Running in the background

Learn to run containers in the background with -d.

Running in the background

By default docker run stays attached to the terminal; Ctrl+C stops the container. -d (detached) starts the container in the background and prints only the container ID:

$ docker run -d --name web nginx
7c2f4a9e1b3d8f6a0e5c2b1d4f7a9c3e5b8d0f2a4c6e8b1d3f5a7c9e1b3d5f7a

You later manage the background container with docker logs, docker exec and docker stop.

A command in a running container

docker exec starts an extra process inside a running container. An interactive shell can be opened with -it:

docker exec web nginx -v              # nginx version: nginx/1.27.2
docker exec -it web sh                # a shell inside the container
# leave with exit; the container keeps running

Leaving a shell opened with exec doesn't stop the container, since the main process is still nginx.

Exercise Background and exec

Start a container named web from the nginx image in the background, then run the nginx -v command inside it.

Show example solution

docker run -d --name web nginx
docker exec web nginx -v

Test yourself

What does docker run -d print as output?
- The container ID
- The application logs
- The image size
- Nothing
Show answer

The container ID — The ID is 64 characters; for most commands the first few characters are enough.

Run a command in a running container.

docker ____ web nginx -v

Show answer

docker exec web nginx -v

Match.

-d

In the background
docker exec

A process in an existing container
-it

Interactive terminal
exit (in the exec shell)

The container keeps running

Next lesson: docker ps

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