Running and stopping containers

Next we will start using a more useful image than hello-world. We can run ubuntu just with the shorthand docker run ubuntu (which is exactly the same as docker container run ubuntu)

$ docker run ubuntu
  Unable to find image 'ubuntu:latest' locally
  latest: Pulling from library/ubuntu
  0509fae36eb0: Pull complete
  Digest: sha256:4b1d0c4a2d2aaf63b37111f34eb9fa89fa1bf53dd6e4ca954d47caebca4005c2
  Status: Downloaded newer image for ubuntu:latest

Anticlimactic as nothing really happened. The image was downloaded and ran and that was the end of that. It actually tried to open a shell but we will need to add a few flags to interact with it. -t will create a tty.

$ docker run -t ubuntu
  root@f83969ce2cd1:/#

Now we're inside the container and if we input ls and press enter... nothing happens. Because our terminal is not sending the messages into the container. The -i flag will instruct to pass the STDIN to the container.

Now you'll realize that you're stuck. Control+C or typing exit won't work.

Just open up another terminal to stop the container. Note that the container's hostname is set to the container name so you don't necessarily have to docker container ls here, just copy the ID from the output after root@<CONTAINER ID>.

Now if you just docker stop f83969ce2cd1 you are stuck again, atleast for 10 seconds. Why exactly 10?

Let's see the --help for docker stop (which is exactly the same as docker container stop)

$ docker stop --help

Usage:  docker stop [OPTIONS] CONTAINER [CONTAINER...]

Stop one or more running containers

Options:
  -t, --time int   Seconds to wait for stop before killing it (default 10)

Stop sends a SIGTERM Signal to the process running in the container and waits for 10 seconds to exist. Our ubuntu image does not listen this signal and then stop will time out and send unconditional SIGKILL instead. You'll notice someday that many images don't handle SIGTERM properly.

So, as it's cumbersome to fix ubuntu image right now, specifying -t 0 would stop the image immediately.

Even better, docker rm -f <id> would not only stop, but remove the container all together which is probably what you often want.

So now let's start the container again with both -t and -i:

$ docker run -it ubuntu
  root@2eb70ecf5789:/# ls
  bin  boot  dev  etc  home  lib  lib32  lib64  libx32  media  mnt  opt  proc  root  run  sbin  srv  sys  tmp  usr  var

Great! Now we know at least 3 useful flags. -i (interactive), -t (tty), -d (detached) and -f (force (or format))

Date print loop

(mac/linux only) This shell script prints the output of command date

$ while true; do date; sleep 1; done
Wed Nov  9 14:04:43 UTC 2022
Wed Nov  9 14:04:44 UTC 2022
Wed Nov  9 14:04:45 UTC 2022
...

Smash control+c a few times to stop the loop.

Now let's start that as the command for a new container named looper out of our ubuntu image

$ docker run -it --name looper ubuntu bash -c 'while true; do date; sleep 1; done'
Wed Nov  9 14:04:43 UTC 2022
Wed Nov  9 14:04:44 UTC 2022
...

So here we gave the name looper so that the name is not autogenerated. And then we started /bin/sh with our -command. When you hit control+c it will exit the process and thus terminate the container.

Now, run the command again and it fails:

$ docker run -it --name looper ubuntu bash -c 'while true; do date; sleep 1; done'
  docker: Error response from daemon: Conflict. The container name "/looper" is already in use by container "3fd613501081b20101bc6df2b5711939b2dcbce7e871b3fdaaf45cbb107b92f6". You have to remove (or rename) that container to be able to reuse that name.
  See 'docker run --help'.

Since we are not autogenerating names, the previous container still exists. So let's remove it

$ docker rm looper
looper

And then start the container as daemonized on the background with -d

$ docker run -d -it --name looper ubuntu bash -c 'while true; do date; sleep 1; done'
  de13ddceec7a7d8f14348ebee739269633e5962b4005c8c4230c2ee68f2884d0

And we notice how a bit confusingly it printed out the container id and returned.

