I am currently using 64 bit Linux Mint 18.03 with the Cinnamon Desktop 3.6.7.

In an attempt to modify my Plymouth startup and shutdown screens, I've had a fair bit of success, but am still attempting to do a few things that, although they seem to be possible based on samples I have seen (and tried), still elude me. Searching the internet has mostly proven to be a waste of time, primarily because the multitude of postings make no mention of the OS and/or Desktops they were using, nor the particular versions of each - and the targets are continually moving. Even with Ubuntu and Mint, file locations have recently changed. Even if what I'm looking for is there, it is probably well camouflaged due to the fact that I don't know precisely what I'm looking for.

So, if anyone can answer (or point to discussions about) the following questions relative to Linux Mint 18.03, I would much appreciate it.

1) In the definition file for each plymouth theme (e.g. /usr/share/plymouth/themes/mint-logo/mint-logo.plymouth for the Mint 18.03 default theme), there is a line that says "ModuleName=script" and below that is a section "[script]" that specifies the name of the script that will be run by that module. Does anyone know if more than one module can be defined in the theme definition files and used simultaneously with the script? In addition to the script.so module, there are nine others on my system that are used by various themes I have looked at in an attempt to figure out how all this is organized.

2) The plymouth scripting language seems to be able to set up handlers for various call backs the operating system might make during the boot process. Among these are SetRefreshFunction(custom_function_name), SetQuitFunction(custom_function_name), SetMessageFunction(custom_function_name), and so forth. Some "fun custom_function_name" is written and followed by one of the aforementioned calls to assign it to a particular callback. My limited blind experimentation suggests to me that not all of these callbacks are made by any given operating system. Can anyone confirm that this is true?

3) Assuming that the answer to #2 is True, does anyone know how to determine which callbacks are used by a particular OS (and specifically by Mint or Ubuntu)?

4) Again assuming that the answer to #2 is True, does anyone know if there is some configuration value (set in Grub perhaps?) that can cause the OS to make specific callbacks? The file "/etc/init.d/plymouth" seems to define messages that COULD BE sent to plymouth by the OS, but doesn't seem to force them to BE SENT. I've wandered through other scripts in the directory but didn't spot anything promising.

Again, thanks for any insights anyone can offer.