probe-file
pathspec => truename
pathspec--a pathname designator.
truename--a physical pathname or nil.
probe-file tests whether a file exists.
probe-file returns false if there is no file named pathspec, and otherwise returns the truename of pathspec.
If the pathspec designator is an open stream, then probe-file produces the truename of its associated file.
If pathspec is a stream, whether open or closed, it is coerced to a pathname as if by the function pathname.
The host computer's file system.
An error of type file-error is signaled if pathspec is wild.
An error of type file-error is signaled if the file system cannot perform the requested operation.
logical-pathname, File System Concepts, File Operations on Open and Closed Streams, Pathnames as Filenames