abspath ⇔ relpath (bash)

path.sh

function abspath() {
  if [[ ! -r "${1}" ]]; then
    echo >&2 "No such file or directory: $1"
    return 1
  fi

  if [[ -d "${1}" ]]; then
    echo "$(cd "${1}" || return 1; pwd)"
  else
    echo "$(cd "$(dirname "${1}")" || return 1; pwd)/$(basename "${1}")"
  fi
}

function relpath() {
  if [[ ! -r "${1}" ]]; then
    echo >&2 "No such file or directory: $1"
    return 1
  fi

  if [[ ! -r "${2}" ]]; then
    echo >&2 "No such file or directory: $2"
    return 1
  fi
  
  local relative_path=""
  local p q pi qi pj qj

  p="$(abspath "${1}")"
  q="$(abspath "${2}")"
  
  pi="$(get_distance_from_root_directory "${p}")"
  qi="$(get_distance_from_root_directory "${q}")"

  pj="${pi}"
  qj="${qi}"

  # Calculate the common part from two given paths.
  while [[ "${p}" != "${q}" ]]; do
    if [[ "${pj}" == "${qj}" ]]; then
      relative_path="$(basename "${q}")/${relative_path}"
      p="$(dirname "${p}")"
      q="$(dirname "${q}")"
      pj="$(( pj - 1 ))"
      qj="$(( qj - 1 ))"
    elif (( pj >= qj )); then
      p="$(dirname "${p}")"
      pj="$(( pj - 1 ))"
    else
      relative_path="$(basename "${q}")/${relative_path}"
      q="$(dirname "${q}")"
      qj="$(( qj - 1 ))"
    fi
  done

  if [[ "${pi}" == "${pj}" ]]; then
    relative_path="./${relative_path}"
  fi
  for _ in $(seq "$(( pi - pj ))"); do
    relative_path="../${relative_path}"
  done
  echo "${relative_path%/}"
}

function get_distance_from_root_directory() {
  local distance="0"
  local p q

  p="$(abspath "${1}")"
  q="$(dirname "${p}")"
  while [[ "${p}" != "${q}" ]]; do
    distance="$(( distance + 1 ))"
    p="${q}"
    q="$(dirname "${q}")"
  done
  echo "${distance}"
}