Copying files to and from containers in Docker - Commands & Configuration

Introduction

Sometimes you need to move files between your computer and a running container. This helps when you want to add files to the container or get files out for inspection or backup.

When you want to add a configuration file to a running container without rebuilding the image

When you need to retrieve logs or output files generated inside a container

When you want to copy a script or data file into a container to run it there

When you want to backup files from a container to your local machine

When you want to inspect or modify files inside a container without using an interactive shell

Commands

Start a new container named 'example-container' running the nginx web server in the background.

Terminal

docker run -d --name example-container nginx:1.23.4

Expected Output

a1b2c3d4e5f67890123456789abcdef0123456789abcdef0123456789abcdef0

→

-d - Run container in detached mode (in background)

→

--name - Assign a custom name to the container

Copy the file 'local-file.txt' from your computer into the container's nginx web folder so it can be served by nginx.

Terminal

docker cp ./local-file.txt example-container:/usr/share/nginx/html/local-file.txt

Expected Output

No output (command runs silently)

Copy the nginx access log file from inside the container to your local machine for inspection.

Terminal

docker cp example-container:/var/log/nginx/access.log ./access.log

Expected Output

No output (command runs silently)

List files in the nginx web folder inside the container to verify the copied file is there.

Terminal

docker exec example-container ls /usr/share/nginx/html

Expected Output

50x.html

index.html

local-file.txt

→

exec - Run a command inside a running container

Stop and remove the container to clean up after the test.

Terminal

docker stop example-container && docker rm example-container

Expected Output

example-container

example-container

→

stop - Stop a running container

→

rm - Remove a container

Key Concept

If you remember nothing else from this pattern, remember: use 'docker cp' to move files between your local machine and a running container easily.

Common Mistakes

Trying to copy files to a container that does not exist

The 'docker cp' command requires the container to exist. It can copy to running or stopped containers, but copying to a non-existent container will fail.

Make sure the container exists. It can be running or stopped, but the name must be correct.

Using relative paths inside the container without verifying the directory exists

If the target directory inside the container does not exist, the copy will fail or create a file with an unexpected name.

Verify the target directory exists inside the container before copying files there.

Confusing source and destination order in the 'docker cp' command

The command syntax is 'docker cp source destination'. Mixing these will copy files in the wrong direction or cause errors.

Always specify the source first, then the destination.

Summary

Use 'docker cp' to copy files to and from containers without rebuilding images.

Verify container names and paths carefully to avoid errors.

Use 'docker exec' to check files inside containers after copying.

Practice

1. What does the docker cp command do?

easy

A. Copies files or folders between your computer and a Docker container

B. Copies Docker images between registries

C. Copies environment variables from a container

D. Copies container logs to a file

Solution

1. Step 1: Understand the purpose of docker cp

   The docker cp command is designed to transfer files or folders between the host machine and a Docker container.

2. Step 2: Differentiate from other Docker commands

   Other options relate to images, environment variables, or logs, which are not handled by docker cp.

3. Final Answer:

   Copies files or folders between your computer and a Docker container -> Option A

4. Quick Check:

   docker cp = copy files/folders [OK]

Hint: Remember: cp means copy files between host and container [OK]

Common Mistakes:

• Confusing docker cp with image transfer commands
• Thinking it copies logs or environment variables
• Assuming it copies container settings

2. Which of the following is the correct syntax to copy a file named app.log from your computer to a container named webapp at path /var/log/?

easy

A. docker cp app.log webapp:/var/log/

B. docker cp webapp:/var/log/ app.log

C. docker copy app.log webapp:/var/log/

D. docker cp /var/log/ app.log webapp

Solution

1. Step 1: Identify source and destination in docker cp

   The syntax is docker cp [source] [destination]. To copy from host to container, source is local file, destination is container:path.

2. Step 2: Check each option's order and command correctness

   docker cp app.log webapp:/var/log/ correctly uses docker cp app.log webapp:/var/log/. docker cp webapp:/var/log/ app.log reverses source and destination. docker copy app.log webapp:/var/log/ uses wrong command docker copy. docker cp /var/log/ app.log webapp has wrong order and extra arguments.

3. Final Answer:

   docker cp app.log webapp:/var/log/ -> Option A

4. Quick Check:

   docker cp source destination [OK]

Hint: Source first, then destination with container:path [OK]

Common Mistakes:

• Swapping source and destination paths
• Using wrong command name like docker copy
• Missing container name or colon separator

3. Given a container named db with a file /data/dump.sql, what will be the result of running docker cp db:/data/dump.sql ./backup.sql?

medium

A. Throws an error because the destination must be a directory

B. Copies the file dump.sql from the container to the current directory as backup.sql

C. Copies the file backup.sql from the current directory to the container at /data/dump.sql

D. Copies the entire /data directory from the container to ./backup.sql

Solution

1. Step 1: Analyze the command syntax

   The command copies from container db path /data/dump.sql to local path ./backup.sql. This means copying a single file from container to host with a new name.

2. Step 2: Understand the effect on local filesystem

   The file dump.sql inside the container is copied and saved locally as backup.sql in the current directory.

3. Final Answer:

   Copies the file dump.sql from the container to the current directory as backup.sql -> Option B

4. Quick Check:

   docker cp container:file localfile [OK]

Hint: Container source first, local destination second [OK]

Common Mistakes:

• Confusing source and destination order
• Assuming destination must be a directory
• Thinking it copies directories when given a file

4. You run docker cp mycontainer:/app/config.json ./config.json but get an error: no such file or directory. What is the most likely cause?

medium

A. You forgot to specify the container ID instead of name

B. The destination path ./config.json is invalid on your host

C. The file /app/config.json does not exist inside the container

D. You need to run the command inside the container shell

Solution

1. Step 1: Understand the error message

   The error "no such file or directory" usually means the source file inside the container does not exist at the specified path.

2. Step 2: Evaluate other options

   Container name or ID can be used interchangeably if correct. Destination path on host is usually valid if relative path is used. Running inside container shell is not needed for docker cp.

3. Final Answer:

   The file /app/config.json does not exist inside the container -> Option C

4. Quick Check:

   Source file missing = error [OK]

Hint: Check source file path inside container first [OK]

Common Mistakes:

• Assuming destination path causes error
• Thinking container name must be ID only
• Trying to run docker cp inside container shell

5. You want to copy a folder named logs from your host to a running container appserver at path /var/app/. Which command correctly copies the entire folder including its contents?

hard

A. docker cp logs/* appserver:/var/app/

B. docker cp logs/ appserver:/var/app/

C. docker cp logs appserver:/var/app/

D. docker cp logs appserver:/var/app/logs

Solution

1. Step 1: Understand folder copy behavior

   To copy a folder and its contents, specify the folder as source and the full destination path including the folder name if you want it placed inside the container.

2. Step 2: Analyze each option

   docker cp logs appserver:/var/app/logs copies the entire logs folder into /var/app/logs, preserving folder structure. docker cp logs/ appserver:/var/app/ with trailing slash copies contents of logs into /var/app/ but does not create logs folder inside container. docker cp logs appserver:/var/app/ copies contents of logs into /var/app/ without creating logs folder. docker cp logs/* appserver:/var/app/ copies only files inside logs, not subfolders.

3. Final Answer:

   docker cp logs appserver:/var/app/logs -> Option D

4. Quick Check:

   Include folder name in destination to copy folder [OK]

Hint: Add folder name in destination path to copy folder [OK]

Common Mistakes:

• Using trailing slash and losing folder structure
• Copying only files inside folder, not subfolders
• Not specifying full destination path including folder name