Commands

Application commands in a pod definition file.
Docker commands, for example running docker run ubuntu will run an Ubuntu image and then exit immediately.
docker ps -a will show all containers including those which have stopped.
Containers are meant to run a specific tasks or process.
When the task is complete, the container exits. Similarly if the container is stopped.
Within a container image, if you see the word CMD, that is the program that will start within the container.
- For example CMD ["bash"]
- If it cannot find a terminal, it exits.
  - Therefore the container itself exits.
To change this, you can append a command to the Docker override command.
- An example is the docker run ubuntu sleep 5 (sleep 5 being the added option).
  - When the container starts, it runs the sleep command for 5 seconds and then exits.
    - To make this permanent, edit the docker image and add CMD sleep 5
The CMD can be placed in a shell form or a json array form as well.
- For JSON, it should look like CMD ["sleep","5"]
The image looks like this:
```
FROM Ubuntu
CMD sleep 5
```
Then we build the image with docker build -t <image-name> . and run it with docker run <image-name>
You can also change the image, by appending the new command to it, in this case docker run <image-name> sleep 10
- Ideally we want to pass in the seconds only and invoke the sleep command automatically like so:
```
docker run <image-name> 10
```
Therefore to do the above, you need the ENTRYPOINT instruction, that being ENTRYPOINT ["sleep"]. The image would then look like:
```
FROM Ubuntu
ENTRYPOINT ["sleep"]
```
How do you configure a value automatically if one is not added in the command by the user. This can be done using:
```
FROM Ubuntu
ENTRYPOINT ["sleep"]
CMD ["5"]
```
Therefore, the above command will be sleep 5, if a operand is not set by the user.
You can also override the command using the --entrypoint option in docker like so:
```
docker run --entrypoint sleep2.0 <image-name> <value>
```