bash - Bash 参数扩展中的 `//` 是什么意思？

Question

//,/ 在此命令中是什么意思？

echo ${foo//,/}

Answer 1

来自 bash(1) 手册页(http://linux.die.net/man/1/bash):

${parameter/pattern/string}

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，${something//, 被扩展为 $something，所有出现的 , 都被移除。