Unix directory structure

In computer science, and particularly in Unix-like operating systems, the Unix directory structure is a convention of organization.

There are attempts at creating an "official" standard Unix filesystem layout, such as the "Filesystem Hierarchy Standard." Also as part of the "Linux Standards Base" a common filesystem layout is defined, to encourage interoperability among different Linux distributions.

What follows is a generalized overview of common locations of files in a Unix system:

• / (root) -- The / notes the "root" of the filesystem, where the entire system is contained. Unlike Microsoft Windows, where each drive has its own root directory named by a letter, such as C:\ or F:\, Unix holds the entire system in this single top-level directory, including each device and document. Thus, it is commonly said that in Unix, "everything is a file." Note that Unix uses the forward slash ( / ) rather than the backslash ( \ ) commonly used in Windows.

• /bin -- Stands for "binaries"; Contains some fundamental utilities needed by a system administrator. As a failsafe, these were placed in a separate directory so that they could be placed on a separate disk or disk partition in case the main drive failed.

• /etc -- Contains configuration files and some system databases.

• /dev -- short for devices. Contains file representations of every peripheral device attached to the system.

• /dev/null -- Also known as the "bit bucket" or "black hole", this virtual file discards all contents written to it. This is typically used to throw away unwanted data streams, such as log files.

• /dev/random -- This is a virtual file which contains random numbers (subject to the limitations of Random Number Generators in Computing). It uses system noise to generate random numbers and blocks if not enough entropy in the noise is available. Random is commonly used more by programs that absolutely need high quality random data (such as SSH to generate an encryption key).

• /dev/urandom -- Same as /dev/random, except it always returns random numbers, even if there is not enough entropy in the system noise available. In the latter case, pseudorandom numbers are generated, which are based on an algorithm, depending on the type of Unix system.

• /home -- contains the home directories for the users. On some Unices, this is under /usr/home

• /include

• /lib

• /root -- the home directory for the superuser root.

• /tmp -- a place for temporary files. Many Unices clear this directory upon start up.

• /usr -- originally the directory holding user home directories, its use has changed, and it now holds executables, libraries, and shared resources that are not system critical: X11, KDE, PERL, etc. (The name "Unix System Resources" is a post hoc backronym.)

• /usr/bin -- This directory stores the executables that live in /usr.

• /usr/include -- /usr/include stores the development headers used throughout the system.

• /usr/lib -- The required libraries for executables within /usr or elsewhere for that matter, live here.

• /var -- short for "variable." A place for files that may change often, such as the storage to a database, the contents of a database, log files (usually stored in /var/log), email stored on a server, etc.