2024-10-04
Traditionally, init on Unix was not a service manager as such
Init (the process) has historically had a number of roles but, perhaps surprisingly, being a 'service
manager' (or a 'daemon manager') was not one of them in traditional
init systems. In V7 Unix and continuing on into traditional 4.x
BSD, init (sort of) started various daemons by running /etc/rc, but
its only 'supervision' was of getty processes for the console and
(other) serial lines. There was no supervision or management of
daemons or services, even in the overall init system (stretching
beyond PID 1, init itself). To restart a service, you killed its
process and then re-ran it somehow; getting even the command line
arguments right was up to you.
(It's conventional to say that init started daemons during boot, even though technically there are some intermediate processes involved since /etc/rc is a shell script.)
The System V init had a more general /etc/inittab that could in
theory handle more than getty processes, but in practice it wasn't
used for managing anything more than them. The System V init system
as a whole did have a concept of managing daemons and services, in
the form of its multi-file /etc/rc.d structure, but stopping and
restarting services was handled outside of the PID 1 init itself.
To stop a service you directly ran its init.d script with 'whatever
stop', and the script used various approaches to find the processes
and get them to stop. Similarly, (re)starting a daemon was done
directly by its init.d script, without PID 1 being involved.
As a whole system the overall System V init system was a significant improvement on the more basic BSD approach, but it (still) didn't have init itself doing any service supervision. In fact there was nothing that actively did service supervision even in the System V model. I'm not sure what the first system to do active service supervision was, but it may have been daemontools. Extending the init process itself to do daemon supervision has a somewhat controversial history; there are Unix systems that don't do this through PID 1, although doing a good job of it has clearly become one of the major jobs of the init system as a whole.
That init itself didn't do service or daemon management is, in my view, connected to the history of (process) daemonization. But that's another entry.
(There's also my entry on how init (and the init system as a whole) wound up as Unix's daemon manager.)