Define realpath in a way that works like 'readlink -f ...' even on macOS

realpath.sh

_realpath() {
    local return=0
    local result=''
    local arg=${1:-}
    [[ $arg ]] || die "ERROR: _realpath: missing operand"
    local pwd0=$(pwd)
    local base tmp
    cd "$arg";
    base=$(basename "$(pwd)")
    while [[ $base != / ]]; do
      if ! cd .. ; then
        echo "ERROR: $?: Failed to 'cd ..' from $PWD." 1>&2
        return=3
        break
      fi ;
      tmp=$(readlink "$base")
      tmp=${tmp:-$base}
      tmp=${tmp%/}
      if ! cd "$tmp" ; then
        echo "ERROR: $?: Failed to 'cd $tmp' from $PWD." 1>&2
        return=4
        break
      fi ;
      tmp=$(basename "$PWD")
      if ! cd .. ; then
        echo "ERROR: $?: Failed to 'cd ..' from $PWD." 1>&2
        return=5
        break
      fi ;
      result="$tmp${result:+/}$result"
      base=$(basename "$(pwd)")
    done ;
    result="/$result"
    cd "$pwd0" && echo "$result" && return $return
}
die() {
    echo "${1:ERROR}"
    exit ${2:-2}
}
type realpath >/dev/null 2>&1 || realpath() {
    if   type grealpath >/dev/null 2>&1 ; then
      grealpath "$@"
    elif type readlink >/dev/null 2>&1 && readlink -f . >/dev/null 2>&1 ; then
      readlink -f "$@"
    elif type greadlink >/dev/null 2>&1 && greadlink -f . >/dev/null 2>&1 ; then
      greadlink -f "$@"
    else
      _realpath "$@"
    fi ;
}