This text will assume you have enough knowledge and domain of Nagios and Linux, your Nagios is already deployed and it is in an external server (different where you have your Tiki deployment)
First you need to copy the check_tiky scripts. You can download it from https://sourceforge.net/p/tikiwiki/code/HEAD/tree/trunk/doc/devtools/check_tiki.php. This script is written in PHP. Put it on a path you will remember, for example /usr/local/bin.
Look for your nagios.cfg file, open it and check what directory is pointing the variable cfg_dir. In this case we are going to use /etc/nagios/conf.d/
Create a file at /etc/nagios/conf.d/check_tiki.cfg with the following content:
define command{
command_name check_tiki_bcc
command_line /usr/local/bin/check_tiki.php -u $ARG1$ -c bcc --bccwarn $ARG2$ --bcccrit $ARG3$
}
define command{
command_name check_tiki_searchindex
command_line /usr/local/bin/check_tiki.php -u $ARG1$ -c searchindex --sirwarn $ARG2$ --sircrit $ARG3$
}
define command{
command_name check_tiki_db
command_line /usr/local/bin/check_tiki.php -u $ARG1$ -c db
}
This is a great way to get great tools and to encourage ClearCenter (the main developers of ClearOS)