Interactive Containers

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

Launch an interactive shell in a new or existing container
Run a child process inside a running container
List containers using more options and filters

Writing to Containers

Create a container using the centos:7 image, and connect to its bash shell in interactive mode using the -i flag (also the -t flag, to request a TTY connection):
```
[centos@node-0 ~]$ docker container run -it centos:7 bash
```

Explore your container's filesystem with ls, and then create a new file:

[root@2b8de2ffdf85 /]# ls -l
[root@2b8de2ffdf85 /]# echo 'Hello there...' > test.txt
[root@2b8de2ffdf85 /]# ls -l

Exit the connection to the container:
```
[root@2b8de2ffdf85 /]# exit
```
Run the same command as above to start a container in the same way:
```
[centos@node-0 ~]$ docker container run -it centos:7 bash
```
Try finding your test.txt file inside this new container; it is nowhere to be found. Exit this container for now in the same way you did above.

Reconnecting to Containers

We'd like to recover the information written to our container in the first example, but starting a new container didn't get us there; instead, we need to restart our original container, and reconnect to it. List all your stopped containers:

[centos@node-0 ~]$ docker container ls -a

CONTAINER ID  IMAGE     COMMAND  CREATED              STATUS                         ...
cc19f7e9aa91  centos:7  "bash"   About a minute ago   Exited (0) About a minute ago  ...
2b8de2ffdf85  centos:7  "bash"   2 minutes ago        Exited (0) About a minute ago  ...
...

We can restart a container via the container ID listed in the first column. Use the container ID for the first centos:7 container you created with bash as its command (see the CREATED column above to make sure you're choosing the first bash container you ran):
```
[centos@node-0 ~]$ docker container start <container ID>
[centos@node-0 ~]$ docker container ls

CONTAINER ID  IMAGE     COMMAND  CREATED         STATUS         ...
2b8de2ffdf85  centos:7  "bash"   5 minutes ago   Up 21 seconds  ...
```
Your container status has changed from Exited to Up, via docker container start.
Run ps -ef inside the container you just restarted using Docker's exec command (exec runs the specified process as a child of the PID 1 process inside the container):
```
[centos@node-0 ~]$ docker container exec <container ID> ps -ef
```
What process is PID 1 inside the container? Find the PID of that process on the host machine by using:
```
[centos@node-0 ~]$ docker container top <container ID>
```
Launch a bash shell in your running container with docker container exec:
```
[centos@node-0 ~]$ docker container exec -it <container ID> bash
```
List the contents of the container's filesystem again with ls -l; your test.txt should be where you left it. Exit the container again by typing exit.

Using Container Listing Options

In the last step, we saw how to get the short container ID of all our containers using docker container ls -a. Try adding the --no-trunc flag to see the entire container ID:
```
[centos@node-0 ~]$ docker container ls -a --no-trunc
```
This long ID is the same as the string that is returned after starting a container with docker container run.

List only the container ID using the -q flag:

[centos@node-0 ~]$ docker container ls -a -q

List the last container to have been created using the -l flag:
```
[centos@node-0 ~]$ docker container ls -l
```
Finally, you can also filter results with the --filter flag; for example, try filtering by exit code:
```
[centos@node-0 ~]$ docker container ls -a --filter "exited=0"
```
The output of this command will list the containers that have exited successfully.

Clean up with:

[centos@node-0 ~]$ docker container rm -f $(docker container ls -aq)

Conclusion

In this demo, you saw that files added to a container's filesystem do not get added to all containers created from the same image; changes to a container's filesystem are local to itself, and exist only in that particular container. You also learned how to restart a stopped Docker container using docker container start, how to run a command in a running container using docker container exec, and also saw some more options for listing containers via docker container ls.