Creating and activating a swap file (path: /swapfile), then adding it to /etc/fstab

For an example of a 1GB swap file (1M * 1024)

sudo dd if=/dev/zero of=/swapfile bs=1M count=1024
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile   
sudo sh -c 'echo "/swapfile swap swap defaults 0 0" >> /etc/fstab'

You can check if it works

free -m

clean swap

sudo swapoff -a

sudo swapon -a