Steps to get FT-891 and rigctld working with udev and systemd

99-usb-radio.rules

# /etc/udev/rules.d/99-usb-radio.rules
# Adds the /dev/ft891 device,  note that ID_USB_INTERFACE_NUM is required because of the weird use of the USB to Dual serial adapter driver
SUBSYSTEM=="tty" ENV{ID_USB_INTERFACE_NUM}=="00" ATTRS{idVendor}=="10c4" ATTRS{idProduct}=="ea70" SYMLINK+="ft891" ENV{SYSTEMD_WANTS}="rigctld.service"

References.txt

# Some info on setting up UDEV rules
https://www.thuben.com/amateur/software/fixed-devices

# SystemD service
https://www.thuben.com/amateur/software/rigctl

# Setting up the ENV{SYSTEMD_WANTS} stuff
https://blog.fraggod.net/2012/06/16/proper-ish-way-to-start-long-running-systemd-service-on-udev-event-device-hotplug.html

rigctld.service

# /etc/systemd/system/rigctld.service
[Unit]
Description=rigctld Hamradio rig controler
After=syslog.target network.target
# Remove the service when udev no longger needs it (i.e. unplugged)
StopWhenUnneeded=yes

[Service]
Type=simple
# Use the /dev/ft891 symlink set up by following UDEV rule
ExecStart=/usr/bin/rigctld -m 1036 -r /dev/ft891 -s 38400 -p 4532
ExecReload=/bin/ill -HUP $MAINPID
RestartSec=60
Restart=always

[Install]
WantedBy=multi-user.target