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Mayank Gupta
Mayank Gupta

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Managing Docker Images and Containers: A Comprehensive Guide

#tutorial #devops #productivity #beginners

Docker is widely used for containerization, enabling efficient application deployment. This guide covers essential Docker commands and concepts related to managing images and containers, understanding interactive mode, viewing logs, handling file transfers, and optimizing workflows.

Getting Help with Docker

Docker provides built-in help for commands. To explore available options, simply run:

docker --help
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For specific commands, use:

docker <command> --help
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This helps in discovering flags and additional functionalities.

Start vs Run Command: Attached vs Detached Mode

When working with containers, understanding how they run is crucial.

Docker Run vs Docker Start

  • docker run <container-id> → Starts a new container in attached mode, meaning the terminal blocks while logs are printed.
  • docker start <container-id> → Starts an existing container in detached mode, meaning logs won’t be printed, and interaction isn’t possible.

Running a Container in Detached Mode

By default, docker run runs in attached mode, but you can force it into detached mode with:

docker run -p 8000:80 -d <container-id>
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Here:

  • -p 8000:80 → Maps container's port 80 to host port 8000.
  • -d → Runs the container without blocking the terminal.

Attaching to a Running Container

Even if a container is running in detached mode, you can attach to it anytime:

docker container attach <container-name>
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Viewing Logs for Containers

To see logs, use:

docker logs <container-name>
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To continuously monitor logs in real-time:

docker logs -f <container-name>
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Here, -f ensures live log streaming in the terminal.

Interactive Mode in Docker Containers

Docker isn't just for web applications—it can also be used for interactive scripts.

Practice Resource

Example: Running a Python Script in Interactive Mode

Consider a Python script (rng.py):

from random import randint

min_number = int(input('Please enter the min number: '))
max_number = int(input('Please enter the max number: '))

if max_number < min_number:
    print('Invalid input - shutting down...')
else:
    rnd_number = randint(min_number, max_number)
    print(rnd_number)
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Dockerfile for the Python Script 

FROM python

WORKDIR /app

COPY . /app

CMD ["python", "rng.py"]
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Running the Container

Trying:

docker run <container-id>
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Would result in an error.

Instead, run in interactive mode:

docker run -i -t <container-id>
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Or equivalently:

docker run -it <container-id>
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Explanation of -i and -t Flags

  • -i → Interactive mode, allowing input from the terminal.
  • -t → Tty (pseudo-terminal) mode, enabling a terminal interface within the container.

Starting an Interactive Container

By default, docker start runs a container in detached mode.

To start in interactive mode, use:

docker start -a -i <container-name>
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Where:

  • -a → Attaches output to the terminal.
  • -i → Allows input interaction.

Deleting Containers

Checking Running Containers 

docker ps
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Stopping a Running Container 

docker stop <container-name>
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Removing Containers 

docker rm <container-name_1> <container-name_2>
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Automatically Removing Stopped Containers

Instead of manually cleaning up stopped containers, Docker provides the --rm flag:

docker run -p 3000:80 -d --rm <container-id>
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  • --rm ensures the container automatically gets deleted once it stops.

Copying Files To and From a Container

Copying Files INTO a Container 

docker cp dummy/. <container-name>:/test
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This command:

  • Copies all files from the local dummy/ folder into /test inside the container.
  • If /test doesn’t exist, Docker automatically creates it.
  • Use Case: Copy configuration files into a Docker container.

Copying Files FROM a Container 

docker cp <container-name>:/test /dummy
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  • Copies files from /test inside the container to dummy/ on the local machine.
  • Use Case: Extract logs generated inside a container to the local system.

Naming and Tagging Containers & Images

Naming a Running Container

When starting a container, you can assign a custom name instead of a random identifier:

docker run -p 3000:80 --name my-container <image-id>
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Here:

  • --name my-container assigns a human-friendly name to the container.

Naming (Tagging) an Image

Images can be tagged for easier identification:

docker build -t goals:latest .
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Where:

  • -t goals:latest tags the image as "goals" with version "latest".

To see all images:

docker images
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Conclusion

  • Attached vs Detached Mode: docker run is attached by default, docker start is detached.
  • Interactive Mode: -i and -t allow input/output within a container.
  • Logs Management: docker logs -f <container> provides real-time logs.
  • File Transfers: Use docker cp to move files to/from a container.
  • Container Cleanup: docker rm removes stopped containers, --rm ensures automatic removal.
  • Naming and Tagging: Use --name for containers and -t for images.

By understanding these commands, managing Docker images and containers becomes effortless. Experiment with these concepts in real-world scenarios to strengthen your containerization expertise.

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