Add -o/--output option to write to file instead of STDERR (fixes #15).

README.md

@@ -22,6 +22,7 @@ $ markdownlint --help
     -h, --help                          output usage information
     -V, --version                       output the version number
     -s, --stdin                         read from STDIN (no files)
+    -o, --output [outputFile]           write issues to file (no console)
     -c, --config [configFile]           configuration file
     -i, --ignore [file|directory|glob]  files to ignore/exclude
 ```

markdownlint.js

@@ -76,22 +76,32 @@ function printResult(lintResult) {
       };
     });
   }));
+  var lintResultString = '';
   if (results.length > 0) {
     results.sort(function (a, b) {
       return a.file.localeCompare(b.file) || a.lineNumber - b.lineNumber ||
         a.names.localeCompare(b.names) || a.description.localeCompare(b.description);
     });
-    var lintResultString = results.map(function (result) {
+    lintResultString = results.map(function (result) {
       return result.file + ': ' + result.lineNumber + ': ' + result.names + ' ' + result.description;
     }).join('\n');
-    console.error(lintResultString);
     // Note: process.exit(1) will end abruptly, interrupting asynchronous IO
     // streams (e.g., when the output is being piped). Just set the exit code
     // and let the program terminate normally.
     // @see {@link https://nodejs.org/dist/latest-v8.x/docs/api/process.html#process_process_exit_code}
     // @see {@link https://github.com/igorshubovych/markdownlint-cli/pull/29#issuecomment-343535291}
     process.exitCode = 1;
   }
+  if (program.output) {
+    try {
+      fs.writeFileSync(program.output, lintResultString);
+    } catch (err) {
+      console.warn('Cannot write to output file ' + program.output + ': ' + err.message);
+      process.exitCode = 2;
+    }
+  } else if (lintResultString) {
+    console.error(lintResultString);
+  }
 }
 
 function concatArray(item, array) {
@@ -104,6 +114,7 @@ program
   .description(pkg.description)
   .usage('[options] <files|directories|globs>')
   .option('-s, --stdin', 'read from STDIN (no files)')
+  .option('-o, --output [outputFile]', 'write issues to file (no console)')
   .option('-c, --config [configFile]', 'configuration file')
   .option('-i, --ignore [file|directory|glob]', 'files to ignore/exclude', concatArray, []);
 

test/test.js

@@ -1,5 +1,6 @@
 'use strict';
 
+import fs from 'fs';
 import path from 'path';
 import test from 'ava';
 import execa from 'execa';
@@ -254,3 +255,61 @@ test('--stdin with invalid input reports violations', async t => {
     t.true(err.stderr.match(errorPattern).length === 2);
   }
 });
+
+test('--output with empty input has empty output', async t => {
+  var input = '';
+  var output = 'outputA.txt';
+  const result = await execa('../markdownlint.js',
+    ['--stdin', '--output', output], {input: input});
+  t.true(result.stdout === '');
+  t.true(result.stderr === '');
+  t.true(fs.readFileSync(output, 'utf8') === '');
+  fs.unlinkSync(output);
+});
+
+test('--output with valid input has empty output', async t => {
+  var input = [
+    '# Heading',
+    '',
+    'Text'
+  ].join('\n');
+  var output = 'outputB.txt';
+  const result = await execa('../markdownlint.js',
+    ['--stdin', '--output', output], {input: input});
+  t.true(result.stdout === '');
+  t.true(result.stderr === '');
+  t.true(fs.readFileSync(output, 'utf8') === '');
+  fs.unlinkSync(output);
+});
+
+test('--output with invalid input outputs violations', async t => {
+  var input = [
+    'Heading',
+    '',
+    'Text '
+  ].join('\n');
+  var output = 'outputC.txt';
+  try {
+    await execa('../markdownlint.js', ['--stdin', '--output', output], {input: input});
+    t.fail();
+  } catch (err) {
+    t.true(err.stdout === '');
+    t.true(err.stderr === '');
+    t.true(fs.readFileSync(output, 'utf8').match(errorPattern).length === 2);
+    fs.unlinkSync(output);
+  }
+});
+
+test('--output with invalid path fails', async t => {
+  var input = '';
+  var output = 'invalid/outputD.txt';
+  try {
+    await execa('../markdownlint.js',
+      ['--stdin', '--output', output], {input: input});
+    t.fail();
+  } catch (err) {
+    t.true(err.stdout === '');
+    t.true(err.stderr.replace(/: ENOENT[^]*$/, '') === 'Cannot write to output file ' + output);
+    t.throws(() => fs.accessSync(output, 'utf8'), Error);
+  }
+});