1
00:00:00,000 --> 00:00:10,320
Let's talk about dependencies in systemd.

2
00:00:10,320 --> 00:00:12,760
So you can manage unit dependencies.

3
00:00:12,760 --> 00:00:18,680
And these dependencies normally are defined in the unit section within the unit file.

4
00:00:18,680 --> 00:00:24,680
An easy way to figure out about the current dependencies is systemctl list dependencies.

5
00:00:24,680 --> 00:00:29,280
That is something that you can do without an argument to see all the dependencies that

6
00:00:29,280 --> 00:00:35,799
have brought you into the current state, or use an argument such as systemctl list dependencies

7
00:00:35,799 --> 00:00:43,360
boot.mount to show the current dependencies that are required by a specific unit.

8
00:00:43,360 --> 00:00:47,759
You can also use options within the unit file to manage dependencies.

9
00:00:47,759 --> 00:00:52,320
Use requires, which means that if this unit loads, the units that are listed here will

10
00:00:52,320 --> 00:00:53,599
load as well.

11
00:00:53,599 --> 00:00:58,919
And if one of the other units is deactivated, this unit will be deactivated as well.

12
00:00:58,919 --> 00:00:59,919
There is a requisite.

13
00:00:59,919 --> 00:01:05,040
A requisite is what you use if you must have the unit already available, otherwise this

14
00:01:05,040 --> 00:01:06,040
unit will fail.

15
00:01:06,040 --> 00:01:09,320
There is once, which is a soft dependency.

16
00:01:09,320 --> 00:01:13,440
It means that this unit wants to load the units listed here, but it won't fail if any

17
00:01:13,440 --> 00:01:15,480
of these units fail.

18
00:01:15,480 --> 00:01:20,800
And we have before, which will start this unit before the unit mentioned with before.

19
00:01:20,800 --> 00:01:26,800
And after, we'll start this unit after the unit that is mentioned with after equals.

20
00:01:26,800 --> 00:01:29,199
Let's do a quick demo.

21
00:01:29,199 --> 00:01:34,239
So let me start by using systemctl list dependencies.

22
00:01:34,239 --> 00:01:35,239
Why not?

23
00:01:35,239 --> 00:01:36,239
Boot.mount.

24
00:01:36,239 --> 00:01:37,239
It's one of the mount targets.

25
00:01:37,239 --> 00:01:42,080
And here we can see it doesn't have very many dependencies.

26
00:01:42,080 --> 00:01:47,120
And if we do that for another service like sshd.service, there you can see everything

27
00:01:47,120 --> 00:01:51,480
that is required to get sshd.service.

28
00:01:51,480 --> 00:01:57,400
Look at the green dots because the green dots indicate what should really be there.

29
00:01:57,400 --> 00:02:02,800
Now I want to show you a demo where I'm going to create a dependency myself.

30
00:02:02,800 --> 00:02:09,080
In order to do so, I need to install the vsftpd service.

31
00:02:09,080 --> 00:02:15,639
And then I'm going to use systemctl edit on httpd.service.

32
00:02:15,639 --> 00:02:17,759
And in the edit, I'm doing it a nice way.

33
00:02:17,759 --> 00:02:22,839
I want to have this in the override file so that my edits are protected from system

34
00:02:22,839 --> 00:02:23,839
updates.

35
00:02:23,839 --> 00:02:37,320
So I'm going to use unit and requires is vsftpd.service, which happens to be the name of my vsftpd service.

36
00:02:37,320 --> 00:02:45,240
Now I'm using psaoxpipegrabftp to show you that currently we don't have any vsftpd.

37
00:02:45,240 --> 00:02:51,080
Now I'm going to use systemctl restart httpd.service.

38
00:02:51,080 --> 00:02:53,160
And I'm doing my psaox again.

39
00:02:53,160 --> 00:02:56,559
And look at that, vsftpd is started as well.

40
00:02:56,559 --> 00:02:59,320
Can we find any information about it?

41
00:02:59,320 --> 00:03:02,839
Systemctl status vsftpd.

42
00:03:02,839 --> 00:03:05,080
Without the service, it defaults to service.

43
00:03:05,080 --> 00:03:11,279
I can see that it is started, but you don't really see anything about the dependency relation,

44
00:03:11,279 --> 00:03:14,000
the reason why it has been started.

45
00:03:14,000 --> 00:03:20,240
And on the httpd, well, we can see that httpd has an override.conf.

46
00:03:20,240 --> 00:03:23,820
But for the rest of it, we don't see the dependency relation either.

47
00:03:23,820 --> 00:03:28,559
So you really need to figure out how it works.

48
00:03:28,559 --> 00:03:32,080
And that is where systemctl list dependencies can come in handy.