语法“ $ {foo ## *。}”如何获取文件扩展名？

看一下man bash（或您正在使用的任何shell）：

   ${parameter##word}
          Remove matching prefix pattern.  The word is expanded to produce
          a pattern just as in pathname expansion.  If the pattern matches
          the  beginning of the value of parameter, then the result of the
          expansion is the expanded value of parameter with  the  shortest
          matching  pattern  (the ``#'' case) or the longest matching pat‐
          tern (the ``##'' case) deleted.  If parameter is  @  or  *,  the
          pattern  removal operation is applied to each positional parame‐
          ter in turn, and the expansion is the resultant list.  If param‐
          eter  is  an array variable subscripted with @ or *, the pattern
          removal operation is applied to each  member  of  the  array  in
          turn, and the expansion is the resultant list.

因此，在您的情况下，它将删除直到最后一个“”的所有内容。并返回剩下的字符串，该字符串恰好是文件的扩展名。