#!/bin/bash

# usage: relpath from to

if [[ "$1" == "$2" ]]
then
    echo "."
    exit
fi

IFS="/"

current=($1)
absolute=($2)

abssize=${#absolute[@]}
cursize=${#current[@]}

while [[ ${absolute[$level]} == ${current[$level]} ]]
do
    (( level++ ))
    if (( level > abssize || level > cursize ))
    then
        break
    fi
done

for ((i = level; i < cursize; i++))
do
    if ((i > level))
    then
        newpath=$newpath"/"
    fi
    newpath=$newpath".."
done

for ((i = level; i < abssize; i++))
do
    if [[ -n $newpath ]]
    then
        newpath=$newpath"/"
    fi
    newpath=$newpath${absolute[i]}
done

echo "$newpath"