docs: function exec(cmd, args, stdin, callback) {
              var stdout = concat();
              var stderr = concat();
  
              var process = child_process.spawn(cmd, args, {
                  stdio: [stdin || 'ignore', 'pipe', 'pipe']
              });
              process.stdout.pipe(stdout);
              process.stderr.pipe(stderr);
              process.on('exit', function(code, signal) {
                var err;
                if (code) {
                  err = new Error(code);
                  err.code = code;
                  err.signal = signal;
                }
                callback(err, stdout, stderr);
              });
              process.on('error', function (err) {
                callback(err, stdout, stderr);
              });
          }