There is no portable way in Common Lisp of determining whether a file

is a directory or not. Calling DIRECTORY on the pathname will not

always work, since the directory could be empty. For UNIX systems

(defun DIRECTORY-P (pathname)

(probe-file (concatenate 'string pathname "/.")))

seems to work fairly reliably. (If "foo" is a directory, then "foo/."

will be a valid filename; if not, it will return NIL.) This won't, of

course, work on the Macintosh, or on other operating systems (e.g.,

MVS, CMS, ITS). On the Macintosh, use DIRECTORYP.

Moreover, some operating systems may not support the concept of

directories, or even of a file system. For example, recent work on

object-oriented technology considers files to be collections of

objects. Each type of collection defines a set of methods for reading

and writing the objects "stored" in the collection.

There's no standard function for finding the current directory from

within a Lisp program, since not all Lisp environments have the

concept of a current directory. Here are the commands from some Lisp

implementations:

Lucid: WORKING-DIRECTORY (which is also SETFable)

PWD and CD also work

Allegro: CURRENT-DIRECTORY (use excl:chdir to change it)

CMU CL: DEFAULT-DIRECTORY

LispWorks: LW:*CURRENT-WORKING-DIRECTORY*

(use LW:CHANGE-DIRECTORY to change it)

Allegro also uses the variable *default-pathname-defaults* to resolve

relative pathnames, maintaining it as the current working directory.

So evaluating (truename "./") in Allegro (and on certain other

systems) will return a pathname for the current directory. Likewise,

in some VMS systems evaluating (truename "[]") will return a pathname

for the current directory.