but用作关系代词的用法问题

but 用作关系代词，等于that (或 who, which)... not。主句中通常有否定词（如 no, none 等）或者疑问词。例如：
No one is in the world but knows the Great Wall. 世上无人不知长城。
but 在从句中作主语。but knows 相当于 who doesn't know 。这样，整个句子就是双重否定，语气强于一般肯定句，有强烈的感染力。又如：
There is no one of them but is ready to help the people in trouble. 他们个个都乐意帮助有困难的人。(but is ready 相当于 who is not ready)
Who but knows it? (=Who [is there] that does not know it?) 谁不知道呢？