What does the for loop do in this bash function?

Question

linux

Good day! Help me understand what is happening in the for loop (implementation of the Luhn algorithm for bank cards). Thanks

function validate {
num=$1 

   len=${#num} 

    is_odd=1 
    sum=0
     
    for((t = len - 1; t >= 0; --t)) {
        digit=${num:$t:1} #????

        if ; then 
            sum=$(( sum + $digit ))
        else
            sum=$(( $sum + ( $digit != 9 ? ( ( 2 * $digit ) % 9 ) : 9 ) ))
        fi

        is_odd=$(( ! $is_odd ))
    }

    # NOTE: returning exit status of 0 on success
    return $(( 0 != ( $sum % 10 ) ))
}

Answer 1

What does the for loop do in this bash function?

It simply takes each character from num in reverse order.

[[email protected] ~]$ s=abcd
[gues[email protected] ~]$ echo "${s:0:3} ${s:0:2} ${s:2:1}"
abc ab c
[[email protected] ~]$

Answer 2

The suma variable is not declared.