diff --git a/shadowsocks b/shadowsocks
index 92fa401..d87fd7a 100644
--- a/shadowsocks
+++ b/shadowsocks
@@ -23,7 +23,7 @@ CONF=/etc/shadowsocks.json
 
 check_running(){
     PID=`ps -ef | grep -v grep | grep -i "${BIN}" | awk '{print $2}'`
-    if [ -n $PID ]; then
+    if [ ! -z $PID ]; then
         return 0
     else
         return 1