ENTRYPOINT vs CMD difference in Docker - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the Dockerfile line to set the default command using CMD.

Docker

CMD ["[1]"]

Drag options to blanks, or click blank then click option'

ACOPY

BRUN

CENTRYPOINT

Dbash

Attempts:

3 left

💡 Hint

Common Mistakes

Using RUN instead of CMD

Confusing ENTRYPOINT with CMD

The CMD instruction sets the default command to run when the container starts, such as bash.

2fill in blank

medium

Complete the Dockerfile line to set the container's main executable using ENTRYPOINT.

Docker

ENTRYPOINT ["[1]"]

Drag options to blanks, or click blank then click option'

ACOPY

BCMD

Cpython

DRUN

Attempts:

3 left

💡 Hint

Common Mistakes

Using CMD instead of ENTRYPOINT

Using Dockerfile commands like COPY here

ENTRYPOINT sets the main executable for the container, like python to run Python scripts.

3fill in blank

hard

Complete the docker run command to override the CMD arguments and echo 'World' instead.

Docker

ENTRYPOINT ["/bin/sh", "-c"]
CMD ["echo Hello"]
# To override CMD, use: docker run myimage [1]

Drag options to blanks, or click blank then click option'

Aecho World

B--entrypoint echo World

C--cmd echo World

D--rm

Attempts:

3 left

💡 Hint

Common Mistakes

Using --entrypoint, which overrides the ENTRYPOINT instead

Using invalid --cmd option

Confusing --rm with command override

CMD provides default arguments to the ENTRYPOINT, which can be overridden by simply passing arguments after the image name in docker run: docker run myimage echo World executes /bin/sh -c echo World.

4fill in blank

hard

Fill both blanks to create a Dockerfile snippet where ENTRYPOINT runs python and CMD passes the script name.

Docker

ENTRYPOINT ["[1]"]
CMD ["[2]"]

Drag options to blanks, or click blank then click option'

Apython

Bapp.py

Cbash

Dscript.sh

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping ENTRYPOINT and CMD values

Using bash instead of python for ENTRYPOINT

ENTRYPOINT runs python and CMD provides the default script app.py to run.

5fill in blank

hard

Fill all three blanks to write a Dockerfile snippet where ENTRYPOINT runs /bin/bash, CMD passes -c, and default command echoes Hello.

Docker

ENTRYPOINT ["[1]"]
CMD ["[2]", "[3]"]

Drag options to blanks, or click blank then click option'

A/bin/bash

B-c

Cecho Hello

D/bin/sh

Attempts:

3 left

💡 Hint

Common Mistakes

Using /bin/sh instead of /bin/bash

Missing the -c option for bash

ENTRYPOINT runs /bin/bash, CMD passes -c and the command echo Hello to execute.

Practice

1. What is the primary role of ENTRYPOINT in a Dockerfile?

easy

A. It provides default arguments for the command.

B. It specifies environment variables for the container.

C. It sets the main command that always runs when the container starts.

D. It defines the working directory inside the container.

Solution

1. Step 1: Understand ENTRYPOINT purpose

   ENTRYPOINT defines the main command that runs every time the container starts, making it fixed.

2. Step 2: Compare with CMD

   CMD provides default arguments or a default command only if ENTRYPOINT is not set, so it is not the main command itself.

3. Final Answer:

   It sets the main command that always runs when the container starts. -> Option C

4. Quick Check:

   ENTRYPOINT = main fixed command [OK]

Hint: ENTRYPOINT fixes the command, CMD sets defaults [OK]

Common Mistakes:

• Confusing CMD as the main command
• Thinking ENTRYPOINT sets environment variables
• Mixing ENTRYPOINT with working directory

2. Which of the following is the correct syntax to set ENTRYPOINT in a Dockerfile?

easy

A. ENTRYPOINT = "/bin/bash -c"

B. ENTRYPOINT ["/bin/bash", "-c"]

C. ENTRYPOINT [/bin/bash, "-c"]

D. ENTRYPOINT: /bin/bash -c

Solution

1. Step 1: Recall ENTRYPOINT syntax

   ENTRYPOINT uses JSON array syntax to specify the executable and its arguments, e.g. ["executable", "arg1"].

