Systemd Command (Demo)
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Video Transcription
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>> Hey there, Cybrarians.
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Welcome back to the Linux plus course here at Cybrary.
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I'm your instructor Rob Gills.
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In today's lesson, we're going
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to talk about the systemd commands.
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Upon completion of this lesson,
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you're going to be able to understand the purpose of
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the systemctl command and then use
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the systemctl command to manage
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services and configure services to boot automatically.
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Let's get after it with some demo time.
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Here we are in our demo environment in Ubuntu.
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What we're going to do here today
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is we're going to start working with the systemctl.
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Now the systemctl command is
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primarily used to manage services,
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so let's run some of those commands.
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We're going to start out with systemctl,
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and then we're going to say that we want
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the status of a service.
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We can get the status of, let's say, ssh.
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Then we can see that the service status
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of this is active running.
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We can also see that it's enabled at boot.
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We can see it's enabled right
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here in terms of the loaded status.
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If we wanted to restart the service, we could do that.
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We're going to do this with a sudo command so that we
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can do this without having to
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constantly type the password.
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We're going to say restart instead of status.
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Now it's going to restart the service.
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When we run systemctl status,
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we see that it is still
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running and we're good to go there.
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That's been started. We see that it got
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restarted a second ago. We're good to go.
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Now what we can also do is we can stop the service.
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We can stop it from running with the systemctl stop.
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We're going to use sudo to become root
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temporarily to do that. There we go.
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Now if we run the systemctl status on ssh again,
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we see that it's inactive or
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dead instead of active and running.
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That just means that the service has stopped.
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Now what we do still see here though,
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is that the service is enabled and
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the systemctl command can also be
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used to manage whether or not a service starts at boot,
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whether it's enabled or disabled.
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Ssh is enabled, so let's disable it.
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We're going to go ahead and do sudo systemctl
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disable ssh.
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Note that when we disable this,
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is actually removing links here.
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It just basically is pointing to
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the multiuser target wants ssh service.
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Basically, what happens is that when we boot into
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this multiuser target and
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we'll talk about targets later,
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there's a link there that says, hey,
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you also need to boot up the ssh service on start.
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That's really how a systemd initialization
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understands what services need to
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come up as child processes,
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which we talked about in the previous lesson.
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Now we can check the status again.
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We can do a systemctl status, ssh,
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and now we'll see that it's actually marked
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disabled and it's also inactive and dead,
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which means it stopped and it's disabled.
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But you know what, ssh is important.
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We want that so we can talk to
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other Linux systems and they can talk to us.
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Let's go ahead and re-enable it.
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We're going to do a sudo systemctl and we'll change
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this disable to an enable ssh.
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Then we see that it creates those symlinks again.
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Let's say that we want sshd to be
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linked to sshd service and we want
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that multiuser target to bring up ssh when we
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boot into multiuser mode
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when we're bringing up this Linux system.
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Then we can also go ahead and start it.
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Let's go ahead and do a sudo systemctl start ssh,
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and now let's get a status again,
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we don't need to run enable again, we already did that.
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Sorry. Let's get the status again.
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We see that it is active
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and running and then it's enabled.
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We just undid everything we did,
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everything's back to normal. We're good to go.
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Let's move on.
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>> Another command that we can sometimes
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>> run in systemctl is the daemon-reload command,
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and that's used to restart
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a service without disrupting users.
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What it does is it rereads
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application configuration files to pick up changes.
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It doesn't actually restart the service.
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This is commonly used with Apache
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>> or HTTPD web services,
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>> so the web page access isn't actually interrupted.
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The way that we can run this command is with systemctl,
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daemon or daemon, however, you want to pronounce this,
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>> reload and we're going to use
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>> sudo to do this so it doesn't
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yell at us and request a password.
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Now it's done.
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>> In many times you might also need to
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>> mask a service to ensure that
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>> it absolutely doesn't run.
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>> This is a service that you completely do
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not want to run on the system,
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you don't want it to start.
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If we were to go ahead and do that,
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let's pick on ssh again, just for fun.
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We're going to use sudo systemctl
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and we'll change this to say mask.
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We're going to mask ssh. There we go.
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What it does is it creates a symlink to /dev/null.
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Basically, it just says, hey,
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this all goes to the bit bucket.
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Anytime you try and start this using systemd,
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it's going to go to /dev/null
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and nothing's going to happen.
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Running systemctl mask changes that link.
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>> It knows the bit bucket.
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>> If we want to fix this,
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we can unmask the same link and turn ssh back on.
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You do sudo systemctl unmask ssh,
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and now we see it removes that link to the ssh service.
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If we run a systemctl status on ssh,
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>> we see it's all fine again.
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>> It's loaded, it's active,
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it's enabled, and we're good to go.
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But with that, we've reached the end of this lesson.
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In this lesson, we covered how to use systemctl
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to manage services with systemctl start,
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restart, stop, reload, daemon-reload and mask.
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We also covered how to configure
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if services boot automatically
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using systemctl enable and systemctl disable.
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Thank you so much for being here and I look
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forward to seeing you in the next lesson.
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