Comment by stoplight | Hacker News Reader

Comment by stoplight | Hacker News Reader

Am I missing something? The author states:

> Try to find the answer to "which units are going to be started on next boot?".

Isn't this just `ls /etc/systemd/{user,system,network}//*` (bah HN formatting is messing this up)

Also the other states:

> Speaking of which, why is systemctl daemon-reload even a thing?

Because configuration changes while things are running. Otherwise we wouldn't need apache2 reload, nginx reload, haproxy reload, etc. Also, according to the man page:

> After the units have been edited, systemd configuration is reloaded (in a way that is equivalent to daemon-reload).

As for systemctl edit, it allows you to add diffs to the unit file. For example, let's say I am using redis and I want to ensure it always restarts (for whatever reason). Rather than manually editing the unit file (which will have issues if an upstream change happens), I can simply use systemctl edit to add a diff for my change. This way I can still maintain compatibility with the upstream unit file instead of having to deal with potential conflicts. It's not so much babysitting the user as it is a potentially better experience.

> Isn't this just `ls /etc/systemd/{user,system,network}//*` (bah HN formatting is messing this up)

Nope.

There are many virtual units for which there are no files. One example is the on-demand getty units. There are two for "plymouth" boot-splash stuff (I see these attempted-and-failed units on archlinux where plymouth never exists). There's some autofs thing. There's auditd.

I'm actually not booted into linux right now so I'm struggling to remember the specifics... but if you also count all the simple mounts and devices which clutter up the output of "systemd --all" as "units which are not files in /etc/systemd" then the count is around 100.

Couldn't/Shouldn't this be solved with some script that traces the dependencies of units?

I'm not familiar enough with systemd to understand how to trace dependencies and create a complete list of all units that will run at next boot but I expect this to be doable by someone with that expertise.

  systemd-analyze dot --order | dot -Tsvg > /tmp/system.svg

The resulting dependency diagram may look complex, but consider that there is no equivalent functionality for SysV init scripts despite the fact that they have the same level of inter-dependent complexity.

Whether using a dependency system at all for the purpose of ordering and expressing relationships between daemons is the best approach is beyond the scope of this reply, but I just want to say that most modern sysvinit deployments did in fact have service dependencies, through the use of LSB initscript headers that were parsed by insserv(8).

It was kludgy, but there.

The complexity of the dependency graph produced by system-analyze is not present in most sysv init systems. That complexity is mostly a reflection of the efforts to make system services boot in parallel. Most sysv init scripts were blocking and executed serially, which is what allowed the simple filename prefixes to be used for ordering.

What to see your entire sysv init, as it will run every single time?

    ls -1 /etc/rc3.d/[SK]* /etc/rc.local | xargs -- cat | less

Obviously, substitute whichever runlevel you're concerned with.

Previously on arch linux I had a list of 7 to 10 services in rc.conf that started serially in less than 2 seconds. That was pretty damn simple. Besides that there was inittab which just had the gettys and the main startup script.

    systemctl list-dependencies

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