The XDG Base Directory Specification is based on the following concepts:
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.
There is a single base directory relative to which
user-specific runtime files and other file objects should
be placed. This directory is defined by the environment
variable $XDG_RUNTIME_DIR.
All paths set in these environment variables must be absolute. If an implementation encounters a relative path in any of these variables it should consider the path invalid and ignore it.