A pathname has six components: a host, a device, a directory, a name, a type, and a version.
The name of the file system on which the file resides, or the name of a logical host.
Corresponds to the “device” or “file structure” concept in many host file systems: the name of a logical or physical device containing files.
Corresponds to the “directory” concept in many host file systems: the name of a group of related files.
The “name” part of a group of files that can be thought of as conceptually related.
Corresponds to the “filetype” or “extension” concept in many host file systems. This says what kind of file this is. This component is always a string, nil, :wild, or :unspecific.
Corresponds to the “version number” concept in many host file systems.
The version is either a positive integer or a symbol from the following list: nil, :wild, :unspecific, or :newest (refers to the largest version number that already exists in the file system when reading a file, or to a version number greater than any already existing in the file system when writing a new file). Implementations can define other special version symbols.
Strings in pathname component values never contain special characters that represent separation between pathname fields, such as slash in Unix filenames. Whether separator characters are permitted as part of a string in a pathname component is implementation-defined; however, if the implementation does permit it, it must arrange to properly “quote” the character for the file system when constructing a namestring. For example,
;; In a TOPS-20 implementation, which uses ^V to quote (NAMESTRING (MAKE-PATHNAME :HOST "OZ" :NAME "<TEST>")) → #P"OZ:PS:^V<TEST^V>" not → #P"OZ:PS:<TEST>"
Namestrings always use local file system case conventions, but Common Lisp functions that manipulate pathname components allow the caller to select either of two conventions for representing case in component values by supplying a value for the :case keyword argument. Figure 19–2 lists the functions relating to pathnames that permit a :case argument:
For the functions in Figure 19–2, a value of :local for the :case argument (the default for these functions) indicates that the functions should receive and yield strings in component values as if they were already represented according to the host file system’s convention for case.
If the file system supports both cases, strings given or received as pathname component values under this protocol are to be used exactly as written. If the file system only supports one case, the strings will be translated to that case.
Mixed case represents itself.
Note that these conventions have been chosen in such a way that translation from :local to :common and back to :local is information-preserving.
The value of any pathname component can be nil.
If :wild is the value of a pathname component, that component is considered to be a wildcard, which matches anything.
When constructing a pathname, a conforming program may use :wild as the value of any or all of the directory, name, type, or version component, but must not use :wild as the value of the host, or device component.
If :wild is used as the value of the directory component in the construction of a pathname, the effect is equivalent to specifying the list
(:absolute :wild-inferiors), or the same as
(:absolute :wild) in a file system that does not support :wild-inferiors.
Whether a value of :unspecific is permitted for any component on any given file system accessible to the implementation is implementation-defined. A conforming program must never unconditionally use a :unspecific as the value of a pathname component because such a value is not guaranteed to be permissible in all implementations. However, a conforming program can, if it is careful, successfully manipulate user-supplied data which contains or refers to non-portable pathname components. And certainly a conforming program should be prepared for the possibility that any components of a pathname could be :unspecific.
If a pathname is converted to a namestring, the symbols nil and :unspecific cause the field to be treated as if it were empty. That is, both nil and :unspecific cause the component not to appear in the namestring.
However, when merging a pathname with a set of defaults, only a nil value for a component will be replaced with the default for that component, while a value of :unspecific will be left alone as if the field were “filled”; see the function merge-pathnames and Section 19.2.3 (Merging Pathnames).
Wildcard pathnames can be used with directory but not with open, and return true from wild-pathname-p. When examining wildcard components of a wildcard pathname, conforming programs must be prepared to encounter any of the following additional values in any component or any element of a list that is the directory component:
The symbol :wild, which matches anything.
The space of possible objects that a conforming program must be prepared to read1 as the value of a pathname component is substantially larger than the space of possible objects that a conforming program is permitted to write1 into such a component.
While the values discussed in the subsections of this section, in Section 220.127.116.11 (Special Pathname Component Values), and in Section 18.104.22.168 (Restrictions on Wildcard Pathnames) apply to values that might be seen when reading the component values, substantially more restrictive rules apply to constructing pathnames; see Section 22.214.171.124 (Restrictions on Constructing Pathnames).
The device might be a string, :wild, :unspecific, or nil.
Note that :wild might result from an attempt to read1 the pathname component, even though portable programs are restricted from writing1 such a component value; see Section 126.96.36.199 (Restrictions on Wildcard Pathnames) and Section 188.8.131.52 (Restrictions on Constructing Pathnames).
The directory might be a string, :wild, :unspecific, or nil.
A list whose car is the symbol :absolute represents a directory path starting from the root directory. The list
(:absolute) represents the root directory. The list
(:absolute "foo" "bar" "baz") represents the directory called
"/foo/bar/baz" in Unix (except possibly for case).
A list whose car is the symbol :relative represents a directory path starting from a default directory. The list
(:relative) has the same meaning as nil and hence is not used. The list (:relative "foo" "bar") represents the directory named "bar" in the directory named "foo" in the default directory.
In place of a string, at any point in the list, symbols can occur to indicate special file notations. Figure 19–3 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.
|: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)|
The following notes apply to the previous figure:
“Syntactic” means that the action of :back depends only on the pathname and not on the contents of the file system.
“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")
In non-hierarchical file systems, the only valid list values for the directory component of a pathname are
(:absolute string) and
(:absolute :wild). :relative directories and the keywords :wild-inferiors, :up, and :back are not used in non-hierarchical file systems.
The name might be a string, :wild, :unspecific, or nil.
The type might be a string, :wild, :unspecific, or nil.
The symbol :newest refers to the largest version number that already exists in the file system when reading, overwriting, appending, superseding, or directory listing an existing file. The symbol :newest refers to the smallest version number greater than any existing version number when creating a new file.
The symbols nil, :unspecific, and :wild have special meanings and restrictions; see Section 184.108.40.206 (Special Pathname Component Values) and Section 220.127.116.11 (Restrictions on Constructing Pathnames).
It is suggested, but not required, that implementations do the following:
Use positive integers starting at 1 as version numbers.
Recognize the symbol :oldest to designate the smallest existing version number.
Use keywords for other special versions.
When constructing a pathname from components, conforming programs must follow these rules:
Any component can be nil. nil in the host might mean a default host rather than an actual nil in some implementations.
The version can be :newest.
Any component can be taken from the corresponding component of another pathname. When the two pathnames are for different file systems (in implementations that support multiple file systems), an appropriate translation occurs. If no meaningful translation is possible, an error is signaled. The definitions of “appropriate” and “meaningful” are implementation-dependent.
An implementation might support other values for some components, but a portable program cannot use those values. A conforming program can use implementation-dependent values but this can make it non-portable; for example, it might work only with Unix file systems.
Merging takes a pathname with unfilled components and supplies values for those components from a source of defaults.
Except as explicitly specified otherwise, for functions that manipulate or inquire about files in the file system, the pathname argument to such a function is merged with *default-pathname-defaults* before accessing the file system (as if by merge-pathnames).
Although the following examples are possible to execute only in implementations which permit :unspecific in the indicated position andwhich permit four-letter type components, they serve to illustrate the basic concept of pathname merging.
(pathname-type (merge-pathnames (make-pathname :type "LISP") (make-pathname :type "TEXT"))) → "LISP" (pathname-type (merge-pathnames (make-pathname :type nil) (make-pathname :type "LISP"))) → "LISP" (pathname-type (merge-pathnames (make-pathname :type :unspecific) (make-pathname :type "LISP"))) → :UNSPECIFIC