shell - shell脚本中变量前的 '#'是什么意思？

Question

有人可以解释一下下面这段 shell 脚本会做什么吗？

END_POS=$((${#column}-$COLON_INDEX))

Answer 1

在此上下文中，它代表该变量值的长度:

$ v="hello"
$ echo ${#v}
5

$ v="bye"
$ echo ${#v}
3

那么这个命令是做什么用的呢？

END_POS=$((${#column}-$COLON_INDEX))

它获取 $column 中值的长度，并使用 $(( )) 语法减去 $COLON_INDEX 中的值执行算术运算:

$ column="hello"
$ colon_index=2
$ r=$((${#column}-$colon_index))   # len("hello") - 2 = 5 - 2
$ echo $r
3

来自 Arithmetic expression :

(( )) without the leading $ is not a standard sh feature. It comes from ksh and is only available in ksh, Bash and zsh. $(( )) substitution is allowed in the POSIX shell. As one would expect, the result of the arithmetic expression inside the $(( )) is substituted into the original command. Like for parameter substitution, arithmetic substitution is subject to word splitting so should be quoted to prevent it when in list contexts.