Programmatically add new product attribute with options to a Magento 2 store and add to Default attribute group

create_attributes.php

<?php
defined('STDIN') or die(_("Access Denied. CLI Only"));

// execute from the command line: php <path to magento root>/cli/create_attributes.php

require __DIR__.'/../app/bootstrap.php'; // assuming this file is in /cli under the root magento dir

$bootstrap = \Magento\Framework\App\Bootstrap::create(BP, $_SERVER);
$installer = $bootstrap->getObjectManager()->create('Magento\Catalog\Setup\CategorySetup');
$objectManager = $bootstrap->getObjectManager();
$attributeRepository = $objectManager->get('\Magento\Catalog\Api\ProductAttributeRepositoryInterface');

$indexAdmin = 0;
$indexEnglish = 12; // be nice to get this dynamically

function labelToSnakeCase($someString) {
    $retval = $someString;
    $retval = strtolower($retval);
    $retval = preg_replace('/\W+/', '_', $retval);
    return $retval;
}

// describe the attributes we want to add; all array fields are optional
$attributes_meta = array(
    'Finish' => array(
//        'admin' => 'color',
        'type' => 'swatch_visual', // boolean, text, textarea, date, select, multiselect, swatch_text, swatch_visual, media_image
        'visible' => true,
        'options' => array(
            'American Chestnut',
            'Black Cherry',
            'Cherry',
            'Espresso',
            'Honey',
            'Mahogany',
            'Natural',
            'Oak',
            'Pecan',
            'Unfinished',
            'Walnut',
        ),
    ),
);

// cycle through and add each attribute
foreach ($attributes_meta as $attribute_label => $attribute_meta) {
    $attribute_label_admin = empty($attribute_meta['admin']) ? labelToSnakeCase($attribute_label) : $attribute_meta['admin'];
    $attribute_type = empty($attribute_meta['type']) ? 'text' : $attribute_meta['type'];
    $attribute_data = array(
        // table eav_attribute
        'entity_type_id' => $installer->getEntityTypeId('catalog_product'),
        'attribute_code' => $attribute_label_admin,
        // 'attribute_model' => NULL,
        // 'backend_model' => NULL,
        'backend_type' => 'int',
        // 'backend_table' => NULL,
        // 'frontend_model' => NULL,
        'frontend_input' => 'swatch_' === substr($attribute_type, 0, strlen('swatch_')) ? 'select' : $attribute_type,
        'frontend_label' => [$attribute_label],
        // 'frontend_class' => NULL,
        // 'source_model' => NULL,
        'is_required' => 0,
        'is_user_defined' => 1,
        // 'default_value' =>
        'is_unique' => 0,

        // table catalog_eav_attribute
        'is_global' => 1,
        'is_visible' => 1,
        'is_searchable' => 0,
        'is_filterable' => 0,
        'is_comparable' => 0,
        'is_visible_on_front' => 0,//empty($attribute_meta['visible']) ? 0 : 1,
        'is_html_allowed_on_front' => 1,
        'is_used_for_price_rules' => 0,
        'is_filterable_in_search' => 0,
        'used_in_product_listing' => 0,
        'used_for_sort_by' => 0,
        // 'apply_to' => NULL,
        'is_visible_in_advanced_search' => 1,
        // 'position' => 0,
        'is_wysiwyg_enabled' => 0,
        'is_used_for_promo_rules' => 0,
        'is_required_in_admin_store' => 0,
        'is_used_in_grid' => 1,
        'is_visible_in_grid' => 1,
        'is_filterable_in_grid' => 1,
        // 'search_weight' => 1,

//        'update_product_preview_image' => 1, // not sure how to control this but this doesn't work
    );
    if (!empty($attribute_meta['options'])) {
        $options = array();
        $options['value'] = array();
        foreach ($attribute_meta['options'] as $option_label) {
            $option_label_admin = labelToSnakeCase($option_label);
            $option[$indexAdmin] = $option_label_admin;
            $option[$indexEnglish] = $option_label;
            $options['value'][$option_label_admin] = $option;
//            $options['order'][$option_label_admin] = $index;
        }
        $attribute_data['option'] = $options;
    }
    if ('swatch_visual' === $attribute_type) {
        // table catalog_eav_attribute
        $attribute_data['additional_data'] = array(
            'swatch_input_type' => 'visual',
            'update_product_preview_image' => 0,
            'use_product_image_for_swatch' => 0,
        );
        // there must be a proper way to set this...
        $attribute_data['additional_data'] = 'a:3:{s:17:"swatch_input_type";s:6:"visual";s:28:"update_product_preview_image";s:1:"0";s:28:"use_product_image_for_swatch";s:1:"0";}';
    }

    $attribute = $bootstrap->getObjectManager()->create('Magento\Catalog\Model\ResourceModel\Eav\Attribute');
    $attribute->setData($attribute_data);
    try {
        $attributeRepository->save($attribute);
    } catch(Magento\Framework\Exception\AlreadyExistsException $e) {
        echo "$attribute_label already exists\n";
        continue;
    }
    $installer->addAttributeToGroup('catalog_product', 'Default', 'General', $attribute->getId());
    echo "$attribute_label added\n";
}

// there must be a proper way to set this...
You can use serialize() for the $attribute_data['additional_data'] to retrieve proper structure.