$ {parameter/pattern/string}
模式替换.就像路径名扩展一样，扩展模式以产生模式.扩展参数，并将模式与其值的最长匹配项替换为字符串.如果pattern以/开头，则pattern的所有匹配项都将替换为字符串.通常只替换第一个比赛.如果pattern以#开头，则它必须与参数的扩展值的开头匹配.如果模式以％开头，则它必须在参数的扩展值的末尾匹配.如果string为null，则将删除模式的匹配项，并且可以省略以下/模式.如果参数是 @ 或 * ，则将替换操作依次应用于每个位置参数，并且扩展名是结果列表.如果parameter是一个用 @ 或 * 下标的数组变量，则将替换操作依次应用于该数组的每个成员，并且扩展为结果列表.
Pattern substitution. The pattern is expanded to produce a pattern just as in pathname expansion. Parameter is expanded and the longest match of pattern against its value is replaced with string. If pattern begins with /, all matches of pattern are replaced with string. Normally only the first match is replaced. If pattern begins with #, it must match at the beginning of the expanded value of parameter. If pattern begins with %, it must match at the end of the expanded value of parameter. If string is null, matches of pattern are deleted and the / following pattern may be omitted. If parameter is @ or *, the substitution 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 substitution operation is applied to each member of the array in turn, and the expansion is the resultant list.
也就是说，将 $ {something//，/} 扩展为 $ something ，并删除所有出现的 .
That is, ${something//,/} is expanded to the $something with all the occurrences of , removed.
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