#!/bin/bash

FOUR=4
FIVE=5

SUM="$FOUR + $FIVE"
let NINE="$FOUR + $FIVE"

echo '$FOUR + $FIVE'
echo "$FOUR + $FIVE"
echo

echo '$(($FOUR + $FIVE))'
echo "$(($FOUR + $FIVE))"
echo

echo '$SUM'
echo "$SUM"
echo

echo '$NINE'
echo "$NINE"
echo