Running & Inspecting Containers

By the end of this exercise, you should be able to:

Start a container
List running and stopped containers

Running Containers

First, let's start a container, and observe the output:
```
[centos@node-0 ~]$ docker container run centos:7 echo "hello world"

Unable to find image 'centos:7' locally
7: Pulling from library/centos
256b176beaff: Pull complete 
Digest: sha256:6f6d986d425aeabdc3a02cb61c02abb2e78e57357e92417d6d58332856024faf
Status: Downloaded newer image for centos:7
hello world
```
The centos:7 part of the command indicates the image we want to use to define this container; it defines a private filesystem for the container. echo "hello world" is the process we want to execute inside the kernel namespaces created when we use docker container run.

Since we've never used the centos:7 image before, first Docker downloads it, and then runs our echo "hello world" process inside a contianer, sending the STDOUT stream of that process to our terminal by default.
Now create another container from the same image, and run a different process inside of it:
```
[centos@node-0 ~]$ docker container run centos:7 ps -ef

UID        PID  PPID  C STIME TTY          TIME CMD
root         1     0  0 14:28 ?        00:00:00 ps -ef
```
No download this time, and we can see that our containerized process (ps -ef in this case) is PID 1 inside the container.
Try doing ps -ef at the host prompt and see what process is PID 1 here.

Listing Containers

Try listing all your currently running containers:
```
[centos@node-0 ~]$ docker container ls
```
There's nothing listed, since the containers you ran executed a single command, and shut down when finished.
List stopped as well as running containers with the -a flag:
```
[centos@node-0 ~]$ docker container ls -a

CONTAINER ID  IMAGE      COMMAND                CREATED              STATUS                          ...
a525daef85ab  centos:7   "ps -ef"               About a minute ago   Exited (0) About a minute ago   ...
db6aabba5157  centos:7   "echo 'hello world'"   3 minutes ago        Exited (0) 3 minutes ago        ...
```
We can see our exited containers this time, with a time and exit code in the STATUS column.

Where did those names come from? We truncated the above output table, but in yours you should also see a NAMES column with some funny names. All containers have names, which in most Docker CLI commands can be substituted for the container ID as we'll see in later exercises. By default, containers get a randomly generated name of the form <adjective>_<scientist / technologist>, but you can choose a name explicitly with the --name flag in docker container run.
Clean up all containers using this command:
```
[centos@node-0 ~]$ docker container rm -f $(docker container ls -aq)
```
Please discuss with your peers what the above command exactly does.

Conclusion

In this exercise you ran your first container using docker container run, and explored the importance of the PID 1 process in a container; this process is a member of the host's PID tree like any other, but is 'containerized' via tools like kernel namespaces, making this process and its children behave as if it was the root of a PID tree, with its own filesystem, mountpoints, and network stack. The PID 1 process in a container defines the lifecycle of the container itself; when one exits, so does the other.