Moves backups from a primary disk to a secondary disk when primary becomes full. Removes the oldest file from the secondary drive when secondary becomes full.

auto_purge.sh

PRIMARY_DIR=/mnt/primary
SECONDARY_DIR=/mnt/secondary

is_almost_full() {
	DIR_NAME=$1
	cd $DIR_NAME

	# Get percent usage of current directory in bytes
	PERCENT=$(df -B1 `pwd` | awk 'END{print $5}')

	# Find the most recent file
	NEWEST=$(find -type f -printf '%p\n' | sort | tail -n 1)

	# Calculate how big the most recent file is
	NEWEST_BYTES=$(stat -c%s "$NEWEST")

	# How many more bytes do we have remaining in this partition
	REMAINING_DISK_SIZE=$(df -B1 . | awk '{ print $4 }' | tail -n 1)

	# If we assume the next file is the exact same size, how much space
	# would remain in the partition?
	# Add an extra buffer just to make sure.
	BUFFER=0

	SIZE_WITH_ANOTHER_FILE=$(($REMAINING_DISK_SIZE - $NEWEST_BYTES - $BUFFER))

	if [ $SIZE_WITH_ANOTHER_FILE -lt 0 ];
	then
		echo "1"
	else
		echo "0"
	fi
}

get_oldest_file() {
	cd $1
	OLDEST=$(find -type f -printf '%p\n' | sort | head -n 1)
	echo $OLDEST
}

PRIMARY_FULL=$(is_almost_full $PRIMARY_DIR)
SECONDARY_FULL=$(is_almost_full $SECONDARY_DIR)

# Primary drive is full
if [ $PRIMARY_FULL == "1" ]
then
	# Secondary drive is also full
	while [ $SECONDARY_FULL == "1" ]
	do
		# Remove the oldest file in the secondary drive
		cd $SECONDARY_DIR
		OLDEST="$(get_oldest_file $SECONDARY_DIR)"
		echo "Removing oldest file: $OLDEST"
		rm $OLDEST
		SECONDARY_FULL=$(is_almost_full $SECONDARY_DIR)
	done

	# Secondary now has enough space. Move file to secondary
	cd $PRIMARY_DIR
	OLDEST="$(get_oldest_file $PRIMARY_DIR)"
	echo "Moving oldest file to other backup dir $OLDEST -> $SECONDARY_DIR"
	mv $OLDEST $SECONDARY_DIR
fi

echo "Completed moving backups."