How can I run a custom .sh script at each startup (without native init.d support)? This looks like a general concern of rookie Android system players who have just rooted their phones. The most prevalent method may be throwing your script into /system/etc/init.d. If the ROM supports init.d then Voila but that's not always the case. So

How can I run my custom script at each startup? What's the user/group my script will be running as if I use your solution?



We welcome all kinds of solutions.
I think that most easy way would be by using app Kernel Adiutor. It has ability to emulate init.d
Open app and swipe from left edge, it will reveal menu, scroll to tools section and than tap on init.d 
There you have a switch to enable or disable init.d emulator, for devices that doesn't support init.d, in bottom right you have a "plus" icon to add scripts to run.