php调用shell程序出现僵尸程序defunct

在php中调用bash shell

system("/home/app/clean_queue.sh", $retval);

clean_queue.sh如下:

#!/bin/bash
sid="/data/httpsqs/queue"
pid=`ps -ef|grep -v grep|grep $sid|sed -n '1P'|awk '{print $2}'`
while [ -n "$pid" ] 
do
echo "queue id=$pid"
kill $pid 
pid=`ps -ef|grep -v grep|grep $sid|sed -n '1P'|awk '{print $2}'`
done

echo "has shutdown queue."
rm -rf /data/httpsqs/queue/httpsqs.db
echo "has delete queue db."

/usr/bin/httpsqs -d -p 1234 -x /data/httpsqs/queue 
echo "has restarted page queue"

pid2=`ps -ef|grep -v grep|grep $sid|sed -n '1P'|awk '{print $2}'`
while [ -z "$pid2" ] 
do
echo "queue has not been started, ready to retry."
/usr/bin/httpsqs -d -p 1234 -x /data/httpsqs/queue 
pid2=`ps -ef|grep -v grep|grep $sid|sed -n '1P'|awk '{print $2}'`
done
echo "end check if started"

但执行的时候此段php代码始终不结束

[root@script ~]# ps -ef|grep defunct
root 11273 21369 0 16:58 pts/0 00:00:00 php test_defunct.php
root 11275 11273 0 16:58 pts/0 00:00:00 [sh] <defunct>
root 11384 13877 0 16:59 pts/2 00:00:00 grep defunct

刚开始看到/usr/bin/httpsqs的背景颜色是红色的，以为是权限的问题，后来chmod u-s /usr/bin/httpsqs去掉suid属性还是问题依旧。

再后来感觉是不是httpsqs启动后是deamon程序，调用的php一直判断这个shell没结果，就一直等，尝试加上nohup后台执行，如下面的写法：

system("nohup /home/app/clean_queue.sh > /home/app/log/clean_queue 2>&1", $retval);

果然就不出现僵尸进程了。