Running a container with docker run - Commands & Configuration

Introduction

Sometimes you want to quickly start a program inside a container without setting up complex files. The docker run command lets you do this by creating and starting a container from an image in one step.

When you want to test a new application image quickly without writing configuration files

When you need to run a simple web server on your local machine for development

When you want to run a database container temporarily for testing

When you want to try out a command-line tool inside a container without installing it on your computer

When you want to expose a container's service port to your local machine to access it via a browser

Commands

This command starts a new container named 'my-nginx' from the nginx image version 1.23.3. It maps port 80 inside the container to port 8080 on your computer, so you can access the web server at localhost:8080.

Terminal

docker run --name my-nginx -p 8080:80 nginx:1.23.3

Expected Output

Unable to find image 'nginx:1.23.3' locally

1.23.3: Pulling from library/nginx

Digest: sha256:...

Status: Downloaded newer image for nginx:1.23.3

2024/06/01 12:00:00 [notice] 1#1: using the "epoll" event method

2024/06/01 12:00:00 [notice] 1#1: nginx/1.23.3

2024/06/01 12:00:00 [notice] 1#1: built by gcc 10.2.1 20210110 (Debian 10.2.1-6)

2024/06/01 12:00:00 [notice] 1#1: OS: Linux 5.10.0-21-amd64

2024/06/01 12:00:00 [notice] 1#1: getrlimit(RLIMIT_NOFILE): 1048576:1048576

2024/06/01 12:00:00 [notice] 1#1: start worker processes

2024/06/01 12:00:00 [notice] 1#1: start worker process 31

→

--name - Assigns a custom name to the container for easier management

→

-p - Maps a port from the container to the host machine

This command lists all running containers so you can verify that 'my-nginx' is running and see its details.

Terminal

docker ps

Expected Output

CONTAINER ID   IMAGE          COMMAND                  CREATED          STATUS          PORTS                  NAMES

abcdef123456   nginx:1.23.3   "/docker-entrypoint.…"   10 seconds ago   Up 9 seconds    0.0.0.0:8080->80/tcp   my-nginx

This command fetches the default web page served by the nginx container through the mapped port to confirm the server is working.

Terminal

curl http://localhost:8080

Expected Output

<!DOCTYPE html>

<html>

<head>

<title>Welcome to nginx!</title>

</head>

<body>

<h1>Welcome to nginx!</h1>

</body>

</html>

This command stops the running 'my-nginx' container when you no longer need it.

Terminal

docker stop my-nginx

Expected Output

my-nginx

This command removes the stopped container to free up system resources.

Terminal

docker rm my-nginx

Expected Output

my-nginx

Key Concept

If you remember nothing else from this pattern, remember: docker run creates and starts a container from an image in one step, optionally mapping ports and naming the container for easy access.

Common Mistakes

Not mapping container ports to host ports with -p flag

Without port mapping, services inside the container cannot be accessed from your computer

Always use -p host_port:container_port to expose container services

Using docker run without --name and then losing track of container ID

It becomes hard to manage or stop the container without a memorable name

Use --name to assign a clear name to your container

Trying to run a container from an image that does not exist locally without internet access

Docker cannot download the image and the command fails

Ensure you have internet or pre-pull the image with docker pull before running

Summary

Use docker run to start a container from an image with one command.

Map container ports to your computer ports with -p to access services.

Name your container with --name to manage it easily later.

Practice

1. What does the docker run command do?

easy

A. Starts a new container from a specified image

B. Builds a new Docker image from a Dockerfile

C. Stops a running Docker container

D. Deletes an existing Docker image

Solution

1. Step 1: Understand the purpose of docker run

   The docker run command is used to start a new container based on a Docker image.

2. Step 2: Differentiate from other Docker commands

   Commands like docker build create images, docker stop stops containers, and docker rmi removes images, so they are not correct here.

