class output_command :Runs the command with?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.
onclose : this function is called when the close_out method is
invoked, just after the underlying descriptor has been closed.