diff --git a/tarback.sh b/tarback.sh
index 6b12d1b..67d6e45 100755
--- a/tarback.sh
+++ b/tarback.sh
@@ -1,27 +1,33 @@
 #!/bin/sh
 
-set -e
+set -eu
 
 XDG_DATA_HOME="${XDG_DATA_HOME:-$HOME/.local/share}"
 XDG_DATA_DIRS="${XDG_DATA_DIRS:-/usr/local/share:/usr/share}"
 
+TARBACK_REMOTE_DEFAULT=
+TARBACK_TRANSFORM_REMOTE_COMMAND_DEFAULT=
+# shellcheck disable=SC2016 # disable variable not expanding hint
+TARBACK_TAR_CREATE_COMMAND_DEFAULT='tar c -C "$(dirname "$1")" "$(basename "$1")"'
+# shellcheck disable=SC2016 # disable variable not expanding hint
+TARBACK_TAR_EXTRACT_COMMAND_DEFAULT='tar x -C "$(dirname "$1")"'
+TARBACK_COMPRESSION_DEFAULT='xz'
+TARBACK_SPLIT_DEFAULT="split -b $((128*1024*1024)) -"
+TARBACK_MERGE_DEFAULT='cat'
+TARBACK_PLUGIN_SEARCH_PATH_DEFAULT="$(dirname "$0"):$XDG_DATA_HOME:$XDG_DATA_DIRS"
+
+
 # example: TARBACK_REMOTE="ssh myuser@myserver" ./tarback ...
-TARBACK_REMOTE="${TARBACK_REMOTE:-}"
+TARBACK_REMOTE="${TARBACK_REMOTE:-$TARBACK_REMOTE_DEFAULT}"
 # maybe the command needs to be transformed before being passed to the
 # remote command, see _transform_ssh_command for an example
-TARBACK_TRANSFORM_REMOTE_COMMAND="${TARBACK_TRANSFORM_REMOTE_COMMAND:-}"
-# shellcheck disable=SC2016 # disable variable not expanding hint
-[ -z "$TARBACK_TAR_CREATE_COMMAND" ] \
-    && TARBACK_TAR_CREATE_COMMAND='tar c -C "$(dirname "$1")" "$(basename "$1")"'
-# shellcheck disable=SC2016 # disable variable not expanding hint
-[ -z "$TARBACK_TAR_EXTRACT_COMMAND" ] \
-    && TARBACK_TAR_EXTRACT_COMMAND='tar x -C "$(dirname "$1")"'
-[ -z "$TARBACK_COMPRESSION" ] && TARBACK_COMPRESSION="xz"
-[ -z "$TARBACK_SPLIT" ] && TARBACK_SPLIT="split -b $((128*1024*1024)) -"
-[ -z "$TARBACK_MERGE" ] && TARBACK_MERGE='cat'
-[ -z "$TARBACK_PLUGIN_SEARCH_PATH" ] \
-    && TARBACK_PLUGIN_SEARCH_PATH="$(dirname "$0"):$XDG_DATA_HOME:$XDG_DATA_DIRS"
-
+TARBACK_TRANSFORM_REMOTE_COMMAND="${TARBACK_TRANSFORM_REMOTE_COMMAND:-$TARBACK_TRANSFORM_REMOTE_COMMAND_DEFAULT}"
+TARBACK_TAR_CREATE_COMMAND="${TARBACK_TAR_CREATE_COMMAND:-$TARBACK_TAR_CREATE_COMMAND_DEFAULT}"
+TARBACK_TAR_EXTRACT_COMMAND="${TARBACK_TAR_EXTRACT_COMMAND:-$TARBACK_TAR_EXTRACT_COMMAND_DEFAULT}"
+TARBACK_COMPRESSION="${TARBACK_COMPRESSION:-$TARBACK_COMPRESSION_DEFAULT}"
+TARBACK_SPLIT="${TARBACK_SPLIT:-$TARBACK_SPLIT_DEFAULT}"
+TARBACK_MERGE="${TARBACK_MERGE:-$TARBACK_MERGE_DEFAULT}"
+TARBACK_PLUGIN_SEARCH_PATH="${TARBACK_PLUGIN_SEARCH_PATH:-$TARBACK_PLUGIN_SEARCH_PATH_DEFAULT}"
 TARBACK_PLUGIN_SEARCH_PATH="$(echo "$TARBACK_PLUGIN_SEARCH_PATH" | tr ':'  '\n')"
 
 _transform_split_name() {