realpath.sh/realpath.sh

#!/bin/sh

case "$1" in
/*)
  left="${1#/}"
  ;;
*)
  left="${PWD#/}/$1"
  ;;
esac

result=""

while [ -n "$left" ]; do
  case "$left" in
  */*)
    # Snip one path segment off $left
    seg="${left%%/*}"
    left="${left#*/}"
    ;;
  *)
    # Last path segment already
    seg="$left"
    left=""
    ;;
  esac

  case "$seg" in
  .)
    # Just ignore this segment
    continue
    ;;
  ..)
    # Eat a segment off result
    result="${result%/*}"
    continue
    ;;
  esac

  result="$result/$seg"
  target="$(readlink "$result")"

  case "$target" in
  /*)
    # Absolute symlink. Resets buildup of $result
    left="${target}/${left}"
    result=""
    ;;
  '')
    # Readlink failed, its not a symlink
    # If file does not exist, exit
    [ -e "$result" ] || exit 1
    ;;
  *)
    # Relative symlink: Keep $result, its the symlink base
    left="${target}/${left}"
    result="${result%/*}"
    ;;
  esac

  # Make sure $left never starts with a slash
  left="${left#/}"
done

printf "%s\n" "$result"