Why CMD over ENTRYPOINT in Dockerfiles

[AceHack]

I'm just trying to understand better why choose CMD over ENTRYPOINT in your docker files?

[AceHack]

It seems like ENTRYPOINT would be a better option but I'm sure I'm just missing something. Thanks.

[elijahgagne]

It seems like CMD is a good choice to me because it's easier to override. To quote Dockerfile: ENTRYPOINT vs CMD:

Given how much easier it is to override the CMD, the recommendation is use CMD in your Dockerfile when you want the user of your image to have the flexibility to run whichever executable they choose when starting the container.
...
In contrast, ENTRYPOINT should be used in scenarios where you want the container to behave exclusively as if it were the executable it's wrapping. That is, when you don't want or expect the user to override the executable you've specified.
...
Of course you can achieve this same thing with CMD, but the use of ENTRYPOINT sends a strong message that this container is only intended to run this one command.

The primary use of these images is certainly to run pwsh, but maybe you want to run pwsh with arguments.

[AceHack]

@elijahgagne powershell is a command line itself, so when you override arguments you are actually creating new commands in the context of powershell. Take the following.

ENTRYPOINT ["pwsh", "-Command", "$ErrorActionPreference = 'Stop'; $ProgressPreference = 'SilentlyContinue';"]

Now if I run

docker run --rm microsoft/powershell:6.0.1-nanoserver-1709 $env:ProgramFiles

with the above entry point, it's like I'm running the $env:ProgramFiles in powershell itself. So the container is acting like powershell itself, this is where entry point really shines in my opinion when you want to use the container similar to a command line tool.

But if I have

CMD ["pwsh"]

when I run the same command in the context of CMD

docker run --rm microsoft/powershell:6.0.1-nanoserver-1709 $env:ProgramFiles

pwsh is stripped off and replaced with $env:ProgramFiles only so it does not execute in the context of PowerShell so it fails.

On a side note it would be great to see SHELL defined as well.
https://docs.docker.com/engine/reference/builder/#shell

Thanks.

[elijahgagne]

@AceHack Thanks for explaining this a bit more. Apologies if I'm thinking about this wrong, but if you have an entrypoint like:

ENTRYPOINT ["pwsh", "-Command", "$ErrorActionPreference = 'Stop'; $ProgressPreference = 'SilentlyContinue';"]

Then wouldn't that make it hard for people to use it as an interactive shell? It seems that you could do what you're looking for by running something like:

docker run --rm microsoft/powershell pwsh -c dir

The main downside is that you have to include pwsh at the beginning of the command

[SteveL-MSFT]

cc @TravisEz13

[iSazonov]

@SteveL-MSFT @TravisEz13 Do we still tracking anything or we can close the issue?