Listing containers (docker ps, docker ps -a) - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of docker ps command

You run docker ps on a system with three containers: one running, two stopped. What will docker ps show?

Docker

docker ps

A Lists only the running container with its details

B Lists all three containers with their details

C Lists only the stopped containers

D Shows an error because some containers are stopped

💡 Hint

Remember, docker ps shows only running containers by default.

The docker ps command lists only containers that are currently running. Stopped containers are not shown unless you add the -a option.

💻 Command Output

intermediate

Output of docker ps -a command

You run docker ps -a on a system with two running and two stopped containers. What will the command output include?

Docker

docker ps -a

A Lists only the two running containers

B Lists all four containers with their status

C Lists only the two stopped containers

D Shows an error because of invalid option

💡 Hint

The -a option means 'all' containers.

The docker ps -a command lists all containers, whether running or stopped, showing their status.

❓ Troubleshoot

advanced

Why does docker ps show no containers?

You run docker ps but it shows no containers, even though you know containers exist on the system. What is the most likely reason?

Docker

docker ps

A You need to run <code>docker ps -a</code> to see containers

B You do not have permission to list containers

C Docker daemon is not running

D No containers are currently running

💡 Hint

Think about what docker ps shows by default.

docker ps shows only running containers. If none are running, it will show no containers.

✅ Best Practice

advanced

Best command to list all containers including stopped ones

Which command should you use to see all containers, including those that have stopped?

A docker container ls

B docker ps

C docker ps -a

D docker container ls -q

💡 Hint

The -a option means 'all'.

docker ps -a lists all containers, running or stopped. docker container ls is equivalent to docker ps and shows only running containers. The -q option shows only IDs.

🧠 Conceptual

expert

Understanding container listing filters

You want to list only containers that exited with a non-zero status. Which command will do this?

A docker ps -a --filter "exited=1"

B docker ps -a --filter "exited!=0"

C docker ps -a --filter "exited=0"

D docker ps -a --filter "status=exited"

💡 Hint

The filter 'exited' takes an exit code number.

The --filter "exited=1" option lists containers that exited with status 1 (non-zero). Using exited=0 lists containers that exited successfully. The syntax exited!=0 is invalid. status=exited lists all exited containers regardless of exit code.

Practice

1. What does the command docker ps show by default?

easy

A. Only running containers

B. All containers, running and stopped

C. Only stopped containers

D. Docker images available locally

Solution

1. Step 1: Understand the default behavior of docker ps

   The docker ps command lists containers that are currently running.

2. Step 2: Compare with other options

   To see all containers, including stopped ones, you use docker ps -a. The command does not list images.

3. Final Answer:

   Only running containers -> Option A

4. Quick Check:

   docker ps = running containers [OK]

Hint: Remember: docker ps shows running containers only [OK]

Common Mistakes:

• Thinking docker ps shows all containers
• Confusing containers with images
• Assuming stopped containers show without -a

2. Which of the following commands correctly lists all containers, including stopped ones?

easy

A. docker ps --stopped

B. docker ps

C. docker ps --running

D. docker ps -a

Solution

1. Step 1: Identify the flag for listing all containers

   The -a flag with docker ps lists all containers, running or stopped.

2. Step 2: Check other options

   docker ps alone shows only running containers. The options --running and --stopped are invalid flags.

3. Final Answer:

   docker ps -a -> Option D

4. Quick Check:

   docker ps -a = all containers [OK]

Hint: Use -a to list all containers, not just running [OK]

Common Mistakes:

• Using docker ps without -a to see stopped containers
• Trying invalid flags like --running
• Confusing container listing with image listing

3. Given the following output from docker ps -a:

CONTAINER ID   IMAGE     STATUS              NAMES
abc123        nginx     Up 2 hours          webserver
def456        redis     Exited (0) 3 days ago cache

What will docker ps show?

medium

A. Both containers: webserver and cache

B. Only the cache container

C. Only the webserver container

D. No containers

Solution

1. Step 1: Analyze the status of containers

   The container named webserver is "Up 2 hours" meaning it is running. The container named cache is "Exited", meaning it is stopped.

2. Step 2: Understand docker ps output

   docker ps shows only running containers, so it will list only webserver.

3. Final Answer:

   Only the webserver container -> Option C

4. Quick Check:

   Running containers = webserver only [OK]

Hint: Check STATUS: 'Up' means running, 'Exited' means stopped [OK]

Common Mistakes:

• Listing stopped containers with docker ps
• Assuming all containers show without -a
• Confusing container names with images

4. You run docker ps -a but get an error: unknown flag: --all. What is the likely cause?

medium

A. You typed --all instead of -a

B. Docker daemon is not running

C. You need to run the command as root

D. The container list is empty

Solution

1. Step 1: Check the flag syntax

   The correct flag to list all containers is -a, not --all. The error shows the flag is unknown.

2. Step 2: Rule out other causes

   If the daemon was not running, the error would be different. Running as root is not required for listing containers. An empty list does not cause a flag error.

3. Final Answer:

   You typed --all instead of -a -> Option A

4. Quick Check:

   Use -a flag, not --all [OK]

Hint: Use short flag -a, not --all [OK]

Common Mistakes:

• Using invalid flag --all
• Assuming root is needed for listing
• Ignoring daemon status errors

5. You want to list all containers but only see their IDs and names, not other details. Which command achieves this?

hard

A. docker ps -a --short

B. docker ps -a --format "{{.ID}} {{.Names}}"

C. docker ps -a --quiet

D. docker ps --filter id --filter name

Solution

1. Step 1: Understand formatting options

   The --format flag lets you customize output. Using {{.ID}} {{.Names}} shows only container IDs and names.

2. Step 2: Evaluate other options

   --quiet shows only IDs, not names. --filter is for filtering, not formatting. --short is not a valid flag.

3. Final Answer:

   docker ps -a --format "{{.ID}} {{.Names}}" -> Option B

4. Quick Check:

   Use --format to customize output [OK]

Hint: Use --format to show only needed fields [OK]

Common Mistakes:

• Using invalid flags like --short
• Confusing --quiet with showing names
• Trying to filter instead of format output