How to process input and output streams in Steel Bank Common Lisp?

Question

I'm trying to figure out how to use the output stream of one program I start with RUN-PROGRAM so it can be used as the input of another program started with RUN-PROGRAM (i.e., the moral and perhaps literal equivalent of piping). I've tried using a number of combinations of the :INPUT, :OUTPUT and :WAIT keyword arguments, but nothing I've hit upon has been productive so far. Any tips would be helpful; for example, how would I go about doing something like ls | grep lisp from the shell?

One of my attempts is

(defun piping-test () 
  (let ((grep-process (run-program "/usr/bin/grep" '("lisp") 
                                  :input :stream 
                                  :output :stream))) 
    (unwind-protect 
        (with-open-stream (s (process-input grep-process)) 
          (let ((ls-process (run-program "/bin/ls" '() 
                                        :output s))) 
            (when ls-process 
              (unwind-protect 
                  (with-open-stream (o (process-output grep-process)) 
                   (loop 
                      :for line := (read-line o nil nil) 
                      :while line 
                      :collect line)) 
               (process-close ls-process))))) 
     (when grep-process (process-close grep-process))))) 

Running this in a SLIME REPL causes everything to hang until I break with C-c C-c, so it's pretty obviously not the right thing, but I'm not sure how to change it so it is the right thing.

EDIT: Adding :WAIT NIL to both RUN-PROGRAM invocations, or to only the invocation for grep, doesn't do the trick. In that case, the function will hang, and breaking with C-c C-c gets a stack trace indicating that there's a local function (defined via FLET) called SB-UNIX:SELECT that has hung.

Answer 1

I got a working answer from Raymond Toy on comp.lang.lisp. His solution was for CMUCL, but it worked with the essentially identical RUN-PROGRAM function on the closely related SBCL, and with minor changes it will work on CCL as well, because CCL's RUN-PROGRAM is basically a clone of the one from CMUCL/SBCL.

The secret, as it were, is to set up the ls process first, and then provide its output stream to the grep process as input, like so:

(defun piping-test2 () 
  (let ((ls-process (run-program "/bin/ls" '() 
                                 :wait nil 
                                 :output :stream))) 
    (unwind-protect 
        (with-open-stream (s (process-output ls-process)) 
          (let ((grep-process (run-program "/usr/bin/grep" '("lisp") 
                                          :input s 
                                          :output :stream))) 
            (when grep-process 
              (unwind-protect 
                  (with-open-stream (o (process-output grep-process)) 
                    (loop 
                       :for line := (read-line o nil nil) 
                       :while line 
                       :collect line)) 
                (process-close grep-process))))) 
      (when ls-process (process-close ls-process))))) 

I also experimented with omitting the :WAIT NIL argument from the RUN-PROGRAM call for ls, and it worked just as well.