Ubuntu 18.04 running script at boot

Ubuntu 18 is using systemd, and there are three things we need to do 

1) Create a service file and put it in /etc/systemd/system/

  let say the file be board-init.service, with the below content

[Unit]
Description=Board initializations
DefaultDependencies=no

[Service]
Type=oneshot
ExecStart=/etc/board-init.sh

[Install]
WantedBy=sysinit.target

2) create your board-init.sh script file, and put it in /etc/

3) Create a soft link in the /etc/systemd/system/sysinit.target.wants to the service file w created

cd /etc/systemd/system/sysinit.target.wants

ln -s /etc/systemd/system/board-init.service board-init.service

reboot the board. To see the output of the script type 

journalctl -u board-init -b