diff --git a/shadowsocksR b/shadowsocksR
index 0647108..bc2e6d2 100644
--- a/shadowsocksR
+++ b/shadowsocksR
@@ -25,7 +25,7 @@ RETVAL=0
 
 check_running(){
     PID=`ps -ef | grep -v grep | grep -i "${BIN}" | awk '{print $2}'`
-    if [ ! -z $PID ]; then
+    if [ ! -z "$PID" ]; then
         return 0
     else
         return 1