diff --git a/setup/firstuser.sh b/setup/firstuser.sh
index d24fc370..71264f3b 100644
--- a/setup/firstuser.sh
+++ b/setup/firstuser.sh
@@ -6,7 +6,7 @@ if [ -z "`tools/mail.py user`" ]; then
 	# If we didn't ask for an email address at the start, do so now.
 	if [ -z "$EMAIL_ADDR" ]; then
 		# In an interactive shell, ask the user for an email address.
-		if [ -z "$NONINTERACTIVE" ]; then
+		if [ -z "${NONINTERACTIVE:-}" ]; then
 			input_box "Mail Account" \
 				"Let's create your first mail account.
 				\n\nWhat email address do you want?" \
diff --git a/setup/functions.sh b/setup/functions.sh
index 75c6821e..1a74edfd 100644
--- a/setup/functions.sh
+++ b/setup/functions.sh
@@ -1,3 +1,9 @@
+# Turn on "strict mode." See http://redsymbol.net/articles/unofficial-bash-strict-mode/.
+# -e: exit if any command unexpectedly fails.
+# -u: exit if we have a variable typo.
+# -o pipefail: don't ignore errors in the non-last command in a pipeline 
+set -euo pipefail
+
 function hide_output {
 	# This function hides the output of a command unless the command fails
 	# and returns a non-zero exit code.
@@ -5,11 +11,14 @@ function hide_output {
 	# Get a temporary file.
 	OUTPUT=$(tempfile)
 
-	# Execute command, redirecting stderr/stdout to the temporary file.
+	# Execute command, redirecting stderr/stdout to the temporary file. Since we
+	# check the return code ourselves, disable 'set -e' temporarily.
+	set +e
 	$@ &> $OUTPUT
+	E=$?
+	set -e
 
 	# If the command failed, show the output that was captured in the temporary file.
-	E=$?
 	if [ $E != 0 ]; then
 		# Something failed.
 		echo
@@ -75,7 +84,7 @@ function get_publicip_from_web_service {
 	#
 	# Pass '4' or '6' as an argument to this function to specify
 	# what type of address to get (IPv4, IPv6).
-	curl -$1 --fail --silent --max-time 15 icanhazip.com 2>/dev/null
+	curl -$1 --fail --silent --max-time 15 icanhazip.com 2>/dev/null || /bin/true
 }
 
 function get_default_privateip {
@@ -131,11 +140,10 @@ function get_default_privateip {
 	fi
 
 	echo $address
-		
 }
 
 function ufw_allow {
-	if [ -z "$DISABLE_FIREWALL" ]; then
+	if [ -z "${DISABLE_FIREWALL:-}" ]; then
 		# ufw has completely unhelpful output
 		ufw allow $1 > /dev/null;
 	fi
@@ -154,10 +162,13 @@ function input_box {
 	# input_box "title" "prompt" "defaultvalue" VARIABLE
 	# The user's input will be stored in the variable VARIABLE.
 	# The exit code from dialog will be stored in VARIABLE_EXITCODE.
+	# Temporarily turn off 'set -e' because we need the dialog return code.
 	declare -n result=$4
 	declare -n result_code=$4_EXITCODE
+	set +e
 	result=$(dialog --stdout --title "$1" --inputbox "$2" 0 0 "$3")
 	result_code=$?
+	set -e
 }
 
 function input_menu {
@@ -167,8 +178,10 @@ function input_menu {
 	declare -n result=$4
 	declare -n result_code=$4_EXITCODE
 	local IFS=^$'\n'
+	set +e
 	result=$(dialog --stdout --title "$1" --menu "$2" 0 0 0 $3)
 	result_code=$?
+	set -e
 }
 
 function wget_verify {
diff --git a/setup/munin.sh b/setup/munin.sh
index 2529ba4c..c581c6d0 100755
--- a/setup/munin.sh
+++ b/setup/munin.sh
@@ -29,7 +29,7 @@ address 127.0.0.1
 
 # send alerts to the following address
 contacts admin
-contact.admin.command mail -s "Munin notification ${var:host}" administrator@$PRIMARY_HOSTNAME
+contact.admin.command mail -s "Munin notification \${var:host}" administrator@$PRIMARY_HOSTNAME
 contact.admin.always_send warning critical
 EOF
 
diff --git a/setup/preflight.sh b/setup/preflight.sh
index 7466b857..d087efe2 100644
--- a/setup/preflight.sh
+++ b/setup/preflight.sh
@@ -41,7 +41,7 @@ if [ $TOTAL_PHYSICAL_MEM -lt 750000 ]; then
 fi
 
