Could not open command file nagios.cmd' for update

Error: Could not open command file '/opt/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.

www-data权限不够导致这个问题，解决的办法是将apache的系统用户添加到nagcmd组中。

/usr/sbin/usermod -a -G nagcmd nagios
/usr/sbin/usermod -a -G nagcmd www-data

root@inner:~# id www-data
uid=33(www-data) gid=33(www-data) groups=33(www-data),1003(nagcmd)

重新提交之后：
Your command request was successfully submitted to Nagios for processing.

Note: It may take a while before the command is actually processed.