The simplest way to extend Cacti's data gathering functionality is through external scripts. Cacti comes with a number of scripts out of the box which are located in the scripts/ directory. These scripts are used by the data input methods that are present in a new installation of Cacti.
To have Cacti call an external script to gather data you must create a new data input method, making sure to specify Script/Command for the Input Type field. See the previous section, Creating a Data Input Method for more information about how to create a data input method. To gather data using your data input method, Cacti simply executes the shell command specified in the Input String field. Because of this, you can have Cacti run any shell command or call any script which can be written in almost any language.
What Cacti is concerned with is the output of the script. When you define your data input method, you are required to define one or more output fields. The number of output fields that you define here is important to your script's output. For a data input method with only one output field, your script should output its value in the following format:
<value_1>
So if I wrote a script that outputs the number of running processes, its output might look like the following:
Data input methods with more than one output field are handled a bit differently when writing scripts. Scripts that output more than one value should be formatted like the following:
<fieldname_1>:<value_1> <fieldname_2>:<value_2> ... <fieldname_n>:<value_n>
Lets say that I write a script that outputs the 1, 5, and 10 minute load average of a Unix machine. In Cacti, I name the output fields '1min', '5min', and '10min', respectively. Based on these two things, the output of the script should look like the following:
One last thing to keep in mind when writing scripts for Cacti is that they will be executed as the user the data gatherer runs as. Sometimes a script may work correctly when executed as root, but fails due to permissions problems when executed as a less privileged user.