The directory might be a string, :wild, :unspecific, or nil.
The directory can be a list of strings and symbols. The car of the list is one of the symbols :absolute or :relative, meaning:
Each remaining element of the list is a string or a symbol.
Each string names a single level of directory structure. The strings should contain only the directory names themselves---no punctuation characters.
In place of a string, at any point in the list, symbols can occur to indicate special file notations. The next figure lists the symbols that have standard meanings. Implementations are permitted to add additional objects of any type that is disjoint from string if necessary to represent features of their file systems that cannot be represented with the standard strings and symbols.
Supplying any non-string, including any of the symbols listed below, to a file system for which it does not make sense signals an error of type file-error. For example, Unix does not support :wild-inferiors in most implementations.
Symbol Meaning :wild Wildcard match of one level of directory structure :wild-inferiors Wildcard match of any number of directory levels :up Go upward in directory structure (semantic) :back Go upward in directory structure (syntactic)
Figure 19-3. Special Markers In Directory Component
The following notes apply to the previous figure:
``Semantic'' means that the action of :up depends on the contents of the file system; to resolve a pathname containing :up to a pathname whose directory component contains only :absolute and strings requires probing the file system.
:up differs from :back only in file systems that support multiple names for directories, perhaps via symbolic links. For example, suppose that there is a directory (:absolute "X" "Y" "Z") linked to (:absolute "A" "B" "C") and there also exist directories (:absolute "A" "B" "Q") and (:absolute "X" "Y" "Q"). Then (:absolute "X" "Y" "Z" :up "Q") designates (:absolute "A" "B" "Q") while (:absolute "X" "Y" "Z" :back "Q") designates (:absolute "X" "Y" "Q")
18.104.22.168.3.1 Directory Components in Non-Hierarchical File Systems