The following code parses the "/etc/passwd" file to
fetch the user's full-name:

   ;; ,open receiving srfi-2 scheme-with-scsh
   (define user-gecos
     (let ((read-passwd (field-reader (infix-splitter ":" 7))))
       (lambda (username)
         (call-with-input-file "/etc/passwd"
           (lambda (port)
             (let loop ()
               (receive (record fields)
                   (read-passwd port)
                 (cond
                  ((eof-object? record)
                   #f)
                  ((string=? (list-ref fields 0) username)
                   (list-ref fields 4))
                  (else
                   (loop))))))))))

   (define user-full-name
     (let ((gecos-splitter (infix-splitter ",")))
       (lambda (username)
         (and-let* ((gecos (user-gecos username)))
           (car (gecos-splitter gecos))))))

Sample usage:

   (user-full-name (user-login-name))