Linux 环境下使用shutdown.sh命令无法关闭tomcat 进程，什么原因？

1.Red Hat 4.8.2-16 版本的Linux系统，安装jdk1.7.0_79和apache-tomcat-8.0.26，使用shutdown.sh命令关闭tomcat:

[root@iZ94pb4l70nZ apache-tomcat-8.0.26_mq]# ./bin/shutdown.sh 
Using CATALINA_BASE:   /home/apache-tomcat-8.0.26_mq
Using CATALINA_HOME:   /home/apache-tomcat-8.0.26_mq
Using CATALINA_TMPDIR: /home/apache-tomcat-8.0.26_mq/temp
Using JRE_HOME:        /home/jdk1.7.0_79
Using CLASSPATH:       /home/apache-tomcat-8.0.26_mq/bin/bootstrap.jar:/home/apache-tomcat-8.0.26_mq/bin/tomcat-juli.jar

2.查看tomcat进程，并没有结束

[root@iZ94pb4l70nZ apache-tomcat-8.0.26_mq]# ps -ef | grep tomcat
root     18821     1 24 16:29 pts/0    00:00:31 /home/jdk1.7.0_79/bin/java -Djava.util.logging.config.file=/home/apache-tomcat-8.0.26_mq/conf/logging.properties -Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager -server -Xms1024m -Xmx2048m -XX:PermSize=256M -XX:MaxPermSize=512M -Djava.endorsed.dirs=/home/apache-tomcat-8.0.26_mq/endorsed -classpath /home/apache-tomcat-8.0.26_mq/bin/bootstrap.jar:/home/apache-tomcat-8.0.26_mq/bin/tomcat-juli.jar -Dcatalina.base=/home/apache-tomcat-8.0.26_mq -Dcatalina.home=/home/apache-tomcat-8.0.26_mq -Djava.io.tmpdir=/home/apache-tomcat-8.0.26_mq/temp org.apache.catalina.startup.Bootstrap start
root     19080 18659  0 16:31 pts/0    00:00:00 grep --color=auto tomcat

3.使用kill命令强制结束tomcat进程

[root@iZ94pb4l70nZ apache-tomcat-8.0.26_mq]# kill -9 18821
[root@iZ94pb4l70nZ apache-tomcat-8.0.26_mq]# ps -ef | grep tomcat
root     19119 18659  0 16:32 pts/0    00:00:00 grep --color=auto tomcat