Ipercube

Copy & Paste:

service nagios start
service nagios stop
service nagios reload
service nagios restart

If you get a configuration error during Nagios startup, the check command shows exactly where the error is:

/usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg 

First create the user with the command htpasswd2 (see section Commands that usually I forget ).
In the front-end configuration file (/usr/local/nagios/etc/cgi.cfg) add the users (comma separated) in the parameter authorized_for_read_only:

authorized_for_read_only=user1,user2,user...