# Unix filesystem > Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Unix_filesystem > Markdown URL: https://mediated.wiki/source/Unix_filesystem.md > Source: https://en.wikipedia.org/wiki/Unix_filesystem > Source revision: 1353355403 > License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) {{Short description|Directory structure used by a Unix-like operating system}} {{Redirect|Unix file system|UFS, a specific file system used by many Unix and Unix-like operating systems|Unix File System}} [[File:Version 7 UNIX SIMH PDP11 Filesystem Layout.png|thumb|[Version 7 Unix](/source/Version_7_Unix) filesystem layout: subdirectories of "/" and "/usr"]] [[File:Standard-unix-filesystem-hierarchy.svg|thumb|An overview of a [Unix](/source/Unix) filesystem layout]] In [Unix](/source/Unix) and [operating system](/source/operating_system)s inspired by it, the [file system](/source/file_system) is considered a central component of the operating system.{{cite journal |last1= Ritchie |first1= D.M. |author-link1= Dennis Ritchie |last2= Thompson |first2= K. |author-link2= Ken Thompson |title= The UNIX Time-Sharing System |journal= Bell System Tech. J. |volume= 57 |issue= 6 |pages= 1905–1929 |date= July 1978 |doi=10.1002/j.1538-7305.1978.tb02136.x|citeseerx= 10.1.1.112.595 }} It was also one of the first parts of the system to be designed and implemented by [Ken Thompson](/source/Ken_Thompson) in the first experimental version of Unix, [dated 1969](/source/History_of_Unix). As in other operating systems, the filesystem provides information storage and retrieval, and one of several forms of [interprocess communication](/source/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 [pipes](/source/Pipeline_(Unix)) complemented it in this role starting with the [Third Edition](/source/Research_Unix). Also, the filesystem provides access to other resources through so-called ''[device file](/source/device_file)s'' that are entry points to [terminals](/source/computer_terminal), [printers](/source/computer_printer), and [mice](/source/computer_mouse). The rest of this article uses ''Unix'' as a [generic name](/source/Generic_trademark) to refer to both the original Unix operating system and its many [workalikes](/source/Unix-like). ==Principles== The filesystem appears as one [rooted tree](/source/rooted_tree) of directories. Instead of addressing separate volumes such as [disk partitions](/source/disk_partitions), removable media, and [network share](/source/network_share)s as separate trees, such volumes can be ''[mounted](/source/mount_(Unix))'' on a directory, causing the volume's file system tree to appear as that directory in the larger tree. The root of the entire tree is denoted /. In the original [Bell Labs Unix](/source/Research_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 disk’s contents viewable at /usr. Directories in Unix file systems do not ''contain'' files. Instead, they contain the names of files paired with references to so-called [inode](/source/inode)s, which in turn contain both the file and its [metadata](/source/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](/source/hard_link)''. The mathematical traits of hard links make the file system a limited type of [directed acyclic graph](/source/directed_acyclic_graph), although the ''directories'' still form a tree, as they typically may not be hard-linked. 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.{{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 }} ===File types=== {{Main |Unix file types}} The original Unix file system supported three types of files: ordinary files, [directories](/source/Directory_(computing)), and "special files", also termed device files. The [Berkeley Software Distribution](/source/Berkeley_Software_Distribution) (BSD) and [System V](/source/System_V) each added a file type to be used for [interprocess communication](/source/interprocess_communication): BSD added [sockets](/source/Berkeley_sockets),{{cite book |last1= Leffler |first1= Samuel J. |author-link1= Samuel J Leffler |last2= McKusick |first2= Marshall Kirk |author-link2= Marshall Kirk McKusick |last3= Karels |first3= Michael J. |author-link3= Michael J. Karels |last4= Quarterman |first4= John S. |author-link4= John Quarterman |title= The Design and Implementation of the 4.3BSD UNIX Operating System |date= October 1989 |publisher= [Addison-Wesley](/source/Addison-Wesley) |isbn= 978-0-201-06196-3 |url-access= registration |url= https://archive.org/details/designimplementa0000unse }} while System V added [FIFO files](/source/Named_pipe). BSD also added [symbolic link](/source/symbolic_link)s (often termed "symlinks") to the range of file types, which are files that refer to other files, and complement hard links. Symlinks were modeled after a similar feature in [Multics](/source/Multics),{{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|access-date=16 November 2016}} and differ from hard links in that they may span filesystems and that their existence is independent of the target object. Other Unix systems may support additional types of files.{{man|2|stat|Linux}} ==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 directories](/source/home_directories). These were first documented in the hier(7) [man page](/source/man_page) since [Version 7 Unix](/source/Version_7_Unix);{{man|7|hier|v7}} subsequent versions, derivatives and clones typically have a similar man page.