Set $BASE_URL or similar env var for subsequent steps

[jcbhmr]

It's my understanding that this action is sort of an optional configuration step in the chain of:

• actions/configure-pages
• (build your site)
• actions/upload-pages-artifact
• actions/deploy-pages

Where actions/configure-pages is supposed to tweak some configs a bit like for Next.js to make it play nice with a https://example.org/sub-project/... base URL.

Since that seems to be the goal, I'd like to propose a feature: auto-setting some kind of $PAGES_BASE_URL or $BASE_URL (idk if there's a common conventional name or standard) or some other environment variable like this:

await appendFile(process.env.GITHUB_ENV, `BASE_URL=${baseURL}`)
// or similar

Why is this a good idea? 1) It's an environment variable, not an output. That means you don't need to do this messiness each time you want to use it:

- id: my-id
  uses: actions/configure-pages@vN
- run: npm run build:docs
  env:
    BASE_URL: ${{ steps.my-id.outputs.base_url }}/

2) It makes it much easier for non-Next.js/Jekyll/whatever supported projects in the magic js file to just pick up on a $BASE_URL env var. Yes, you can do this now but it's a bit clunky with adding the outputs to an env var manually.

This proposed $BASE_URL idea is more implicit than the current explicit ${{ steps.my-id.outputs.base_url }} is right now. In my opinion that's OK similar to how there's already a slew of $GITHUB_* env vars but my opinion isn't the right answer.

If this is out-of-scope, feel free to close this. This is just a QoL feature that would make it easier to do configs without mucking with outputs in my-pages-workflow.yml. 🤷‍♀️

[jcbhmr]

This would also be a way around issues like #74 so that you can just do something like this in your config as opposed to wrangling with manually computing process.env.GITHUB_REPOSITORY.split("/")[1] and handling if (process.env.GITHUB_REPOSITORY.split("/")[1].endsWith(".github.io")) ... edge cases

// https://nextjs.org/docs/pages/api-reference/next-config-js
module.exports = {
  ...process.env.BASE_URL && { basePath: new URL(process.env.BASE_URL).pathname },
}

- uses: actions/configure-pages@vN
- run: npm run build:docs # gets the BASE_URL env var from configure-pages

[JamesMGreene]

BASE_URL is a far too generic environment variable name for GitHub Pages to lay claim to. If we were going to do something like this, it would be very explicitly named, like GITHUB_PAGES_BASE_URL.

As it is, I would still suggest utilizing the action's outputs.base_url to set the value of the relevant environment variable for your own usage as outlined in your original post. 🤷