Running Your First Container — Docker CLI

Overview

Use the core Docker CLI to run, inspect, exec into, log, stop, and remove containers confidently.

Daily ops is CLI fluency: run, ps, logs, exec, stop, rm. Lifecycle awareness prevents orphan containers and surprise disk use.

This is a core tutorial in Module 3 · Docker CLI of the REBASH Academy Docker for Cloud & DevOps Engineers series — written for Cloud, DevOps, Platform, and SRE engineers.

Prerequisites

• Docker Installation and Setup

Learning Objectives

By the end of this tutorial, you will be able to:

• docker run with ports and names
• List running/stopped containers
• Follow logs and exec a shell
• Inspect JSON config
• Clean up with stop / rm

Architecture

This topic’s control points and relationships are shown below.

Theory

What

docker run creates and starts a container from an image. Day-one operations also include listing (ps), stopping, reading logs, executing a shell in a running container (exec), and inspecting low-level metadata. These commands are the foundation of every later Compose and Kubernetes skill.

Why

If you cannot start a container, read its logs, and confirm the process exit code, you cannot debug CI or production. Learning graceful stop versus kill, and when to use --rm, prevents leftover containers filling disks during labs.

How it works

docker run [options] image [cmd] pulls the image if needed, creates a container, and starts it. Common flags publish ports (-p), set environment variables (-e), mount volumes, and name the container. Foreground vs detached (-d) changes whether your terminal attaches to the process. docker logs reads the configured logging driver (often json-file capturing stdout/stderr). docker exec starts an additional process in the same namespaces — useful for debugging, not as the main entrypoint. docker inspect prints JSON; Go-template --format strings use double braces and must be escaped in MkDocs, for example {{.State.Status}}.

Command	Use
docker run	Create + start (often --rm)
docker ps -a	List containers
docker stop / kill	Graceful vs SIGKILL
docker logs -f	Stdout/stderr
docker exec -it	Shell in running container
docker inspect	Low-level details

Key concepts

• PID 1 — the container’s main process; its exit stops the container
• Published ports — host:container mapping is not the same as EXPOSE
• Ephemeral vs named — --rm for experiments; names for labs you revisit
• Exit codes — distinguish pull failures from app crashes

Common pitfalls

• Forgetting -d and thinking the container “died” when you closed the terminal wrongly
• Using kill before allowing graceful stop
• exec into a crashed container (it must be running)
• Publishing 0.0.0.0 ports on shared runners without care

Hands-on Lab

Create a workspace for this tutorial.

mkdir -p ~/rebash-docker/module-03 && cd ~/rebash-docker/module-03

Focus: run, publish ports, view logs, and remove the container

Step 1 – Foreground then detached

docker run --rm alpine:3.20 echo "hello from alpine"
docker run -d --name rebash-first -p 18081:80 nginx:alpine
curl -s http://127.0.0.1:18081 | head -n 3
docker logs rebash-first --tail 20

Step 2 – Stop and remove

docker stop rebash-first
docker rm rebash-first
docker ps -a --filter name=rebash-first

Final step – Cleanup note

docker rm -f rebash-first 2>/dev/null || true
# Keep ~/rebash-docker/ for later tutorials

Validation

• Lab commands run under ~/rebash-docker/module-03/
• You can explain each Theory section in your own words
• You used modern tooling where it applies to this topic
• You can describe one production failure mode for this topic

Code Walkthrough

Production practice for Running Your First Container — Docker CLI always combines:

1. Inspect before you change (status, plan, logs, dry-run)
2. Prefer reversible, documented changes (Git, IaC, drop-ins, version pins)
3. Capture evidence (command output, pipeline logs) for handovers
4. Prefer current tools and APIs over legacy shortcuts
5. Least privilege — escalate credentials only when required

Keep runbooks short enough to follow under pressure. Automate checks; keep humans for judgement.

Security Considerations

• Treat credentials and tokens for docker as privileged — never commit them
• Prefer short-lived auth (OIDC, roles, SSO) over long-lived keys
• Validate blast radius before apply/deploy/delete operations
• Restrict who can approve production changes
• Collect audit logs; limit who can read sensitive traces

Common Mistakes

Forgetting -d and thinking the container “died” when you closed the terminal wrongly

Validate assumptions against the Theory section and official docs before changing production.

Using kill before allowing graceful stop

Lab shortcuts (open security groups, admin roles, skip approvals) must not ship unchanged.

Changing production without a rollback path

Always know how to revert (previous artefact, prior release, state rollback, DNS failback).

Best Practices

• Encode Running Your First Container — Docker CLI changes as code and review them in pull requests
• Pin versions (images, modules, actions, provider plugins)
• Separate environments with clear promotion gates
• Alert on symptoms with runbooks attached
• Destroy lab resources; tag everything with owner and expiry where possible

Troubleshooting

Symptom	Likely cause	Fix
Auth / permission denied	Wrong identity, policy, or scope	Check caller identity, roles, and least-privilege policies
Timeout / no route	Network, DNS, security group, or endpoint	Trace path, DNS, and allow-lists before retrying
Drift / unexpected plan	Manual change or wrong state/workspace	Reconcile desired vs actual; avoid click-ops on managed resources
Pipeline/job red	Flaky step, cache, or missing secret	Read failing step logs; bisect recent workflow/config changes
Cost spike	Idle load balancer, NAT, oversized compute	Inventory billable resources; stop/delete labs promptly

Summary

Running Your First Container — Docker CLI is essential for Cloud and DevOps engineers working with docker. Practise the lab until the inspection and change path is muscle memory, then continue the track.

Interview Questions

1. What do -d, --name, and -p do?
2. Port publish fails — typical causes?
3. How do you get a shell in a running container?
4. Difference between stop and rm?
5. Why prefer --rm for ephemeral experiments?

Sample answer — question 2

Confirm the container is running, the published port mapping, and that nothing else bound the host port.

Sample answer — question 4

Do not publish administrative ports to 0.0.0.0 on untrusted networks.

Related Tutorials

• Course overview
• Working with Docker Images

References

• docker run