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When using m4 to configure Nagios, great advantages can be realized.  One of the easiest places to gain an advantage by using m4 is when defining a new host.

Typically, a new host not only has a host definition but a number of fairly standardized services – such as ping, FTP, telnet, SSH, and so forth.  Thus, when defining a new host configuration, you not only have to add a new host, but all of the relevant services as well – and may also include host extra info and service extra info also.

#----------------------------------------
# HOST: marco
#----------------------------------------
define host{
        use                     hpux-host               ; Name of host template
        host_name               marco
        address                 192.168.4.1
        }
define hostextinfo{
        host_name               marco
        action_url              http://marco-mp/
}
define service{
        use                             passive-service          ; Name of servi
        host_name                       marco
        service_description             System Load
        servicegroups                   Load
        }
define service{
        use                             hpux-service          ; Name of service
        host_name                       marco
        service_description             PING
        check_command                   check_ping!100.0,20%!500.0,60%
        }
define service{
        use                             hpux-service          ; Name of service
        host_name                       marco
        service_description             TELNET
        servicegroups                   TELNET
        check_command                   check_telnet
        }
define serviceextinfo{
        host_name                       marco
        service_description             TELNET
        action_url                      telnet://marco
}
define service{
        use                             hpux-service          ; Name of service
        host_name                       marco
        service_description             FTP
        servicegroups                   FTP
        check_command                   check_ftp
        }
define service{
        use                             hpux-service          ; Name of service
        host_name                       marco
        service_description             NTP
        servicegroups                   NTP
        check_command                   check_ntp
        }
define service{
        use                             hpux-service          ; Name of service
        host_name                       marco
        service_description             SSH
        servicegroups                   SSH
        check_command                   check_ssh
        }

Compare that output from the m4 code that generated it:

DEFHPUX(`marco',`192.168.4.1')

Another benefit is that if DEFHPUX is coded correctly (with each service independent – such as an m4 macro DOSSH for SSH) – then a single change to the m4 file, propogated to the Nagios config file, can alter a service for every HP-UX host (in this example).

Here is a possible definition of DEFHPUX:

define(`DEFHPUX',`
#----------------------------------------
# HOST: $1
#----------------------------------------
define host{
        use                     hpux-host               ; Name of host template
        host_name               $1
        address                 $2
        }
define hostextinfo{
        host_name               $1
        action_url              http://$1-mp/
}'
DOLOAD(`$1')
DOPING(`$1')
DOTELNET(`$1')
DOFTP(`$1')
DONTP(`$1')
DOSSH(`$1')

There is a lot more that m4 can do; this is just the tip of the iceberg.

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