wget https://github.com/Radarr/Radarr/releases/download/v3.0.1.4259/Radarr.master.3.0.1.4259.linux.tar.gz
tar -xzvf Radarr.*.linux.tar.gz
rm -rf /opt/Radarr/*
mv Radarr/* /opt/Radarr
chown -R pi:pi /opt/Radarr
rm -rf Radarr*

old :

cd /opt
curl -L -O $( curl -s https://api.github.com/repos/Radarr/Radarr/releases | grep linux.tar.gz | grep browser_download_url | head -1 | cut -d \" -f 4 )
tar -xvzf Radarr.v*.linux.tar.gz
rm -rf Radarr.v*.linux.tar.gz
chown -R pi:pi /opt/Radarr/

• https://github.com/Radarr/Radarr/wiki/Autostart-on-Linux

  User=pi
  Group=pi

• https://github.com/joshualyon/radarr-sub-downloader