2. Step 2: Identify correct option

   ENTRYPOINT ["/bin/bash", "-c"] uses the correct JSON array format. Options A, B, and D use invalid syntax for Dockerfile ENTRYPOINT.

3. Final Answer:

   ENTRYPOINT ["/bin/bash", "-c"] -> Option B

4. Quick Check:

   ENTRYPOINT uses JSON array syntax [OK]

Hint: Use JSON array syntax for ENTRYPOINT [OK]

Common Mistakes:

• Using shell form without brackets for ENTRYPOINT
• Adding equal sign or colon incorrectly
• Using plain string without quotes

3. Given this Dockerfile snippet:

ENTRYPOINT ["echo"]
CMD ["Hello", "World"]

What will be the output when running the container without extra arguments?

medium

A. Hello World

B. echo Hello World

C. No output

D. Error: command not found

Solution

1. Step 1: Understand ENTRYPOINT and CMD interaction

   ENTRYPOINT sets the fixed command 'echo'. CMD provides default arguments 'Hello' and 'World'.

2. Step 2: Combine command and arguments

   Running the container runs 'echo Hello World', printing 'Hello World'.

3. Final Answer:

   Hello World -> Option A

4. Quick Check:

   ENTRYPOINT + CMD = echo Hello World [OK]

Hint: ENTRYPOINT command + CMD args run together [OK]

Common Mistakes:

• Thinking CMD overrides ENTRYPOINT command
• Expecting full command string printed
• Assuming no output without arguments

4. You wrote this Dockerfile:

ENTRYPOINT echo
CMD Hello World

But when you run the container, it produces unexpected output. What is the problem?

medium

A. Dockerfile must end with CMD, not ENTRYPOINT.

B. CMD cannot have multiple words without quotes.

C. ENTRYPOINT cannot be used with CMD.

D. ENTRYPOINT must be in JSON array form to avoid shell parsing errors.

Solution

1. Step 1: Check ENTRYPOINT syntax

   ENTRYPOINT without JSON array uses shell form, but 'echo' alone may cause parsing issues or unexpected behavior.

2. Step 2: Correct syntax requirement

   Using JSON array form like ["echo"] ensures proper command execution without shell interpretation errors.

3. Final Answer:

   ENTRYPOINT must be in JSON array form to avoid shell parsing errors. -> Option D

4. Quick Check:

   ENTRYPOINT needs JSON array for safety [OK]

Hint: Use JSON array for ENTRYPOINT to avoid errors [OK]

Common Mistakes:

• Using plain string ENTRYPOINT causing errors
• Misunderstanding CMD syntax rules
• Thinking ENTRYPOINT and CMD cannot be combined

5. You want a Docker container that always runs python app.py, but allows users to add extra arguments like --debug. How should you write ENTRYPOINT and CMD in the Dockerfile?

hard

A. ENTRYPOINT ["python", "app.py"] and CMD ["--debug"]

B. ENTRYPOINT ["python"] and CMD ["app.py", "--debug"]

C. CMD ["python", "app.py", "--debug"] only

D. ENTRYPOINT ["python", "app.py", "--debug"] only

Solution

1. Step 1: Fix main command with ENTRYPOINT

   Set ENTRYPOINT to ["python", "app.py"] so this command always runs.

2. Step 2: Provide default extra arguments with CMD

   Set CMD to ["--debug"] so users can override or add arguments when running the container.

3. Step 3: Understand combined behavior

   Docker runs ENTRYPOINT command plus CMD arguments, allowing flexibility with fixed main command.

4. Final Answer:

   ENTRYPOINT ["python", "app.py"] and CMD ["--debug"] -> Option A

5. Quick Check:

   ENTRYPOINT fixes command, CMD adds default args [OK]

Hint: ENTRYPOINT fixes command, CMD adds default args [OK]

Common Mistakes:

• Putting all commands in CMD only
• Using ENTRYPOINT without CMD for defaults
• Mixing order causing command override