 # Check that tempfs is mounted with exec
-MOUNTED_TMP_AS_NO_EXEC=$(grep "/tmp.*noexec" /proc/mounts)
+MOUNTED_TMP_AS_NO_EXEC=$(grep "/tmp.*noexec" /proc/mounts || /bin/true)
 if [ -n "$MOUNTED_TMP_AS_NO_EXEC" ]; then
 	echo "Mail-in-a-Box has to have exec rights on /tmp, please mount /tmp with exec"
 	exit
diff --git a/setup/questions.sh b/setup/questions.sh
index 3d227d81..3ad7eead 100644
--- a/setup/questions.sh
+++ b/setup/questions.sh
@@ -1,4 +1,4 @@
-if [ -z "$NONINTERACTIVE" ]; then
+if [ -z "${NONINTERACTIVE:-}" ]; then
 	# Install 'dialog' so we can ask the user questions. The original motivation for
 	# this was being able to ask the user for input even if stdin has been redirected,
 	# e.g. if we piped a bootstrapping install script to bash to get started. In that
@@ -25,8 +25,8 @@ if [ -z "$NONINTERACTIVE" ]; then
 fi
 
 # The box needs a name.
-if [ -z "$PRIMARY_HOSTNAME" ]; then
-	if [ -z "$DEFAULT_PRIMARY_HOSTNAME" ]; then
+if [ -z "${PRIMARY_HOSTNAME:-}" ]; then
+	if [ -z "${DEFAULT_PRIMARY_HOSTNAME:-}" ]; then
 		# We recommend to use box.example.com as this hosts name. The
 		# domain the user possibly wants to use is example.com then.
 		# We strip the string "box." from the hostname to get the mail
@@ -86,7 +86,7 @@ fi
 # If the machine is behind a NAT, inside a VM, etc., it may not know
 # its IP address on the public network / the Internet. Ask the Internet
 # and possibly confirm with user.
-if [ -z "$PUBLIC_IP" ]; then
+if [ -z "${PUBLIC_IP:-}" ]; then
 	# Ask the Internet.
 	GUESSED_IP=$(get_publicip_from_web_service 4)
 
@@ -105,11 +105,11 @@ if [ -z "$PUBLIC_IP" ]; then
 		PUBLIC_IP=$GUESSED_IP
 	fi
 
-	if [ -z "$PUBLIC_IP" ]; then
+	if [ -z "${PUBLIC_IP:-}" ]; then
 		input_box "Public IP Address" \
 			"Enter the public IP address of this machine, as given to you by your ISP.
 			\n\nPublic IP address:" \
-			$DEFAULT_PUBLIC_IP \
+			${DEFAULT_PUBLIC_IP:-} \
 			PUBLIC_IP
 
 		if [ -z "$PUBLIC_IP" ]; then
@@ -121,27 +121,27 @@ fi
 
 # Same for IPv6. But it's optional. Also, if it looks like the system
 # doesn't have an IPv6, don't ask for one.
-if [ -z "$PUBLIC_IPV6" ]; then
+if [ -z "${PUBLIC_IPV6:-}" ]; then
 	# Ask the Internet.
 	GUESSED_IP=$(get_publicip_from_web_service 6)
 	MATCHED=0
-	if [[ -z "$DEFAULT_PUBLIC_IPV6" && ! -z "$GUESSED_IP" ]]; then
+	if [[ -z "${DEFAULT_PUBLIC_IPV6:-}" && ! -z "$GUESSED_IP" ]]; then
 		PUBLIC_IPV6=$GUESSED_IP
-	elif [[ "$DEFAULT_PUBLIC_IPV6" == "$GUESSED_IP" ]]; then
+	elif [[ "${DEFAULT_PUBLIC_IPV6:-}" == "$GUESSED_IP" ]]; then
 		# No IPv6 entered and machine seems to have none, or what
 		# the user entered matches what the Internet tells us.
 		PUBLIC_IPV6=$GUESSED_IP
 		MATCHED=1
-	elif [[ -z "$DEFAULT_PUBLIC_IPV6" ]]; then
+	elif [[ -z "${DEFAULT_PUBLIC_IPV6:-}" ]]; then
 		DEFAULT_PUBLIC_IP=$(get_default_privateip 6)
 	fi
 
-	if [[ -z "$PUBLIC_IPV6" && $MATCHED == 0 ]]; then
+	if [[ -z "${PUBLIC_IPV6:-}" && $MATCHED == 0 ]]; then
 		input_box "IPv6 Address (Optional)" \
 			"Enter the public IPv6 address of this machine, as given to you by your ISP.
 			\n\nLeave blank if the machine does not have an IPv6 address.
 			\n\nPublic IPv6 address:" \
-			$DEFAULT_PUBLIC_IPV6 \
+			${DEFAULT_PUBLIC_IPV6:-} \
 			PUBLIC_IPV6
 
 		if [ ! $PUBLIC_IPV6_EXITCODE ]; then
@@ -154,10 +154,10 @@ fi
 # Get the IP addresses of the local network interface(s) that are connected
 # to the Internet. We need these when we want to have services bind only to
 # the public network interfaces (not loopback, not tunnel interfaces).
-if [ -z "$PRIVATE_IP" ]; then
+if [ -z "${PRIVATE_IP:-}" ]; then
 	PRIVATE_IP=$(get_default_privateip 4)
 fi
-if [ -z "$PRIVATE_IPV6" ]; then
+if [ -z "${PRIVATE_IPV6:-}" ]; then
 	PRIVATE_IPV6=$(get_default_privateip 6)
 fi
 if [[ -z "$PRIVATE_IP" && -z "$PRIVATE_IPV6" ]]; then
@@ -186,11 +186,11 @@ fi
 
