I use this. This makes the "shell" command work in SBCL, and does nothing in CLISP or any other implementation.

  #+sbcl
  (defun shell (x)
    (run-program "/bin/sh" (list "-c" x) :output t))

Tell me if you want it to work in an implementation other than CLISP or SBCL, and if I can install it on my computer, then I'll figure it out.