Systemd Targets (Demo)

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Time
21 hours 25 minutes
Difficulty
Intermediate
CEU/CPE
21
Video Transcription
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>> Hello, Cybrarians. Welcome back to
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the Linux+ course here at Cybrary.
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I'm your instructor Rob Gayles,
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and in today's lesson we're going to be
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talking about systemd targets.
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Now upon completion of today's lesson,
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you're going to be able to explain
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the purpose of the systemd target files,
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which are actually just unit files
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like we talked about in the previous lesson.
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We're going to talk about how
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the systemd target files work
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and use the systemctl command
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to interact with the target files.
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In systemd you can control services
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and system capabilities that come up as boot time,
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and this is done through a target file.
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Previously in SysVInit this was called a runlevel,
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but here in systemd this is referred to as a target.
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Now, there are several targets to
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know about and I'm going to try to relate
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these to SysVInit because we're going to talk
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about that later in this module
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and we want to have a common
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understanding of what things do.
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Poweroff.target does exactly what it sounds like,
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it shuts off the system,
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shuts down the system, and that's equivalent
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to runlevel 0 in SysVInit.
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The rescue.target configures a rescue shell session
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that's equivalent to runlevel 1 in SysVInit.
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Multi-user target boots the system into
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a non-graphical multi-user environment
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that's equivalent to runlevel 2,
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3, and 4 in SysVInit,
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and graphical target boots the system
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into a graphical multi-user environment.
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That's pretty much what we've been using in
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our demos as we've gone through all of these lessons.
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That's going to be equivalent to what's called
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runlevel 5 in SysVInit.
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Then the final target that we need to know about here
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>> is the reboot target that reboots the system which
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>> is equivalent to runlevel 6 in SysVInit.
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Let's go ahead and have a look at all of
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these target files with some demo time.
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Here we are over in our demo environment and
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today we're going to be using CentOS.
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The first thing we'll do with systemctl related to
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targets is that we'll display the default target,
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and the way that we do that is by running
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the systemctl command called git.default,
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and we can see that it displays graphical target.
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That's makes sense, we're in
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graphical user environment here we can see that we
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have a background and a desktop is not
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just the command line with just black and white cursor,
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so this is a graphical environment.
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We can look at this target file by actually running
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the systemctl cat command
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and then specifying the target that we want to view,
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which is really nice, we don't need to go
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hunt down the location of that unit file.
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We can just tell systemctl, hey,
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display this file for us,
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and there we go, we see the content of that unit file,
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of that target file.
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Now the target file has a couple
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of lines that are really important.
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These two are the requires in want line,
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requires means that before
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>> this target can be processed,
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>> all of the services that are in
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the multi-user target must be started.
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Then the wants statement says that all
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of this gets processed after
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the require statements fully processed,
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here this means that
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the display manager services started,
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which brings up the graphical user interface.
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In essence, what all this means is that
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>> the kernel boots,
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>> then systemd initialization takes over
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>> it processes the multiuser target,
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and once it's done with that then it moves on to
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the want line where it brings up the graphical target.
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Really the graphical target
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its only purpose is to bring up
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this display manager service and
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give us the display manager to work.
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Now, one of the other things that we can
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do here is we can look
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in the target file and see that
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there's a line that says "allow isolate yes,"
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>> what does that mean? Well, it's cool.
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>> The isolate command is how we actually change
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between targets in systemd.
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For example, if we want to change from
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the graphical target and
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go to just a command line or multiuser target,
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we can do that using the systemctl command,
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and we say isolate,
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and we're going to put us into,
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let's say we want to go into multi-user.target.
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Actually I'm going to run this as pseudo,
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I'm going to elevate my privileges
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as pseudo and temporarily become root
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>> because we need to do that to change targets.
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>> Now when I hit "Enter" with
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hopefully the right password, there we go.
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We boot into just this
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non-graphical multi-user target environment
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which is just a command line.
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I'm going to log back in as me.
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Now that I'm logged in here I can use systemctl,
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get default, and
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we can see that I'm still set
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the default the graphical target.
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Well, that's weird.
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What do you think's going to happen? Well, let's see.
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Let's do a systemctl, isolate,
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and we're going to go to the reboot target.
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[NOISE] I'm just going
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to actually use pseudo as well because again,
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we're going to need to elevate
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privileges in order to do this,
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change our targets and hit "Enter" and we reboot.
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We can see that this is a VirtualBox VM,
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so we're actually just going to use that to
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reboot into CentOS once again.
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We'll wait a little while for
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that to come up and bring us
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to a login session.
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Again, that's going to bring us to
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that user-space to login,
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so we'll wait a couple of seconds for
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that to come up and we get logged in here.
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We can already see that this is coming
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up in a graphical target.
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When the system comes up,
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it comes up in a graphical target,
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and so we might be asking ourselves, why is that?
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Well, again, like I just showed you a
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moment ago when we went in and we ran that systemctl,
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get default, we saw that the system was
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set to be the graphical target.
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We'll do that again here in just a second.
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Let's wait for this to finish loading,
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>> and there we are.
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>> Let's go ahead and open up a terminal window,
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and we'll maximize the terminal
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window and then we'll do a systemctl,
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get default and we
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can see that it's still set to graphical target.
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Now if we wanted to change this so
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that every time we reboot,
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we boot into that multi-user target
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instead of graphical, you can do that.
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You do multi-task user target,
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and now if we do,
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we're going to provide a password remember,
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in order to change targets,
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you need to be root or you need
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to elevate your privileges with pseudo.
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We can see what it actually does is it creates a same
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like it removes the same like that
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exists for default target,
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and then it recreates that same link,
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it's default target just
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points to one of the target files.
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That's really all it does.
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We see that it points from
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systemd system default target to
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user lib systemd system multi-user target.
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That's where that unit file actually lives,
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and we talked about that previously.
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But we don't really want to be in multi-user,
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I want to use this graphical environment just
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as a little nicer to show things during the demo.
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Let me change this back.
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I'm going to change this from
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multi-user target to graphical target,
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[NOISE] and then I'm going
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to do this as pseudo and
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put in my pseudo password again,
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so you can elevate privileges, and there we go.
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It removes the default target and we set
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>> the default target to now point to graphical target.
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>> Just changed it similarly, it's
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all similarly tricks, all similarly video.
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But with that, we wish to end this lesson,
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>> and in this lesson we cover the purpose of target
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>> unit files versus
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>> convenient how those target files work.
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>> We also use systemctl to change
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between targets and to set the default target.
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Thanks so much for being here and I look
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forward to seeing you in our next lesson.
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