Allow users to provide default values for unset environment variables

[aioobe]

Reading up on Environment Variables I don't see a way to specify default values for environment variables. This is a common feature in similar types of configuration files. Docker for example supports the standard bash syntax:

${variable:-word} indicates that if variable is set then the result will be that value. If variable is not set then word will be the result.

I would very much like to have a self contained Caddyfile with sane defaults so you can safely do caddy in prod. (I don't want to say "oh, when you deploy in prod you need to follow instruction A, B and C", I would like it to be "Just run caddy".) And then something like DOMAIN=staging caddy / DOMAIN=dev caddy to override relevant parts in staging and dev.

[mholt]

Will look into this for Caddy 2, probably for all placeholder values spawning from the new Replacer type.

[aioobe]

Where can I follow the work on Caddy 2? Is there a list of features, breaking changes, estimated release date, etc somewhere?

[mholt]

@aioobe It's not public yet, but when we get around to the public beta period, we'll move it into this repo! That's the plan as of now, anyway.

Watch for announcements at OpenGo 2019.

[mholt]

Related to / same as #1793

[francislavoie]

I'll close this as a duplicate of #1793

[francislavoie]

Actually, derp, they're not really the same. #1793 is for defaults for placeholders in general, whereas this is defaults for environment variables specifically.

I'll reopen this but I'm also submitting a PR for this specific issue now 😛