This is a short overview of standards for directories.
TODO: Overlaps with Configuration Locations.
There are 2 important standards, which have their own scope:
The Filesystem Hierarchy Standard (FHS) defines the general directory layout of a computer system. It is currently maintained by The Linux Foundation at https://wiki.linuxfoundation.org/lsb/fhs. Before it was at http://www.pathname.com/fhs/ maintained by the Linux Standard Base (LSB) workgroup of freestandards.org.The different BSD variants are no longer part. They use only
|$XDG_CONFIG_HOME||$HOME/.config||the base directory relative to which user specific configuration files should be stored.|
|$XDG_CONFIG_DIRS||/etc/xdg||the colon ':' separated preference-ordered set of base directories to search for configuration files in addition to $XDG_CONFIG_HOME|
|$XDG_DATA_HOME||$HOME/.local/share||the base directory relative to which user specific data files should be stored.|
|$XDG_DATA_DIRS||usr/local/share/:/usr/share/||the colon ':' separated preference-ordered set of base directories to search for data files in addition to $XDG_DATA_HOME|
|$XDG_CACHE_HOME||$HOME/.cache||the base directory relative to which user specific non-essential data files should be stored.|
|$XDG_RUNTIME_DIR||If it is not set applications should fall back to a replacement directory with similar capabilities and print a warning message.||Applications should use this directory for communication and synchronization purposes and should not place larger files in it, since it might reside in runtime memory and cannot necessarily be swapped out to disk...|
Partly the home directory uses similar subdirectories as the system has, some with the same name like
~/.share, others with a different name like
~/.config. This will allow among others a fallback: if no value is found in ~/.config check /etc for the system wide default.