Restarting containers in Docker - Step-by-Step Execution

Process Flow - Restarting containers

Identify container

↓

Run restart command

↓

Stop container

↓

Start container

↓

Container running again

The flow shows how Docker stops a container and then starts it again to restart.

Execution Sample

Docker

docker restart my_container

This command stops and then starts the container named 'my_container'.

Process Table

Step	Action	Container State Before	Container State After	Output
1	Identify container 'my_container'	running	running	Container found
2	Stop container	running	stopped	Container stopped
3	Start container	stopped	running	Container started
4	Restart complete	running	running	Container restarted successfully

💡 Restart finishes after container is running again.

Status Tracker

Variable	Start	After Step 2	After Step 3	Final
container_state	running	stopped	running	running

Key Moments - 2 Insights

Why does the container state change from running to stopped before starting again?

Because the restart command first stops the container to reset its state, then starts it fresh. See execution_table steps 2 and 3.

What happens if the container is already stopped before restart?

Docker will start the container directly without stopping it first, but the restart command always ensures the container ends up running.

Visual Quiz - 3 Questions

Look at the execution table, what is the container state after step 2?

A running

B paused

C stopped

D exited

💡 Hint

Check the 'Container State After' column for step 2 in the execution_table.

At which step does the container start running again?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Container State After' column and find when it changes back to running.

If the container was stopped before restart, what would change in the execution table?

A Step 1 would show 'stopped' state

B Step 2 would be skipped

C Step 3 would be skipped

D No steps would change

💡 Hint

Check the 'Container State Before' column at step 1 and think about initial container state.

Concept Snapshot

docker restart <container_name>
- Stops the container if running
- Starts the container again
- Ensures container ends up running
- Useful to apply changes or recover
- Simple one-command restart

Full Transcript

Restarting a Docker container means stopping it first and then starting it again. The command 'docker restart my_container' does this in two steps: it stops the container if it is running, then starts it fresh. This resets the container's state and helps apply changes or fix issues. The container state changes from running to stopped, then back to running. If the container was already stopped, Docker just starts it. This process ensures the container is running at the end.

Practice

1. What does the docker restart <container_name> command do?

easy

A. Only starts the specified container

B. Only stops the specified container

C. Deletes the specified container

D. Stops and then starts the specified container

Solution

1. Step 1: Understand the restart command

   The docker restart command is designed to stop a running container and then start it again.

2. Step 2: Compare with other commands

   Stopping only halts the container, starting only launches it, and deleting removes it. Restart does both stop and start.

3. Final Answer:

   Stops and then starts the specified container -> Option D

4. Quick Check:

   Restart = Stop + Start [OK]

Hint: Restart means stop then start container [OK]

Common Mistakes:

• Thinking restart only stops or only starts
• Confusing restart with remove or delete
• Assuming restart creates a new container

2. Which of the following is the correct syntax to restart a container named webapp?

easy

A. docker restart -c webapp

B. docker restart webapp

C. docker restart --container webapp

D. docker restart container webapp

Solution

1. Step 1: Recall the basic restart syntax

   The correct syntax is docker restart <container_name> without extra flags for container name.

2. Step 2: Evaluate the options

   Options B, C, and D add invalid flags or keywords not recognized by Docker CLI for restart.

3. Final Answer:

   docker restart webapp -> Option B

4. Quick Check:

   Simple restart command = docker restart container_name [OK]

Hint: Use 'docker restart' plus container name only [OK]

Common Mistakes:

• Adding unnecessary flags like -c or --container
• Using extra keywords after restart
• Confusing restart syntax with other commands

3. What will be the output of the command docker restart myapp if the container myapp is currently stopped?

medium

A. myapp

B. Error: Container is not running

C. Container myapp restarted successfully

D. No output

Solution

1. Step 1: Understand restart behavior on stopped container

   If the container is stopped, docker restart will start it and print the container name.

2. Step 2: Check Docker CLI output format

   The command outputs the container name on success, not a success message or error if stopped.

3. Final Answer:

   myapp -> Option A

4. Quick Check:

   Restart prints container name on success [OK]

Hint: Restart prints container name even if stopped [OK]

Common Mistakes:

• Expecting an error if container is stopped
• Expecting a success message instead of container name
• Thinking restart does nothing if container stopped

4. You run docker restart 123abc but get an error: Error: No such container: 123abc. What is the likely cause?

medium

A. Docker daemon is not running

B. The container is already running

C. The container ID or name is incorrect or does not exist

D. You need to add --force flag to restart

Solution

1. Step 1: Analyze the error message

   The error says no such container, meaning Docker cannot find a container with ID or name '123abc'.

2. Step 2: Check other options

   Running container does not cause this error, Docker daemon not running causes different errors, and --force is not a valid restart flag.

3. Final Answer:

   The container ID or name is incorrect or does not exist -> Option C

4. Quick Check:

   No such container = wrong ID/name [OK]

Hint: Check container ID/name spelling if 'No such container' error [OK]

Common Mistakes:

• Assuming container must be stopped to restart
• Trying to force restart with invalid flags
• Ignoring Docker daemon status

5. You want a container to automatically restart if it crashes or Docker restarts. Which command correctly starts a container named db with this behavior?

hard

A. docker run --restart=always db

B. docker run --restart=never db

C. docker run --restart=on-failure db

D. docker run --restart=unless-stopped db

Solution

1. Step 1: Understand restart policies

   The --restart=always policy restarts the container whenever it stops, including crashes and Docker daemon restarts.

2. Step 2: Compare other policies

   never disables restart, on-failure restarts only on errors, and unless-stopped restarts except when manually stopped.

3. Final Answer:

   docker run --restart=always db -> Option A

4. Quick Check:

   Always restart = auto restart on crash and daemon restart [OK]

Hint: Use --restart=always for auto restart on crash and reboot [OK]

Common Mistakes:

• Using --restart=never which disables restart
• Confusing on-failure with always
• Not setting restart policy at container start