(If we were to remove the image from our machine, it would be downloaded again.) Time to experiment. Since we already have this image on our machine, it will be easy for Docker to grab it. We are using bash instead of the Node REPL. Nothing special, right? That means the Node REPL. You can type exit to leave the interactive shell and close the container. To close the Node REPL, you can type .exit in the REPL. If we run $ docker images in the command line again, we'll even see our new image. The script runs, ends, and then the process is finished and the container closes. Last updated July 31, 2022. Both of these configuration files can get a lot more complicated. Then script.js was copied again, the image was successfully built, and Docker ran our container with the image. Next, we indicate the services we are using. This is where winpty comes in. What gives? It takes a little time but it's not the end of the world. This is the base layer. We will just call it app here. Hey, wait a minute. Try out this command: If we run this command, we'll enter an interactive bash shell. It's common to put each part of that command in an array. Well, we are writing our script in VS Code. It did that very faithfully. But what's actually running here is the Node REPL inside of a Docker container. We don't have to download node again because we already have the image on our machine. Next, we set a working directory with WORKDIR. As you'll see, the container will stop running once we leave the REPL. We'll start by adding a Dockerfile to our project with several lines: A Dockerfile always starts with FROM. Well, it may not seem like it if you already have Node installed on your machine. Make sure you have Docker installed and running. What do you think? However, usually our containers will be running local servers and databases. This is the working directory inside the Docker container. In this case, the base layer is a Node development environment. It'll look something like this: We now have two images here: one for node and one for our newly created image. Remember, we can't run a container without an image. These are continuously running processes and we can use $ docker-compose down to stop those containers from running. The second element in the array is what node will run (/script.js). Docker provides public images that we can pull and use. First, let's actually use this one. So now we're ready for the Docker magic to happen. specified here. That is on our local machine. We'll discuss how to remove images soon. When Docker spins up an interactive terminal, it sometimes runs into this exact communication issue and needs the help of winpty. Now run $ docker-compose up again. Well, let's start by creating a small Node script which we'll store in a directory called docker-example: This will just print Hello world! So let's try something out. It just means look in the current directory for the Dockerfile, any files that need to be copied over, and so on. But if you try to run $ node script.js inside this interactive shell, you'll get the following error: The reason for this error should be easy to guess. We won't need to pull it again if we need to use it. Well, if the script had come with a Dockerfile configuration, you wouldn't have needed to spend all that time installing that software. We won't need any configuration for this. That's potentially very inefficient and a waste of time, especially in production. We are inside a container now and it's walled off from the contents of our local machine. It's an image that no longer has any use and is just wasting space. And the script is absolutely essential to your work. As long as you have Node on your machine, it will output Hello world! Install Docker, run $ docker-compose up, and you're done. Node is being too formal. For our purposes, it'll be common to have one service for a web application (running a local server) and another for a database. We'll need to use a Dockerfile and docker-compose.yaml so that we can configure our Docker container to load and run our script. Lesson 4 of 10 The solution to this issue is exactly what the message suggests: prefix the command with winpty. Once again, Docker is running a container with the Node image but this time, we are using a different process. There can be multiple services specified in a docker-compose.yaml file. Did you get this error message after running docker run -it node? We've added just one instruction: You'll usually see a . We can use this command instead: This command will remove all stopped containers and unused images in addition to dangling images. If you are ever in a situation where you've updated your code on your local machine and it's not updating in Docker, you probably omitted the --build flag and you need to run $ docker-compose up --build. At the very end, our application runs Hello world! Using our example of creating a container with an interactive Node terminal, the command should look like this: So, what's happening here? Our test example will be extremely simple so we can really focus on Docker itself. Let's make an update to our script in VS Code. Can we run this script from inside a Docker interactive terminal? to the console. So as we can see, Docker builds our image and then runs that image inside of a container. We could run a container with this image by just running the following command: However, it looks like nothing happens. The latest update isn't on our image. This happens to some Windows users when they try to start an interactive terminal with Docker. Why would it? Let's start by looking at a very simple example of Docker in action. It will clean up things a lot more nicely, which you can verify by running $ docker images. As you'll