3. Final Answer:

   Starts a new container from a specified image -> Option A

4. Quick Check:

   docker run = start container [OK]

Hint: Remember: run = start container from image [OK]

Common Mistakes:

• Confusing 'run' with 'build' command
• Thinking 'run' deletes images
• Mixing 'run' with stopping containers

2. Which of the following is the correct syntax to run an interactive container with a bash shell using the image ubuntu?

easy

A. docker run ubuntu -it bash

B. docker run -d ubuntu bash

C. docker run -it ubuntu bash

D. docker run ubuntu -d -it bash

Solution

1. Step 1: Identify flags for interactive mode

   The -it flag is used to run a container interactively with a terminal attached.

2. Step 2: Check command order and flags

   The correct syntax places flags immediately after docker run, then the image name, then the command to run inside the container.

3. Final Answer:

   docker run -it ubuntu bash -> Option C

4. Quick Check:

   Flags before image, -it for interactive [OK]

Hint: Put flags right after 'docker run', then image, then command [OK]

Common Mistakes:

• Placing flags after image name
• Using -d for interactive shell
• Mixing order of command and flags

3. What will be the output of the following command?

docker run --rm alpine echo Hello World

medium

A. Hello World

B. Container ID printed

C. Error: alpine image not found

D. No output, container runs in background

Solution

1. Step 1: Understand the command components

   docker run --rm alpine echo Hello World runs the echo Hello World command inside an Alpine container and removes the container after it finishes.

2. Step 2: Predict the output

   The command prints Hello World to the terminal, then the container stops and is removed automatically.

3. Final Answer:

   Hello World -> Option A

4. Quick Check:

   Run with echo prints text, --rm cleans up [OK]

Hint: Echo prints text, --rm removes container after run [OK]

Common Mistakes:

• Expecting container ID output
• Thinking container stays running
• Assuming image is missing

4. You run docker run -p 8080:80 nginx but cannot access the web server on port 8080. What is a likely cause?

medium

A. Port mapping syntax is incorrect

B. The container is not running

C. You forgot to add -d to run detached

D. Port 80 inside container is not exposed

Solution

1. Step 1: Check container running status

   If the container is not running, port mapping won't work and you can't access the server.

2. Step 2: Verify port exposure and syntax

   By default, nginx listens on port 80 and the syntax -p 8080:80 is correct. -d is optional but recommended for background.

3. Final Answer:

   The container is not running -> Option B

4. Quick Check:

   Container must run for port mapping to work [OK]

Hint: Check if container is running before checking ports [OK]

Common Mistakes:

• Assuming port 80 is not exposed by nginx
• Thinking -d is required for port mapping
• Misunderstanding port mapping syntax

5. You want to run a container from the image myapp that needs environment variables API_KEY and DEBUG set, and expose port 5000 on the host to port 80 in the container. Which command is correct?

hard

A. docker run -e API_KEY=123 DEBUG=true -p 5000:80 myapp

B. docker run -p 80:5000 -e API_KEY=123 -e DEBUG=true myapp

C. docker run myapp -e API_KEY=123 -e DEBUG=true -p 5000:80

D. docker run -e API_KEY=123 -e DEBUG=true -p 5000:80 myapp

Solution

1. Step 1: Set environment variables correctly

   Use -e VAR=value for each environment variable before the image name.

2. Step 2: Map ports correctly

   Host port 5000 maps to container port 80 with -p 5000:80. The order is flags, then image name.

3. Step 3: Verify command order and syntax

   Options must come before the image name. docker run -p 80:5000 -e API_KEY=123 -e DEBUG=true myapp reverses ports, C places options after image, D misses -e for second variable.

4. Final Answer:

   docker run -e API_KEY=123 -e DEBUG=true -p 5000:80 myapp -> Option D

5. Quick Check:

   Flags before image, correct port and env syntax [OK]

Hint: Put all flags before image; use -e for each env var [OK]

Common Mistakes:

• Placing options after image name
• Reversing port mapping order
• Missing -e for environment variables