Bash brace expansion not working on Dockerfile RUN command

Question

I'm running the following RUN command in my Dockerfile, expecting a "logs" directory to be created under each of the listed subdirectories:

RUN mkdir -p /opt/seagull/{diameter-env,h248-env,http-env,msrp-env,octcap-env,radius-env,sip-env,synchro-env,xcap-env}/logs

But when I check the image, I see a directory literally called "{diameter-env,h248-env,http-env,msrp-env,octcap-env,radius-env,sip-env,synchro-env,xcap-env}" created under /opt/seagull, instead of brace expansion taking place.

What could I be doing wrong?

Ministrant

I don't know one way or the other, but does RUN run under bash? – Klinger

It turns out, it depends on whether you're using the shell form or the exec form. See my answer below. – Ministrant

Answer 1

You're not using brace expansion, because you're not using Bash. If you look at the documentation for the RUN command:

RUN (shell form, the command is run in a shell, which by default is /bin/sh -c on Linux or cmd /S /C on Windows)

And also:

Note: To use a different shell, other than ‘/bin/sh’, use the exec form passing in the desired shell. For example, RUN ["/bin/bash", "-c", "echo hello"]

So, just change the command to use the exec form and explicitly use a Bash shell:

RUN [ "/bin/bash", "-c", "mkdir -p /opt/seagull/{diameter-env,h248-env,http-env,msrp-env,octcap-env,radius-env,sip-env,synchro-env,xcap-env}/logs" ]

Ministrant

Thanks! I can't believe I spent 45 minutes on this, trying different escaping techniques, checking default bash settings, etc. – Ministrant

Wait, what? You've answered you own question, and you're thanking yourself for it...? – Klinger

Yes! :) It's been an exhausting day, and I needed to thank myself for finally RTFM. Also, stackoverflow.com/help/self-answer – Ministrant

It's not the answering own question that confused the hell out of me, it was just the comment, but hey, if it's deserved ;D – Klinger

Could you not ln -f /bin/bash /bin/sh, I believe the reason is that typically nowadays /bin/sh is a symlink to /bin/dash. – Plethora

Important note from the official documentation: The exec form is parsed as a JSON array, which means that you must use double-quotes (“) around words not single-quotes (‘). – Yen

Answer 2

If /bin/bash is available in your image, you can change the shell that the docker build system uses to execute your RUN command, like this:

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

Now, your RUN command should work unchanged.

Eulogium

So worth the scroll past the accepted answer. – Orenorenburg