实现文件复制：完成一个目录复制命令，包括目录下的文件和子目录。学习和掌握操作系统中与文件操作相关的概念，并使用系统提供的API编程实现。-Implementation file: a directory to complete the copy command, including the directory files and subdirectories. Learn and master the operating system and file operation related concepts, and using the system provided API programming.