which -- check if a command is available on the working environment

which.lisp

(defun which (command)
  (handler-case
      (let* ((result (with-output-to-string (s)
                       (uiop:run-program `("which" ,command)
                                         :output s
                                         :error-output *error-output*)))
             (newline-pos
               (position-if (lambda (char)
                              (or (char= char #\Newline)
                                  (char= char #\Return)))
                            result)))
        (if newline-pos
            (subseq result 0 newline-pos)
            result))
    (uiop:subprocess-error ()
      nil)))