There is a single base directory relative to which user-specific data files should be written. This directory is defined by the environment variable $XDG_DATA_HOME.

There is a single base directory relative to which user-specific configuration files should be written. This directory is defined by the environment variable $XDG_CONFIG_HOME.

There is a set of preference ordered base directories relative to which data files should be searched. This set of directories is defined by the environment variable $XDG_DATA_DIRS.

There is a set of preference ordered base directories relative to which configuration files should be searched. This set of directories is defined by the environment variable $XDG_CONFIG_DIRS.

There is a single base directory relative to which user-specific non-essential (cached) data should be written. This directory is defined by the environment variable $XDG_CACHE_HOME.