Minimal way to always have screen on on Android?

Set the 'keep awake while plugged in' setting

(Screen will never sleep while charging)

Settings - Developer options - Stay awake: on

or from terminal

svc power stayon true

if the user can press the power button, set the screen lock to "none" (not even swipe) and

use Tasker/MacroDroid/Automate to watch the power button. or run a watchdog from shell

the following example depends on three android binaries

/system/bin/toolbox

/system/bin/toybox

/system/bin/input

# (replace first line with while 1)

until [ $(($(toybox cat /sys/class/power_supply/ac/online)+$(toybox cat /sys/class/power_supply/*usb/online))) -eq 0 ]

  do


 case "$(toolbox getevent -lc4 /dev/input/event2 | toybox head -n1)" in


*KEY_POWER*)


  input keyevent KEYCODE_WAKEUP


;;


 esac

done  

the above code checks for charger is connected as entry point for the loop. the loop exit when charger is disconnected. /dev/input/event2 may vary for your device. you can check which input event has only 3x buttons [Vol+|Vol-|Pwr] from getevent -il (for more generic code look here)