diff --git a/mpbb-cleanup b/mpbb-cleanup
index e403225..7f6431f 100644
--- a/mpbb-cleanup
+++ b/mpbb-cleanup
@@ -10,15 +10,15 @@ cleanup-help() {
 
 cleanup() {
     echo "----> Deactivating ports"
-	# $option_prefix is set by mpbb
-	# shellcheck disable=SC2154
+    # $option_prefix is set by mpbb
+    # shellcheck disable=SC2154
     if [ -n "$("${option_prefix}/bin/port" -q installed active)" ]; then
         "${option_prefix}/bin/port" -fp deactivate active
     fi
     echo
     echo "----> Uninstalling obsolete ports"
-	# $thisdir is set by mpbb and points to the directory in which this script resides
-	# shellcheck disable=SC2154
+    # $thisdir is set by mpbb and points to the directory in which this script resides
+    # shellcheck disable=SC2154
     "${option_prefix}/bin/port-tclsh" "${thisdir}/tools/uninstall-old-ports.tcl"
     echo
     echo "----> Deleting distfiles"
diff --git a/mpbb-gather-archives b/mpbb-gather-archives
index c7f6d84..a771428 100644
--- a/mpbb-gather-archives
+++ b/mpbb-gather-archives
@@ -11,8 +11,12 @@ gather-archives-help() {
 
 gather-archives() {
     curl=/usr/bin/curl
+    # $option_prefix is set in mpbb
+    # shellcheck disable=SC2154
     tclsh=${option_prefix}/bin/port-tclsh
 
+    # $option_staging_dir is set in mpbb
+    # shellcheck disable=SC2154
     if [ -d "${option_staging_dir}" ]; then
         find "${option_staging_dir}" -type f -delete -print | sed -E -e "s|^.*/||" -e 's/^/Deleting previously staged archive: /'
         rm -rf "${option_staging_dir}"
@@ -26,7 +30,11 @@ gather-archives() {
         archive_port=$(basename "$(dirname "${archive_path}")")
         archive_basename=$(basename "${archive_path}")
 
+        # $option_archive_site is set in mpbb
+        # shellcheck disable=SC2154
         if ! ${curl} -fIsL "${option_archive_site}/${archive_port}/${archive_basename}" > /dev/null; then
+            # $option_workdir is set in mpbb
+            # shellcheck disable=SC2154
             if "${tclsh}" "${option_workdir}/tools/port_binary_distributable.tcl" -v "${archive_port}"; then
                 echo "Staging archive for upload: ${archive_basename}"
                 mkdir -p "${option_staging_dir}/${archive_port}" || { status=$?; break; }
diff --git a/mpbb-install-dependencies b/mpbb-install-dependencies
index e99d695..f3701d2 100644
--- a/mpbb-install-dependencies
+++ b/mpbb-install-dependencies
@@ -13,10 +13,12 @@ install-dependencies() {
     local dependencies
     local dependencies_count
     local dependencies_counter
+    # $option_logdir is set in mpbb
+    # shellcheck disable=SC2154
     local log_status_dependencies="${option_logdir}/dependencies-progress.txt"
     local log_subports_progress="${option_logdir}/ports-progress.txt"
 
-    if [ -z "${option_port}" ]; then
+    if [ -z "${option_port:-}" ]; then
         errmsg "--port is required"
         return 1
     fi
@@ -26,6 +28,8 @@ install-dependencies() {
     > "$log_status_dependencies"
 
     # calculate list of dependencies in-order
+    # $option_prefix and $thisdir are set in mpbb
+    # shellcheck disable=SC2154
     dependencies=$("${option_prefix}/bin/port-tclsh" "${thisdir}/tools/dependencies.tcl" "${option_port}")
     if [ $? -ne 0 ]; then
         echo "Calculating dependencies for '${option_port}' failed, aborting." >&2
@@ -51,7 +55,7 @@ install-dependencies() {
 
     echo >> "$log_status_dependencies"
 
