Starting and stopping containers in Docker - Step-by-Step Execution

Process Flow - Starting and stopping containers

Start Container Command

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Docker checks container status

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Stop Container Command

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Docker sends stop signal

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Container stops gracefully

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Exit

This flow shows how Docker starts a container if it's not running, or notes if it is already running, and how it stops a container by sending a stop signal.

Execution Sample

Docker

docker start mycontainer
# Starts the container named 'mycontainer'
docker stop mycontainer
# Stops the container named 'mycontainer'

This code starts a Docker container named 'mycontainer' if it is stopped, and stops it if it is running.

Process Table

Step	Command	Container Status Before	Action Taken	Container Status After	Output
1	docker start mycontainer	stopped	Start container	running	mycontainer
2	docker start mycontainer	running	No action (already running)	running	mycontainer
3	docker stop mycontainer	running	Send stop signal	stopped	mycontainer
4	docker stop mycontainer	stopped	Error (already stopped)	stopped	Error: Container mycontainer is not running

💡 Commands stop executing after confirming container state or error

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
container_status	stopped	running	running	stopped	stopped

Key Moments - 2 Insights

Why does 'docker start' do nothing if the container is already running?

Because Docker checks the container status first (see execution_table step 2) and skips starting if it's already running to avoid errors.

What happens if you try to stop a container that is already stopped?

Docker returns an error message (see execution_table step 4) because the container is not running and cannot be stopped again.

Visual Quiz - 3 Questions

Look at the execution table, what is the container status after step 1?

A paused

B stopped

C running

D exited

💡 Hint

Check the 'Container Status After' column in row for step 1.

At which step does Docker send a stop signal to the container?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Action Taken' column to find when the stop signal is sent.

If the container is running, what will 'docker start' do according to the table?

A Do nothing and show container name

B Start the container again

C Stop the container

D Show an error

💡 Hint

See step 2 in the execution table under 'Action Taken' and 'Output'.

Concept Snapshot

Starting and stopping containers in Docker:
- Use 'docker start <name>' to start a stopped container.
- If container is already running, 'docker start' does nothing.
- Use 'docker stop <name>' to stop a running container gracefully.
- Stopping a stopped container returns an error.
- Always check container status before commands.

Full Transcript

This visual execution shows how Docker handles starting and stopping containers. When you run 'docker start' on a stopped container, Docker changes its status to running and outputs the container name. If the container is already running, Docker does nothing and just outputs the name again. When you run 'docker stop' on a running container, Docker sends a stop signal and the container status changes to stopped. Trying to stop a container that is already stopped results in an error message. The variable tracker shows the container status changing step by step. Key moments clarify why Docker skips starting an already running container and why stopping a stopped container causes an error. The quiz tests understanding of container status changes and Docker's behavior.

Practice

1. Which command is used to start a container that has been stopped in Docker?

easy

A. docker run <container_name>

B. docker stop <container_name>

C. docker start <container_name>

D. docker restart <container_name>

Solution

1. Step 1: Understand the purpose of docker start

   This command resumes a container that was previously stopped without creating a new one.

2. Step 2: Differentiate from other commands

   docker run creates a new container, docker stop stops a running container, and docker restart stops then starts a container.

3. Final Answer:

   docker start <container_name> -> Option C

4. Quick Check:

   Start stopped container = docker start [OK]

Hint: Use 'docker start' to resume stopped containers fast [OK]

Common Mistakes:

• Confusing 'docker run' with 'docker start'
• Using 'docker stop' to start containers
• Thinking 'docker restart' only starts containers

2. Which of the following is the correct syntax to stop a running Docker container named webapp?

easy

A. docker terminate webapp

B. docker kill webapp

C. docker pause webapp

D. docker stop webapp

Solution

1. Step 1: Identify the command to safely stop a container

   docker stop sends a signal to gracefully stop the container.

2. Step 2: Check other options

   docker kill forcefully stops, docker pause suspends, and docker terminate is invalid.

3. Final Answer:

   docker stop webapp -> Option D

4. Quick Check:

   Stop container safely = docker stop [OK]

Hint: Use 'docker stop' to safely end container processes [OK]

Common Mistakes:

• Using 'docker kill' instead of 'docker stop'
• Confusing 'docker pause' with stopping
• Using non-existent commands like 'docker terminate'

3. What will be the output of the following commands if the container db is currently stopped?

docker start db
docker stop db

medium

A. Container 'db' starts, then stops successfully

B. Error: container 'db' not found

C. Container 'db' remains stopped

D. Container 'db' restarts automatically

Solution

1. Step 1: Analyze docker start db

   This command starts the stopped container named 'db'.

2. Step 2: Analyze docker stop db

   This command stops the running container 'db' safely.

3. Final Answer:

   Container 'db' starts, then stops successfully -> Option A

4. Quick Check:

   Start then stop container = success [OK]

Hint: Start then stop a container runs commands without error [OK]

Common Mistakes:

• Assuming 'docker start' creates a new container
• Expecting errors if container exists
• Thinking container restarts automatically without command

4. You run docker start myapp but get the error: Error response from daemon: No such container: myapp. What is the most likely cause?

medium

A. Docker daemon is not running

B. The container 'myapp' does not exist

C. You need to use docker run instead

D. The container 'myapp' is already running

Solution

1. Step 1: Understand the error message

   The error says 'No such container', meaning 'myapp' is not found in Docker's container list.

2. Step 2: Check other options

   If it was running, 'docker start' would not error; 'docker run' creates new containers but error is about missing container; daemon not running causes different errors.

3. Final Answer:

   The container 'myapp' does not exist -> Option B

4. Quick Check:

   No such container error = container missing [OK]

Hint: Check container existence before starting [OK]

Common Mistakes:

• Assuming container is running when error says missing
• Trying 'docker run' without checking container list
• Ignoring Docker daemon status

5. You want to stop a container named api-server but ensure it has 10 seconds to finish its work before stopping. Which command should you use?

hard

A. docker stop -t 10 api-server

B. docker kill -t 10 api-server

C. docker pause api-server

D. docker stop api-server -timeout 10

Solution

1. Step 1: Understand the timeout option in docker stop

   The -t or --time flag sets seconds to wait before killing the container.

2. Step 2: Evaluate the options

   docker kill does not support -t, docker pause only suspends, and -timeout is invalid syntax.

3. Final Answer:

   docker stop -t 10 api-server -> Option A

4. Quick Check:

   Stop with timeout = docker stop -t [OK]

Hint: Use 'docker stop -t seconds' to allow graceful shutdown [OK]

Common Mistakes:

• Using 'docker kill' with timeout flag
• Using invalid flags like '-timeout'
• Confusing 'pause' with stopping