first

.github/problem-matcher.json

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+{
+    "problemMatcher": [
+        {
+            "owner": "markdownlint",
+            "pattern": [
+                {
+                    "regexp": "^([^:]*):(\\d+):?(\\d+)?\\s([\\w-\\/]*)\\s(.*)$",
+                    "file": 1,
+                    "line": 2,
+                    "column": 3,
+                    "code": 4,
+                    "message": 5
+                }
+            ]
+        }
+    ]
+}

.github/workflows/main.yml

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+name: Main
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - run: shellcheck entrypoint.sh
+      - uses: ./

.markdownlint.json

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+{
+    "default": true,
+    "MD003": {
+        "style": "atx"
+    },
+    "MD004": {
+        "style": "asterisk"
+    },
+    "MD013": {
+        "code_blocks": false,
+        "tables": false
+    },
+    "MD033": {
+        "allowed_elements": [
+            "details",
+            "summary"
+        ]
+    },
+    "MD046": false
+}

Dockerfile

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+FROM node:lts-alpine
+
+COPY entrypoint.sh .github/problem-matcher.json .markdownlint.json /
+
+RUN npm install --global --production markdownlint-cli
+
+ENTRYPOINT ["/entrypoint.sh"]

README.md

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+# markdownlint action
+
+This action lints your Markdown files with [markdownlint-cli](https://github.com/igorshubovych/markdownlint-cli).
+
+## Usage
+
+Including the action without any defaults will find a local markdownlint config
+file (`.markdownlint.{yaml,yml,json}`) or use our included default and scan all
+files with the `md` or `markdown` extension.
+
+```yaml
+- uses: articulate/actions-markdownlint@main
+```
+
+You can set the config file, the files it scans, or files/directories to ignore.
+
+```yaml
+- uses: articulate/actions-markdownlint@main
+  with:
+    config: markdownlint-config.json
+    files: 'docs/**/*.md'
+    ignore: node_modules
+```

action.yml

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+name: 'markdownlint'
+author: 'Articulate'
+description: 'Lint your markdown'
+inputs:
+  config:
+    description: 'markdownlint config file'
+    required: false
+  files:
+    description: 'Markdown files to scan'
+    required: false
+    default: '**/*.{md,markdown}'
+  ignore:
+    description: 'Files or directories to ignore'
+    required: false
+  fix:
+    description: 'Try to fix basic errors'
+    required: false
+runs:
+  using: 'docker'
+  image: 'Dockerfile'
+branding:
+  color: 'blue'
+  icon: 'check-circle'

entrypoint.sh

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+#!/bin/sh
+
+# Copy the matcher to a shared volume with the host; otherwise "add-matcher"
+# can't find it.
+cp /problem-matcher.json "${HOME}/markdownlint-problem-matcher.json"
+echo "::add-matcher::${HOME}/markdownlint-problem-matcher.json"
+
+MARKDOWNLINT=
+
+if [ -z "$INPUT_CONFIG" ]; then
+    echo "::debug::no config given, finding suitable config"
+    INPUT_CONFIG=/.markdownlint.json
+    for cfg in .markdownlint.yaml .markdownlint.yml .markdownlint.json .markdown-lint.yml .markdown-lint.json; do
+        if [ -f "$cfg" ]; then
+            INPUT_CONFIG="$cfg"
+            continue
+        fi
+    done
+fi
+
+echo "::debug::using config ${INPUT_CONFIG}"
+MARKDOWNLINT="${MARKDOWNLINT} --config ${INPUT_CONFIG}"
+
+if [ -n "$INPUT_IGNORE" ]; then
+    echo "::debug::ignoring ${INPUT_IGNORE}"
+    MARKDOWNLINT="${MARKDOWNLINT} --ignore ${INPUT_IGNORE}"
+fi
+
+if [ -n "$INPUT_FIX" ]; then
+    echo "::debug::attempting to fix input"
+    MARKDOWNLINT="${MARKDOWNLINT} --fix"
+fi
+
+echo "::debug::linting ${INPUT_FILES}"
+# shellcheck disable=SC2086
+markdownlint ${MARKDOWNLINT} ${INPUT_FILES}