diff --git a/scripts/config b/scripts/config
index 567120a..2283be2 100755
--- a/scripts/config
+++ b/scripts/config
@@ -62,15 +62,52 @@
 	fi
 }
 
+txt_append() {
+	local anchor="$1"
+	local insert="$2"
+	local infile="$3"
+	local tmpfile="$infile.swp"
+
+	# sed append cmd: 'a\' + newline + text + newline
+	cmd="$(printf "a\\%b$insert" "\n")"
+
+	sed -e "/$anchor/$cmd" "$infile" >"$tmpfile"
+	# replace original file with the edited one
+	mv "$tmpfile" "$infile"
+}
+
+txt_subst() {
+	local before="$1"
+	local after="$2"
+	local infile="$3"
+	local tmpfile="$infile.swp"
+
+	sed -e "s/$before/$after/" "$infile" >"$tmpfile"
+	# replace original file with the edited one
+	mv "$tmpfile" "$infile"
+}
+
+txt_delete() {
+	local text="$1"
+	local infile="$2"
+	local tmpfile="$infile.swp"
+
+	sed -e "/$text/d" "$infile" >"$tmpfile"
+	# replace original file with the edited one
+	mv "$tmpfile" "$infile"
+}
+
 set_var() {
 	local name=$1 new=$2 before=$3
 
 	name_re="^($name=|# $name is not set)"
 	before_re="^($before=|# $before is not set)"
 	if test -n "$before" && grep -Eq "$before_re" "$FN"; then
-		sed -ri "/$before_re/a $new" "$FN"
+		txt_append "^$before=" "$new" "$FN"
+		txt_append "^# $before is not set" "$new" "$FN"
 	elif grep -Eq "$name_re" "$FN"; then
-		sed -ri "s:$name_re.*:$new:" "$FN"
+		txt_subst "^$name=.*" "$new" "$FN"
+		txt_subst "^# $name is not set" "$new" "$FN"
 	else
 		echo "$new" >>"$FN"
 	fi
@@ -79,7 +116,8 @@
 undef_var() {
 	local name=$1
 
-	sed -ri "/^($name=|# $name is not set)/d" "$FN"
+	txt_delete "^$name=" "$FN"
+	txt_delete "^# $name is not set" "$FN"
 }
 
 if [ "$1" = "--file" ]; then