The Filesystem Hierarchy Standard (FHS) defines the directory structure and directory contents in Linux distributions. It is maintained by the Linux Foundation. The latest version is 3.0, released on 3 June 2015.
In the FHS, all files and directories appear under the root directory /, even if they are stored on different physical or virtual devices. Some of these directories only exist on a particular system if certain subsystems, such as the X Window System, are installed.
Most 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.