COPY instruction for adding files in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the Dockerfile line to copy a file named app.py to the container's /app directory.

Docker

COPY [1] /app/

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

Aapp/

Bapp.py

C./app.py/

D/app.py

Attempts:

3 left

💡 Hint

Common Mistakes

Adding a slash before the filename

Using a directory name instead of the file

The COPY instruction copies files from the build context. Use the exact filename app.py without slashes.

2fill in blank

medium

Complete the Dockerfile line to copy the entire config folder into the container's /etc/config directory.

Docker

COPY [1] /etc/config/

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

Aconfig

Bconfig/

C/config

D./config

Attempts:

3 left

💡 Hint

Common Mistakes

Adding a leading slash

Adding a trailing slash unnecessarily

To copy a folder, specify the folder name without trailing slash. Docker treats config as the folder to copy.

3fill in blank

hard

Fix the error in this Dockerfile line that tries to copy requirements.txt to /app but fails.

Docker

COPY [1] /app/

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

Arequirements.txt

B/requirements.txt

C./requirements.txt/

Drequirements

Attempts:

3 left

💡 Hint

Common Mistakes

Using a leading slash

Adding a trailing slash

Using a folder name instead of file

The source must be the exact filename without slashes. Leading or trailing slashes cause errors.

4fill in blank

hard

Fill both blanks to copy index.html and style.css into the container's /usr/share/nginx/html directory.

Docker

COPY [1] [2] /usr/share/nginx/html/

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

Aindex.html

Bstyle.css

Cindex.htm

Dstyles.css

Attempts:

3 left

💡 Hint

Common Mistakes

Typos in filenames

Using wrong file extensions

Use the exact filenames to copy both files correctly.

5fill in blank

hard

Fill all three blanks to copy app.py, requirements.txt, and Dockerfile into the container's /app directory.

Docker

COPY [1] [2] [3] /app/

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

Aapp.py

Brequirements.txt

CDockerfile

Ddockerfile

Attempts:

3 left

💡 Hint

Common Mistakes

Wrong capitalization

Using lowercase 'dockerfile' instead of 'Dockerfile'

Use the exact filenames with correct capitalization to copy all files.

Practice

1. What does the COPY instruction do in a Dockerfile?

easy

A. It copies files or folders from your computer into the Docker image.

B. It copies files from the Docker image to your computer.

C. It copies environment variables into the container.

D. It copies running containers to new images.

Solution

1. Step 1: Understand the purpose of COPY

   The COPY instruction is used to transfer files or folders from the build context (your computer) into the Docker image during build time.

2. Step 2: Differentiate from other options

   Options A, B, and D describe actions not performed by COPY. COPY does not move files out of the image or handle environment variables or containers directly.

3. Final Answer:

   It copies files or folders from your computer into the Docker image. -> Option A

4. Quick Check:

   COPY moves files into image = C [OK]

Hint: COPY moves files from host to image during build [OK]

Common Mistakes:

• Thinking COPY moves files from image to host
• Confusing COPY with environment variable setting
• Assuming COPY copies running containers

2. Which of the following is the correct syntax to copy a file named app.py from your local folder to /usr/src/app/ inside the Docker image?

easy

A. COPY app.py > /usr/src/app/

B. COPY /usr/src/app/ app.py

C. COPY app.py /usr/src/app/

D. COPY app.py : /usr/src/app/

Solution

1. Step 1: Understand COPY syntax

   The correct syntax is COPY <source> <destination>. The source is the file on your computer, and the destination is the path inside the image.

2. Step 2: Check each option

   COPY app.py /usr/src/app/ correctly uses COPY app.py /usr/src/app/. Options B, C, and D use incorrect order or invalid symbols.

3. Final Answer:

   COPY app.py /usr/src/app/ -> Option C

4. Quick Check:

   COPY source destination = A [OK]

Hint: COPY syntax is COPY source destination [OK]

Common Mistakes:

• Reversing source and destination
• Using symbols like > or : instead of space
• Adding slashes incorrectly

3. Given this Dockerfile snippet:

FROM python:3.12
COPY src/ /app/
RUN ls /app/

What will be the output of RUN ls /app/ if the local folder src contains files main.py and utils.py?

medium

A. src

B. main.py

C. No such file or directory

D. main.py utils.py

Solution

1. Step 1: Understand COPY with folder source

   Using COPY src/ /app/ copies all contents inside src into /app/ inside the image.

2. Step 2: Predict RUN ls output

   Since src contains main.py and utils.py, listing /app/ will show these two files.

3. Final Answer:

   main.py utils.py -> Option D

4. Quick Check:

   COPY folder copies contents = B [OK]

Hint: COPY folder/ dest copies folder contents, not folder itself [OK]

Common Mistakes:

• Expecting folder name to appear inside destination
• Thinking COPY copies folder as a subfolder
• Assuming error if folder exists

4. You wrote this Dockerfile line:

COPY --chown=appuser:appgroup config.yaml /etc/config/

But during build, you get an error: invalid flag: --chown. What is the likely cause?

medium

A. Docker version is too old and does not support --chown flag.

B. The source file config.yaml does not exist.

C. The destination path /etc/config/ is invalid.

D. The --chown flag must be placed after the destination path.

Solution

1. Step 1: Understand --chown flag usage

   The --chown flag sets ownership of copied files but requires Docker version 17.09 or newer.

2. Step 2: Analyze error message

   An 'invalid flag' error usually means the Docker version does not recognize the flag, indicating it is too old.

3. Final Answer:

   Docker version is too old and does not support --chown flag. -> Option A

4. Quick Check:

   Old Docker lacks --chown support = D [OK]

Hint: Check Docker version if --chown flag errors appear [OK]

Common Mistakes:

• Assuming source file missing causes invalid flag error
• Thinking --chown position matters
• Blaming destination path for flag errors

5. You want to copy multiple files index.html and style.css from your local folder into /var/www/html/ inside the Docker image using a single COPY instruction. Which is the correct way?

hard

A. COPY ["index.html", "style.css"] /var/www/html/

B. COPY index.html style.css /var/www/html/

C. COPY index.html /var/www/html/ style.css

D. COPY index.html; style.css /var/www/html/

Solution

1. Step 1: Understand multiple source files syntax

   To copy multiple files, list them separated by spaces before the destination path.

2. Step 2: Evaluate options

   COPY index.html style.css /var/www/html/: COPY index.html style.css /var/www/html/ correctly lists files followed by destination. COPY ["index.html", "style.css"] /var/www/html/ mixes JSON array for sources with space-separated destination, which is invalid syntax. Options C and D have incorrect syntax.

3. Final Answer:

   COPY index.html style.css /var/www/html/ -> Option B

4. Quick Check:

   Multiple files listed before destination = A [OK]

Hint: List multiple files separated by spaces before destination [OK]

Common Mistakes:

• Using JSON array syntax incorrectly
• Placing destination between source files
• Using semicolons instead of spaces