Brendan Burns has some code to interface with the BSD subsystem of OS X from MCL.

  hilights include:
  support for fopen, fclose, fwrite and chdir (see the man pages)

  and I've added "cd" and "ls".
  cd calls (set-mac-default-directory) so you can:

  ? (cd "/Volumes/Vogon/src/utils/dev")
  "/Volumes/Vogon/src/utils/dev/"
  ? (ls)
  anaphoric.lisp 
  containers.lisp
  ...