test error from empty id attribute in update-checks-doc script

scripts/update-checks-doc/README.md

@@ -8,11 +8,21 @@ This script does:
 - update the outputs of the example inputs; the code blocks after `Output:` header
 - update the links to the [playground](https://rhysd.github.io/actionlint/) for the example inputs
 
-For making the implementation simple, this script does not support Windows. Please run this script
-on Linux or macOS.
+For making the implementation simple, this script does not support Windows.
+
+## Prerequisites
+
+- Go
+- Linux or macOS
+- `shellcheck` command
+- `pyflakes` command
 
 ## Usage
 
+```
+go run ./scripts/update-checks-doc [-check] FILE
+```
+
 Update the document. This command directly modifies the file.
 
 ```sh

scripts/update-checks-doc/main.go

@@ -113,6 +113,9 @@ func Update(in []byte) ([]byte, error) {
 		}
 		if strings.HasPrefix(l, `<a id="`) && strings.HasSuffix(l, `"></a>`) {
 			anchor = strings.TrimSuffix(strings.TrimPrefix(l, `<a id="`), `"></a>`)
+			if len(anchor) == 0 {
+				return nil, fmt.Errorf("id for <a> tag is empty at line %d", lnum)
+			}
 		}
 		if l == "Example input:" {
 			log.Printf("Found example input header for %q at line %d", section, lnum)

scripts/update-checks-doc/testdata/err/empty_anchor_id.md

@@ -0,0 +1,23 @@
+<a id=""></a>
+## Hello
+
+Example input:
+
+```yaml
+on: push
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo ${{ unknown }}
+```
+
+Output:
+
+```
+This section will be generated
+```
+
+[Playground](https://rhysd.github.io/actionlint/#THIS_URL_WILL_BE_UPDATED)
+
+The `id` attribute of the `<a>` is an empty string.