{{man|7|hier|FreeBSD}}{{man|7|hier|OpenBSD}}{{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}}{{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}}{{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}}{{man|7|hier|Linux}} The details of the directory layout have varied over time. Although the file system layout is not part of the [Single UNIX Specification](/source/Single_UNIX_Specification), several attempts exist to standardize (parts of) it, such as the [System V](/source/System_V) [Application Binary Interface](/source/Application_Binary_Interface), the [Intel Binary Compatibility Standard](/source/Intel_Binary_Compatibility_Standard), the Common Operating System Environment, and [FreeDesktop](/source/FreeDesktop)'s [Filesystem Hierarchy Standard](/source/Filesystem_Hierarchy_Standard) (FHS).{{cite web |title=Where to Install My Products on Linux? |url=http://www.linuxjournal.com/article/4121 |website=[Linux Journal](/source/Linux_Journal) |date=1 November 2000 |author=George Kraft IV |access-date=13 November 2014}} Here is a generalized overview of common locations of files on a Unix operating system: {|class="wikitable" !Directory or file !Description |- |/ |The slash / character alone denotes the root of the filesystem tree. |- | /bin |Stands for ''[binaries](/source/computer_program)'' and contains certain fundamental utilities, such as ls or cp, that are needed to mount /usr, when that is a separate filesystem, or to run in one-user (administrative) mode when /usr cannot be mounted. In System V.4, this is a [symlink](/source/symlink) to /usr/bin. Otherwise, it needs to be on the root filesystem itself. |- | [/boot](/source/%2Fboot) |Contains all the files needed for successful booting process. In [Research Unix](/source/Research_Unix), this was one file rather than a directory. Nowadays usually on the root filesystem itself, unless the system, bootloader etc. require otherwise. |- | [/dev](/source/%2Fdev) |Stands for ''devices''. Contains [file representations](/source/device_node) of peripheral devices and [pseudo-devices](/source/Device_file). See also: [Linux Assigned Names and Numbers Authority](/source/Linux_Assigned_Names_and_Numbers_Authority). Needs to be on the root filesystem itself. |- | /etc |Contains system-wide configuration files and system databases; the name stands for ''[et cetera](/source/et_cetera)''{{cite book |first1=Brian W. |last1=Kernighan |author-link1=Brian Kernighan |first2=Rob |last2=Pike |author-link2=Rob Pike |title=The UNIX Programming Environment |publisher=Prentice-Hall |year=1984 |pages=[https://archive.org/details/unixprogramminge0000kern/page/63 63–65]|title-link=The UNIX Programming Environment |bibcode=1984upe..book.....K }} but now a better expansion is '''e'''ditable-'''t'''ext-'''c'''onfigurations. Originally also contained "dangerous maintenance utilities" such as init, but these have typically been moved to /sbin or elsewhere. Needs to be on the root filesystem itself. |- | /home |Contains user home directories on Linux and some other systems. In the original version of Unix, home directories were in /usr instead.{{cite web|last=Ritchie|first=Dennis|title=Unix Notes from 1972|url=https://www.nokia.com/bell-labs/about/dennis-m-ritchie/notes.html|access-date=7 September 2025}} Some systems use or have used different locations still: [macOS](/source/macOS) has home directories in /Users, older versions of BSD put them in /u, [FreeBSD](/source/FreeBSD) has /usr/home. |- | /lib |Originally ''essential libraries'': [C](/source/C_(programming_language)) libraries, but not [Fortran](/source/Fortran) ones. On modern systems, it contains the shared libraries needed by programs in /bin, and possibly [loadable kernel module](/source/loadable_kernel_module), [device driver](/source/device_driver)s or [binary blob](/source/binary_blob)s. Linux distributions may have variants /lib32 and /lib64 for multi-architecture support. |- | /media |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. |- | /mnt |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. |- | /opt |Contains locally installed software. Originated in [System V](/source/System_V), which has a [package manager](/source/package_manager) that installs software to this directory (one subdirectory per package).''[http://www.sco.com/developers/devspecs/gabi41.pdf System V Application Binary Interface]'' Edition 4.1 (1997-03-18) |- | /proc |[procfs](/source/procfs) [synthetic file system](/source/synthetic_file_system) showing information about [processes](/source/process_(computing)) as files. |- | /root |The home directory for the [superuser](/source/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 failures and cannot be properly mounted. |- | /sbin |Stands for "[system (or superuser) binaries](/source/computer_program)" and contains fundamental utilities, such as init, usually needed to start, maintain and recover the system. Needs to be on the root partition itself. |- | /srv |Server data (data for services provided by system). |- | /sys |In some [Linux distribution](/source/Linux_distribution)s, contains a [sysfs](/source/sysfs) [synthetic file