说明：

hostname是Linux系统下的一个内核参数，它保存在/proc/sys/kernel/hostname下，但是它的值是Linux启动时从rc.sysinit读取的。

/etc/rc.d/rc.sysinit中HOSTNAME的取值来自与/etc/sysconfig/network下的HOSTNAME

在/etc/rc.d/rc.sysinit中，有逻辑判断，当hostname为localhost.localdomain时，将会使用接口IP地址对应的hostname来重新设置系统的hostname。
修改了hostname后，如何使其立即生效而不用重启操作系统。

1、 修改了/etc/sysconfig/network下的HOSTNAME后，

使用echo  servername > /proc/sys/kernel/hostname。

使用sysctl kernel.hostname=hostname命令使其立即生效

使用hostname hostname命令使其生效  例 hostname test

当前会话不会变化，只有后续新建会话会生效。  1、下面的这三种方法如果不加1，重启都会失效。
hostname的修改、变更完全不依赖hosts文件。hosts文件的作用相当如DNS，提供IP地址到hostname的对应。Linux系统在向DNS服务器发出域名解析请求之前会查询/etc/hosts文件，如果里面有相应的记录，就会使用hosts里面的记录。
/etc/hosts文件通常里面包含这一条记录  127.0.0.1 localhost.localdomain localhost  分别是IP地址 、hostname、 aliases，三者用空白字符分隔，aliases可选。
hostname   在广域网中标识这台机器的名称

hosts              指定了你访问域名时的一个域名和ip对应。

 Main part from: http://www.cpqlinux.com/hostname.html

Checking your Linux host name
First, see if your host name is set correclty using the following commands:
uname -n
hostname -a
hostname -s
hostname -d
hostname -f
hostname

If the above commands return correctly with no errors then all may be well;  however, you may want to read on to verify that all settings are correct.
--------------------------------------------------------------------------------------------------------------------------------
Configuring /etc/hosts
If your IP address is assigned to you by a DHCP server, then /etc/hosts is configured as follows:
127.0.0.1	mybox.mydomain.com	localhost.localdomain localhost mybox

If you have a static IP address, then /etc/hosts is configured as follows:
127.0.0.1	localhost.localdomain localhost
192.168.0.10	mybox.mydomain.com	mybox
------------------------------------------------------------------------

Setting the Host Name using "hostname"
After updating the /etc/hosts file correctly, the "hostname" command should be run as follows to set your hostname:

 hostname mybox.mydomain.com
-------------------------------------------------------------------------------------------------------------------------------
Checking /etc/HOSTNAME (if present)
You may or may not have the file /etc/HOSTNAME:
mybox.mydomain.com
-------------------------------------------------------------------------

Checking /proc/sys/kernel/hostname
This is checked with the following command: cat /proc/sys/kernel/hostname   If you need to set this file, you can either reboot or set it now with the following command: 

echo mybox.mydomain.com > /proc/sys/kernel/hostname
------------------------------------------------------------------------------------------------------------------------------
It is not reliable to use X tool for my experience.

What is relationship in these files? I will discover in the future.

查看hostname   ：   hostname
修改hostname   ：   hostnamectl set-hostname  master (比如要修改为master)
修改完重启生效 ：reboot

1.查看 hostname
2.修改

1.centos6

 vim /etc/sysconfig/network
 HOSTNAME=new_hostname


centos7

hostnamectl set-hostname new_hostname


vim /etc/hosts  127.0.0.1添加hostname