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- [Narrator] In this video
we'll talk about dependencies,

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dependencies between
different systemd units.

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Different statements can
be used in the Unit section

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to define unit file dependencies.

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There is Before, which
means that this unit starts

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before the specified unit.

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And After, which obviously
means that this unit starts

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when the specified unit is started.

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There is a Requires, which
means that when this unit starts

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the unit listed here is also started.

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And if the specified unit
fails, this one also fails.

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And there is Wanted.

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When this unit starts, the unit
listed here is also started.

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Now even if you can manage
dependencies in units

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you should consider using targets

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for more consistent dependency management.

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It's easy to have an
overview of everything

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that is going on,

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but for now let's create
some dependencies.

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Okay, let me start by
using dnf install vsftpd

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(keyboard clicks)

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And systemctl status httpd.

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As you can see, loaded,
disabled, inactive.

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That's exactly what we need.

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Loaded means that systemd
knows which unit file to use

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and that is about it.

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For the rest of it, this unit
currently is doing nothing.

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Same case for vsftpd.

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Now I'm going to use systemctl
edit httpd dot service

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And I'm going to edit
the unit configuration.

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Requires is vsftpd dot service.

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And I'm using systemctl start.

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No, before doing that let
me use ps aux pipe grep ftp.

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Just so we agree that there
is no vsftpd service running

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at this moment

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and now I can use systemctl start httpd.

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This is a systemd ctl start,

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systemctl start doesn't make it persistent

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that would need systemctl enabled,

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but I don't need it to be
persistent at this moment.

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What I need is httpd to be starting

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and what we also need is to see

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that suddenly vsftpd is running as well

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and that's what you obtain
by using a Requires.

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Now as I was mentioning a
Requires isn't very clear

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to analyze dependency relations

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so it's probably much better
to make sure that vsftpd,

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as well as httpd, are
just in the same target

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and they're also WantedBy section.

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WantedBy is not as strong as Requires

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but by putting them in the same target

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you make it easier to
see the logical relation

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between the two of them.