evaluator.sh

#!/bin/bash

#disable globbing of the *
set -f

INPUT=$1

SUBBED=($(
  sed \
  -e "s,multiply,*,g" \
  -e "s,divide,/,g" \
  -e "s,add,+,g" \
  -e "s,subtract,-,g" \
  -e "s,apply,,g" \
  -e "s, *,,g" \
  $INPUT
))

EXPR=""

for (( idx=${#SUBBED[@]}-1 ; idx>=0 ; idx-- )) ; do
  LINE="${SUBBED[idx]}"
  EXPR="($EXPR $LINE)"
done

echo $EXPR
echo $(($EXPR))