function detectTabs(dir, next) {
        exec('grep -R -l --include=*.{js,scss,html,dot} "\t" ' + dir, function(err, stdout, stderr) {
            var files = stdout.split('\n').filter(notExcluded);

            assert.ok(files.length == 0, 'tab characters should be replaced with 4 spaces (use ./expandTabs) in the following files: ' + files);