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Docker: Exposing Ports

When running Docker containers, you may need to expose ports to allow communication between the container and the host machine or other containers. This is especially important for web applications and services.

We will try to expose a port for a simple nginx web server.

Step 1: Exposing Ports at Runtime​

To expose a port when starting a container, use the -p flag followed by the port mapping in the format <host_port>:<container_port>. For example, to expose port 80 of a web server running in a container to port 8080 on the host, you would run:

docker run -p 8090:80 nginx

This command will pull the nginx image from Docker Hub (if not already present) and run a new container with port 8090 exposed.

Step 2: Interacting with the Web Server​

Once the container is running, you can interact with the web server by opening a web browser and navigating to http://localhost:8090. You should see the default nginx welcome page.

You can also use curl to test the web server from the command line:

curl http://localhost:8090

Step 3: Stopping the Web Server​

To stop the running nginx container, you can use CTRL+C in the terminal where the container is running. This will gracefully stop the nginx web server running in the container.

Step 4: Removing the Container​

If you want to remove the container completely after stopping it, you first need to get the containers ID:

docker ps -a

This will list all containers, including stopped ones. Find the container ID or name of the nginx container you want to remove.

Once you have the container ID or name, you can use the docker rm command:

docker rm <container_id_or_name>

This will delete the stopped container from your system.

Step 5: Running in detached mode​

To run a container in detached mode (in the background), you can use the -d flag:

docker run -d -p 8090:80 nginx

This command will start the nginx container in detached mode, allowing you to continue using your terminal while the container runs in the background.

Try accessing the web server again by navigating to http://localhost:8090 in your web browser or using curl:

curl http://localhost:8090

Step 6: Removing a container in detached mode​

When a container is running in detached mode, you need to first stop the container before removing it. You can stop the container using the docker stop command followed by the container ID or name:

docker stop <container_id_or_name>

Once the container is stopped, you can remove it using the docker rm command:

docker rm <container_id_or_name>