Magento2 custom script export configurable product with associated

<?php
require __DIR__ . '/app/bootstrap.php';
$bootstrap = \Magento\Framework\App\Bootstrap::create(BP, $_SERVER);

$objectManager = $bootstrap->getObjectManager();
$state = $objectManager->get('Magento\Framework\App\State');
$state->setAreaCode('adminhtml');
$registry = $objectManager->get('Magento\Framework\Registry');
$registry->register('isSecureArea', true);

// Store id of exported products, This is useful when we have multiple stores. 
$store_id = 1;

$fp = fopen("export.csv","w+");
$collection = $objectManager
    ->create('Magento\Catalog\Model\ResourceModel\Product\CollectionFactory')
    ->create()->addStoreFilter($store_id)
    ->addAttributeToFilter('type_id', 'configurable')
    ->addAttributeToSelect(array('name','sku','associated_skus'));

foreach ($collection as $product) {
    $data = array();
    $data[] = $product->getTypeId();
    $data[] = $product->getName();
    $data[] = $product->getSku();

    $child_products = $product->getTypeInstance()->getUsedProducts($product, null);
    if (count($child_products) > 0) {
        $resultstr  = array();
        foreach($child_products as $child) {
            $resultstr[] = $child->getSku();
        }
        echo $data[] = implode(',', $resultstr);
    }  
    fputcsv($fp, $data);    
}

fclose($fp);

Create one php file inside your magento root directory with below code.

<?php
require './app/bootstrap.php';
$bootstrap = \Magento\Framework\App\Bootstrap::create(BP, $_SERVER);
$_objectManager = $bootstrap->getObjectManager();
$state = $_objectManager->get('Magento\Framework\App\State');
$state->setAreaCode('admin');
$registry = $_objectManager->get('Magento\Framework\Registry');
$registry->register('isSecureArea', true);

//Store id of exported products, This is useful when we have multiple stores. 
$store_id = 1;

$fp = fopen("export.csv","w+");
$collection = $_objectManager->create('Magento\Catalog\Model\ResourceModel\Product\CollectionFactory')->create()->addStoreFilter($store_id)->addAttributeToFilter('type_id', 'configurable')->addAttributeToSelect(array('name','sku'));
foreach ($collection as $product)
{
    $data = array();
    $data[] = $product->getTypeId();
    $data[] = $product->getName();
    $data[] = $product->getSku();
    fputcsv($fp, $data);

    $child_products = $product->getTypeInstance()->getUsedProducts($product, null);
    if(count($child_products) > 0)
    {
        foreach($child_products as $child)
        {
            $data = array();
            $data[] = $child->getTypeId();
            $data[] = $child->getName();
            $data[] = $child->getSku();
            fputcsv($fp, $data);
        }
    }   
}
fclose($fp);