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Let's talk about dependencies in systemd.

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So you can manage unit dependencies.

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And these dependencies normally are defined in the unit section within the unit file.

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An easy way to figure out about the current dependencies is systemctl list dependencies.

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That is something that you can do without an argument to see all the dependencies that

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have brought you into the current state, or use an argument such as systemctl list dependencies

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boot.mount to show the current dependencies that are required by a specific unit.

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You can also use options within the unit file to manage dependencies.

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Use requires, which means that if this unit loads, the units that are listed here will

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load as well.

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And if one of the other units is deactivated, this unit will be deactivated as well.

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There is a requisite.

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A requisite is what you use if you must have the unit already available, otherwise this

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unit will fail.

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There is once, which is a soft dependency.

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It means that this unit wants to load the units listed here, but it won't fail if any

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of these units fail.

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And we have before, which will start this unit before the unit mentioned with before.

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And after, we'll start this unit after the unit that is mentioned with after equals.

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Let's do a quick demo.

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So let me start by using systemctl list dependencies.

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Why not?

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Boot.mount.

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It's one of the mount targets.

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And here we can see it doesn't have very many dependencies.

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And if you do that for another service like sshd.service, there you can see everything

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that is required to get sshd.service.

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Look at the green dots because the green dots indicate what should really be there.

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Now I want to show you a demo where I'm going to create a dependency myself.

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In order to do so, I need to install the vsftpd service.

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And then I'm going to use systemctl edit on httpd.service.

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And in the edit, I'm doing it a nice way.

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I want to have this in the override file so that my edits are protected from system

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updates.

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So I'm going to use unit and requires is vsftpd.service, which happens to be the name of my vsftpd service.

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Now I'm using psaox.pipe.grep.ftp to show you that currently we don't have any vsftpd.

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Now I'm going to use systemctl restart httpd.service.

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And I'm doing my psaox again.

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And look at that.

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vsftpd is started as well.

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Can we find any information about it?

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Systemctl status vsftpd.

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Without the service, it defaults to service.

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I can see that it is started, but you don't really see anything about the dependency relation,

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the reason why it has been started.

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And on the httpd, well, we can see that httpd has an override.conf.

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But for the rest of it, we don't see the dependency relation either.

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So you really need to figure out how it works.

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And that is where systemctl list dependencies can come in handy.