The following config will discuss a basic example on how to execute shell script during a boot time on systemd Linux. There maybe various reason why you might want to execute shell script during Linux startup like for example to start a particular custom service, check disk space, create a backup etc.
This is a companion discussion topic for the original entry at https://linuxconfig.org/easy-way-to-install-firefox-browser-on-debian-7-wheezy/920-example-on-how-to-execute-shell-script-at-startup-boot-on-systemd-linux