365 Days of Code - Day 047

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#!/usr/bin/env bash
set -Eeuo pipefail

REMOTE_USER="archivepull"
REMOTE_HOST="webserver.example.com"
REMOTE_DIR="/var/backups/local"
SSH_KEY="/home/user/.ssh/archive_pull"
LOCAL_DIR="/srv/archive/webserver"
LOCK_FILE="/var/lock/pull_web_backup.lock"

log() {
    echo "[$(date '+%Y-%m-%d %H:%M:%S')] [INFO] $*"
}

warn() {
    echo "[$(date '+%Y-%m-%d %H:%M:%S')] [WARN] $*" >&2
}

error() {
    echo "[$(date '+%Y-%m-%d %H:%M:%S')] [ERROR] $*" >&2
}

require_command() {
    local cmd="$1"
    command -v "${cmd}" >/dev/null 2>&1 || {
        error "Required command not found: ${cmd}"
        exit 1
    }
}

create_lock() {
    mkdir -p "$(dirname "${LOCK_FILE}")"
    exec 9>"${LOCK_FILE}"
    flock -n 9 || {
        error "Another archive pull job is already running"
        exit 1
    }
}

main() {
    local latest_archive
    local latest_file
    local latest_checksum

    require_command ssh
    require_command rsync
    require_command sha256sum
    require_command find
    require_command sort
    require_command tail
    require_command basename

    mkdir -p "${LOCAL_DIR}"
    create_lock

    log "Locating newest remote backup"

    latest_archive="$(ssh -i "${SSH_KEY}" -o BatchMode=yes "${REMOTE_USER}@${REMOTE_HOST}" \
        "find '${REMOTE_DIR}' -maxdepth 1 -type f -name 'backup_*.tar.gz' | sort | tail -n 1")"

    if [[ -z "${latest_archive}" ]]; then
        error "No remote backup archive found"
        exit 1
    fi

    latest_file="$(basename "${latest_archive}")"
    latest_checksum="${latest_file}.sha256"

    log "Newest backup identified: ${latest_file}"

    if [[ -f "${LOCAL_DIR}/${latest_file}" && -f "${LOCAL_DIR}/${latest_checksum}" ]]; then
        log "Latest backup already exists locally, validating checksum"
        (
            cd "${LOCAL_DIR}"
            sha256sum -c "${latest_checksum}" >/dev/null
        )
        log "Backup already archived and verified; skipping copy"
        exit 0
    fi

    log "Copying backup and checksum from remote server"
    rsync -av --partial -e "ssh -i ${SSH_KEY} -o BatchMode=yes" \
        "${REMOTE_USER}@${REMOTE_HOST}:${REMOTE_DIR}/${latest_file}" \
        "${REMOTE_USER}@${REMOTE_HOST}:${REMOTE_DIR}/${latest_checksum}" \
        "${LOCAL_DIR}/"

    log "Validating checksum"
    (
        cd "${LOCAL_DIR}"
        sha256sum -c "${latest_checksum}" >/dev/null
    )

    log "Archive pull completed successfully"
}

main "$@"