报错处理：
1、Could not open command file '/usr/local/nagios/var/rw/nagios.cmd' for update!
The permissions on the external command file and/or directory may be incorrect. Read the FAQs on how to setup proper permissions.
An error occurred while attempting to commit your command for processing.
2、Error: Could not stat() command file '/usr/local/nagios/var/rw/nagios.cmd'!

设置过nagios.cmd的权限，发现仍然不要

最终解决方法：

# vim /usr/local/nagios/etc/nagios.cfg
将nagios_group=nagios改成
nagios_group=nagcmd
 

为了验证错误，我又将nagios_group=nagcmd改成nagios_group=nagios，发现又可以了，真是一个奇怪的问题啊！