yacc grammar for a simple shell

%token  STRING
%token  RIGHT_APPEND

%%

cmd:    fg_cmd
   |    bg_cmd
   ;

bg_cmd: fg_cmd '&'
      ;

fg_cmd:    simple_cmd
      |    pipe_cmd
      ;

simple_cmd:     /* empty command */
          |     executable
          |     executable input_redirection
          |     input_redirection executable
          |     executable output_redirection
          |     output_redirection executable
          |     executable input_redirection   output_redirection
          |     input_redirection   executable  output_redirection
          |     input_redirection   output_redirection  executable
          |     executable  output_redirection   input_redirection
          |     output_redirection   executable  input_redirection
          |     output_redirection   input_redirection  executable
          ;

executable: STRING
          | STRING executable;

input_redirection:  '<' STRING;

output_redirection:     overwrite_output_redirection
                  |     append_output_redirection
                  ;

overwrite_output_redirection:   '>'  STRING ;

append_output_redirection:      RIGHT_APPEND STRING ;

pipe_cmd:   left_cmd '|' right_cmd
        |   left_cmd '|' middle_cmd right_cmd
        ;

left_cmd:   executable
        |   executable input_redirection
        |   input_redirection executable 
        ;

middle_cmd: executable '|'
          | middle_cmd executable '|'
          ;

right_cmd:  executable  
         |  executable output_redirection
         |  output_redirection executable 
         ;

%%

int main(int argc, char *argv[])
{
    yyparse();

    return 0;
}

int yywrap()
{
    return 1;
}

int yyerror(char *msg)
{
    printf("Error encountered: %s\n", msg);
}