How to check if a customer is logged in to Magento 2 or not?

There are two ways by which you can check if a customer is logged in or not:

1) By using Object Manager

It is always a bad practice to use ObjectManager directly.

    $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
    $customerSession = $objectManager->get('Magento\Customer\Model\Session');
    if($customerSession->isLoggedIn()) {
       // customer login action
    }

2) By Injecting Class (Dependency Injection)

As said earlier, it is one of the worst practices to use the ObjectManager directly. Therefore an alternate way to check if a user is logged in or not is by using the following code in any class except the template files:

You first need to inject the following class in your constructor: 
/Magento/Customer/Model/Session

    protected $_session;

    public function __construct(
        ...
        \Magento\Customer\Model\Session $session,
        ...
    ) {
        ...
        $this->_session = $session;
        ...
    }

Then you need to use Magento\Customer\Model\Session::isLoggedIn() to check if the customer is logged in or not.

if ($this->_session->isLoggedIn()) {
   // Customer is logged in 
} else {
   // Customer is not logged in
}