Why Container states (created, running, paused, stopped) in Docker? - Purpose & Use Cases

The Big Idea

Discover how knowing container states saves you from endless guessing and frustration!

The Scenario

Imagine you have many boxes (containers) with your important stuff (applications). You want to know if a box is packed, opened, temporarily closed, or completely shut. Without clear labels, you waste time guessing and opening each box.

The Problem

Manually checking each container's status is slow and confusing. You might try to start a container that is already running or pause one that is stopped. This causes errors and wastes your time.

The Solution

Docker container states clearly tell you if a container is created but not started, running, paused temporarily, or stopped. This helps you manage containers easily and avoid mistakes.

Before vs After

✗ Before

docker ps
# Guess container status by looking at logs or trial and error

✓ After

docker ps -a
# See exact container states like created, running, paused, exited

What It Enables

Knowing container states lets you control your applications smoothly and fix issues faster.

Real Life Example

When deploying a website, you can quickly see if the web server container is running or paused, so you know if visitors can access the site or if you need to restart it.

Key Takeaways

Containers have clear states to show their current condition.

This helps avoid confusion and errors in managing containers.

You can quickly check and control containers using simple commands.

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