polybar: Improve headset battery script

home-manager/modules/polybar/default.nix

@@ -44,16 +44,21 @@ with lib; let
     };
 
   headset-battery = pkgs.writeShellScript "headset-battery" ''
-    battery_level=$(${pkgs.headsetcontrol}/bin/headsetcontrol -c -b)
+    battery_level=$(${pkgs.headsetcontrol}/bin/headsetcontrol -c -b 2> /dev/null)
+
+    if [[ $? -ne 0 ]]; then
+      printf "󰂑"
+      exit
+    fi
 
     battery_icons=(󰁺 󰁻 󰁼 󰁽 󰁾 󰁿 󰂀 󰂁 󰂂 󰁹 )
     battery_charging="󰂄"
 
-    if [ "''${battery_level}" -eq -1 ]; then
+    if [[ "''${battery_level}" -eq -1 ]]; then
       printf "%s " "''${battery_charging}"
     else
       idx=$(((battery_level / 10)))
-      if [ "''${idx}" -eq 10 ]; then
+      if [[ "''${idx}" -eq 10 ]]; then
         idx=9
       fi