podman build produces "SHELL is not supported for OCI image format"

[svdHero]

Hi there,

I have Dockerfiles containing the command

SHELL ["/bin/bash", "-c"] 

for setting the default shell to bash.

When I try to build images from said Dockerfiles with podman version 2.1.1, I get the error message:
"SHELL is not supported for OCI image format".

What should I do? Do I have to modify the Dockerfiles or is there another way to do this? I thought podman was a drop-in replacement for docker. Am I missing something or doing something wrong?

[rhatdan]

You can use the --format docker flag, to support this option. This option did not make it into the OCI specification.
Podman defaults to building OCI images.

[rhatdan]

If you use buildah bud, you would get this message.
WARN SHELL is not supported for OCI image format, [/bin/bash -c] will be ignored. Must use docker format

[svdHero]

Great. I did not know about the --format option. Thank you.
I will try to avoid non-OCI commands for new Containerfiles.

[Ferryistaken]

Is there some kind of alternative to the SHELL command with OCI containers? I would like to build an OCI image but I don't really know how to transition.

[mheon]

I can't find any documentation on what the SHELL command actually does - it's been removed from all current Dockerfile syntax references, which makes me think it's deprecated.

I think it may be similar to the ENTRYPOINT instruction?

[Ferryistaken]

I tried building with buildah instead of podman build and it worked. It makes me think that I now have a docker formate image instead of an OCI image. How do I check if my image is OCI? Is there some tool that automatically does that?

[mheon]

I believe ManifestType in podman inspect is what you're looking for. application/vnd.docker.distribution.manifest.v2+json for DockerV2, application/vnd.oci.image.manifest.v1+json for OCIv1

[nezartarbin]

I can't find any documentation on what the SHELL command actually does - it's been removed from all current Dockerfile syntax references, which makes me think it's deprecated.

I think it may be similar to the ENTRYPOINT instruction?

@mheon I am late to this, but the SHELL command changes the default shell used by commands such as RUN, which defaults to /bin/sh by default.

Is there a OCI alternative to this? the closest equivalent would be RUN bash -c [commands] but I would have to insert bash -c everywhere, and wrapping string around commands messes up syntax highlighting, which is especially bad for long or multiple &&-chained commands in a single RUN.

[akostadinov]

How to request this feature in OCI?

[dusansimic]

@mheon sorry for the ping but I came across this same issue and researched it a bit.

I've found some info in Dockerfile reference. It seems like it's used to reduce the number of arguments for RUN instruction. Since when building an image in oci mode, this instruction is ignored, I'm guessing one could recreate it by looking at what the SHELL instruction arguments are and putting it into the RUN instruction instead. I'm not sure if that is really the case tho.

My assumption is that since Docker images are already built (I'm using one for example) and using them as a base image won't affect their contents (since they are already built), podman and buildah are just warning users that some image (in my case a base image) is built with those unsupported instructions. If a user would want to have the same behavior of the RUN instruction tho, they would need to use the run it through the shell specified in the SHELL instruction manually. Am I wrong thinking that?