Filesystem Hierarchy Standard
The Filesystem Hierarchy Standard, or FHS, is a specification that defines the filesystem hierarchy of a Linux operating system. It specifies minimum requirements for top-level subdirectories, such as /usr and /bin, and what files and symbolic links must be located in those directories. In an FHS-compliant operating system, all files and directories are rooted in a top-level directory named / (a single forward slash).
Originally named FSSTND ("filesystem standard"), the first version of the standard was released on February 14, 1994. Today, FHS is maintained and published by the Linux Foundation. The most recent version, FHS 3.0, was published on March 18, 2015.
For more information about the directories and files in an FHS-compliant operating system, see: Linux file hierarchy.
Most Linux operating systems are FHS-compliant, but some are not. For example, NixOS uses a unique hierarchy to install Linux in the file system of the Nix package manager.