#!/bin/sh
# $1 * $2
# count the total decimal places
countd() {
  m="$1"; n=${2:-0}; while [ "$m" ]; do
    : $((n+=1))
    next="${m%?}"
    [ "${m#$next}" = "." ] && {
      break
    } || {
      m="$next"
    }
  done
  echo "$n"
}
dn=$(countd "$2" $(countd "$1")); [ "$dn" -gt 0 ] && : $((dn-=2)) # decimal count
# remove all decimal places
set -- "${1%.*}${1#*.}" "${2%.*}${2#*.}"
m="$2"; n=0; while [ "$m" ]; do
  next="${m%?}" # current is ${m#$next}
  z=""; zn=0; until [ "$zn" -eq "$n" ]; do
    z="${z}0"
    : $((zn+=1))
  done
  o="$((o+$((${m#$next}*$1))$z))"
  m="$next"
  : $((n+=1))
done # long multiplication
# add back decimal point using $dn
n=0; m="$o"; [ "$dn" -gt 0 ] && while [ "$m" ]; do
  next="${m%?}"
  : $((n+=1))
  [ "$n" -eq "$dn" ] && o="$next.${o#${m%?}}"
  m="$next"
done
echo "$o"