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In the last lecture

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we saw how we can inspect an image.

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Okay that's nice to have

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but what if you want to look into a container

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or maybe not just look into it

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but add something to it or extract something from it

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whilst it is already running?

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Docker has a command for this

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and that's the CP command which stands for copy.

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It allows you to copy files or folders

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into a running container or out of a running container.

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And here's how you could use this.

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Let's say we have a folder named Dummy here

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with a test text file

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where I simply say, hello.

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This is on my local host project folder

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not inside of the container.

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Now we want to copy that into an already running container.

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And for this, I already started a container here

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which has this name.

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So we can now run Docker copy

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and first of all specify the source.

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So the folder or file we want to copy somewhere else.

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In this case, I want to copy the Dummy folder

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and now you could either point at a specific file

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or add a dot to copy everything in that folder

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which is what I will do.

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Then you add a blank and then the destination

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of this copy operation

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so the path you want to copy to.

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And data would be inside of your container.

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For this you need the container name

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because Docker of course needs to know

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which container to copy this into.

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You could have multiple running containers after all

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so you specify your container name then a colon

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and then the path inside of the container

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you want a copy to.

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Let's say Test but this name is up to you.

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This path will be created if it doesn't exist yet.

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If I now hit enter,

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the content of Dummy should have been copied

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into the Test folder inside of the container.

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Okay, but how can we now see it?

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Well, let's delete test text on our local host machine

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and let's run this command again but differently now.

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Now the first argument is the path and the container

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and the second argument is our local folder.

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So the destination is now our local folder.

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If I now hit enter you see this test folder

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which was created in the container

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was now copied into Dummy.

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And the test text file in there is all the back.

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And if we would have wanted

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to target this file specifically we could of course done so.

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We can also run the command like this

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and now it copies just the text file into the Dummy folder

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without the Test folder.

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This folder is only there because of the previous command,

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this command.

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And that's how you can copy files

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between your local host machine and running containers.

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Now this command can be useful for a couple of things.

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It of course would allow you to add something to a container

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without restarting the container and rebuilding the image.

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Let's say our source code changed.

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Normally we need to rebuild the image because of that

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and restart the container.

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We could of course also just copy the changed code

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into the container.

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Typically that is not what you do though

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because it's pretty error prone.

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It would be easy to forget a file which you changed

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and you might then have a strange behavior

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or a broken application in your container.

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It's also not really possible to replace files

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which are currently executed like the server JS file.

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So copying a file into a container

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is not a good solution for that.

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We will instead learn about a better way

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of updating code in a container

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without rebuilding the image later.

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Nonetheless there could be scenarios

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where copying something into a container

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could be interesting for example,

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configuration files for a web server

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which you want to change.

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But in my opinion,

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copying something out of a container

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can also be quite interesting

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because if your container, for example,

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would generate a bunch of log files,

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you could use the Docker CP command to copy these log files

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out of your container onto your local host machine

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so that you can reach them there.

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So all of a sudden this container black box

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into which you normally can't look

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becomes more transparent

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because you can copy files and folders

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from inside the container to your local host system.

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And that could be an interesting use case

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for the Docker CP command

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which is why I also wanted to show this here.

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It's not a command we'll use very often in this course

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but still a command you should be aware of.