The first part, docker run -d. Should be familiar by now, run container detached.
Followed by -it is short for -i and -t. Also familiar, -it allows you to interact with the container by using the command line.
Because we ran the container with --name looper, we can now reference it easily.
The image is ubuntu and what follows it is the command given to the container.

And to check that it's running, run docker container ls

Let's follow -f the output of logs with

$ docker logs -f looper
  Thu Feb  4 15:51:29 UTC 2021
  Thu Feb  4 15:51:30 UTC 2021
  Thu Feb  4 15:51:31 UTC 2021
  ...

Keep the logs open and attach to the running container from the second terminal using 'attach':

$ docker attach looper
  Mon Jan 15 19:26:54 UTC 2018
  Mon Jan 15 19:26:55 UTC 2018
  ...

Now you have process logs (STDOUT) running in two terminals. Now press control+c in the attached window. The container is stopped because the process is no longer running.

If we want to attach to a container while making sure we don't close it from the other terminal we can specify to not attach STDIN with --no-stdin option. Let's start the stopped container with docker start looper and attach to it with --no-stdin.

Then try control+c.

$ docker start looper

$ docker attach --no-stdin looper
  Mon Jan 15 19:27:54 UTC 2018
  Mon Jan 15 19:27:55 UTC 2018
  ^C

The container will continue running. Control+c now only disconnects you from the STDOUT.

To enter a container, we can start a new process in it.

$ docker exec -it looper bash

  root@2a49df3ba735:/# ps aux

  USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
  root         1  0.2  0.0   2612  1512 pts/0    Ss+  12:36   0:00 sh -c while true; do date; sleep 1; done
  root        64  1.5  0.0   4112  3460 pts/1    Ss   12:36   0:00 bash
  root        79  0.0  0.0   2512   584 pts/0    S+   12:36   0:00 sleep 1
  root        80  0.0  0.0   5900  2844 pts/1    R+   12:36   0:00 ps aux

From the ps aux listing we can see that our bash process got PID (process ID) of 64.

Now that we're inside the container it behaves as you'd expect from ubuntu, and we can exit the container with exit and then either kill or stop the container.

Our looper won't stop for a SIGTERM signal sent by a stop command. To terminate the process, stop follows the SIGTERM with a SIGKILL after a grace period. In this case, it's simply faster to use kill.

docker kill looper
docker rm looper

Running the previous two commands is basically equivalent to running docker rm --force looper

Let's start another process with -it and add --rm in order to remove it automatically after it has exited. The --rm ensures that there are no garbage containers left behind. It also means that docker start can not be used to start the container after it has exited.

docker run -d --rm -it --name looper ubuntu bash -c 'while true; do date; sleep 1; done'

Now let's attach to the container and hit control+p, control+q to detach us from the STDOUT (from https://docs.docker.com/engine/reference/commandline/attach/)

$ docker attach looper

  Mon Jan 15 19:50:42 UTC 2018
  Mon Jan 15 19:50:43 UTC 2018
  ^P^Qread escape sequence

Instead, if we had used ctrl+c, it would have sent a kill signal followed by removing the container as we specified --rm in docker run command.

Start a ubuntu image with the process bash -c 'echo "Input website:"; read website; echo "Searching.."; sleep 1; curl http://$website;'

You will notice that a few things required for proper execution are missing. Be sure to remind yourself which flags to use so that the read actually waits for input.

Note also that curl is NOT installed in the container yet. You will have to install it from inside of the container.

Test inputting helsinki.fi into the application. It should respond with something like

<html>
  <head>
    <title>301 Moved Permanently</title>
  </head>

  <body>
    <h1>Moved Permanently</h1>
    <p>The document has moved <a href="http://www.helsinki.fi/">here</a>.</p>
  </body>
</html>

This time return the command you used to start process and the command(s) you used to fix the ensuing problems.

This exercise has multiple solutions, if the curl for helsinki.fi works then it's done. Can you figure out other (smart) solutions?

You have reached the end of this section! Continue to the next section:

3. In-depth dive to images