Script to keep differential backups of your Dropbox with a maximum storage quota using rclone. Uses hardlinks to save space.

dropbox-differential-backup.sh

#!/bin/bash

# Configuration
RCLONE_REMOTE="dropbox:/"                    # Adjust this if you don't want the full Dropbox
BACKUP_DIR="/path/to/backups"                # Adjust to your local backup directory
SNAPSHOT_LIMIT=500000                        # Max usage in MB (500 GB)

# Create backup directory if it doesn't exist
mkdir -p "$BACKUP_DIR"

# Function to calculate disk usage (including 'backup-main')
calculate_usage() {
    du -sm "$BACKUP_DIR" 2>/dev/null | awk '{print $1}'
}

# Step 1: Sync Dropbox to backup-main
echo "Starting rclone sync..."
if ! rclone sync "$RCLONE_REMOTE" "$BACKUP_DIR/backup-main"; then
    echo "Error: rclone sync failed"
    exit 1
fi
echo "rclone sync completed."

# Step 2: Create a snapshot with hardlinks
DATE=$(date -I)  # ISO format (YYYY-MM-DD)
SNAPSHOT_DIR="$BACKUP_DIR/backup-$DATE"

echo "Creating snapshot for $DATE..."
if ! rsync -a --link-dest="$BACKUP_DIR/backup-main" "$BACKUP_DIR/backup-main/" "$SNAPSHOT_DIR"; then
    echo "Error: rsync snapshot creation failed"
    exit 1
fi
echo "Snapshot $SNAPSHOT_DIR created successfully."

# Step 3: Check disk usage and delete old snapshots if necessary
current_usage=$(calculate_usage)
echo "Current total backup usage: $current_usage MB"

if [ "$current_usage" -gt "$SNAPSHOT_LIMIT" ]; then
    echo "Storage limit exceeded. Cleaning up old snapshots..."
    while [ "$current_usage" -gt "$SNAPSHOT_LIMIT" ]; do
        # Find the oldest snapshot (excluding 'backup-main')
        oldest_snapshot=$(ls -1d "$BACKUP_DIR"/backup-* 2>/dev/null | grep -v "backup-main" | sort | head -n 1)
        
        if [ -z "$oldest_snapshot" ]; then
            echo "No snapshots found to delete. Exiting cleanup."
            break
        fi

        # Failsafe: Ensure $oldest_snapshot is within $BACKUP_DIR
        if [[ "$oldest_snapshot" != "$BACKUP_DIR/"* ]]; then
            echo "Error: $oldest_snapshot is outside the backup directory. Aborting deletion."
            exit 1
        fi

        echo "Deleting oldest snapshot: $oldest_snapshot"
        if ! rm -rf "$oldest_snapshot"; then
            echo "Error: Failed to delete $oldest_snapshot"
            exit 1
        fi

        current_usage=$(calculate_usage)
        echo "Updated total backup usage: $current_usage MB"
    done
else
    echo "Storage usage is within limit."
fi

echo "Backup process completed successfully."
exit 0