Basic Docker Run Command

The Foundation of Docker Containers

The docker run command is the most fundamental Docker command. It creates a new container from a specified image and starts it. At its simplest, the command takes just an image name:

docker run ubuntu

This pulls the ubuntu image (if not already cached locally), creates a new container from it, and runs the default command specified in the image's CMD or ENTRYPOINT instruction.

What Happens Behind the Scenes

When you execute docker run, Docker performs several steps in sequence:

1. Image lookup: Docker checks the local image cache. If the image is not found, it pulls from Docker Hub (or a configured registry).
2. Container creation: A new read-write filesystem layer is created on top of the image's read-only layers.
3. Network setup: Docker assigns the container a virtual network interface and an IP address on the default bridge network.
4. Process execution: The container's main process (PID 1) starts. This is either the image's default CMD or a command you specify.

Overriding the Default Command

You can append a custom command after the image name:

docker run ubuntu echo "Hello from container"

This replaces the image's default command with echo "Hello from container". The container runs the command, prints the output, and exits.

Interactive Mode

For an interactive shell session, combine the -i (interactive) and -t (pseudo-TTY) flags:

docker run -it ubuntu bash

This gives you a bash prompt inside the container. Without -it, the shell would exit immediately because it has no input stream attached.

Key Points for Beginners

• Each docker run creates a new container. It does not reuse existing containers.
• When the main process exits, the container stops but is not removed (use --rm to auto-remove).
• Container names are auto-generated unless you specify one with --name.
• The container's filesystem is ephemeral by default; changes are lost when the container is removed.