diff --git a/demo/quick_start/preprocess.sh b/demo/quick_start/preprocess.sh
index 516b655e4bb25eec1952631e536a40449ee16c97..fb2bee98beb268e88d82b64332273aa10399ff42 100755
--- a/demo/quick_start/preprocess.sh
+++ b/demo/quick_start/preprocess.sh
@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 # Copyright (c) 2016 Baidu, Inc. All Rights Reserved
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
@@ -33,7 +33,7 @@ test_num=$((min_len/10))
 if [ $test_num -gt 12500 ];then
  test_num=12500
 fi
-train_num=((min_len-test_num))
+train_num=$((min_len-test_num))
 
 head -n$train_num pos.shuffed >train.pos
 head -n$train_num neg.shuffed >train.neg