class output_command :?onclose:unit -> unit -> string ->out_obj_channel
/bin/sh, and data written to the channel is
 piped to stdin of the command.
 The method pos_out returns the number of written octets.
 When close_out is invoked, the subprocess is waited for. If the
 process exits with code 0, the method returns normally. Otherwise,
 the exception Command_failure is raised. (The channel is closed
 even if this exception is raised.)
onclose : this function is called when the close_out method is
 invoked, just after the underlying descriptor has been closed.