Interactive vs Detached Containers

LEVEL 0

The Problem

Some containers run a quick task and exit (like hello-world). Some containers run services that should stay running (like a web server). Some containers you want to interact with (like a shell).

How do you control this? How do you run containers in the background? How do you attach to them?

LEVEL 2

The Mechanics — Run Modes

The Mechanics

Default (Foreground, Non-Interactive)

docker run nginx

This runs Nginx in the foreground. The container’s output goes to your terminal. Press Ctrl+C to stop.

Detached (Background)

docker run -d nginx

The -d flag runs the container in the background. Docker prints the container ID and returns control to you immediately.

To see what’s happening:

docker logs <container_id>

Interactive

docker run -it ubuntu bash

The -i flag keeps stdin open (interactive). The -t flag allocates a pseudo-TTY (terminal). Together, they let you interact with a shell inside the container.

You’ll get a bash prompt inside the Ubuntu container. Type exit to leave (and stop the container).

LEVEL 3

Combining Flags

Common patterns:

# Run a web server in background, name it, map port
docker run -d --name web -p 8080:80 nginx

# Run interactive shell, remove container when done
docker run -it --rm ubuntu bash

# Run detached, but follow logs immediately
docker run -d --name api my-api && docker logs -f api

The --rm flag: Automatically remove the container when it exits. Great for one-off commands to avoid container accumulation.

LEVEL 4

Attaching and Detaching

What if you ran something in the background and want to see its output?

# Attach to a running container's output
docker attach <container_id>

Be careful: if you Ctrl+C from attach, it can stop the container.

What if you want to get into a running container?

# Run a new process inside a running container
docker exec -it <container_id> bash

exec is different from attach:

attach: Connect to the container’s main process (PID 1)
exec: Start a new process inside the container

You almost always want exec when you want a shell in a running container.