Debugging volume mount issues in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a common cause of volume mount failures in Docker?

A common cause is incorrect file or directory paths on the host system that do not exist or have wrong permissions.

beginner

How can you check if a volume is correctly mounted inside a running Docker container?

Use docker exec -it <container_name> ls <mount_path> to list files inside the mounted directory.

intermediate

Why might a Docker volume mount show empty inside the container even though the host directory has files?

This can happen if the container path is overwritten by another Docker volume or if the mount path is incorrect.

intermediate

What Docker command option helps you see detailed error messages when starting containers with volume mounts?

Use docker run --rm -it -v <host_path>:<container_path> <image> and check the error output for permission or path issues.

intermediate

How do file permissions affect Docker volume mounts?

If the host directory or files have restrictive permissions, the container user may not access them, causing mount issues.

What command lets you inspect the contents of a mounted volume inside a running Docker container?

A docker exec -it <container> ls <mount_path>

B docker volume ls

C docker ps

D docker inspect <container>

Use 'docker exec' to run commands inside the container and check the mounted directory contents.

If a volume mount shows empty inside the container, what is a likely cause?

A The container is not running

B Docker daemon is stopped

C The image is corrupted

D The container path is overwritten by another volume

An existing volume or mount can overwrite the path, hiding the host files.

Which of these can cause permission denied errors on volume mounts?

A Dockerfile missing CMD

B Container image is too large

C Host directory has restrictive permissions

D Network issues

If the host directory permissions do not allow container access, mounts fail.

What is the correct syntax to mount a host directory to a container path in Docker run?

A docker run -v /host/path:/container/path image

B docker run -v /container/path:/host/path image

C docker run --mount /container/path:/host/path image

D docker run --volume image /host/path:/container/path

The syntax is host path first, then container path separated by a colon.

How can you verify if a host directory exists before mounting it in Docker?

A Use docker volume ls

B Use ls /host/path on the host machine

C Run docker ps

D Restart Docker daemon

Checking the directory on the host ensures the path exists for mounting.

Explain the steps you would take to debug a Docker volume mount that appears empty inside the container.

Think about verifying paths, permissions, and container inspection.

Describe how file permissions on the host affect Docker volume mounts and how to fix permission issues.

Focus on permission settings and user access.

Practice

1. What is the main purpose of using volume mounts in Docker?

easy

A. To share files between the host machine and the container

B. To increase the container's CPU usage

C. To change the container's network settings

D. To update the Docker engine version

Solution

1. Step 1: Understand volume mounts

   Volume mounts allow sharing files or folders between your computer (host) and the Docker container.

2. Step 2: Identify the purpose

   This sharing helps containers access or save data on the host, making data persistent or accessible.

3. Final Answer:

   To share files between the host machine and the container -> Option A

4. Quick Check:

   Volume mounts = file sharing [OK]

Hint: Volume mounts connect host and container files [OK]

Common Mistakes:

• Confusing volume mounts with CPU or network settings
• Thinking volume mounts update Docker itself
• Assuming volume mounts only work inside containers

2. Which of the following is the correct syntax to mount a host folder /data to a container folder /app/data using docker run?

easy

A. docker run --mount /data:/app/data image_name

B. docker run -v /app/data:/data image_name

C. docker run -v /data:/app/data image_name

D. docker run -volume /data:/app/data image_name

Solution

1. Step 1: Recall volume mount syntax

   The correct syntax uses -v host_path:container_path to mount volumes.

2. Step 2: Match paths correctly

   The host folder /data should be before the colon, and container folder /app/data after.

3. Final Answer:

   docker run -v /data:/app/data image_name -> Option C

4. Quick Check:

   Host path before colon, container path after [OK]

Hint: Use -v host_path:container_path exactly [OK]

Common Mistakes:

• Swapping host and container paths
• Using incorrect flags like --mount without full syntax
• Typing -volume instead of -v

3. You run this command:
docker run -v /host/data:/container/data busybox ls /container/data
But the output is empty even though /host/data has files. What is the likely cause?

medium

A. The host path /host/data does not exist or is empty

B. The container image busybox does not support volume mounts

C. The container path /container/data is incorrect

D. The Docker daemon is not running

Solution

1. Step 1: Check host path existence

   If the host folder /host/data does not exist or is empty, the container will see nothing.

2. Step 2: Confirm container path and image

   The container path and busybox image support volume mounts, so they are unlikely the issue.

3. Final Answer:

   The host path /host/data does not exist or is empty -> Option A

4. Quick Check:

   Empty mount means host folder missing or empty [OK]

Hint: Check host folder exists and has files [OK]

Common Mistakes:

• Blaming the container image for mount issues
• Assuming container path must exist before mount
• Ignoring host folder permissions or existence

4. You tried to mount a folder with docker run -v /host/folder:/container/folder but got a permission denied error inside the container. What should you check first?

medium

A. If the Docker image supports the mounted folder

B. If the Docker CLI version is up to date

C. If the container folder path is a valid Linux path

D. If the host folder has correct read/write permissions

Solution

1. Step 1: Understand permission errors

   Permission denied usually means the container user cannot access the host folder due to host permissions.

2. Step 2: Verify host folder permissions

   Check if the host folder allows read/write access for Docker or the container user.

3. Final Answer:

   If the host folder has correct read/write permissions -> Option D

4. Quick Check:

   Permission denied = check host folder permissions [OK]

Hint: Check host folder permissions first on errors [OK]

Common Mistakes:

• Assuming container image causes permission errors
• Ignoring host file system permissions
• Updating Docker CLI instead of fixing permissions

5. You want to mount a host folder /data into a container at /app/data but only want the container to read files, not write. Which command correctly sets a read-only volume mount?

hard

A. docker run -v /data:/app/data:rw image_name

B. docker run -v /data:/app/data:ro image_name

C. docker run -v /data:/app/data:read image_name

D. docker run -v /data:/app/data:readonly image_name

Solution

1. Step 1: Recall volume mount options

   Docker uses :ro to mark a volume as read-only and :rw for read-write (default).

2. Step 2: Identify correct option

   Only :ro is valid for read-only; others like :read or :readonly are invalid.

3. Final Answer:

   docker run -v /data:/app/data:ro image_name -> Option B

4. Quick Check:

   Use :ro suffix for read-only mounts [OK]

Hint: Add :ro suffix for read-only mounts [OK]

Common Mistakes:

• Using :rw or invalid suffixes for read-only
• Forgetting the colon before options
• Assuming read-only is default