Comment by vngzs

I have the following unit file saved for that purpose:

    [Unit]
    # https://stackoverflow.com/questions/36729207/trigger-event-on-aws-ec2-instance-stop-terminate
    Description=unlink agent from remote server
    Before=shutdown.target
    [Service]
    Type=oneshot
    EnvironmentFile=-/etc/environment
    KillMode=none
    ExecStart=/bin/true
    ExecStop=/opt/service-name/shutdown-unlink
    RemainAfterExit=yes
    User=root
    [Install]
    WantedBy=multi-user.target

If I recall correctly, the KillMode=none is important as it causes the shutdown-unlink binary to escape systemd process supervision. Without it, you may deal with systemd immediately halting your shutdown unit (and killing the process) when it hits the shutdown target.