Update backup.sh

In case of making a backup to format the card and shoot new photos/videos, I need to be sure to have a valid backup.
I check if there is enough space on storage to make the backup, else shutting down the Pi.
User feedback in that case is not perfect, ACT led is turned off, it should exist a better way to warn the user.

backup.sh

@@ -61,6 +61,15 @@ if [ ! -z $CARD_READER ]; then
   # and use it as a directory name in the backup path
   read -r ID < $CARD_MOUNT_POINT/CARD_ID
   BACKUP_PATH=$STORAGE_MOUNT_POINT/"$ID"
+  
+# Shutdown if there is not enough free space on storage
+STORAGEFREE=$(df -P /dev/sda1 | awk 'NR==2 {print $4}')
+TRANSFERTAMOUNT=$(($(rsync -an --stats $CARD_MOUNT_POINT/ $BACKUP_PATH | awk '/Total transferred file size:/ {print $5}' | sed 's/,//g') /1024 ))
+if (($STORAGEFREE < $TRANSFERTAMOUNT)); then
+sudo sh -c "echo 0 > /sys/class/leds/led0/brightness"
+shutdown -h now
+fi
+  
 # Perform backup using rsync
 rsync -avh $CARD_MOUNT_POINT/ $BACKUP_PATH
 # Turn off the ACT LED to indicate that the backup is completed