Add --stdin flag

[alypeng]

Clearly describe the bug

If a.css is an empty file then cat a.css | stylelint outputs help instead of an error.

Which rule, if any, is the bug related to?

no-empty-source

What CSS is needed to reproduce the bug?

An empty file (e.g. touch a.css)

What stylelint configuration is needed to reproduce the bug?

{
  "rules": {
    "no-empty-source": true
  }
}

Which version of stylelint are you using?

13.0.0

How are you running stylelint: CLI, PostCSS plugin, Node.js API?

CLI

Does the bug relate to non-standard syntax (e.g. SCSS, Less etc.)?

No

What did you expect to happen?

Same output as stylelint a.css

a.css
 1:1  ✖  Unexpected empty source   no-empty-source

What actually happened (e.g. what warnings or errors did you get)?

Same output as stylelint --help

  A mighty, modern CSS linter.

  Usage: stylelint [input] [options]

  Input: Files(s), glob(s), or nothing to use stdin.

    If an input argument is wrapped in quotation marks, it will be passed to
    globby for cross-platform glob support. node_modules are always ignored.
    You can also pass no input and use stdin, instead.

  Options:

...

Possibly related to #1557

[jeddy3]

@alypeng Thanks for the report and for using the template.

What did you expect to happen?

Same output as stylelint a.css

That does seem like the best behaviour. I suspect ESLint has its --stdin flag to support this behaviour. Without the flag ESLint, like stylelint, prints help when an empty file is piped to it.

Ideally, we can achieve this without a new flag because turning off stdin support by default would be a breaking change.

I'll label as a discussion to see if anyone else has an opinion on this.

[jeddy3]

I suggest we implement this as a non-breaking change. We can keep the existing default behaviour, but add a new --stdin flag that changes the behaviour of the CLI to not trigger help if an empty file it piped in.

We would document the flag in the Developer guide > Usage > CLI (options), perhaps as:

### `--stdin`

Force accepting of the standard input stream even if it is empty

I've labelled the issue as ready to implement. Please consider contributing if you have time.