Create linux systemd service

Create the file
/etc/systemd/system/routes.service


File content:

[Unit]
Description=Routes
After=multi-user.target

[Service]
Type=idle
ExecStart=/etc/init.d/routes.sh

[Install]
WantedBy=multi-user.target


Now lets enable service:
systemctl enable routes.service

If your script have start/stop options, change the ExecStart:
ExecStart=/etc/init.d/routes.sh start
ExecStop=/etc/init.d/routes.sh stop