Bash script as altering the mass element

[Trenton]

How can you replace the element in bash? For example, if the third element in the volume number 3 needs to be replaced by 2 equal to 3-1, it makes a mistake.

[Kadyn]

#! /bin/bash

$ declare -a arr=(a b c)

$ echo ${arr[*]@A}
declare -a arr=([0]="a" [1]="b" [2]="c")

$ echo ${arr[2]}
c

$ declare -i IDX=3
$ echo ${arr[$(( IDX - 1 ))]}
c

$ arr[$(( IDX - 1 ))]="d"
$ echo ${arr[$(( IDX - 1 ))]}
d
$ echo ${arr[*]@A}
declare -a arr=([0]="a" [1]="b" [2]="d")