Listing containers (docker ps, docker ps -a) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the command docker ps do?

It lists all running Docker containers on your system, showing details like container ID, image, status, and ports.

beginner

What extra information does docker ps -a show compared to docker ps?

docker ps -a lists all containers, including those that are stopped, exited, or paused, not just the running ones.

beginner

How can you see the status of a container using Docker commands?

Use docker ps or docker ps -a. The status column shows if a container is running, exited, or paused.

intermediate

Why might you want to use docker ps -a instead of just docker ps?

To see all containers including those that have stopped or exited, which helps in troubleshooting or cleaning up unused containers.

beginner

What information is typically shown in the output of docker ps?

Container ID, image name, command run, creation time, status, ports, and container name.

Which command lists only running Docker containers?

A docker ps -a

B docker list

C docker ps

D docker containers

docker ps shows only running containers. docker ps -a shows all containers.

What does the -a option do in docker ps -a?

A Shows all containers including stopped ones

B Shows only active containers

C Shows containers with active ports

D Shows containers in alphabetical order

The -a option lists all containers, not just running ones.

If you want to check why a container stopped, which command helps you see its status?

A docker start

B docker ps -a

C docker run

D docker logs

docker ps -a shows the status of all containers including stopped ones.

Which column in docker ps output tells you the container's current state?

A PORTS

B IMAGE

C COMMAND

D STATUS

The STATUS column shows if a container is running, exited, or paused.

What command would you use to see all containers including those that exited hours ago?

A docker ps -a

B docker ps

C docker inspect

D docker rm

docker ps -a lists all containers regardless of their state.

Explain the difference between docker ps and docker ps -a.

Think about when you want to see containers that are not running.

Describe what information you get from the output of docker ps.

Look at the columns shown when you run the command.

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