runtime: command: add an arg to begin_processing() to indicate pipeline position

bshell.runtime/runtime/pipeline.c

@@ -7,6 +7,7 @@
 #include <fx/vector.h>
 
 struct bshell_pipeline_p {
+	bool p_input_values_provided;
 	bshell_pipeline *p_self;
 	fx_array *p_in, *p_out;
 	FX_VECTOR_DECLARE(bshell_cmdcall *, p_cmds);
@@ -72,6 +73,7 @@ static enum bshell_status pipeline_add_input_value(
 	fx_value value,
 	bool enumerate)
 {
+	pipeline->p_input_values_provided = true;
 	return write_value(pipeline, pipeline->p_in, value, enumerate);
 }
 
@@ -118,10 +120,26 @@ static enum bshell_status pipeline_pump_record(
 
 	size_t i = 0;
 	for (i = 0; i < pipeline->p_cmds.count;) {
+		enum bshell_command_position position;
+		if (pipeline->p_cmds.count == 1) {
+			position = pipeline->p_input_values_provided
+			                 ? BSHELL_COMMAND_POSITION_END
+			                 : BSHELL_COMMAND_POSITION_SINGLE;
+		} else if (i == 0) {
+			position = pipeline->p_input_values_provided
+			                 ? BSHELL_COMMAND_POSITION_END
+			                 : BSHELL_COMMAND_POSITION_BEGINNING;
+		} else if (i == pipeline->p_cmds.count - 1) {
+			position = BSHELL_COMMAND_POSITION_END;
+		} else {
+			position = BSHELL_COMMAND_POSITION_MIDDLE;
+		}
+
 		status = bshell_cmdcall_process_record(
 			pipeline->p_cmds.items[i],
 			pipeline->p_self,
-			rt);
+			rt,
+			position);
 
 		if (status != BSHELL_SUCCESS) {
 			break;