Bash-Scripting-Cheatsheet

```bash
#!/bin/bash

# Backup Script for Critical Directories
# Usage: ./backup_script.sh /path/to/source /path/to/destination

# Validate arguments
if [ "$#" -ne 2 ]; then
    echo "Error: Invalid number of arguments. Usage: $0 <source_dir> <destination_dir>" >&2
    exit 1
fi

SOURCE_DIR="$1"
DEST_DIR="$2"
LOG_FILE="/var/log/directory_backup.log"
TIMESTAMP=$(date '+%Y-%m-%d %H:%M:%S')

# Create log directory if it doesn't exist
mkdir -p "$(dirname "$LOG_FILE")"

# Check if source directory exists
if [ ! -d "$SOURCE_DIR" ]; then
    echo "[$TIMESTAMP] ERROR: Source directory $SOURCE_DIR does not exist" >> "$LOG_FILE"
    exit 1
fi

# Check if destination directory exists, create if not
if [ ! -d "$DEST_DIR" ]; then
    echo "[$TIMESTAMP] INFO: Creating destination directory $DEST_DIR" >> "$LOG_FILE"
    mkdir -p "$DEST_DIR"
fi

# Perform backup using rsync with compression and archive mode
if rsync -avz --delete "$SOURCE_DIR/" "$DEST_DIR/"; then
    echo "[$TIMESTAMP] SUCCESS: Backup completed from $SOURCE_DIR to $DEST_DIR" >> "$LOG_FILE"
    echo "Backup completed successfully. Files transferred: $(rsync -avz --delete "$SOURCE_DIR/" "$DEST_DIR/" --dry-run | grep -c '^deleting')"
else
    echo "[$TIMESTAMP] ERROR: Backup failed from $SOURCE_DIR to $DEST_DIR" >> "$LOG_FILE"
    exit 1
fi

echo "Backup operation completed at $TIMESTAMP"
```

**Example Execution:**
```
$ ./backup_script.sh /home/user/documents /mnt/backup/documents
Backup completed successfully. Files transferred: 42
```

**Log Output:**
```
[2023-11-15 14:30:45] INFO: Creating destination directory /mnt/backup/documents
[2023-11-15 14:30:47] SUCCESS: Backup completed from /home/user/documents to /mnt/backup/documents
```