#!/bin/sh
relativepath() (
  # both $1 and $2 are absolute paths beginning with /
  # returns relative path to $2 from $1
  source="$1"
  target="$2"

  commonPart="$source"
  result=""

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

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

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

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

  echo "$result"
)
relativepath "$@"