system](/source/synthetic_file_system), containing information related to hardware and the operating system. On BSD systems, commonly a [symlink](/source/symlink) to the kernel sources in /usr/src/sys. |- | /tmp |A place for temporary files not expected to survive a reboot. Many systems clear this directory upon startup or use [tmpfs](/source/tmpfs) to implement it. |- | /unix |The Unix [kernel](/source/kernel_(operating_system)) in Research Unix and [System V](/source/System_V). With the addition of [virtual memory](/source/virtual_memory) support to [3BSD](/source/Berkeley_Software_Distribution), this got renamed /vmunix. |- | /usr |In the earliest versions of Unix, /usr was the directory holding user home directories; the directory name reflected that. By the Third Edition of [Research Unix](/source/Research_Unix), ca. 1973, the operating system's programs overflowed the 256K fixed-head drive containing /bin, so a /usr/bin directory was created on the disk containing /usr to hold some commands; commands and user home directories both resided on /usr.{{cite web |author-link=Doug McIlroy |author=M. D. McIlroy |date=1987 |url=http://www.cs.dartmouth.edu/~doug/reader.pdf |title=A Research Unix reader: annotated excerpts from the Programmer's Manual, 1971–1986 |id=CSTR 139 |publisher=Bell Labs}} Additional directories, such as /usr/lib and /usr/tmp, were also created under /usr; it now holds executables, libraries, and shared resources that are not system critical, such as the [X Window System](/source/X_Window_System), [window manager](/source/window_manager)s, [scripting language](/source/scripting_language)s, etc. In older Unix systems, user home directories might still appear in /usr alongside directories containing those items; some systems have moved user home directories under a directory other than /usr. |- | /usr/include |Stores the development headers used throughout the system. [Header file](/source/Header_file)s are mostly used by the #include directive in [C](/source/C_(programming_language)) language, which historically is how the name of this directory was chosen. |- | /usr/lib |Stores the needed libraries and data files for programs stored within /usr or elsewhere. |- | /usr/libexec |Holds programs meant to be executed by other programs rather than by users directly. E.g., the [Sendmail](/source/Sendmail) executable may be found in this directory.{{cite web |url=http://docs.cray.com/books/S-2341-22/html-S-2341-22/z1028736068smg.html |website=[UNICOS](/source/UNICOS)/mp Networking Facilities Administration |publisher=[Cray](/source/Cray) |title=Chapter 7. sendmail |access-date=14 September 2013 |archive-date=9 November 2017 |archive-url=https://web.archive.org/web/20171109075438/http://docs.cray.com/books/S-2341-22/html-S-2341-22/z1028736068smg.html |url-status=dead }} Not present in the FHS until 2011;{{cite web |url=http://bzr.linuxfoundation.org/loggerhead/lsb/devel/fhs-spec/revision/44 |title=fhs-spec revision 44 |access-date=2016-07-14 |archive-date=2017-03-05 |archive-url=https://web.archive.org/web/20170305015621/http://bzr.linuxfoundation.org/loggerhead/lsb/devel/fhs-spec/revision/44 |url-status=dead }} Linux distributions have traditionally moved the contents of this directory into /usr/lib, where they also resided in 4.3BSD. |- | /usr/local |Resembles /usr 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](/source/BSD) [Ports collection](/source/Ports_collection). Usually has subdirectories such as /usr/local/lib or /usr/local/bin. |- | /usr/share |Architecture-independent program data. On Linux and modern BSD derivatives, this directory has subdirectories such as man for [manpage](/source/manpage)s, that used to appear directly under /usr in older versions. |- | /var |Stands for ''variable''. A place for files that might change frequently - especially in size, for example e-mail sent to users on the system, or process-ID [lock file](/source/lock_file)s. |- | /var/log |Contains system log files. |- | /var/mail |The place where all incoming mail is stored. Users (other than root) can access their own mail only. Often, this directory is a [symbolic link](/source/symbolic_link) to /var/spool/mail. |- | /var/spool |[Spool](/source/spooling) directory. Contains print jobs, mail spools and other queued tasks. |- | /var/src |The place where the uncompiled source code of some programs is. |- | /var/tmp |The /var/tmp directory is a place for temporary files which should be preserved between system reboots. |} ==See also== *[Btrfs](/source/Btrfs) *[ext2](/source/ext2) *[ext3](/source/ext3) *[ext4](/source/ext4) *[Filesystem Hierarchy Standard](/source/Filesystem_Hierarchy_Standard) *[HAMMER](/source/HAMMER_(file_system)) *[JFS (file system)](/source/JFS_(file_system)) *[Unix File System](/source/Unix_File_System) *[Veritas File System](/source/Veritas_File_System) *[ZFS](/source/ZFS) ==References== *{{Citizendium}} {{Reflist|30em}} {{Unix}} {{DEFAULTSORT:Unix Directory Structure}} Category:Unix file system technology Category:File system management --- Adapted from the Wikipedia article [Unix filesystem](https://en.wikipedia.org/wiki/Unix_filesystem) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Unix_filesystem?action=history)). 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