Detached Containers and Logging

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

Run a container detached from the terminal
Fetch the logs of a container
Attach a terminal to the STDOUT of a running container

Running a Container in the Background

First try running a container as usual; the STDOUT and STDERR streams from the main containerized process are directed to the terminal:

PS: node-0 Administrator> docker container run microsoft/nanoserver ping 8.8.8.8 -n 2

Pinging 8.8.8.8 with 32 bytes of data:
Reply from 8.8.8.8: bytes=32 time=1ms TTL=113
Reply from 8.8.8.8: bytes=32 time=1ms TTL=113

Ping statistics for 8.8.8.8:
    Packets: Sent = 2, Received = 2, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 1ms, Maximum = 1ms, Average = 1ms

The same process can be run in the background with the -d flag:

PS: node-0 Administrator> docker container run -d microsoft/nanoserver ping 8.8.8.8 -t

5505012d74b3480a0a05ebd0ca1d256d08edc1916ef19d56fbe728bc3cecc502

This time, we only see the container's ID; its STDOUT isn't being sent to the terminal.

Use this second container's ID to inspect the logs it generated:
```
PS: node-0 Administrator> docker container logs <container ID>
```
These logs correspond to STDOUT and STDERR from all processes running in the container. Also note when using container IDs: you don't need to specify the entire ID. Just enough characters from the start of the ID to uniquely identify it, often just 2 or 3, is sufficient.

Attaching to Container Output

We can attach a terminal to a container's main process output with the attach command; try it with the last container you made in the previous step:
```
PS: node-0 Administrator> docker container attach <container ID>
```
We can leave attached mode by then pressing CTRL+C. Note that the container still happily runs in the background as you can confirm with docker container ls.

Using Logging Options

We saw previously how to read the entire log of a container's main process; we can also use a couple of flags to control what logs are displayed. --tail n limits the display to the last n lines; try it with the container that should be running from the last step:
```
PS: node-0 Administrator> docker container logs --tail 5 <container ID>
```
You should see the last 5 pings from this container.

Conclusion

In this exercise, we saw our first detached containers. Almost all containers you ever run will be running in detached mode; you can use container attach to interact with their main processes, as well as container logs to fetch their logs. Note that both attach and logs interact with the main process only - if you launch child processes inside a container, it's up to you to manage their STDOUT and STDERR streams.