List active swap areas

# procfs
cat /proc/swaps

# cli tool
swapon --show

Manual swapfile setup

# One time:
#   Create and initialize swapfile.
#   mkswap will initialize swap area over full filesize by default.
sudo dd if=/dev/zero of=/swapfile bs=1G count=1
mkswap /swapfile

# Enable swap file (until next reboot).
swapon /swapfile

# Persistent setup of swap file.
echo "/swapfile none swap sw 0 0" | sudo tee -a /etc/fstab

# Disable swap file (until next reboot).
swapoff /swapfile

Recommended file permissions 0600 and file owner uid=0 (root).

Using dphys-swapfile service.

Dynamically computes size of swap file based on installed RAM.

# Setup and enable swap based on /etc/dphys-swapfile.
dphys-swapfile setup
dphys-swapfile swapon

# Disable swap on configured file.
dphys-swapfile swapoff

Usually comes with a script to be automatically run at system startup and shutdown. For example as systemd service:

systemctl status dphys-swapfile