FR: Environment variables in templates/configs

[algmyr]

I've run into some cases where having configs conditional on env variables would be useful. Access in templates would give some of the powers I would like, but maybe a --when for env variables would be the more natural thing.

One example for a thing I'm playing around with:

Some terminals have fancy capabilities that will not work in other terminals, being able to adapt based on TERM would allow writing a config that works more generally.

Describe the solution you'd like

Provide access to lookup env variables in templates, and/or allow config scoped on env variable values.

Describe alternatives you've considered

Writing a jj wrapper or similar could accomplish the same thing, but it doesn't feel desirable.

[yuja]

and/or allow config scoped on env variable values.

Adding --when.environments = ["KEY=VALUE"] wouldn't be difficult, but we might need a proper expression language to express negative patterns, etc.