Writing a munin plugin


A munin plugin is a small executable. Usually, it is written in some interpreted language.

In its simplest form, when the plugin is executed with the argument “config”, it outputs metadata needed for generating the graph. If it is called with no arguments, it outputs the data which is to be collected, and graphed later.

Plugin output

The minimum plugin output when called with “config” it must output the graph title.

It should also output a label for at least one datasource.

graph_title Some title for our plugin
something.label Foobar per second

When the plugin is executed with no arguments, it should output a value for the datasource labelled in “config”. It must not output values for which there are no matching labels in the configuration output.

something.value 42

For a complete description of the available fields, see the Plugin reference.

Example shell plugin

The base of a plugin is a small option parser, ensuring the plugin is called with the correct argument, if any.

Two main functions are defined: One for printing the configuration to the standard output, and one for printing the data. In addition, we have defined a function to generate the data itself, just to keep the plugin readable.

The “output_usage” function is there just to be polite, it serves no other function. :)


output_config() {
    echo "graph_title Example graph"
    echo "plugins.label Number of plugins"

output_values() {
    printf "plugins.value %d\n" $(number_of_plugins)

number_of_plugins() {
    find /etc/munin/plugins -type l | wc -l

output_usage() {
    printf >&2 "%s - munin plugin to graph an example value\n" ${0##*/}
    printf >&2 "Usage: %s [config]\n" ${0##*/}

case $# in
        case $1 in
                exit 1
        exit 1

Activating the plugin

Place the plugin in the /etc/munin/plugins/ directory, and make it executable. Note that most distributions place plugins in a different directory, and ‘activate’ them by symlinking htem to /etc/munin/plugins. New module development should use a similar approach so that in-process development doesn’t get run by mistake.

Any time a new plugin is placed or symlinked into /etc/munin/plugins, munin-node should be restarted.

Debugging the plugin

Plugins are just small programs or scripts, and just like small programs, are prone to problems or unexpected behaviour. When either developing a new plugin, or debugging an already existing one, use the following information to help track down the problem:

  • A plugin may be tested ‘by hand’ by using the command ‘munin-run’. Note the plugin needs to have been activated before this will work (see above).
  • If an error occurs, error messages will be written to STDERR, and exit status will be non-zero.
  • If a plugin is already activated, any errors that may happen when the ‘munin-node’ cron job is executed will be logged, via stderr, to /var/log/munin/munin-node.log