WEBVTT

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-: In the last section,

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we downloaded and installed Docker for Windows

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or Docker for Mac on your personal computer.

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You should now be able to ran the command Docker version

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at the command line

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and see a little bit of text like this appear.

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I now want to write out

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our very first kind of meaningful command

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with the Docker client or the Docker CLI.

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We're going to run a very quick command here,

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and then we're gonna go

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through a very specific flow of actions that occurred

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when that command got executed.

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So, here's the command.

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We're gonna write docker run hello-world.

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Yes, it is kind of a hello world thing of sorts.

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Don't worry, it's gonna be rather interesting.

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I'll then press enter,

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and we're gonna very quickly see

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a lot of text start to scroll along the screen.

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Now, I'm gonna scroll up a little bit

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and you'll see

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a little hello from Docker message right here.

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And then you'll notice underneath that

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it list out these series of steps that just occurred

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when you ran that command.

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Now, we're going to go over the series of steps right here

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in great detail.

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So, if you want to,

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you can kind of ignore that text for right now.

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I'm gonna scroll up a little bit further

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and I want you to see up here it says,

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"Unable to find image hello world locally."

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All right, so with that mind,

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let's go take a look at a couple of diagrams

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that are gonna help explain what just occurred

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when we run that command.

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All right, here we go.

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So, at the Terminal,

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you and I executed the command, docker run hello-world.

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That starts up that Docker client or the Docker CLI.

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Again, the Docker CLI

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is in charge of taking commands from you,

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kind of doing a little bit of processing on them,

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and then communicating the commands

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over to something called the Docker server.

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And it's that docker server that is really in charge

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of the heavy lifting.

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When we ran the command, docker run hello-world,

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that meant that we wanted to start up a new container

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using the image with the name of hello world.

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The hello world image

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has a tiny little program inside of it

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whose sole purpose, sole job,

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issue print-out the message that you see right here.

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That's the only purpose of that image.

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Now, when we ran that command

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and it was issued over to the Docker server,

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a series of actions very quickly occurred in the background.

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The Docker server

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saw that we were trying to start up a new container

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using an image called hello world.

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The first thing that the Docker server did

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was check to see if it already had a local copy.

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Like a copy on your personal machine

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of the hello world image or that hello world file.

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So, the Docker server

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looked into something called the ImageCash.

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Now, because you and I just installed Docker

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on our personal computers,

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the ImageCache is currently empty.

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We have no images that have already been downloaded before.

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So, because the ImageCache was empty,

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the Docker server decided to reach out

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to a free service called Docker Hub.

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The Docker Hub is a repository of free public images

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so you can freely download and run

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on your personal computer.

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So, Docker server reached out to Docker Hub and said,

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"Hey, I'm looking for an image called hello world.

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Do you have one?"

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Of course, the Docker hub does.

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So, Docker server downloaded this hello world file

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and stored it on your personal computer in this ImageCache

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where it can now be reran

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at some point in the future very quickly

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without having to re-download it from the Docker Hub.

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So, essentially, we downloaded it over here like some.

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After that, the Docker server then said,

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"Okay, great. I've got this image

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and now, it's time to use it

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to create an instance of a container."

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And I remember

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what we just said about a container a moment ago.

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An instant container is an instance of an image.

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Its sole purpose is to run one very specific program.

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So, the Docker server

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then essentially took that single file,

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loaded it up into memory,

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created a container out of it,

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and then ran a single program inside of it.

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And that single program's purpose

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was to print out the message that you see right here.

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So, that's pretty much it.

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That's what happens when you run this Docker run command.

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It reaches out to Docker Hub,

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it grabs the image,

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and then it creates a container out of that image.

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Now, one thing that you'll notice that's kind of interesting

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if we run the docker run hello-world command a second time,

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you'll notice that we are not going to see the message

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of downloading or container image not found locally

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that we saw the first time.

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So, scroll back up to the very top.

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Here's the first time that we ran the command.

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You'll notice it up here it said,

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"Unable to find image locally."

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But now the second time,

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we did not see that message

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and that is because we already downloaded it

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to our ImageCache on our personal computer.

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So, the big lesson here

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is that the first time that you try

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to make use of any of these public images over here,

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you're gonna have to do a little bit of a download.

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But then in the future after that,

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you can start up a container

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using that image much more quickly

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because the image has already been downloaded

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to your computer.

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All right, now in this entire flow,

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one thing that we've been kind of light on at this point

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is talking about exactly what a container is.

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So, let's take a quick pause right here.

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We're gonna come back the next section

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and go into a little bit greater detail

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on exactly what a container is.

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So, quick break and I'll see you in just a minute.