diff --git a/shadowsocks b/shadowsocks
index 33c2549..9c1e9b6 100644
--- a/shadowsocks
+++ b/shadowsocks
@@ -28,8 +28,8 @@ fi
 RETVAL=0
 
 check_running(){
-    PID=`ps -ef | grep -v grep | grep -i "${BIN}" | awk '{print $2}'`
-    if [ ! -z "$PID" ]; then
+    PID=$(ps -ef | grep -v grep | grep -i "${BIN}" | awk '{print $2}')
+    if [ -n "$PID" ]; then
         return 0
     else
         return 1
diff --git a/shadowsocks-debian b/shadowsocks-debian
index 3dfea7d..7292380 100644
--- a/shadowsocks-debian
+++ b/shadowsocks-debian
@@ -26,8 +26,8 @@ fi
 RETVAL=0
 
 check_running(){
-    PID=`ps -ef | grep -v grep | grep -i "${BIN}" | awk '{print $2}'`
-    if [ ! -z "$PID" ]; then
+    PID=$(ps -ef | grep -v grep | grep -i "${BIN}" | awk '{print $2}')
+    if [ -n "$PID" ]; then
         return 0
     else
         return 1