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.