Session 4 · Running your first containers

The big idea

docker run is the command you will type most. It creates a container from an image and runs a command in it. The container lives exactly as long as its main process: when that command finishes, the container stops. That one rule explains most beginner surprises.

Everyday example: a taxi booked for a single trip. It comes, drives where you asked, and the booking ends when the ride ends. A container is booked for one command; when the command is done, it is done. (If you want a “taxi on standby”, run a command that keeps going, like a web server.)

docker run creates and starts a container. It runs as long as its main process runs. Stopped containers stay until you remove them.

Run, look around, run more

docker rm -f dm-shell >/dev/null 2>&1
echo '--- run a command and let the container disappear (--rm)'
docker run --rm alpine echo "Hello, Docker!"
echo '--- the same container has its own hostname and its own process list'
docker run --rm alpine sh -c 'hostname; ps'
echo '--- a container lives only as long as its main process'
docker run --name dm-shell alpine sh -c 'echo working; sleep 1; echo done'
docker ps -a --filter name=dm-shell --format '{{.Names}}: {{.Status}}'
echo '--- run a second command inside a running container (exec)'
docker run -d --name dm-shell2 alpine sleep 30 >/dev/null
docker exec dm-shell2 sh -c 'echo "I am inside: $(hostname)"; cat /etc/alpine-release'
docker rm -f dm-shell dm-shell2 >/dev/null

--- run a command and let the container disappear (--rm)
Hello, Docker!
--- the same container has its own hostname and its own process list
ab7c3a046a65
PID   USER     TIME  COMMAND
    1 root      0:00 ps
--- a container lives only as long as its main process
working
done
dm-shell: Exited (0) Less than a second ago
--- run a second command inside a running container (exec)
I am inside: 1f30554a3e2c
3.24.1

What happened:

• --rm deleted the container as soon as it exited, so nothing was left to clean up.

• A private world. Inside, hostname is a random id and ps shows only one process (PID 1), not your host’s hundreds. Containers have their own process list.

• Main process = lifetime. The sh -c '... sleep 1 ...' ran for a second, then the container showed Exited (0). Exit code 0 means success.

• docker exec runs an extra command in a container that is already running. We started one with sleep 30, then ran another shell command inside it.

• Both hostname values differ because those are two different containers.

Interactive mode

For a shell you can type into, add -it: docker run -it alpine sh. -i keeps input open and -t gives a terminal. Type exit to leave; the container stops because its main process (the shell) ended.

Reading docker ps

docker ps lists running containers; docker ps -a also lists the stopped ones. The columns you will use most are NAMES (the handle you type in other commands), IMAGE (what it was made from) and STATUS (Up 5 minutes, or Exited (0) 2 minutes ago). If you do not give --name, Docker invents a funny name like quirky_hopper. Naming containers yourself makes commands readable, and you’ll do that properly in the next session.

Common mistakes

• Expecting a container to stay up. docker run alpine exits immediately because the default command has nothing to do. Give it a long-running command or a server.

• Forgetting -it for a shell. Without it sh sees no input and exits at once.

• Typing exec on a stopped container. docker exec needs a running container. Use docker run or docker start first.

• Never cleaning up. Each docker run without --rm leaves a stopped container. Check docker ps -a.

• Reading exit codes wrongly. 0 is success, anything else is an error (137 means it was killed).

Hands-on exercises

Exercise 1. Run a container that prints today’s date and then disappears.

Solution

docker run --rm alpine date

It prints the date (in UTC) and leaves nothing behind, since --rm removed it.

Exercise 2. Start an Alpine container that sleeps for 60 seconds in the background, run cat /etc/alpine-release inside it, then remove it.

Solution

docker run -d --name nap alpine sleep 60
docker exec nap cat /etc/alpine-release
docker rm -f nap

On this machine the version printed was 3.24.1 (yours may differ). rm -f stops and removes in one step.

Exercise 3. Run a container whose command fails and read its exit code.

Solution

docker run --name bad alpine sh -c 'exit 3'
docker ps -a --filter name=bad --format '{{.Status}}'
docker rm bad

The status shows Exited (3): the container’s exit code is the command’s exit code.

Practice

Take the Session 4 quiz. The original BusyBox “Hello, World” adventure is in the reference section.