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In this lesson, you will learn about systemd targets.

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So what is a target?

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Well, technically, a target is a group of units

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that can be managed as one.

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So you can use systemctl start on a target

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and systemctl stop on a target to manage the current state

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of all the units that are included.

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Some targets are isolatable, and that

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means that they can be used to define

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a state in which the system should be started.

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These are emergency target, which

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is your minimal troubleshooting target,

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residual target, which is a little bit more

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troubleshooting target, multi-user target,

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which is the current state of a server that doesn't run

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a graphical user interface, and graphical.target,

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which is bringing a complete graphical interface.

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You can use systemctl get default

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to see the default target in which your system will boot.

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And you can use systemctl set default

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to set the default target to something else.

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Let me show you.

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So I'm starting with systemctl list units minus t target.

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Minus t is for type, and it allows

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you to show unit files of a specific type.

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Here you can see all the units that are available,

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including, for instance, a unit like sound.target.

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So what happens if you do systemctl status sound.target?

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Well, we can see that the sound target is active.

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Doesn't provide any more details about it.

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Finding what is in a target is not always obvious.

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I like to go into etc systemd system.

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And in there, I'm looking for the once directory.

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And oh, boy, we don't have a once directory

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for the sound.target.

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Well, that means that the sound.target is empty.

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But I'm going to use the printer.target as an example.

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So if we use systemctl status on printer.target, we see what?

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We see that it is loaded, which means

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that systemd knows about the configuration file,

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but it's currently inactive.

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But if I use ls on printer.target once,

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then I can see symbolic links referring

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to the units that should be included in this target.

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More interesting are the targets that are isolatable.

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Let me use systemctl cat on graphical.target.

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And what do we see?

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We see that it has an allow isolate is yes.

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We also see that there is an after,

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which is defining when the graphical target should

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be loaded.

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So this should be loaded after multi-user target.

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And even after the display manager.service,

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because it needs display manager.service to be available.

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You want to know what is going on in this target?

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Well, use ls on graphical.target.once.

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And you can see everything that is started in this target.

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When you use systemctl enable, every unit

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has the install section, where it defines the target

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that it should be installed in.

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And when you use systemctl enable on a unit,

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the unit creates a symbolic link in this directory.

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And that makes sure that when you are processing your target,

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you will start all of these units.

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Now, your system has a default target.

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And you can find out using systemctl get default.

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There we can see that my default is graphical.target.

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You can use systemctl set default

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followed by anything else to change the default target.

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You can also specify your targets

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while you are rebooting.

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At the grub command prompt.

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I'm going to reboot so that I can show you.

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Now, you don't see the grub shell by default.

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It requires a little bit of practice

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to press escape at the right moment if you want to see it.

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And if you don't see it, it also helps

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if you reset your system while it is booting,

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because then on the next start,

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it will detect that the last time

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it wasn't started correctly.

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And it will automatically give you the grub shell.

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I'm going to enter this grub shell by using E,

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which puts me in edit mode.

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And there I'm looking up the line that starts with Linux

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and it ends with RHDB and quiet.

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First, I'm going to remove the RHDB and quiet.

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And I'm using systemd.unit

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is emergency.target.

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Emergency target is what you can use if you are in trouble.

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There is even more minimal modes,

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but we will talk about that later

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when we talk about the system boot procedure.

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For now, I want to show you emergency target.

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Likewise, you can also use systemd.unit is rescue.target.

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Use control X to start in the target,

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and then this will happen.

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So here we see what?

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Oh, no, I see cannot open access to console.

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The root account is locked.

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See, as you log in man page for more details

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and press enter to continue.

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You know what that means?

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That means that I can't enter emergency targets right now

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because my root account is protected.

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Fortunately, later in this course,

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you will learn how you can unlock your root account

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if it is locked.

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Consult lesson 12.6 in this lesson for more information

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on how to reset the root user account.

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For now, as I'm in a bootable state,

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I can use a different approach

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and show you emergency target anyway.

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So here I'm back on my system,

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and I'm going to use sudo minus I to open my sudo shell,

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and I'm using passwd root or even passwd.

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I'm in a root shell anyway.

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New password going once, new password going twice.

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At this point, my root user is enabled.

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I can log in as a root user

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because root user now has a password.

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So I'm going to reboot,

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and I need to enter the grub boot prompt again.

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Oops, that was too fast.

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If you see this, just type exit to get back into the grub shell.

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Then you type escape again,

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and you need to edit this line that starts with Linux.

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So once again, I'm using systemd.unit is emergency.target

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and control X to start in the emergency target.

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And this time you see that it is asking for the root password.

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Give root password for maintenance.

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It's doing that because emergency target is protected

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by the SU login program,

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and that means that I need to enter the root password

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to get access to a root shell

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that I can use for further troubleshooting.

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Now, if you want to investigate,

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it's interesting to see systemctl list units.

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That will show you all the units

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that you have in emergency target.

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And as you can see, there is 60 only.

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Now, once done in your emergency target,

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use a reboot to get back in a normal operational state.