Sync a Directory to a Remote Host via rsync over SSH

#!/usr/bin/env bash
#
# remote_sync.sh - Mirror a local directory to a remote host over SSH.
#
# Uses rsync over SSH for efficient, resumable, encrypted off-site backups.
#
# Usage: ./remote_sync.sh <local_dir> <user@host> <remote_dir> [ssh_port]
#
set -euo pipefail

# --- Argument handling --------------------------------------------------------
if [[ $# -lt 3 || $# -gt 4 ]]; then
    echo "Usage: $0 <local_dir> <user@host> <remote_dir> [ssh_port]" >&2
    exit 1
fi

LOCAL_DIR="$1"
REMOTE="$2"
REMOTE_DIR="$3"
SSH_PORT="${4:-22}"

[[ -d "$LOCAL_DIR" ]] || { echo "Error: local dir '$LOCAL_DIR' not found." >&2; exit 1; }

# --- Run the sync -------------------------------------------------------------
# -a archive, -z compress in transit, -h human-readable, --partial resumes
# interrupted transfers, --delete keeps the remote mirrored to the source.
echo "Syncing '$LOCAL_DIR' -> ${REMOTE}:${REMOTE_DIR} (port $SSH_PORT)"
rsync -azh --partial --progress --delete \
    -e "ssh -p ${SSH_PORT} -o StrictHostKeyChecking=accept-new" \
    "${LOCAL_DIR%/}/" "${REMOTE}:${REMOTE_DIR%/}/"

echo "Remote sync complete."