Problem Statement

Select certain elements from an Array into a new Array in a generic way.

To demonstrate, select all even numbers from an Array. As an option, give a second solution which filters destructively, by modifying the original Array rather than creating a new Array.

Let's start with the solution:

a=(1 2 3 4 5)
unset e[@]
for ((i=0;i<${#a[@]};i++)); do
  [ $((a[$i]%2)) -eq 0 ] && e[$i]="${a[$i]}"
done

a=(1 2 3 4 5)
read -a e -d\n < <(printf '%s\n' "${a[@]}" | grep '[02468]$')

echo "${a[@]}"
echo "${e[@]}"