Common container startup failures in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the error 'OCI runtime create failed' usually indicate when starting a Docker container?

It means the container runtime failed to create the container. This often happens due to permission issues, missing files, or invalid configurations.

beginner

Why might a Docker container exit immediately after starting?

Because the main process inside the container finished or crashed. Containers stop when their main process ends.

beginner

What is a common cause of 'port already allocated' error in Docker?

Trying to start a container that wants to use a port already in use by another container or process on the host.

intermediate

How can missing environment variables cause container startup failures?

If the container expects certain environment variables to configure its app and they are missing, the app may fail to start.

beginner

What does a 'no such file or directory' error mean when starting a container?

It means the container tried to run a command or access a file that does not exist inside the container image or volume.

What happens if the main process inside a Docker container exits immediately?

A The container restarts automatically

B The container stops running

C The container runs in the background

D The container ignores the exit

Docker containers stop when their main process ends, so if it exits immediately, the container stops.

Which error indicates a port conflict when starting a Docker container?

A port already allocated

B no such file or directory

C permission denied

D container not found

The 'port already allocated' error means the port is in use by another container or process.

What is a common reason for 'permission denied' errors when starting containers?

A Port conflicts

B Missing environment variables

C Trying to access files or directories without proper rights

D Container image not found

Permission denied errors happen when the container or Docker daemon lacks rights to access needed files or directories.

If a container fails because it cannot find a command to run, what is likely the cause?

A The command is missing inside the container image

B The container has no network

C The container is out of memory

D The Docker daemon is down

A missing command inside the container image causes 'no such file or directory' errors at startup.

How can missing environment variables affect container startup?

A The container will restart repeatedly

B The container will ignore them and start normally

C The container will automatically set default values

D The app inside the container may fail to start

Many apps rely on environment variables for configuration; missing them can cause startup failure.

Explain three common reasons why a Docker container might fail to start.

Think about what the container needs to run properly.

Describe how environment variables can impact container startup and what happens if they are missing.

Consider the role of environment variables in app configuration.

Practice

1. What is a common reason a Docker container might fail to start immediately after running docker run?

easy

A. The container tries to use a port already in use on the host

B. The Docker daemon is running

C. The container image is very large

D. The host has enough free disk space

Solution

1. Step 1: Understand port conflicts in Docker

   If a container tries to bind to a port already used by another process on the host, it cannot start because the port is busy.

2. Step 2: Check other options for relevance

   Options B, C, and D do not prevent container startup immediately; only port conflict causes failure here.

3. Final Answer:

   The container tries to use a port already in use on the host -> Option A

4. Quick Check:

   Port conflict = container startup failure [OK]

Hint: Check if the port is free before starting container [OK]

Common Mistakes:

• Assuming large image size stops container start
• Thinking Docker daemon running causes failure
• Ignoring port conflicts

2. Which of the following docker run commands will correctly set an environment variable named APP_ENV to production?

easy

A. docker run --env APP_ENV production myimage

B. docker run -e APP_ENV production myimage

C. docker run -env APP_ENV=production myimage

D. docker run -e APP_ENV=production myimage

Solution

1. Step 1: Recall correct syntax for environment variables

   The correct flag is -e or --env followed by VAR=VALUE without spaces.

2. Step 2: Analyze each option

   docker run -e APP_ENV=production myimage uses correct syntax: -e APP_ENV=production. Options A, C, and D have syntax errors or missing '=' sign.

3. Final Answer:

   docker run -e APP_ENV=production myimage -> Option D

4. Quick Check:

   Use -e VAR=VALUE for env vars [OK]

Hint: Use -e VAR=VALUE with no spaces for env vars [OK]

Common Mistakes:

• Omitting '=' between variable and value
• Using incorrect flag like -env
• Adding spaces between variable and value

3. Given this command:

docker run -p 8080:80 nginx

What will happen if port 8080 on the host is already used by another service?

medium

A. Docker will automatically choose a different host port

B. The container will fail to start with a port binding error

C. The container will start normally and ignore the port conflict

D. The container will start but the service inside will not work

Solution

1. Step 1: Understand port binding behavior

   If the host port (8080) is already in use, Docker cannot bind the container port to it, causing failure.

2. Step 2: Evaluate other options

   Docker does not auto-select ports unless specified with -P. The container won't start ignoring conflicts or silently fail.

3. Final Answer:

   The container will fail to start with a port binding error -> Option B

4. Quick Check:

   Port in use causes start failure [OK]

Hint: Check host ports before binding with -p [OK]

Common Mistakes:

• Assuming Docker auto-changes ports
• Thinking container starts ignoring conflicts
• Believing service inside fails but container runs

4. You run a container with docker run -v /host/data:/app/data myimage but the container fails to start. What is the most likely cause?

medium

A. The image name is incorrect

B. The container does not have enough CPU resources

C. The host directory /host/data does not exist or has wrong permissions

D. The Docker daemon is not running

Solution

1. Step 1: Understand volume mount requirements

   Docker requires the host directory to exist and be accessible for volume mounts; otherwise, container startup fails.

2. Step 2: Check other options

   Incorrect image name causes image pull errors, not volume mount failure. CPU or daemon issues cause different errors.

3. Final Answer:

   The host directory /host/data does not exist or has wrong permissions -> Option C

4. Quick Check:

   Volume mount fails if host path missing [OK]

Hint: Verify host path exists and permissions before mounting [OK]

Common Mistakes:

• Ignoring host directory existence
• Confusing image name errors with volume errors
• Assuming CPU limits cause mount failures

5. You have a container that fails immediately after starting. You run docker logs container_id and see an error about a missing environment variable DB_HOST. How should you fix this?

hard

A. Add -e DB_HOST=value to the docker run command

B. Increase the container's CPU and memory limits

C. Change the container image to a different one

D. Restart the container without changes

Solution

1. Step 1: Interpret the error message from logs

   The logs indicate the container expects an environment variable DB_HOST which is missing, causing failure.

2. Step 2: Provide the missing environment variable

   Use -e DB_HOST=value in the docker run command to supply the required variable.

3. Step 3: Understand why other options fail

   Restarting without changes won't fix missing vars. Changing image or resources doesn't address environment variables.

4. Final Answer:

   Add -e DB_HOST=value to the docker run command -> Option A

5. Quick Check:

   Missing env var fixed by -e flag [OK]

Hint: Use docker logs to find missing env vars, then add -e [OK]

Common Mistakes:

• Restarting without fixing environment variables
• Changing image unnecessarily
• Increasing resources instead of fixing config