if [ "${1##*.}" = "tar" ] 如何理解这句话？

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代码:

       ${parameter#word}
       ${parameter##word}
              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  pattern  (the
              ``##''  case) deleted.  If parameter is @ or *, the
              pattern removal operation is applied to each  posi-
              tional  parameter in turn, and the expansion is the
              resultant list.  If parameter is an array  variable
              subscripted with @ or *, the pattern removal opera-
              tion is applied to each  member  of  the  array  in
              turn, and the expansion is the resultant list.