realpath.sh/realpath.sh

#!/bin/sh
p="${1%/}"
p="${p#/}"

r=""

while [ -n "$p" ]; do
  case "$p" in
  */*)
    seg="${p%%/*}"
    p="${p#*/}"
    ;;
  *)
    seg="$p"
    p=""
    ;;
  esac
  case "$seg" in
  .)
    r="${r%/*}"
    continue
    ;;
  ..)
    r="${r%/*/*}"
    continue
    ;;
  esac
  r="$r/$seg"
  target="$(readlink "$r")"
  case "$target" in
    /*) p="${target}"
        r="";;
    '') ;;
    *) p="${p%/*}/${target}";;
  esac
  p="${p#/}"
done

echo "$r"