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composer require magento/product-community-edition=2.3.0 --no-update;
composer require --dev phpunit/phpunit:~6.2.0 friendsofphp/php-cs-fixer:~2.10.1 lusitanian/oauth:~0.8.10 pdepend/pdepend:2.5.2 sebastian/phpcpd:~3.0.0 squizlabs/php_codesniffer:3.2.2 --no-update;
composer remove --dev sjparkinson/static-review fabpot/php-cs-fixer --no-update;

#update composer.json autoload section with zend mvc controller
"autoload": {
    "psr-4": {
        "Magento\\Framework\\": "lib/internal/Magento/Framework/",
        "Magento\\Setup\\": "setup/src/Magento/Setup/",
        "Magento\\": "app/code/Magento/",
        "Zend\\Mvc\\Controller\\": "setup/src/Zend/Mvc/Controller/"
    },
    ...
}

composer update;
rm -rf ./var/cache/*;
rm -rf ./var/page_cache/*;
rm -rf ./generated/code/*;
php bin/magento setup:upgrade;
php bin/magento maintenance:disable;