Don't write to stdout if `--output` is specified

Fixes #436.

lib/cli.js

@@ -162,7 +162,7 @@ module.exports = function(args) {
   options.src = argv._[0];
   options.dest = argv.o || argv._[1];
 
-  if (!options.dest) {
+  if (!options.dest && (process.stdout.isTTY || process.env.isTTY)) {
     var suffix = '.css';
     if (/\.css$/.test(options.src)) {
       suffix = '';

lib/render.js

@@ -28,7 +28,7 @@ function render(options, emitter) {
       }
     };
 
-    if (options.stdout || (!process.stdout.isTTY && !process.env.isTTY)) {
+    if (options.stdout || (!options.dest && (!process.stdout.isTTY || !process.env.isTTY))) {
       emitter.emit('log', css);
       return done();
     }

test/cli.js

@@ -47,6 +47,23 @@ describe('cli', function() {
     src.pipe(emitter.stdin);
   });
 
+  it('should write to disk when using --output', function(done) {
+    this.timeout(6000);
+    var src = fs.createReadStream(sampleScssPath);
+    var emitter = spawn(cliPath, ['--output', sampleCssOutputPath], {
+      stdio: [null, 'ignore', null]
+    });
+
+    emitter.on('close', function() {
+      fs.exists(sampleCssOutputPath, function(exists) {
+        assert(exists);
+        fs.unlink(sampleCssOutputPath, done);
+      });
+    });
+
+    src.pipe(emitter.stdin);
+  });
+
   it('should print help when run with no arguments', function(done) {
     var env = assign(process.env, { isTTY: true });
     exec('node ' + cliPath, {