diff --git a/modules/fasd/functions/fasd b/modules/fasd/functions/fasd
index fc16ee7..b6b33be 100644
--- a/modules/fasd/functions/fasd
+++ b/modules/fasd/functions/fasd
@@ -41,20 +41,20 @@ case $1 in
         fi
 
         # set default options
-        [ -z "$_FASD_DATA" ] && _FASD_DATA="$HOME/.fasd"
-        [ -z "$_FASD_BLACKLIST" ] && _FASD_BLACKLIST="--help"
-        [ -z "$_FASD_SHIFT" ] && _FASD_SHIFT="sudo busybox"
-        [ -z "$_FASD_IGNORE" ] && _FASD_IGNORE="fasd ls echo"
-        [ -z "$_FASD_SINK" ] && _FASD_SINK=/dev/null
-        [ -z "$_FASD_TRACK_PWD" ] && _FASD_TRACK_PWD=1
-        [ -z "$_FASD_MAX" ] && _FASD_MAX=2000
-        [ -z "$_FASD_BACKENDS" ] && _FASD_BACKENDS=native
-        [ -z "$_FASD_FUZZY" ] && _FASD_FUZZY=2
-        [ -z "$_FASD_VIMINFO" ] && _FASD_VIMINFO="$HOME/.viminfo"
-        [ -z "$_FASD_RECENTLY_USED_XBEL" ] && \
+        (( ! "${+_FASD_DATA}" )) && _FASD_DATA="$HOME/.fasd"
+        (( ! "${+_FASD_BLACKLIST}" )) && _FASD_BLACKLIST="--help"
+        (( ! "${+_FASD_SHIFT}" )) && _FASD_SHIFT="sudo busybox"
+        (( ! "${+_FASD_IGNORE}" )) && _FASD_IGNORE="fasd ls echo"
+        (( ! "${+_FASD_SINK}" )) && _FASD_SINK=/dev/null
+        (( ! "${+_FASD_TRACK_PWD}" )) && _FASD_TRACK_PWD=1
+        (( ! "${+_FASD_MAX}" )) && _FASD_MAX=2000
+        (( ! "${+_FASD_BACKENDS}" )) && _FASD_BACKENDS=native
+        (( ! "${+_FASD_FUZZY}" )) && _FASD_FUZZY=2
+        (( ! "${+_FASD_VIMINFO}" )) && _FASD_VIMINFO="$HOME/.viminfo"
+        (( ! "${+_FASD_RECENTLY_USED_XBEL}" )) && \
           _FASD_RECENTLY_USED_XBEL="$HOME/.local/share/recently-used.xbel"
 
-        if [ -z "$_FASD_AWK" ]; then
+        if (( ! "${+_FASD_AWK}" )); then
           # awk preferences
           local awk; for awk in mawk gawk original-awk nawk awk; do
             $awk "" && _FASD_AWK=$awk && break