do not rely on ${parameter:offset:length} substring expansion not supported by dash

.tmux.conf

@@ -410,7 +410,7 @@ run 'cut -c3- ~/.tmux.conf | sh -s apply_configuration'
 #
 #   if [ x"$battery_status" = x"1" ] || [ x"$battery_status" = x"true" ] || [ x"$battery_status" = x"enabled" ] ; then
 #     if [ x"$discharging" = x"true" ]; then
-#       [ x"${uname_s:0:6}" != x"CYGWIN" ] && printf "%s " 🔋
+#       echo "$uname_s" | grep -q -v "CYGWIN" && printf "%s " 🔋
 #     else
 #       printf "%s " ⚡
 #     fi