diff --git a/shadowsocks b/shadowsocks
index b1a0e19..33c2549 100644
--- a/shadowsocks
+++ b/shadowsocks
@@ -29,7 +29,7 @@ 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
diff --git a/shadowsocks-debian b/shadowsocks-debian
index 5079efe..3dfea7d 100644
--- a/shadowsocks-debian
+++ b/shadowsocks-debian
@@ -27,7 +27,7 @@ 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