Basic Docker commands

The container lifecycle — run, ps, logs, exec, stop, rm — and the flags you use most.

Container lifecycle

stateDiagram-v2
  [*] --> Running: docker run
  Running --> Stopped: docker stop
  Stopped --> Running: docker start
  Stopped --> [*]: docker rm

The command you use most is docker run; it pulls the image (if missing), creates and starts the container:

docker run -d --name web -p 8080:80 nginx:1.27

-d (--detach) runs in the background and prints the container ID.
--name web sets a name, so later commands can use web instead of the ID.
-p 8080:80 publishes a port in host:container order: http://localhost:8080 reaches port 80 inside the container.
All flags go before the image name; anything after the image name is the command run inside the container.

Command	Meaning
`docker ps` / `docker ps -a`	Running containers / all, including stopped
`docker logs web`	Show the container's stdout/stderr
`docker exec -it web sh`	Open a shell in a running container
`docker stop web` / `docker start web`	Stop / start the container again
`docker rm web`	Remove a stopped container (`-f` also removes a running one)
`docker rmi nginx:1.27`	Remove an image (= `docker image rm`)

Management commands are the newer, equivalent spelling: docker container ls = docker ps, docker image ls = docker images, docker container run = docker run.

Common traps

Reversing it as -p 80:8080 sends you to the wrong port.
You cannot remove an image while a container (even a stopped one) uses it: docker rm the container first.
Copying commands from slides with – (en dash) instead of -: Docker does not see a flag.
To push to Docker Hub: docker tag as <user>/<repo>:<tag>, docker login, then docker push.

More `docker run` flags you will use

Flag	Meaning
`-e KEY=value`, `--env-file .env`	Set environment variables
`-v data:/var/lib/app`	Mount a volume
`--rm`	Remove the container when it exits
`-it`	Interactive terminal, for shells
`--restart unless-stopped`	Restart after crashes and daemon restarts
`--memory 512m --cpus 1`	Limit memory and CPU
`-u 1000:1000`	Run as a specific user

Looking inside

docker logs -f --tail 100 web follows the last 100 lines.
docker inspect web prints full JSON; docker inspect -f '{{.State.Status}}' web extracts one field.
docker stats shows live CPU, memory, and network per container; docker top web lists its processes.
docker cp web:/etc/nginx/nginx.conf ./nginx.conf copies files out of (or into) a container.
docker port web shows published ports.

Exit codes tell a story

Code	Usual meaning
0	The main process finished normally
1	Application error
125	`docker run` itself failed (bad flag, name conflict)
127	Command not found inside the image
137	Killed by SIGKILL, often the out-of-memory killer or `docker kill`
143	Stopped by SIGTERM, a normal `docker stop`

docker ps -a shows the exit code in the STATUS column, and docker inspect -f '{{.State.OOMKilled}}' web confirms a memory kill.

Cleaning up

Stopped containers, unused images, and build cache pile up quickly. docker system df shows how much space each uses. docker container prune, docker image prune -a, and docker builder prune remove them selectively; docker system prune removes stopped containers, unused networks, dangling images, and build cache in one go (add --volumes only when you are sure no data is needed).