TclFile *inPipePtr,		/* If non-NULL, input to the pipeline comes
				 * from a pipe (unless overridden by
				 * redirection in the command). The file id
				 * with which to write to this pipe is stored
				 * at *inPipePtr. NULL means command specified
				 * its own input source. */
    TclFile *outPipePtr,	/* If non-NULL, output to the pipeline goes to
				 * a pipe, unless overridden by redirection in
				 * the command. The file id with which to read
				 * frome this pipe is stored at *outPipePtr.
				 * NULL means command specified its own output
				 * sink. */
    TclFile *errFilePtr)	/* If non-NULL, all stderr output from the
				 * pipeline will go to a temporary file
				 * created here, and a descriptor to read the