Working inside a container

Opening a shell in a new container

Sometimes you want to poke around inside a container - look at its files, run commands, see what's installed. You can start a container and drop straight into its shell with two flags:

docker run -it ubuntu bash

• -i keeps the input open so you can type.
• -t gives you a proper terminal.
• ubuntu is the image, and bash is the shell we want to run.

Together, -it ... bash means "start an Ubuntu container and give me a shell inside it". Your prompt changes - you're now typing inside the container. Try a few commands:

ls
cat /etc/os-release
whoami

When you're done, type exit to leave. Because the shell was the container's main process, leaving it stops the container.

Enter a running container with docker exec

What if a container is already running (like our Nginx server) and you want to look inside it without stopping it? Use docker exec:

docker exec -it my-web bash

This runs a new bash shell inside the existing my-web container. You can inspect files, check configuration, and so on. Type exit to leave - this time the container keeps running, because you only exited the extra shell you started, not the main process.

Why this is useful

Being able to open a shell inside a container is one of the most useful debugging skills in Docker. When something isn't working, you can go in and check:

• Are the files where you expect them?
• Is the config correct?
• Can the app reach the things it needs?

You now know how to run, manage and inspect containers and images. In the next chapter we'll build our own image from scratch.

A gotcha: not every image has bash

You'll eventually run docker exec -it somecontainer bash and get "executable file not found". That's because small images like alpine don't ship bash - they include the lighter sh shell instead. When bash fails, try sh:

docker exec -it my-container sh

It's a tiny thing, but it confuses almost everyone the first time.

FAQ

What is the difference between docker run -it and docker exec -it?

docker run -it starts a new container and drops you into its shell. docker exec -it runs a shell inside a container that is already running, without disturbing it. Use run to start fresh, exec to look inside something live.

Why do I get "executable file not found" when I run bash?

The image probably doesn't include bash. Minimal images such as alpine ship only sh. Run docker exec -it my-container sh instead.

Does exiting the shell stop the container?

It depends. If the shell was the container's main process (a docker run -it ... bash), exiting stops the container. If you attached with docker exec, exiting only ends that extra shell and the container keeps running.