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In Unix and inspired by it, the file system is considered a central component of the operating system. It was also one of the first parts of the system to be designed and implemented by Ken Thompson in the first experimental version of Unix, dated 1969.
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 pipes complemented it in this role starting with the Research Unix. Also, the filesystem provides access to other resources through so-called that are entry points to terminals, computer printer, and computer mouse.
The rest of this article uses Unix as a generic name to refer to both the original Unix operating system and its many Unix-like.
In the original 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 , 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. 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. 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.
BSD also added (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, 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.
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 System V Application Binary Interface, the Intel Binary Compatibility Standard, the Common Operating System Environment, and FreeDesktop's Filesystem Hierarchy Standard (FHS).
Here is a generalized overview of common locations of files on a Unix operating system:
| / | The slash / character alone denotes the root of the filesystem tree. |
| /binStands for 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 Symbolic link to /usr/bin. Otherwise, it needs to be on the root filesystem itself. | |
| /bootContains all the files needed for successful booting process. In Research Unix, this was one file rather than a directory. Nowadays usually on the root filesystem itself, unless the system, bootloader etc. require otherwise. | |
| /devStands for devices. Contains device node of peripheral devices and pseudo-devices. See also: Linux Assigned Names and Numbers Authority. Needs to be on the root filesystem itself. | |
| /etcContains system-wide configuration files and system databases; the name stands for et cetera but now a better expansion is editable- text- configurations. 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. | |
| /homeContains user home directories on Linux and some other systems. In the original version of Unix, home directories were in /usr instead. Some systems use or have used different locations still: macOS has home directories in /Users, older versions of BSD put them in /u, FreeBSD has /usr/home. | |
| /libOriginally essential libraries: C libraries, but not Fortran ones. On modern systems, it contains the shared libraries needed by programs in /bin, and possibly loadable kernel module, or . Linux distributions may have variants /lib32 and /lib64 for multi-architecture support. | |
| /mediaDefault 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. | |
| /mntStands 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. | |
| /optContains locally installed software. Originated in System V, which has a package manager that installs software to this directory (one subdirectory per package). System V Application Binary Interface Edition 4.1 (1997-03-18) | |
| /procprocfs virtual File system showing information about processes as files. | |
| /rootThe 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 failures and cannot be properly mounted. | |
| /sbinStands for "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. | |
| /srvServer data (data for services provided by system). | |
| /sysIn some Linux distributions, contains a sysfs virtual File system, containing information related to hardware and the operating system. On BSD systems, commonly a Symbolic link to the kernel sources in /usr/src/sys. | |
| /tmpA place for temporary files not expected to survive a reboot. Many systems clear this directory upon startup or use tmpfs to implement it. | |
| /unixThe Unix kernel in Research Unix and System V. With the addition of virtual memory support to 3BSD, this got renamed /vmunix. | |
| /usrIn 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, 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. 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, , scripting languages, 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/includeStores the development headers used throughout the system. are mostly used by the #include directive in C language, which historically is how the name of this directory was chosen. | |
| /usr/libStores the needed libraries and data files for programs stored within /usr or elsewhere. | |
| /usr/libexecHolds programs meant to be executed by other programs rather than by users directly. E.g., the Sendmail executable may be found in this directory. Not present in the FHS until 2011; Linux distributions have traditionally moved the contents of this directory into /usr/lib, where they also resided in 4.3BSD. | |
| /usr/localResembles /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 Ports collection. Usually has subdirectories such as /usr/local/lib or /usr/local/bin. | |
| /usr/shareArchitecture-independent program data. On Linux and modern BSD derivatives, this directory has subdirectories such as man for , that used to appear directly under /usr in older versions. | |
| /varStands 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 . | |
| /var/logContains system log files. | |
| /var/mailThe place where all incoming mail is stored. Users (other than root) can access their own mail only. Often, this directory is a symbolic link to /var/spool/mail. | |
| /var/spoolspooling directory. Contains print jobs, mail spools and other queued tasks. | |
| /var/srcThe place where the uncompiled source code of some programs is. | |
| /var/tmpThe /var/tmp directory is a place for temporary files which should be preserved between system reboots. |
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