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Docker Commands in Labs: Pull, Run, Exec, and Container Management

Last updated on Sep 08, 2026

Docker Commands in Labs: Pull, Run, Exec, and Container Management

The Working With Docker Command exercise introduces core Docker operations that appear across 5 courses. This article covers common failures with docker pull, docker run, docker exec, and container lifecycle management.

Table of Contents

  1. Container Exits Immediately
  2. Container Name Conflict
  3. Port Binding Must Use -p 80:80, Not 127.0.0.1:80:80
  4. Environment Variables with -e
  5. docker exec on Stopped Containers
  6. Common Questions

Container Exits Immediately

No Foreground Process Means Instant Exit

If you run docker run -d --name myubuntu ubuntu without the -i flag, the container starts and immediately exits:

$ docker ps -a
CONTAINER ID   IMAGE     STATUS
abc123         ubuntu    Exited (0) 3 seconds ago

This is expected Docker behavior: a container runs only as long as its main process (PID 1) is alive. The ubuntu image has no default foreground process.

Fix: Add -i (interactive/keep stdin open) to keep the container running:

docker run -d --name myubuntu -i ubuntu

Or run a long-lived command:

docker run -d --name myubuntu ubuntu sleep infinity

Container Name Conflict

Can't Reuse a Container Name

If you try to create a container with a name that already exists:

docker: Error response from daemon: Conflict. The container name "/myubuntu" is already in use by container "abc123". You have to remove (or rename) that container to be able to reuse that name.

Fix: Remove the existing container first:

docker rm -f myubuntu
docker run -d --name myubuntu -i ubuntu

The -f flag forces removal even if the container is running.

Port Binding Must Use -p 80:80, Not 127.0.0.1:80:80

Port Must Bind to All Interfaces

The exercise verification checks that port 80 is exposed on 0.0.0.0 (all interfaces). If you bind to localhost only, the verification fails:

# WRONG - binds to localhost only:
docker run -d --name webserver -p 127.0.0.1:80:80 nginx:1.21.3

# CORRECT - binds to all interfaces:
docker run -d --name webserver -p 80:80 nginx:1.21.3

Fix: Use -p 80:80 to bind to 0.0.0.0:80.

Verification Checks

The exercise verifies port binding via:

docker inspect webserver | jq '.[].NetworkSettings.Ports."80/tcp"[] | select(.HostIp=="0.0.0.0" and .HostPort=="80")'

Environment Variables with -e

Passing Environment Variables to Containers

Use -e to set environment variables:

docker run -d --name webserver -e APP=nginx nginx:1.21.3

Verify the environment variable was set:

docker inspect webserver | jq -r ".[].Config.Env" | grep "APP=nginx"

Common mistake: Forgetting -e or using --env-file when the exercise expects inline -e KEY=VALUE.

docker exec on Stopped Containers

Can't Exec Into an Exited Container

ERROR: Cannot execute in a non-running container

This happens when you try to docker exec into a container that has already exited.

Fix: Check if the container is running first:

docker ps | grep myubuntu

If it's not listed, restart it or recreate it with -i to keep it running:

docker rm -f myubuntu
docker run -d --name myubuntu -i ubuntu
docker exec -it myubuntu bash

Common Questions

Question Answer
My container shows "Exited (0)" immediately. The image has no foreground process. Add -i or run a long-lived command like sleep infinity.
docker run says the container name is already in use. Run docker rm -f <name> first, then create the container.
docker exec says "Cannot execute in a non-running container." The container has stopped. Check docker ps and restart if needed.
docker pull isn't needed before docker run. Correct. docker run automatically pulls the image if it's not present locally.
How do I see all containers, including stopped ones? Use docker ps -a (the -a flag shows all containers).
How do I check what process is running inside a container? Use docker top <container-name>.

Wrap-Up

Docker container management boils down to keeping the main process alive, managing container names, binding ports correctly, and using the right flags. Key takeaways:

  • -i keeps containers running when the image has no default foreground process.
  • docker rm -f removes containers so you can reuse names.
  • -p 80:80 binds to all interfaces — 127.0.0.1:80:80 will fail verification.
  • -e KEY=VALUE sets environment variables.
  • docker exec only works on running containers.

The DevSecOps Box is stateless — any Docker containers you create will be lost on page refresh.