Last tested: Jan 21, 2019
If you use a System5 style startup system the script is looker_init and it is here... https://github.com/looker/customer-scripts/tree/master/startup_scripts
If you use systemd startups then here is the answer https://github.com/looker/customer-scripts/tree/master/startup_scripts/systemd
And if cron is running on the system one could always use the crontab file to restart looker https://www.cyberciti.biz/faq/linux-execute-cron-job-after-system-reboot/
This post applies if you host your own instance - Looker-hosted instances automatically start on reboot.