see, it now correctly says "Hey world!". We can even check the version of Node that's being used inside our interactive shell (node -v) and it'll quite likely be different from the version you have installed on your local machine. The following command will generally clean up all dangling images. Next, let's add a little bit to docker-compose.yaml: We are keeping this very simple. Note that this is the copy inside the container in our working directory not the copy of the script on our local machine. We can add other layers on top of that but we need that base layer. If we moved our Dockerfile and script.js file into another directory called some-directory, it would be build: ./some-directory instead. No Dockerfile, no docker-compose.yaml in fact, no project at all. Or rather, hello, wait a minute. In this case, we only need to copy over script.js. Nothing can get in or get out. winpty is a program that provides an interface so that software like MinTTY can communicate with Windows console programs. $ docker-compose down has no effect on the images on our machine. We can name our service whatever we want. Note that this isn't the Docker version we are using, it's the version of docker-compose we are using. It is isolated from your machine's environment. By the way, if we ever want to stop a container from running, we use the following command in the root directory of the project where we ran $ docker-compose up: Note that this will do nothing in this particular case because our process isn't continuously running. That means we need to trigger a build all over again so we can copy the latest updates to script.js. We use the $ docker run command to create a container. It works on someone else's computer but not on yours. What if we wanted to run a Node script within Docker? The Dockerfile specifies the build instructions so all of those commands run again. But what is a dangling image? You'll notice that the node image isn't there anymore. And yeah, they are big. It's a Node command. We need to use the --build flag to do that: We triggered a build with the --build flag. instead of "Hello world!. In this case, the process is the Node REPL. This will set the groundwork to using containers that are specific for that section's development environment. Remember that COPY command? Then run the following command: It will take a little time for your machine to download this image, but once it's done, we can take a look at a list of all our images with the following command: So this image now lives on our machine. But imagine if you'd spent the last three hours trying to install software on your machine simply because you wanted to run a script. It's good to run this command sometimes just to clean things up but it's not going to remove the images we've just created. Next, we need to COPY over any files we need from our local machine. This interactive terminal could be for Node, Ruby, Rails or another technology. In the process, we'll also go over some basic concepts and commands that you'll be using with Docker containers and images. That's because a Docker container runs a process. Without this command, our Docker container won't include this file. So what about those images on our machine? Note the name of the new one docker-example_app. We can get everything running with the following command: The output will look something like this: As you can see, all the commands in our Dockerfile are run. That's because there's no continuous process involved. cd into the docker-example directory and test it with the following command (outside of Docker): node script.js. The one situation where you'd want to be careful with docker system prune is if you want to have database persistence with your volumes. Anytime you see the message about prefixing your Docker command with winpty, do just that. Once we close the REPL, the process is done. It should say "Hey world!" Finally, we run a specified command with CMD. Here, we specify the version of docker-compose we are using. What we name it doesn't matter. Under the hood, Git Bash uses MinTTY as a terminal emulator, and MinTTY is not compatible with Windows console programs. Note that our Dockerfile automatically used a cache where possible to make the process faster. In this lesson, we'll use Docker to build an image and run a container. And also, as you can see by the size of the image, you won't want to leave it on your machine if you aren't using it. Then node is the image that is actually running inside the container. Okay, so how do we get this script working? The first element in the array is the command we will run (node). Just to clarify, this isn't a Docker command. The images we have so far were very big and we don't need them anymore now that we are done with this brief Docker tutorial. This is a very simple Docker configuration and it's common to copy over entire directories, more configuration files, and so on. Well, docker-compose is for running our container. It's the name of our directory with the name of the service appended to it (app). This may seem like a lot of work to run Hello world! Our script still says "Hello world!" In the next lesson, we'll learn about using aliases, which will automate the process of prefixing your Docker commands with winpty. If we were to run $ docker-compose up, Docker would have to download the public image from scratch instead of using the image on our machine. The -it flag specifies that Docker should open an interactive Node terminal. As you'll see, a REPL for Node opens up. But it didn't rebuild it. However, having database persistence isn't expected while you are a student at Epicodus so you don't need to worry about it.
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