-    echo "$dependencies" | while read dependency; do
+    echo "$dependencies" | while read -r dependency; do
         # Split portname +variant1+variant2 into portname and variants, where
         # the variants are optional.
         depname=${dependency%% *}
@@ -60,6 +64,8 @@ install-dependencies() {
         text="Installing dependency ($dependencies_counter of $dependencies_count) '${depname}' with variants '${depvariants}'"
         echo "----> ${text}"
         echo -n "${text} ... " >> "$log_status_dependencies"
+        # $depvariants isn't quoted on purpose
+        # shellcheck disable=SC2086
         if ! "${option_prefix}/bin/port" -d install --unrequested "$depname" $depvariants; then
             echo "Build of dependency '${depname}' failed, aborting." >&2
             echo "[FAIL]" >> "$log_status_dependencies"
diff --git a/mpbb-install-port b/mpbb-install-port
index eb5155f..67abcb8 100644
--- a/mpbb-install-port
+++ b/mpbb-install-port
@@ -10,12 +10,14 @@ install-port-help() {
 }
 
 install-port() {
+    # $option_logdir is set in mpbb
+    # shellcheck disable=SC2154
     local log_port_contents="${option_logdir}/port-contents.txt"
     local log_port_stats="${option_logdir}/port-statistics.txt"
     local log_port_main="${option_logdir}/main.log"
     local log_subports_progress="${option_logdir}/ports-progress.txt"
 
-    if [ -z "${option_port}" ]; then
+    if [ -z "${option_port:-}" ]; then
         errmsg "--port is required"
         return 1
     fi
@@ -28,13 +30,17 @@ install-port() {
     # log: summary for the portwatcher
     echo -n "- Installing ${option_port} ... " >> "$log_subports_progress"
 
-    local time_start=$(date +%s)
+    local time_start
+    local time_stop
+    time_start=$(date +%s)
+    # $option_prefix is set in mpbb
+    # shellcheck disable=SC2154
     if ! "${option_prefix}/bin/port" -dk install "${option_port}"; then
         echo "Build of '${option_port}' failed."
         echo "[FAIL]" >> "$log_subports_progress"
         return 1
     fi
-    local time_stop=$(date +%s)
+    time_stop=$(date +%s)
 
     echo "[OK]" >> "$log_subports_progress"
 
@@ -44,22 +50,41 @@ install-port() {
     # TODO: printing statistics (and installing the port + dependencies)
     #       only makes sense when the port hasn't been installed previously
     # log: statistics
-    echo "time:    $(($time_stop - $time_start))s" >> "$log_port_stats"
+    echo "time:    $((time_stop - time_start))s" >> "$log_port_stats"
 
-    local port_workdir=$("${option_prefix}/bin/port" work "${option_port}")
-    local port_workdir_size=$(du -ks "$port_workdir" | sed 's/^ *//' | tr '\t' '\n' | head -n 1)
-    echo "workdir: ${port_workdir_size}k" >> "$log_port_stats"
-    local port_destdir="$port_workdir/destroot"
-    if [ -d "$port_destdir" ]; then 
-        local port_destdir_size=$(du -ks "$port_destdir" | sed 's/^ *//' | tr '\t' '\n' | head -n 1)
-        echo "destdir: ${port_destdir_size}k" >> "$log_port_stats"
-    else
-        echo "destdir: -" >> "$log_port_stats"
+    local port_workdir
+    local port_workdir_size=""
+    local port_destdir_size=""
+    local print_arg_workdir="ERROR"
+    local print_arg_destdir="ERROR"
+    # First, compute port_workdir_size and port_destdir_size
+    port_workdir=$("${option_prefix}/bin/port" work "${option_port}")
+    if [ -n "$port_workdir" ]; then
+        port_workdir_size=$(du -ks "$port_workdir" | sed 's/^ *//' | tr '\t' '\n' | head -n 1)
+        if [ $? -eq 0 ] && [ -n "$port_workdir_size" ]; then
+            print_arg_workdir="${port_workdir_size}k"
+        fi
+
+        local port_destdir="$port_workdir/destroot"
+        # if we arrive here, 'port work $option_port' was successful, so we're
+        # at least going to print 'destdir: -'
+        print_arg_destdir="-"
+        if [ -d "$port_destdir" ]; then 
+            port_destdir_size=$(du -ks "$port_destdir" | sed 's/^ *//' | tr '\t' '\n' | head -n 1)
+            if [ $? -eq 0 ] && [ -n "$port_destdir_size" ]; then
+                print_arg_destdir="${port_destdir_size}k"
+            fi
+        fi
     fi
+    # Then print them, or on error (or if destdir doesn't exist), print the
+    # appropriate message
+    echo "workdir: $print_arg_workdir" >> "$log_port_stats"
+    echo "destdir: $print_arg_destdir" >> "$log_port_stats"
 
