Managing containers

Usually we want some feedback on how our containers are doing. In this section, we'll look into a few ways we can examine and manage Docker containers.

Listing containers

There are two ways to list the currently running containers. One is to use docker container ls and the other docker ps. Both commands produce the same output.

$ docker ps
CONTAINER ID    IMAGE    COMMAND                  CREATED         STATUS          PORTS                  NAMES
624a8853afe5    nginx    "nginx -g 'daemon of…"   3 seconds ago   Up 2 seconds    0.0.0.0:8080->80/tcp   web

If you still have your web server from the previous exercise running, you should see it listed.

You can also list all past containers with the help of the -a flag.

$ docker ps -a
CONTAINER ID    IMAGE           COMMAND                  CREATED          STATUS                     PORTS                  NAMES
624a8853afe5    nginx           "nginx -g 'daemon of…"   2 minutes ago    Up 2 minutes               0.0.0.0:8080->80/tcp   web
ee73eeb1a2e3    nginx           "nginx -g 'daemon of…"   3 minutes ago    Exited (0) 2 minutes ago                          festive_mestorf
8c6a15958a2e    ubuntu:20.04    "uname -a"               5 minutes ago    Exited (0) 5 minutes ago                          jovial_sinoussi
4cbcaffa8ca5    hello-world     "/hello"                 6 minutes ago    Exited (0) 6 minutes ago                          suspicious_pascal

We can later use the container IDs and names to refer to these containers for further operations. Both of them are automatically generated for us, but we can also set the name as done in the earlier exercise.

Printing logs

When we ran the NGINX container in foreground earlier, we could see the logs printed on the terminal directly. Now that NGINX is running in the background, we need another way to view the logs.

This is where the docker logs command comes in handy. We can use container name we defined earlier or the container ID to get logs from the web server.

$ docker logs web
172.17.0.1 - - [01/Feb/2019:13:12:58 +0000] "GET / HTTP/1.1" 200 612 "-" "Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:64.0) Gecko/20100101 Firefox/64.0" "-"
...

$ docker logs 624a8853afe5
172.17.0.1 - - [01/Feb/2019:13:12:58 +0000] "GET / HTTP/1.1" 200 612 "-" "Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:64.0) Gecko/20100101 Firefox/64.0" "-"
...

You can also follow the flow of logs with the help of the -f flag.

$ docker logs -f web

Press the keyboard combination Ctrl+C to exit the log follow command.

Examining container processes

We can use the docker top command to examine the processes running inside a container. Let's see what's running in the NGINX container.

$ docker top web
UID     PID      PPID     C     STIME     TTY    TIME       CMD
root    32541    32524    0     20:00     ?      00:00:00   nginx: master process nginx -g daemon off;
101     32583    32541    0     20:00     ?      00:00:00   nginx: worker process
...

The docker top command accepts Linux ps command parameters, which you can use to tune the top output. See man ps for more ps options.

Executing commands on containers

Sometimes we may want to inspect the container by running arbitrary commands inside it. We can use docker exec for that.

For example, if we want to check the NGINX web server configuration used in the NGINX container, we can display it in the terminal window by running the cat command using the docker exec command. We provide the path to the configuration file as a parameter to cat.

$ docker exec web cat /etc/nginx/nginx.conf

user  nginx;
worker_processes  1;

error_log  /var/log/nginx/error.log warn;
pid        /var/run/nginx.pid;

...

We can also combine docker exec with the -it flags we used earlier to get an interactive shell inside the container.

$ docker exec -it web bash
root@1aabce753f41:/# ls
bin  boot  dev  etc  home  lib  lib64  media  mnt  opt  proc  root  run  sbin  srv  sys  tmp  usr  var
root@1aabce753f41:/# exit

While in the interactive shell in the container, type exit or press they keyboard combination Ctrl+D to return back to the host command-line interface.

Stopping a container

Once we are done with the container, we can stop it with docker stop.

$ docker stop web

If the container is stuck, we can also stop it by force using docker kill.

$ docker kill web

Listing images

Containers are created from Docker images. The appropriate images are automatically downloaded when you run a container. You can list the downloaded images with docker images or docker image ls.

$ docker images
REPOSITORY      TAG        IMAGE ID         CREATED        SIZE
nginx           latest     42b4762643dc     1 hour ago     109MB
ubuntu          18.04      20bb25d32758     1 hour ago     87.5MB
hello-world     latest     fce289e99eb9     1 hour ago     1.84kB

Cleaning your Docker environment

After the container process stops, the container is still left around for further inspection. You can view the stopped containers using docker ps -a.

To get rid of the stopped containers, you can use the docker rm or docker container rm command.

$ docker rm web

If you want the container to be automatically removed after it has stopped, you can add the --rm flag to docker run. It's considered a good idea to use --rm by default.

$ docker run --rm --name web -p 8080:80 nginx

$ docker stop web

If you want to also get rid of the Docker image, you can run the docker rmi or docker image rm command.

$ docker rmi nginx

If you get a message that says a repository reference can't be removed when attempting to remove a particular image, that means that there's a container that still uses the image. You must first remove that container before you can remove the image. See the example error message below:

Error response from daemon: conflict: unable to remove repository reference "nginx" (must force) - container e01c97f610b3 is using its referenced image f8f4ffc8092c

To automatically get rid of unused containers and images, you can use the following commands.

$ docker container prune
$ docker image prune