Init just a more or less normal program that Linux starts by default and by convention. You can make it boot straight into bash if you want. I created a little programming language with the ultimate goal of booting Linux directly into it and bringing up the entire system from inside it.
It's just a normal process really. Two special cases that I can think of: no default signal handling, and it can't ever exit. Init will not get interrupted by signals unless it explicitly configures the signal dispositions, even SIGKILL will not kill it. Linux will panic if PID 1 ever exits so it can't do that.
Traditionally, it's also the orphaned child process reaper. Process descriptors and their IDs hang around in memory until something calls wait on them. Parent processes are supposed to do that but if they don't it's up to init to do it. Well, that's the way it works traditionally on Unix. On Linux though that's customizable with prctl and PR_SET_CHILD_SUBREAPER so you actually can factor that out to a separate process. As far as I know, systemd does just that, making it more modular and straight up better than traditional Unix, simply because this separate process won't make Linux panic if it ever crashes.
As for the service manager, this page explains process and service management extremely well:
Systemd does it right. It does everything that's described in there, does it correctly, uses powerful Linux features like cgroups for even better process management and also solves the double forking problem described in there. It's essentially a solved problem with systemd. Even the people who hate it love the unit files it uses and for good reason.
There is a secondary feature to run per-user managers, though I'm unsure whether it does run doesn't run without systemd PID1. Though it might only rely on logind.
Wow, I remember reading that PID != 1 line years ago. Had no idea they changed it. I stand corrected then. Given the existence of user service managers as well as flags like --system and --user, I inferred that they were all entirely separate processes.
Makes no sense to me why the service manager part would require running as PID 1. The maintainer just says this:
> PID 1 is very different from other processes, and we rely on that.
He doesn't really elaborate on the matter though.
Every time this topic comes up I end up searching for those so called PID 1 differences. I come up short every time aside from the two things I mentioned above. Is this information buried deep somewhere?
Just asked ChatGPT about PID 1 differences. It gave me the aforementioned two differences, completely dismissed Linux's prctl child subreaper feature "because PID 1 often assumes this role in practice" as well as some total bullshit about process group leaders and regular processes not being special enough to interact with the kernel which is just absolute nonsense.
So I really have no idea what it is about PID 1 that systemd is supposedly relying on that makes it impossible to split off the service manager from it. Everything I have read up until now suggests that it is not required, especially on Linux where you have even more control and it's not like systemd is shy about using Linux exclusive features.
