Error: Could not stat() command file '/var/lib/nagios2/rw/nagios.cmd'!

stop nagios2
/etc/nagios2# dpkg-statoverride --update --add nagios www-data 2710 /var/lib/nagios2/rw
/etc/nagios2# dpkg-statoverride --update --add nagios nagios 751 /var/lib/nagios2
start nagios2

Table 2. Plugin Return Codes

Numeric Value	Service Status	Status Description
0	OK	The plugin was able to check the service and it appeared to be functioning properly
1	Warning	The plugin was able to check the service, but it appeared to be above some "warning" threshold or did not appear to be working properly
2	Critical	The plugin detected that either the service was not running or it was above some "critical" threshold
3	Unknown	Invalid command line arguments were supplied to the plugin or low-level failures internal to the plugin (such as unable to fork, or open a tcp socket) that prevent it from performing the specified operation. Higher-level errors (such as name resolution errors, socket timeouts, etc) are outside of the control of plugins and should generally NOT be reported as UNKNOWN states.

link: http://nagiosplug.sourceforge.net/developer-guidelines.html#AEN35

Error: Could not stat() command file '/var/lib/nagios2/rw/nagios.cmd'!

Oops.

All it needed was
/etc/nagios2#/etc/init.d/nagios2 stop
Stopping nagios2 monitoring daemon: nagios2.
/etc/nagios2# dpkg-statoverride --update --add nagios www-data 2710 /var/lib/nagios2/rw
/etc/nagios2# dpkg-statoverride --update --add nagios nagios 751 /var/lib/nagios2
/etc/nagios2# /etc/init.d/nagios2 start
Starting nagios2 monitoring daemon: nagios2.

ref. http://ubuntuforums.org/showthread.php?t=458375