Most people (including most Unix greybeards!) really don't understand Unix's 'init' (AKA "the init daemon", "the init process", etc., etc.) -- much less any of its massively-increasing-in-LOC (and complexity!) successor programs...

So we need to start with 'init'.

'init' -- even in its absolute first, simplest incarnation -- is still too complex to understand correctly!

You see, we need to shift perspectives!

We need to shift perspectives from a longtime System Administrator -- to that of a new barebone OS programmer.

What is 'init'?

Is 'init' a program that handles runlevels, starts and stops services, that mounts filesystems, that processes messages, that captures dead processes, that waits for hardware to become available, that logs and maintains informational/database/etc files, that starts audio, that starts X11, that stars the GUI, that acts as a proxy for sockets, or does anything else with the system?

No!

From the point of view of a new barebone OS programmer (as Dennis Richie and Ken Thompson were when they invented Unix and invented 'init') -- 'init' is NONE of these things!

'init' is only THE FIRST PROGRAM, THE FIRST COMPUTER CODE THAT RUNS IN USER SPACE.

And that's it!

That is all that 'init' ever is, or ever was!

(User space, to recap, is the unprotected AKA "unprivileged" AKA "non-supervisor" memory running unprotected (AKA "user-land") code: https://en.wikipedia.org/wiki/User_space_and_kernel_space)

'init' (and every single 'init' successor program, i.e., OpenRC, systemd, etc.) -- are the first program, the first set of computer code OUTSIDE OF KERNEL CODE (which has been running and is currently still running) to be run by the system.

Now, what should that first program do?

See, that's the magic question -- which gives rise to all that is to follow!

In theory you could have an OS where the 'init' program, or its equivalent -- did absolutely nothing! But that wouldn't be very productive!

If the 'init' program isn't itself a shell program (i.e., sh, bash, etc.) -- then (because there's no GUI at this point) the computer will not be able to accept typed command-line commands -- which is the first thing that you want a new OS to do!

So now our 'init' expands in scope (and lines of code)!

Our 'init' could be hardcoded to launch 'sh' or 'bash' (or whatever shell program exists) -- but what if the user wants to change that?

OK, so now we need our first configuration file. Where to put that exactly?

Oh, it's on a filesystem that hasn't been mounted yet?

Well, maybe init should mount that filesystem!

Point is, there's a set of problems (and sub-problems!) -- which give rise to increasing and increasing init's functionality over time!

init, as the first user-space program for an OS to run, on whatever OS it is ran on, in whatever form it is in -- could simply be written to run and 'outsource' all of its functionality to other programs...

But init (as it evolved into its very large LOC complex descendants) -- became a "dumping ground" -- for functionality that was inconvenient to go in other places and/or to be outsourced to other programs.

See, all of the code in all userland Linux utilities -- could in theory be grafted together into one big super program in userspace.

It would have the same functionality as all of the individual Unix/Linux command-line programs put together (and maybe that would be desirable to some people). But from a Software Engineering "separation of concerns" AKA dependency reduction AKA modularity AKA "do one thing and do it right" AKA loose-coupling perspective -- doing that might not be so desirable!

And yet, with the complexity brought about by 'init' descendants -- it seems like we're going down that exact route!

Which leads us full circle (because history always repeats itself!) -- back to the reason why Unix was created -- because of the complexity and problems brought about by the complexity of its predecessor, Multics!

https://en.wikipedia.org/wiki/Multics

Point is -- 'init' in whatever form it takes -- is by no means obligated to do anything -- although if it is to do nothing, then it should at least launch one other program which will do something! If that's the case, then why not put that under user control? But wait, if we're doing that, why not make it launch multiple other programs! OK, now we need a file to tell init where that should be! But what if the filesystem for that file is not mounted?

Anyway, you see how the "rabbit hole" of problems (and increasing LOC complexity) forms!

Related: https://www.joelonsoftware.com/2002/11/11/the-law-of-leaky-a...