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Darshan Vasani
Darshan Vasani Subscriber

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What happens when you run `docker run ubuntu`

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🔍 What happens when you run docker run ubuntu?

  1. Check for the Image Locally: Docker first checks if the ubuntu image exists on your local machine.
  • If not found, Docker will pull the latest Ubuntu image from Docker Hub (official registry).
  • If found, it skips pulling and uses the local version.
  1. Create and Start a Container: Docker creates a new container instance from the Ubuntu image and runs the default command.

  • The default command in the Ubuntu image is usually:

     CMD ["bash"]

  • But since you didn't pass -it, you won't see an interactive shell, and the container will likely exit immediately because there's nothing to keep it running.

🧠 So why does it exit immediately?

  • Containers need a foreground process (like a shell or server).
  • Without -it, the container runs bash, but since no input/output is attached, it just runs and exits right away.

✅ The Role of -it (Very Important!)

Here's what happens when you run:

docker run -it ubuntu
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🧩 Breaking down -it:

Flag Full Form What it does
-i --interactive Keeps STDIN open, so you can send input (like typing into a shell).
-t --tty Allocates a pseudo-TTY (like a terminal), so the output looks human-readable and interactive.

🌀 Together, -it means:

  • You get an interactive terminal session inside the container.
  • It attaches your keyboard and screen to the container’s terminal.

✅ Example with -it: 

docker run -it ubuntu
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  • This will:

    • Pull Ubuntu image if not already present.
    • Create a container.
    • Run the default shell (bash) in interactive mode.
    • Give you a terminal prompt like: 
    root@a1b2c3d4e5:/#

🧪 You can also run specific commands: 

docker run -it ubuntu bash
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  • Explicitly tells Docker to run bash shell interactively.
  • Useful if you want to ensure you're in a shell and not relying on image defaults.

🛠 Other Common docker run Flags:

Flag Purpose
--rm Automatically remove the container when it exits.
-d Run container in detached mode (in the background).
-p Publish container port to host (e.g., -p 8080:80).
--name Give the container a specific name.
-v Mount volumes (e.g., -v /host/path:/container/path).

📦 Summary:

Command What it does
docker run ubuntu Runs Ubuntu container with default CMD, but exits immediately (no -it).
docker run -it ubuntu Launches Ubuntu in an interactive terminal (bash).
docker run -it ubuntu bash Explicitly runs bash interactively.
docker run -it --rm ubuntu bash Interactive session that cleans up the container after exit.

Want to try it?

docker run -it ubuntu bash
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Then type ls, pwd, apt update etc. — you're inside a real Linux container now! 🐳💻

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