Filesystem Hierarchy Standard

The Filesystem Hierarchy Standard (FHS) defines the main directories and directory contents in the Linux operating system.FHS is maintained by the Linux Foundation.The current version is version 3.0, released in 2015.

documents are not much, about 50 pages in English, so you can read them quickly.

Look at the abstract first:

This standard consists of a set of requirements and guidelines for file and directory placement under UNIX-like
operating systems. The guidelines are intended to support interoperability of applications, system administration tools,
development tools, and scriptsas well as greater uniformity of documentation for these systems.

Generally speaking, it means to determine a specification for the location of files and directories for UNIX-like systems.

The next article is divided into the following sections:

1. The Root Filesystem - The Root Filesystem
   specifies which directory files need to be in the root directory.
   The following directories (whether linked or not) need to exist in the root directory:

2. /usr directory structure - The /usr Hierarchy
   specifies which files are required in the usr directory.
   It should be noted that **/usr is not the abbreviation of user, in fact, usr is the abbreviation of Unix Software Resource, that is, the directory where the software resources of the Unix operating system are placed, not the user's data; all system default software will be placedTo /usr, when the system is installed, this directory will occupy the most hard disk capacity**

3. /var directory structure - The /var Hierarchy
   specifies which files are required in the var directory.
   /var contains the data to be changed during normal operation of the system.Usually the size of the directory where the data resides is frequently changed or expanded.

4. Some other specific components of the operating system

reference

https://refspecs.linuxfoundation.org/