...
  ${parameter%word}
  ${parameter%%word}
              The word is expanded to produce a pattern just  as  in  pathname
              expansion.   If  the  pattern  matches a trailing portion of the
              expanded value of parameter, then the result of the expansion is
              the  expanded value of parameter with the shortest matching pat-
              tern (the ``%'' case)  or  the  longest  matching  pattern  (the
              ``%%''  case)  deleted.   If  parameter  is  @ or *, the pattern
              removal operation is applied to  each  positional  parameter  in
              turn,  and the expansion is the resultant list.  If parameter 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.
...