Allow the default value of expression to refer to environment variables

[sschuberth]

Describe the problem you'd like to solve

I'd like to specify a sub-directory of the host user's home directory as the local source directory for a volume. As a ~ prefix did not seem to work, I was trying to use an expression like local: ${HOME:-$USERPROFILE}/.gradle (so $HOME is tried first, assuming the host is a Linux system; otherwise assume the host is Windows, and use $USERPROFILE). But that does not seem to work either, the "nested" $USERPROFILE environment variable does not seem to get resolved.

Describe the solution you'd like

Environment variables that are part of the default value of an expression should be resolved.

Describe alternatives you've considered

Alternatively / additionally, a dedicated syntax to refer to the current user's home on the host system in a platform-agnostic way, like using ~, would be nice.

Additional context

Note that in Bash, echo ${HOME:-$USERPROFILE} actually works, so I was assuming it to work with Batect, too.

[sschuberth]

Environment variables that are part of the default value of an expression should be resolved.

This seems to be the code that would need to get touched for this:

batect/app/src/main/kotlin/batect/config/Expression.kt

Line 255 in 8498dde

default != null -> default

[charleskorn]

Thanks for the suggestion @sschuberth. Have you seen #990? I'm planning on replacing the current expression syntax with something that would support this far more naturally.

[charleskorn]

a dedicated syntax to refer to the current user's home on the host system in a platform-agnostic way, like using ~, would be nice

This is supported in places that accept local file paths (eg. the local side of a volume mount). Are there other places that you'd like to use it?

[sschuberth]

Have you seen #990?

Ah, no, I didn't connect "expression" with "environment variable" in that context. Thanks for pointing this out to me.

This is supported in places that accept local file paths (eg. the local side of a volume mount).

Hmm, when I tried it did not seem to work for me on Windows. Is ~-resolution maybe implemented only on platforms where the shell usually supports it (i.e. on non-Windows)?

Are there other places that you'd like to use it?

Nope, not yet 😉

[charleskorn]

Is ~-resolution maybe implemented only on platforms where the shell usually supports it (i.e. on non-Windows)?

It should be happening on all platforms - and if it's not, that's a bug :)

[sschuberth]

Is there a way to see in the log what local paths Batect resolves to?

[charleskorn]

You should be able to see the resolved paths for volume mounts in the creation request sent to the Docker daemon, which is logged here.

[sschuberth]

Ok, I'm getting

{"type":"batect.docker.DockerVolumeMountSource.LocalPath","formatted":"C:\\Users\\sebastian\\.gradle","path":"C:\\Users\\sebastian\\.gradle"}

which actually looks good, and using ~ in front of a local path on Windows seems to resolve to the user home correctly.

[sschuberth]

Have you seen #990?

Ah, no, I didn't connect "expression" with "environment variable" in that context. Thanks for pointing this out to me.

So, since the table in the RFC has an entry for "Reference to environment variable with default from other environment variable" (and my immediate use-case would be solved with ~) I'm closing this in favor of waiting for the new expression syntax.