Official client to connect to ProtonVPN from the command-line. More information: https://protonvpn.com/support/linux-vpn-setup/.
protonvpn-cli connect
protonvpn-cli connect --fastest
protonvpn-cli connect {{server_name}} --protocol {{udp|tcp}}
protonvpn-cli connect --random --protocol {{udp|tcp}}
protonvpn-cli connect --tor
protonvpn-cli connect --help