parent-dir.sh

#!/usr/bin/env bash
function relativepath() {
    source=$(realpath "$1")
    target=$(realpath "$2")

    common_part=$source # for now
    result="" # for now

    while [[ "${target#$common_part}" == "${target}" ]]; do
        # no match, means that candidate common part is not correct
        # go up one level (reduce common part)
        common_part="$(dirname $common_part)"
        # and record that we went back, with correct / handling
        if [[ -z $result ]]; then
            result=".."
        else
            result="../$result"
        fi
    done

    if [[ $common_part == "/" ]]; then
        # special case for root (no common path)
        result="$result/"
    fi

    # since we now have identified the common part,
    # compute the non-common part
    forward_part="${target#$common_part}"

    # and now stick all parts together
    if [[ -n $result ]] && [[ -n $forward_part ]]; then
        result="$result$forward_part"
    elif [[ -n $forward_part ]]; then
        # extra slash removal
        result="${forward_part:1}"
    fi

    echo $result
}
DIR="$1"
PARENT="$DIR/.."
relativepath "$PWD" "$PARENT"