source: vendor/bash/3.1/examples/functions/array-stuff

# usage: reverse arrayname
reverse()
{
        local -a R
        local -i i
        local rlen temp

        # make r a copy of the array whose name is passed as an arg
        eval R=\( \"\$\{$1\[@\]\}\" \)

        # reverse R
        rlen=${#R[@]}

        for ((i=0; i < rlen/2; i++ ))
        do
                temp=${R[i]}
                R[i]=${R[rlen-i-1]}
                R[rlen-i-1]=$temp
        done

        # and assign R back to array whose name is passed as an arg
        eval $1=\( \"\$\{R\[@\]\}\" \)
}

A=(1 2 3 4 5 6 7)
echo "${A[@]}"
reverse A
echo "${A[@]}"
reverse A
echo "${A[@]}"

# unset last element of A
alen=${#A[@]}
unset A[$alen-1]
echo "${A[@]}"

# ashift -- like shift, but for arrays

ashift()
{
        local -a R
        local n

        case $# in
        1)      n=1 ;;
        2)      n=$2 ;;
        *)      echo "$FUNCNAME: usage: $FUNCNAME array [count]" >&2
                exit 2;;
        esac

        # make r a copy of the array whose name is passed as an arg
        eval R=\( \"\$\{$1\[@\]\}\" \)

        # shift R
        R=( "${R[@]:$n}" )

        # and assign R back to array whose name is passed as an arg
        eval $1=\( \"\$\{R\[@\]\}\" \)
}

ashift A 2
echo "${A[@]}"

ashift A
echo "${A[@]}"

ashift A 7
echo "${A[@]}"

# Sort the members of the array whose name is passed as the first non-option
# arg.  If -u is the first arg, remove duplicate array members.
array_sort()
{
        local -a R
        local u

        case "$1" in
        -u)     u=-u ; shift ;;
        esac

        if [ $# -eq 0 ]; then
                echo "array_sort: argument expected" >&2
                return 1
        fi

        # make r a copy of the array whose name is passed as an arg
        eval R=\( \"\$\{$1\[@\]\}\" \)

        # sort R
        R=( $( printf "%s\n" "${A[@]}" | sort $u) )

        # and assign R back to array whose name is passed as an arg
        eval $1=\( \"\$\{R\[@\]\}\" \)
        return 0
}

A=(3 1 4 1 5 9 2 6 5 3 2)
array_sort A
echo "${A[@]}"

A=(3 1 4 1 5 9 2 6 5 3 2)
array_sort -u A
echo "${A[@]}"