In-Class Exercise - Docker Logs and Container Management¶

Step-by-Step Solution¶

1. Run the cowsay container with a custom message

docker run --name my-cowsay docker/whalesay cowsay "Hello from Docker!"

1. Verify the container has stopped

The container stops automatically after execution. You can verify with:

docker ps -a | grep my-cowsay

1. Grab the logs and save to host machine

docker logs my-cowsay > cowsay-logs.txt

1. View the saved logs

cat cowsay-logs.txt

Expected output in cowsay-logs.txt:

 ______________________
< Hello from Docker! >
 ----------------------
    \
     \
      \     
                    ##        .            
              ## ## ##       ==            
           ## ## ## ##      ===            
       /""""""""""""""""___/ ===        
  ~~~ {~~ ~~~~ ~~~ ~~~~ ~~ ~ /  ===- ~~~   
       \______ o          __/            
        \    \        __/             
          \____\______/   

1. Clean up (optional)

# Remove the container
docker rm my-cowsay

# Remove the log file
rm cowsay-logs.txt

Alternative: Using a Different Cowsay Image¶

If docker/whalesay is not available, you can build your own:

Dockerfile:

FROM alpine:latest
RUN apk add --no-cache cowsay
ENTRYPOINT ["/usr/bin/cowsay"]
CMD ["Hello Docker!"]

Build and run:

docker build -t my-cowsay .
docker run --name cowsay-test my-cowsay "Hello from Docker!"
docker logs cowsay-test > cowsay-logs.txt

Explanation¶

• docker run –name: Assigns a name to the container for easy reference
• cowsay “message”: Passes the message to the cowsay command
• docker logs: Retrieves all stdout/stderr output from the container
• > cowsay-logs.txt: Redirects the log output to a file on the host machine
• The container stops automatically after the command completes

Bonus: Running in Detached Mode¶

For containers that run longer:

# Run in detached mode
docker run -d --name my-cowsay-bg docker/whalesay /bin/sh -c "cowsay 'Background task' && sleep 30"

# Get logs while running
docker logs my-cowsay-bg

# Follow logs in real-time
docker logs -f my-cowsay-bg

# Stop the container
docker stop my-cowsay-bg

# Save logs after stopping
docker logs my-cowsay-bg > cowsay-bg-logs.txt