 # Set STORAGE_USER and STORAGE_ROOT to default values (user-data and /home/user-data), unless
 # we've already got those values from a previous run.
-if [ -z "$STORAGE_USER" ]; then
-	STORAGE_USER=$([[ -z "$DEFAULT_STORAGE_USER" ]] && echo "user-data" || echo "$DEFAULT_STORAGE_USER")
+if [ -z "${STORAGE_USER:-}" ]; then
+	STORAGE_USER=$([[ -z "${DEFAULT_STORAGE_USER:-}" ]] && echo "user-data" || echo "$DEFAULT_STORAGE_USER")
 fi
-if [ -z "$STORAGE_ROOT" ]; then
-	STORAGE_ROOT=$([[ -z "$DEFAULT_STORAGE_ROOT" ]] && echo "/home/$STORAGE_USER" || echo "$DEFAULT_STORAGE_ROOT")
+if [ -z "${STORAGE_ROOT:-}" ]; then
+	STORAGE_ROOT=$([[ -z "${DEFAULT_STORAGE_ROOT:-}" ]] && echo "/home/$STORAGE_USER" || echo "$DEFAULT_STORAGE_ROOT")
 fi
 
 # Show the configuration, since the user may have not entered it manually.
diff --git a/setup/start.sh b/setup/start.sh
index 671f4449..0b145022 100755
--- a/setup/start.sh
+++ b/setup/start.sh
@@ -60,8 +60,8 @@ source setup/questions.sh
 # Run some network checks to make sure setup on this machine makes sense.
 # Skip on existing installs since we don't want this to block the ability to
 # upgrade, and these checks are also in the control panel status checks.
-if [ -z "$DEFAULT_PRIMARY_HOSTNAME" ]; then
-if [ -z "$SKIP_NETWORK_CHECKS" ]; then
+if [ -z "${DEFAULT_PRIMARY_HOSTNAME:-}" ]; then
+if [ -z "${SKIP_NETWORK_CHECKS:-}" ]; then
 	source setup/network-checks.sh
 fi
 fi
diff --git a/setup/system.sh b/setup/system.sh
index 2fecac8e..2db48298 100755
--- a/setup/system.sh
+++ b/setup/system.sh
@@ -37,9 +37,9 @@ hostname $PRIMARY_HOSTNAME
 # for reference
 
 SWAP_MOUNTED=$(cat /proc/swaps | tail -n+2)
-SWAP_IN_FSTAB=$(grep "swap" /etc/fstab)
-ROOT_IS_BTRFS=$(grep "\/ .*btrfs" /proc/mounts)
-TOTAL_PHYSICAL_MEM=$(head -n 1 /proc/meminfo | awk '{print $2}')
+SWAP_IN_FSTAB=$(grep "swap" /etc/fstab || /bin/true)
+ROOT_IS_BTRFS=$(grep "\/ .*btrfs" /proc/mounts || /bin/true)
+TOTAL_PHYSICAL_MEM=$(head -n 1 /proc/meminfo | awk '{print $2}' || /bin/true)
 AVAILABLE_DISK_SPACE=$(df / --output=avail | tail -n 1)
 if
 	[ -z "$SWAP_MOUNTED" ] &&
@@ -143,8 +143,8 @@ fi
 # section) and syslog (see #328). There might be other issues, and it's
 # not likely the user will want to change this, so we only ask on first
 # setup.
-if [ -z "$NONINTERACTIVE" ]; then
-	if [ ! -f /etc/timezone ] || [ ! -z $FIRST_TIME_SETUP ]; then
+if [ -z "${NONINTERACTIVE:-}" ]; then
+	if [ ! -f /etc/timezone ] || [ ! -z ${FIRST_TIME_SETUP:-} ]; then
 		# If the file is missing or this is the user's first time running
 		# Mail-in-a-Box setup, run the interactive timezone configuration
 		# tool.
@@ -239,7 +239,7 @@ EOF
 # Various virtualized environments like Docker and some VPSs don't provide #NODOC
 # a kernel that supports iptables. To avoid error-like output in these cases, #NODOC
 # we skip this if the user sets DISABLE_FIREWALL=1. #NODOC
-if [ -z "$DISABLE_FIREWALL" ]; then
+if [ -z "${DISABLE_FIREWALL:-}" ]; then
 	# Install `ufw` which provides a simple firewall configuration.
 	apt_install ufw