diff --git a/setup/preflight.sh b/setup/preflight.sh
index bd6d65b7..2105ebec 100644
--- a/setup/preflight.sh
+++ b/setup/preflight.sh
@@ -1,3 +1,4 @@
+#!/bin/bash
 # Are we running as root?
 if [[ $EUID -ne 0 ]]; then
 	echo "This script must be run as root. Please re-run like this:"
@@ -26,16 +27,16 @@ fi
 #
 # Skip the check if we appear to be running inside of Vagrant, because that's really just for testing.
 TOTAL_PHYSICAL_MEM=$(head -n 1 /proc/meminfo | awk '{print $2}')
-if [ $TOTAL_PHYSICAL_MEM -lt 490000 ]; then
+if [ "$TOTAL_PHYSICAL_MEM" -lt 490000 ]; then
 if [ ! -d /vagrant ]; then
-	TOTAL_PHYSICAL_MEM=$(expr \( \( $TOTAL_PHYSICAL_MEM \* 1024 \) / 1000 \) / 1000)
+	TOTAL_PHYSICAL_MEM=$(( TOTAL_PHYSICAL_MEM * 1024 / 1000 / 1000 ))
 	echo "Your Mail-in-a-Box needs more memory (RAM) to function properly."
 	echo "Please provision a machine with at least 512 MB, 1 GB recommended."
 	echo "This machine has $TOTAL_PHYSICAL_MEM MB memory."
 	exit
 fi
 fi
-if [ $TOTAL_PHYSICAL_MEM -lt 750000 ]; then
+if [ "$TOTAL_PHYSICAL_MEM" -lt 750000 ]; then
 	echo "WARNING: Your Mail-in-a-Box has less than 768 MB of memory."
 	echo "         It might run unreliably when under heavy load."
 fi