Nagios3 – Could not stat() command file 

On Debian and Ubuntu I have seen this error a lot of times with Nagios3.

Error: Could not stat() command file ‘/var/lib/nagios3/rw/nagios.cmd’!

To solve it, simply run

/etc/init.d/nagios3 stop
dpkg-statoverride –update –add nagios www-data 2710 /var/lib/nagios3/rw
dpkg-statoverride –update –add nagios nagios 751 /var/lib/nagios3
/etc/init.d/nagios3 start