I use a Raspberry Pi for a control application. It doesn't run an X interface. My script (Python) runs automatically at boot using the systemctl method.
The machine's job is to display some measurements on an LCD screen, but it doesn't do anything until the Pi OS has finished booting. I understand the need for this: there are certain things the computer simply cannot do before the boot process is finished.
How can I find out what operations the machine is capable of at various stages of its boot process? More specifically, is it possible for me to program the GPIO ports or execute my Python script before the OS has finished booting?
Even more specifically, I wish to display a splash screen on the LCD indicating that the OS is booting and that the measurements will start in the next 20 or 30 seconds.
If I even had a few keywords I could Google it would be helpful. Part of my struggle is not knowing what the question is!!!