etc/nagios/checkcommands.cfg is used to define local service commands, for example:
define command{ command_name check_privoxy command_line /usr/lib/nagios/plugins/check_http -I $ARG1$ -p $ARG2$ - u $ARG3$ } ; $ARG1$ and $ARG2$ are replaced by the arguments you ; specify in the service definition with `!' (bang) define command{ command_name check_ifstatus_router command_line /usr/lib/nagios/plugins/check_ifstatus -H $HOSTADDRESS$ -x $ARG1$ } ; $HOSTADDRES$ is always a single ip address, derived from ; the service definition and hostgroup.cfg and host.cfg files