How to fix Nagios3 external commands error

After installing nagios3 and trying to send it a command to reschedule a check or do some other external activity you may get the following error:

Error: Could not stat() command file '/var/lib/nagios3/rw/nagios.cmd'!
The external command file may be missing, Nagios may not be running, and/or Nagios may not be checking external commands.
An error occurred while attempting to commit your command for processing.

In order to fix it, do the following actions:

Check that /etc/nagios3/nagios.cfg has:

check_external_commands=1

Also check that

command_check_interval=15s is uncommented and
command_check_interval=-1 is commented like this:

command_check_interval=15s
#command_check_interval=-1

Check the path for command_file is OK. It usually looks like this:

command_file=/var/lib/nagios3/rw/nagios.cmd

Make sure that the user www-data is part of the nagios group - this is located in /etc/group

Check permissions on the command file that we looked at above:

# ls -l /var/lib/nagios3/rw/nagios.cmd
prw-rw---- 1 nagios nagios 0 Mar  7 11:56 /var/lib/nagios3/rw/nagios.cmd

If it looks like this, we're good.

The next thing to check is that the directory that nagios.cmd resides in has executable rights for the nagios group:

# ls -l /var/lib/nagios3/
total 180
-rw------- 1 nagios www-data 176049 Mar  7 11:58 retention.dat
drwx------ 2 nagios www-data   4096 Mar  7 11:56 rw
drwxr-x--- 3 nagios nagios     4096 Jun 14  2013 spool

Uh oh - rw has no group rights! Fix it with this command:

# chmod g+x /var/lib/nagios3/rw

and then

# service nagios3 restart

And the crowd goes wild!