Restarting containers in Docker - Commands & Configuration

Introduction

Sometimes a container stops working correctly or needs to apply new settings. Restarting a container stops it and starts it again, fixing temporary issues or applying changes without deleting it.

When a web server container becomes unresponsive and you want to refresh it quickly.

When you update environment variables and want the container to use the new values.

When a container crashes and you want to try running it again without recreating it.

When you want to apply new network settings to a running container.

When you want to clear temporary glitches in a container without losing its data.

Commands

List all running containers to find the container ID or name you want to restart.

Terminal

docker ps

Expected Output

CONTAINER ID   IMAGE          COMMAND                  CREATED         STATUS         PORTS                    NAMES

b7c9f1a2d3e4   nginx:1.23.3   "/docker-entrypoint.…"   2 hours ago     Up 2 hours     0.0.0.0:8080->80/tcp     my-nginx

Restart the container named 'my-nginx'. This stops and then starts the container again.

Terminal

docker restart my-nginx

Expected Output

my-nginx

Verify that the container 'my-nginx' is running again after the restart.

Terminal

docker ps

Expected Output

CONTAINER ID   IMAGE          COMMAND                  CREATED         STATUS         PORTS                    NAMES

b7c9f1a2d3e4   nginx:1.23.3   "/docker-entrypoint.…"   2 hours ago     Up 5 seconds   0.0.0.0:8080->80/tcp     my-nginx

Key Concept

If you remember nothing else from this pattern, remember: restarting a container quickly fixes temporary problems by stopping and starting it without deleting it.

Common Mistakes

Trying to restart a container using its image name instead of its container name or ID.

Docker restart requires the container identifier, not the image name, so the command fails.

Use the container name or ID shown by 'docker ps' to restart the container.

Expecting 'docker restart' to apply changes made to the Dockerfile or image automatically.

Restarting a container does not update the container image; it only restarts the existing container instance.

Rebuild the image and recreate the container to apply image changes.

Summary

Use 'docker ps' to find the container you want to restart.

Run 'docker restart container_name' to stop and start the container again.

Verify the container is running with 'docker ps' after restarting.

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