在用rsync备份时出现错误

我的系统是debian11，用systemd设置了个关机自动备份的程序，但运行后查看状态时提示：

3月 16 08:53:56 debian bash[2705]: rsync error: received SIGINT, SIGTERM, or SIGHUP (code 20) at rsync.c(703) [sender=3.2.3]
3月 16 08:53:58 debian bash[2706]: rsync error: received SIGUSR1 (code 19) at main.c(1595) [generator=3.2.3]

请问这是怎么回事？应该怎么改？

shell代码如下：

#用UUID挂载U盘
if mountpoint -q "${BACKUP_DIR_ROOT}"
then :
    #echo "已经挂载"
else
    #echo "没有挂载"
    echo "${PASSWORD}" | sudo -S mount -U "${U_NAME}" "${BACKUP_DIR_ROOT}"
fi

mkdir -p "${BACKUP_DIR}"

#删除最早的文件夹
sudo -S ls "${BACKUP_DIR}" -rt | head -1 | xargs -I rpl rm -rf "${BACKUP_DIR}/"rpl

rsync -av --delete \
  "${SOURCE_DIR}/" \
  --link-dest "${LATEST_LINK}" \
  --exclude=".cache" \
  --exclude=".local/share/Trash/*" \
  "${BACKUP_PATH}"

rm -rf "${LATEST_LINK}"
ln -s "${BACKUP_PATH}" "${LATEST_LINK}"

#卸载U盘
echo "${PASSWORD}" | sudo -S umount "${BACKUP_DIR_ROOT}"