Support using stdout

src-test/test.js

@@ -177,6 +177,26 @@ describe('mermaid-cli', () => {
     expect(stderr).not.toContain('No input file specified, reading from stdin.')
   }, timeout)
 
+  test('should warn when outputing to stdout with missing --outputFormat', async () => {
+    const execFilePromise = promisify(execFile)('node', ['src/cli.js', '--output', '-'])
+    await promisify(pipeline)(
+      createReadStream('test-positive/flowchart1.mmd'),
+      execFilePromise.child.stdin
+    )
+    const { stderr } = await execFilePromise
+    expect(stderr).toContain('No output format specified, using svg.')
+  }, timeout)
+
+  test('should not warn when outputing to stdout with --outputFormat', async () => {
+    const execFilePromise = promisify(execFile)('node', ['src/cli.js', '--output', '-', '--outputFormat', 'svg'])
+    await promisify(pipeline)(
+      createReadStream('test-positive/flowchart1.mmd'),
+      execFilePromise.child.stdin
+    )
+    const { stderr } = await execFilePromise
+    expect(stderr).not.toContain('No output format specified, using svg.')
+  }, timeout)
+
   test('should error on mermaid syntax error', async () => {
     await expect(
       compileDiagram('test-negative', 'invalid.expect-error.mmd', 'svg')

src/index.js

@@ -111,7 +111,7 @@ async function cli () {
     .addOption(new Option('-w, --width [width]', 'Width of the page').argParser(parseCommanderInt).default(800))
     .addOption(new Option('-H, --height [height]', 'Height of the page').argParser(parseCommanderInt).default(600))
     .option('-i, --input <input>', 'Input mermaid file. Files ending in .md will be treated as Markdown and all charts (e.g. ```mermaid (...)``` or :::mermaid (...):::) will be extracted and generated. Use `-` to read from stdin.')
-    .option('-o, --output [output]', 'Output file. It should be either md, svg, png or pdf. Optional. Default: input + ".svg"')
+    .option('-o, --output [output]', 'Output file. It should be either md, svg, png, pdf or use `-` to output to stdout. Optional. Default: input + ".svg"')
     .addOption(new Option('-e, --outputFormat [format]', 'Output format for the generated image.').choices(['svg', 'png', 'pdf']).default(null, 'Loaded from the output file extension'))
     .addOption(new Option('-b, --backgroundColor [backgroundColor]', 'Background color for pngs/svgs (not pdfs). Example: transparent, red, \'#F0F0F0\'.').default('white'))
     .option('-c, --configFile [configFile]', 'JSON configuration file for mermaid.')
@@ -150,10 +150,22 @@ async function cli () {
     } else {
       output = input ? (`${input}.svg`) : 'out.svg'
     }
-  }
-  if (!/\.(?:svg|png|pdf|md|markdown)$/.test(output)) {
+  } else if (output === '-') {
+    // `--output -` means write to stdout.
+    output = '/dev/stdout'
+    quiet = true
+
+    if (!outputFormat) {
+      outputFormat = 'svg'
+      warn('No output format specified, using svg. ' +
+        'If you want to specify an output format and supress this warning, ' +
+        'please use `-e <format>.` '
+      )
+    }
+  } else if (!/\.(?:svg|png|pdf|md|markdown)$/.test(output)) {
     error('Output file must end with ".md"/".markdown", ".svg", ".png" or ".pdf"')
   }
+
   const outputDir = path.dirname(output)
   if (!fs.existsSync(outputDir)) {
     error(`Output directory "${outputDir}/" doesn't exist`)
@@ -390,7 +402,7 @@ function markdownImage ({ url, title, alt }) {
  * path to a markdown file containing mermaid.
  * If this is a string, loads the mermaid definition from the given file.
  * If this is `undefined`, loads the mermaid definition from stdin.
- * @param {`${string}.${"md" | "markdown" | "svg" | "png" | "pdf"}`} output - Path to the output file.
+ * @param {`${string}.${"md" | "markdown" | "svg" | "png" | "pdf"}` | "/dev/stdout"} output - Path to the output file.
  * @param {Object} [opts] - Options
  * @param {import("puppeteer").LaunchOptions} [opts.puppeteerConfig] - Puppeteer launch options.
  * @param {boolean} [opts.quiet] - If set, suppress log output.
@@ -437,6 +449,10 @@ async function run (input, output, { puppeteerConfig = {}, quiet = false, output
 
     const definition = await getInputData(input)
     if (input && /\.(md|markdown)$/.test(input)) {
+      if (output === '/dev/stdout') {
+        throw new Error('Cannot use `stdout` with markdown input')
+      }
+
       const imagePromises = []
       for (const mermaidCodeblockMatch of definition.matchAll(mermaidChartsInMarkdownRegexGlobal)) {
         if (browser === undefined) {
@@ -497,7 +513,9 @@ async function run (input, output, { puppeteerConfig = {}, quiet = false, output
       info('Generating single mermaid chart')
       browser = await puppeteer.launch(puppeteerConfig)
       const data = await parseMMD(browser, definition, outputFormat, parseMMDOptions)
-      await fs.promises.writeFile(output, data)
+      await output !== '/dev/stdout'
+        ? fs.promises.writeFile(output, data)
+        : process.stdout.write(data)
     }
   } finally {
     await browser?.close?.()