this post was submitted on 29 Jun 2023
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How you would replace those in non-SystemD setup? Asking for learning purposes.
In fact, this is a difficult question.
In Linux, it is usually customary to use the K.I.S.S. methodology, In any case, it was once customary to use it. This in some way meant that there were a huge number of applications performing exactly one task. For example, chron only started timers, ntpd only adjusted the time, grub only loaded the system and nothing else. It also allowed you to change the components at your discretion. With systemd this principle was somewhat lost, since one service with a huge number of its own daemons absorbs more and more functions. This is what causes concern. In some sense, if systemd at some point becomes even more monolithic, it will no longer be possible to change only part of its functionality. For example, I'm not sure if it's possible to disable journald and leave only rsyslog.
On the other hand, the now-forgotten init.V fully adhered to the principle of K.I.S.S. since he was literally the initiator of a set of scripts that could contain anything. If you want, change the user at startup via exec, if you want via su. Isolate the application with any available program. It was as flexible as possible, but on the other hand, the entry threshold was quite high. The complexity of writing scripts for init.V was much higher than systemd.
Therefore, there is no single answer. On the one hand, init.V have maximum modularity, on the other hand, systemd have ease of use.
Sys V init systems were really not K.I.S.S. since it was anything but simple to write an init script in a way that worked without e.g. having the environment of the calling user leak into your script and influence its behaviour or breaking things when called by the wrong user,... Not to mention all the re-implementations of the same functionality and the difficulty of writing an init script that worked on more than one exact OS, distro and version.
Right. But on something more complicated than initializing the usual daemon, systemd has all the same problems. For example, if you have a java application and you want to dynamically manage java parameters and application parameters, the script will look like a pain. something like bash -c 'java ...' or you will have to call a separate script in the initiator. And then to turn off the shell and switch to the application itself, there will be a whole adventure with pid generation.
But sure systemd really more easy then system V.
If you mean you want to auto-detect appropriate values that is no more complicated than the init script was before. You just call that wrapper script.
If you mean you want to turn those on and off as part of your local configuration that is actually quite easy with drop-ins in systemd, much easier than modifying the init script and then having issues with the package overwriting your script with a new copy.
I don't deny that systemd is easier than SysV. I say that on complex configurations it is not slightly simpler. Moreover, what I could do just in the sysV script, I now have to divide by tmpfiles.d and systemd. And sometimes even include processing both there and there, because depending on the version systemd has different behavior with parameters LogsDirectory= and RuntimeDirectory=. As a result, the dependence on the system has not completely disappeared for the package maintainer. Although of course there are a little less problems with systemd.
On other side as a user, I don't really like to guess exactly how a folder was created in /run, via tmpfiles or via systemd.
UPD: On SysV I have one complex, heavy script. Now I have the systemd service, the tmpfiles configuration, the /etc/conf.d parameters file and there is still a shell script to run. But if user wants reconfig something he need look 4 files instead one.
It's never been customary to adhere to KISS in Linux. This whole explanation reads like it came out of a game of Chinese whispers.
or maybe I didn't understand the question. If you about that change daemons to non systemd, then:
systemd-boot -> grub, lilo, efistub
systemd-networkd -> some system scripts (different for different distributions), netplan, NetworkManager
systemd-resolved -> dnsmasq, bind, set directly on 8.8.8.8
systemd-timesyncd -> chrony, ntp
journal -> rsyslog
systemd -> init.V, openRC