     # log: main.log
-    local port_mainlog=$("${option_prefix}/bin/port" logfile "${option_port}")
-    if [ -f $port_mainlog ]; then
+    local port_mainlog
+    port_mainlog=$("${option_prefix}/bin/port" logfile "${option_port}")
+    if [ $? -eq 0 ] && [ -f "$port_mainlog" ]; then
         cp -f "$port_mainlog" "$log_port_main"
     fi
 }
diff --git a/mpbb-list-subports b/mpbb-list-subports
index bff3edc..d4fa80e 100644
--- a/mpbb-list-subports
+++ b/mpbb-list-subports
@@ -18,10 +18,14 @@ print-subports() {
     local exclude
     local exclude_reasons
     local reason
+    # $option_logdir is set in mpbb
+    # shellcheck disable=SC2154
     local log_subports_progress="${option_logdir}/ports-progress.txt"
 
     os_version="$(sw_vers -productVersion | cut -d . -f 1-2)"
     is_64bit_capable="$(sysctl -n hw.cpu64bit_capable)"
+    # $optoin_prefix is set in mpbb
+    # shellcheck disable=SC2154
     ports=$("${option_prefix}/bin/port" -q info --index --line --name "${portname}" "subportof:${portname}" 2>/dev/null) || return $?
     for port in $ports; do
         exclude=0
@@ -31,6 +35,8 @@ print-subports() {
             exclude=1
             exclude_reasons+=("its name contains 'graveyard'")
         else
+            # $thisdir is set in mpbb
+            # shellcheck disable=SC2154
             for portgroup in $("${option_prefix}/bin/port-tclsh" "${thisdir}/tools/portgroups.tcl" "$port"); do
                 if [ "$portgroup" = "obsolete-1.0" ]; then
                     exclude=1
@@ -41,7 +47,7 @@ print-subports() {
 
         if [[ $exclude -eq 0 && "${os_version}" = "10.6" && "${is_64bit_capable}" = "0" ]]; then
             supported_archs=$("${option_prefix}/bin/port-tclsh" "${thisdir}/tools/supported-archs.tcl" "${port}")
-            if [[ -n "${supported_archs}" && !("${supported_archs}" == *"x86_64"* && "${supported_archs}" == *"i386"*) ]]; then
+            if [[ -n "${supported_archs}" && ! ("${supported_archs}" == *"x86_64"* && "${supported_archs}" == *"i386"*) ]]; then
                 exclude=1
                 exclude_reasons+=("the ${os_version}_x86_64 builder will build it")
             fi
@@ -69,7 +75,7 @@ print-subports() {
 list-subports() {
     local log_subports_progress="${option_logdir}/ports-progress.txt"
 
-    if [ $# -le 0 -a -z "${option_port}" ]; then
+    if [ $# -le 0 ] && [ -z "${option_port:-}" ]; then
         errmsg "Either --port or a list of positional arguments with port names is required."
         return 1
     fi
diff --git a/mpbb-selfupdate b/mpbb-selfupdate
index 6f51ab6..fb3001a 100644
--- a/mpbb-selfupdate
+++ b/mpbb-selfupdate
@@ -9,6 +9,8 @@ selfupdate-help() {
 }
 
 selfupdate() {
+    # $option_prefix is set in mpbb
+    # shellcheck disable=SC2154
     if [ ! -f "${option_prefix}/bin/port" ]; then
         macports_version=2.3.4
         macports_distname=MacPorts-${macports_version}
@@ -20,8 +22,8 @@ selfupdate() {
             tar -xjf ${macports_distfile} || return $?
         fi
         cd ${macports_distname} || return $?
-        ./configure --prefix=${option_prefix} --enable-readline || return $?
-        make -j$(sysctl -n hw.activecpu) || return $?
+        ./configure --prefix="${option_prefix}" --enable-readline || return $?
+        make -j"$(sysctl -n hw.activecpu)" || return $?
         make install || return $?
         cd .. || return $?
         rm -rf ${macports_distfile} ${macports_distname} || return $?