Allow for Default Environment Variable Value in `${localEnv:...}` and `${containerEnv:...}`

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@celestialorb microsoft/vscode-remote-release#1108 at 08/06/2019 11:25:45:

I wasn't able to find anything suggesting this was possible, nor did it seem to work when I tried it myself.

I'd like to see the ability to specify a default value for local environment variable values in the runArgs property of the devcontainer.json file. I'm trying to use the value of a local env var to determine the filesystem source of a bind mount into the remote environment container, or default to a named volume if the variable does not exist.

I propose the ability to use the standard way of defining a default value for a shell/environment variable, with the <variable-name>:-<default-value>. I have tried this and it does not seem to work. An example is shown below for the runArgs property of the devcontainer.json file.

runArgs: [
    -v, ${env:DOES_NOT_EXIST:-name-of-volume-or-default-path}:/path/in/container/filesystem,
],

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@vscode-triage-bot commented at 10/21/2021 12:40:27:

This issue is being closed to keep the number of issues in our inbox on a manageable level, we are closing issues that are not going to be addressed in the foreseeable future: We look at the number of votes the issue has received and the number of duplicate issues filed. More details here. If you disagree and feel that this issue is crucial: We are happy to listen and to reconsider.

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Thanks for your understanding and happy coding!

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@MShekow commented at 11/09/2021 16:20:34:

We also would need this feature. What exactly is meant by "out of scope"?

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@chrmarti commented at 11/10/2021 08:32:30:

"out of scope" because we never got to consider doing this. Reopening to reconsider.

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@MShekow commented at 11/10/2021 09:58:21:

Thank you for reopening this ticket. Since the issue is quite old, having just consulted the docs, I suppose it makes sense to have this default-value mechanism implemented for localEnv and containerEnv, e.g. to be able to define something like ${localEnv:DOES_NOT_EXIST:-some default value}.

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@firdacz commented at 05/03/2022 18:34:59:

Not just runArgs but everywhere - e.g. in launch.json:

\"serverpath\": \"${env:jlink:\"c:\Program Files (x86)\SEGGER\JLink_V...\"}/JLinkGDBServerCL.exe\",
\"armToolchainPath\": \"${env:gnuarm:\"c:\devel\gnuarm\"}/bin\",

...would probably need to be quoted (if containing : ...for future extensions, e.g. list of alternatives - checked for exists? I myself have that toolchain on either D: or E:, using company/home PC)

[Chuxel]

@chrmarti Given the help wanted message - should this be in the CLI repo? Or separated into a spec update and then the reference implementation?