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[Intro Music]

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I have just explained to you what is

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default bridge network. And you

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will now understand that containers

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may communicate between each other

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using own IP addresses assigned

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randomly by Docker host. And we have

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verified that using two busybox

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containers. Next important concept

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is environment variable. Let me

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explain first what is an

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environment variable, and

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afterwards I'll explain you how

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you're able to set custom

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environment variable for specific

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container. Great. We are now still

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running two busybox containers. Let me

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clear terminal in one of them,

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and here simply type command 'env'

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like so, and you'll get a list

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of let's say key-value pairs. Here

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are keys or names of environment

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variables - HOSTNAME, SHLVL, HOME,

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TERM, PATH, and PWD. And that's our

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default variables that are set out

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of the box for every busybox

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container. And again, set of

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environment variables is different

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for different containers. And we

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could easily verify that. Let's in

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another step launch another

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container, for example, ubuntu

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container. 'docker run -it

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ubuntu', and let's type 'env' here,

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and you'll see a bunch of different

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environment variables that were set

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for this specific container. For

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example, you see here a variable

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called LS_COLORS, or

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you see another HOSTNAME variable,

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it is actually the same variable as

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was set for those containers, but

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HOSTNAME here is different, of

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course, from this HOSTNAME. Also,

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you may see that such variables

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as PWD, HOME, are the same for

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this container and this container.

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But again, those variables are set

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for each specific container

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individually. Also, you are able to

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verify environment variables

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without logging into 'sh' or 'bash'

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shell by doing following. Let me

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go out from this container, clear

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terminal, let's list the containers

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that are currently running. There

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are two busybox containers. And you

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are able to use 'exec' command in

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order to execute 'env' command inside

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of the container. Let's do so.

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'docker exec', and here will be ID of

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the container, for example, this

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one, 'a5', and next type 'env',

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again here is command that will

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override default command, and we

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will actually create additional

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process that will be executed

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inside of specific container with

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specific ID. Let's press Enter, and

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you'll also see environment

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variables. But notice that with

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this command, you'll see less

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environment variables than in this

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case, because in this case, we are

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looking at environment variables

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inside of the 'sh' shell that

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actually added some additional

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environment variables when we have

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logged into 'sh' shell. That's the

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reason why here was set of

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environment variables that is

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larger than here. Wonderful. But

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what is the essence actually of using

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those environment variables? Very

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often applications require some

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initial setting parameters, like

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password, the username or some

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connection details or something

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else. And you are able to set those

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values before the start of the

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application. For example, let's say

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that your application needs to

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connect to some external server and

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for that it needs to use

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credentials - username and password.

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And it is not safe to store this

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username and password inside of the

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application files. It is not safe

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at all, because anyone else may

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easily read this username and

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password and use for its own

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purposes. That's why you are able

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to initialize those parameters,

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username and password when

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application is started. And you are

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able to use for that environment

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variables. Also you are able to

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override any default environment

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variables if you want. Again, it's

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a pretty common concept to use

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environment variables in different

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applications. And we will use those

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variables multiple times throughout

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this course. For now, that's all

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for this lecture. And let's proceed

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and next as promised, let's create

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mysql container and see whether it

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will be started or not by default

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without any additional

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environmental variables. I will see

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you next. Bye-Bye.

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[no audio]