在其他函数中调用mu函数时，如何使它们停止发挥联合作用？

我计划在以后的函数中使用read_command。我该如何做，以使我的read_command不会继续打印“ Enter a command：”。我希望将来在if语句中使用此功能。

    #include <stdio.h>
    #include <stdbool.h>

    char read_command()
    {
        char command;
        char result;

        printf("Enter a command: ");
        scanf("%c", &command);
        if (command  == 'I' || command == 'i' || command == 'P' || command == 'p' || command == 'D' || command == 'd' || command == 'L' ||        command == 'l' || command == 'Q'|| command == 'q')

            result = command;
        else

            result = 'V';

        return result;

    }

    void print_music_lib()
    {
       if (read_command == 'P'|| read_command == 'p')
       {
          printf("Only prints when case is P or p");
       }
    }



  void evaluate_command()
  {


       while (true)

       {
           char valid_command = read_command();
           scanf("%c", &valid_command);
           switch(valid_command)

           {
               case 'I':
               case 'i':
                     printf("This is the letter I or i\n");
               case 'P':
               case 'p':
                     printf("This is the letter P or p\n");
               case 'D':
               case 'd':
                     printf("This is the letter D or d\n");
               case 'L':
               case 'l':
                     printf("This is the letter L or l\n");
               case 'Q':
               case 'q':

                     printf("This is the letter Q or q\n");
            //on_quit = valid_command;

    }
    if (valid_command == 'Q' || valid_command == 'q')
         break;
    //return on_quit;
    }
}



   int main()
   {
       read_command();
       //printf("%c", read_command());
       evaluate_command();

       return 0;
   }