Container states (created, running, paused, stopped) in Docker - Mini Project: Build & Apply

Container states (created, running, paused, stopped)

📖 Scenario: You are learning how to manage Docker containers. Containers can be in different states like created, running, paused, or stopped. Understanding these states helps you control your applications better.

🎯 Goal: Build a simple Docker workflow to create a container, start it, pause it, and then stop it. You will check the container's state at each step.

📋 What You'll Learn

Use the docker command line tool

Create a container from the alpine image

Start, pause, and stop the container using Docker commands

Check the container state using docker ps -a

💡 Why This Matters

🌍 Real World

Managing container states is essential for running applications reliably in development and production environments.

💼 Career

DevOps engineers and system administrators use these commands daily to control application lifecycles and troubleshoot issues.

1

Create a Docker container

Use the command docker create --name mycontainer alpine sleep 3600 to create a container named mycontainer from the alpine image.

Docker

# Use docker create to make a container named mycontainer from alpine
# Your code here

Hint

This command creates a container but does not start it. The container will be in the 'created' state.

2

Start the Docker container

Use the command docker start mycontainer to start the container named mycontainer.

Docker

docker create --name mycontainer alpine sleep 3600
# Start the container named mycontainer
# Your code here

Hint

Starting the container changes its state to 'running'.

3

Pause the Docker container

Use the command docker pause mycontainer to pause the running container named mycontainer.

Docker

docker create --name mycontainer alpine sleep 3600
docker start mycontainer
# Pause the container named mycontainer
# Your code here

Hint

Pausing a container freezes its processes but keeps it in memory.

4

Stop the Docker container and check its state

Use the command docker stop mycontainer to stop the container named mycontainer. Then use docker ps -a --filter name=mycontainer --format "{{.Names}} {{.Status}}" to display the container's name and current state.

Docker

docker create --name mycontainer alpine sleep 3600
docker start mycontainer
docker pause mycontainer
# Stop the container named mycontainer
# Show the container's name and status
# Your code here

Hint

Stopping the container ends its processes and changes its state to 'exited' or 'stopped'. The docker ps -a command shows all containers and their states.

Practice

1. Which Docker container state means the container is currently executing processes?

easy

A. stopped

B. created

C. paused

D. running

Solution

1. Step 1: Understand container states

   The 'running' state means the container is actively executing its processes.

2. Step 2: Compare other states

   'Created' means container is made but not started, 'paused' means temporarily halted, 'stopped' means container is not running.

3. Final Answer:

   running -> Option D

4. Quick Check:

   Active container = running [OK]

Hint: Running means container is actively working [OK]

Common Mistakes:

• Confusing 'created' with 'running'
• Thinking 'paused' means running
• Mixing 'stopped' with 'running'

2. Which command shows all Docker containers including stopped ones?

easy

A. docker ps -a

B. docker pause

C. docker start

D. docker ps

Solution

1. Step 1: Recall docker ps usage

   'docker ps' shows only running containers by default.

2. Step 2: Use option to show all containers

   'docker ps -a' lists all containers, including stopped and paused ones.

3. Final Answer:

   docker ps -a -> Option A

4. Quick Check:

   Show all containers = docker ps -a [OK]

Hint: Add -a to see all containers, not just running [OK]

Common Mistakes:

• Using 'docker ps' alone shows only running containers
• Confusing 'docker start' with listing containers
• Thinking 'docker pause' lists containers

3. What is the state of the container after running these commands?

docker create nginx
docker start <container_id>
docker pause <container_id>

medium

A. paused

B. created

C. running

D. stopped

Solution

1. Step 1: Analyze commands sequence

   'docker create' makes container in 'created' state, 'docker start' moves it to 'running'.

2. Step 2: Effect of 'docker pause'

   'docker pause' suspends container processes without stopping it, changing state to 'paused'.

3. Final Answer:

   paused -> Option A

4. Quick Check:

   Pause command sets state to paused [OK]

Hint: Pause command changes running to paused state [OK]

Common Mistakes:

• Assuming container stays running after pause
• Confusing created with paused
• Thinking pause stops the container

4. You ran docker pause mycontainer but got an error saying container is not running. What is the likely cause?

medium

A. Container is already paused

B. Container is stopped or not started

C. Container name is incorrect

D. Docker daemon is not running

Solution

1. Step 1: Understand pause command requirements

   'docker pause' only works on running containers to suspend them temporarily.

2. Step 2: Interpret error message

   Error about container not running means it is stopped or never started, so pause cannot apply.

3. Final Answer:

   Container is stopped or not started -> Option B

4. Quick Check:

   Pause needs running container [OK]

Hint: Pause only works on running containers [OK]

Common Mistakes:

• Trying to pause stopped containers
• Ignoring container state before pausing
• Assuming pause works on any container

5. You want to temporarily stop a container without losing its state and then resume it later. Which sequence of commands should you use?

hard

A. docker kill <container_id> then docker start <container_id>

B. docker stop <container_id> then docker start <container_id>

C. docker pause <container_id> then docker unpause <container_id>

D. docker restart <container_id> then docker pause <container_id>

Solution

1. Step 1: Understand stopping vs pausing

   'docker stop' stops container and loses running state; 'docker pause' suspends without stopping.

2. Step 2: Identify commands to pause and resume

   'docker pause' suspends container, 'docker unpause' resumes it preserving state.

3. Final Answer:

   docker pause & docker unpause -> Option C

4. Quick Check:

   Pause/unpause = temporary stop and resume [OK]

Hint: Pause to stop temporarily, unpause to resume [OK]

Common Mistakes:

• Using stop loses container state
• Confusing kill with pause
• Restarting instead of unpausing