Running a container with docker run - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the docker run command do?

It creates and starts a new container from a specified Docker image.

beginner

How do you run a container in the background (detached mode)?

Use the -d option with docker run, like docker run -d image_name.

beginner

What option allows you to map a container port to a host port?

The -p option, for example docker run -p 8080:80 image_name maps host port 8080 to container port 80.

beginner

How can you give a container a custom name when running it?

Use the --name option, like docker run --name mycontainer image_name.

beginner

What happens if you run docker run without specifying a command?

Docker runs the default command defined in the image's CMD or ENTRYPOINT.

Which option runs a Docker container in detached mode?

A -p

B --name

C -d

D -it

The -d option runs the container in the background (detached mode).

How do you map port 5000 on your computer to port 80 in the container?

A docker run -p 5000:80 image

B docker run -p 80:5000 image

C docker run --port 5000:80 image

D docker run -port 80:5000 image

The syntax is -p hostPort:containerPort, so -p 5000:80 maps host port 5000 to container port 80.

What does docker run --name myapp image do?

A Names the image as myapp

B Names the container as myapp

C Runs the container with default name

D Deletes the container named myapp

The --name option assigns a custom name to the container.

If you want to interact with a container's terminal, which option should you add?

A -it

B -p

C -d

D --name

The -it option allows interactive terminal access to the container.

What is the default behavior if no command is given in docker run?

A Throws an error

B Container will not start

C Runs an interactive shell

D Runs the image's default command

Docker runs the default command specified in the image's configuration.

Explain how to run a Docker container in detached mode with a custom name and port mapping.

Think about options for background, naming, and ports.

Describe what happens when you run docker run without specifying a command or options.

Focus on Docker image defaults.

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