How to reverse array in bash onliner FOR loop?

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You can use the C-style for loop:

for (( idx=${#MYARRAY[@]}-1 ; idx>=0 ; idx-- )) ; do
    echo "${MYARRAY[idx]}"
done

For an array with “holes”, the number of elements ${#arr[@]} doesn’t correspond to the index of the last element. You can create another array of indices and walk it backwards in the same way:

#! /bin/bash
arr[2]=a
arr[7]=b

echo ${#arr[@]}  # only 2!!

indices=( ${!arr[@]} )
for ((i=${#indices[@]} - 1; i >= 0; i--)) ; do
    echo "${arr[indices[i]]}"
done

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