How Do I Run a Command on an Already Existing Docker Container?

When working with Docker, you might have an existing container running (or stopped) and want to execute a command inside it without creating a new container. Docker provides a straightforward way to do this using the docker exec and docker run commands.

🔹 When You Might Need This

You may need to:

  • Debug an application by running diagnostic commands.
  • Install additional tools inside a running container.
  • Open a shell session for manual inspection.
  • Check logs, configurations, or environment variables.

1️⃣ Using docker exec for Running Commands in a Running Container

The docker exec command allows you to run commands inside a currently running container.

Syntax:

docker exec [OPTIONS] CONTAINER COMMAND [ARG...]

Example – List files in / inside the container:

docker exec my_container ls /

Example – Start an interactive bash shell:

docker exec -it my_container bash

Use -it for interactive mode (-i = keep STDIN open, -t = allocate a pseudo-TTY).

If your container doesn’t have bash, you can try:

docker exec -it my_container sh

2️⃣ Using docker run for New Containers From the Same Image

If the original container is stopped and you just want to run a command using its image (not the exact container state):

docker run -it my_image bash

This creates a new container from the same image, which is different from exec (which works inside an existing one).

3️⃣ Running a Command in a Stopped Container

docker exec works only on running containers. If your container is stopped:

  1. Start it again: docker start my_container
  2. Then run your command: docker exec -it my_container bash

If you want to inspect files from a stopped container without starting it, you can:

docker cp my_container:/path/in/container /path/on/host

Or commit the container to an image:

docker commit my_container my_new_image
docker run -it my_new_image bash

4️⃣ Practical Examples

Check environment variables:

docker exec my_container printenv

View application logs inside the container:

docker exec my_container cat /var/log/app.log

Install curl inside the container:

docker exec my_container apt-get update && apt-get install -y curl

📌 Summary

  • Use docker exec to run a command inside a running container.
  • Use docker start to restart a stopped container before running commands.
  • For a fresh container from the same image, use docker run.

Example workflow:

docker ps                      # Find container name or ID
docker exec -it my_container bash
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