Filesystem Hierarchy Standard
The Filesystem Hierarchy Standard (FHS) defines the directory structure and directory contents in Unix and Unix-like operating systems. It is maintained by the Linux Foundation. The latest version is 3.0, released on 3 June 2015.
Currently it is only used by Linux distributions.
Directory structure
In the FHS all files and directories appear under the root directory /, even if they are stored on different physical or virtual devices. Note however that some of these directories may or may not be present on a Unix system depending on whether certain subsystems, such as the X Window System, are installed.
The majority of these directories exist in all UNIX operating systems and are generally used in much the same way; however, the descriptions here are those used specifically for the FHS, and are not considered authoritative for platforms other than Linux.
FHS compliance
Most Linux distributions follow the Filesystem Hierarchy Standard and declare it their own policy to maintain FHS compliance.GoboLinux and NixOS provide examples of intentionally non-compliant filesystem implementations.