notes72"/> but already by the Third Edition of [[Research Unix]], ca. 1973, reused to split the operating system's programs over two disks (one of them a 256K fixed-head drive) so that basic commands would either appear in <code>/bin</code> or <code>/usr/bin</code>.<ref name="reader">
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|Stores the development headers used throughout the system. [[Header file]]s are mostly used by the <code>#include</code> directive in [[C (programming language)|C]] language, which historically is how the name of this directory was chosen.
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|Stores the needed libraries and data files for programs stored within <code>/usr</code> or elsewhere.
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|Holds programs meant to be executed by other programs rather than by users directly. E.g., the [[Sendmail]] executable may be found in this directory.<ref>{{cite web |url=http://docs.cray.com/books/S-2341-22/html-S-2341-22/z1028736068smg.html |website=[[UNICOS]]/mp Networking Facilities Administration |publisher=[[Cray]] |title=Chapter 7. sendmail |
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|Resembles <code>/usr</code> in structure, but its subdirectories are used for additions not part of the operating system distribution, such as custom programs or files from a [[BSD]] [[Ports collection]]. Usually has subdirectories such as <code>/usr/local/lib</code> or <code>/usr/local/bin</code>.
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|Architecture-independent program data. On Linux and modern BSD derivatives, this directory has subdirectories such as <code>man</code> for [[manpage]]s, that used to appear directly under <code>/usr</code> in older versions.
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|Stands for ''variable''. A place for files that
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|Contains system log files.
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|The place where all incoming mail is stored. Users (other than <code>root</code>) can access their own mail only. Often, this directory is a [[symbolic link]] to <code>/var/spool/mail</code>.
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|[[spooling|Spool]] directory. Contains print jobs, mail spools and other queued tasks.
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|The place where the uncompiled source code of some programs is.
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|The <code>/var/tmp</code> directory is a place for temporary files which should be preserved between system reboots.
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*{{Citizendium}}
{{Reflist|30em}}
{{Unix}}
{{DEFAULTSORT:Unix Directory Structure}}
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