A Ruby DSL for writing Nagios plugins
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Failed to load latest commit information.



A Ruby DSL for writing Nagios plugins. It handles the tedium of argument parsing, exit statuses, output formatting and such, and lets you focus on writing the actual check.

Written by Erik Grinaker <erik@bengler.no>, and licensed under the GNU GPL v3.


A very simple plugin looks like this:

#!/usr/bin/env ruby
require 'nagi'

Nagi do
  name 'check_true'
  version '0.1'
  prefix 'TRUE'

  check do |args|
    if true
      ok 'True is still true'
      critical 'Uh oh'

The first few methods just set some basic info for the plugin - the name, version, and a prefix for the check output. These are all optional.

Next, the check block is defined, which runs the actual Nagios check. args will contains any options (see command-line parsing below), and the methods ok, warning, critical, and unknown are used to return the check status. A missing status or uncaught exception will result in an 'unknown' status.

The plugin is run as any regular script - save it as a file, make it executable and run it:

$ ./test.rb 
TRUE OK: True is still true

Command-line options

Nagi can automatically parse command-line arguments when requested, and provide them as input to the check block. Here is a simple example:

#!/usr/bin/env ruby
require 'nagi'
require 'resolv'

Nagi do
  name 'check_dns'
  version '0.1'
  prefix 'DNS'
  argument :hostname
  switch :ip, '-i', '--ip IP', 'Expected IP address'

  check do |args|
      ip = Resolv.getaddress(args[:hostname])
    rescue Resolv::ResolvError => e
      critical e.message

    if args[:ip] and ip != args[:ip]
      critical "#{args[:hostname]} resolves to #{ip}, expected #{args[:ip]}"
      ok "#{args[:hostname]} resolves to #{ip}"

The argument method specifies a required positional argument, and only takes the name of the argument. switch specifies an optional switch, which may or may not take an argument of its own. The first switch parameter is its name, while the rest are passed to the standard Ruby OptionParser.on method - see its documentation for details.

The parsed arguments are passed to the check block via the args parameter, which is a hash containing argument names and values. In the case of a switch with no argument of its own, the value will be true.

A few usage examples of this plugin:

$ ./check_dns.rb 
Error: Argument 'hostname' not given

Usage: ./check_dns.rb [options] <hostname>
    -i, --ip IP                      Expected IP address
    -h, --help                       Display this help message
    -V, --version                    Display version information

$ ./check_dns.rb www.google.com
DNS OK: www.google.com resolves to

$ ./check_dns.rb -i www.google.com
DNS CRITICAL: www.google.com resolves to, expected

Nagi will automatically set up the -h/--help and -V/--version switches and handle them appropriately.

Handling multiple statuses

In some cases it is useful to collect several statuses and return the most severe, rather than returning the first status encountered. This is typically used when monitoring multiple resources with a single check.

The collect setting tells Nagi to continue running the check when a status is given. It can be set to either :severe, in which case the most severe status will be returned at the end, or :all which will use the most severe status code but also combine all status messages into one.

However, one can still force a status to return from the check, and ignore the collected statues, by setting the second parameter (force) to true for the status methods.

This example checks used space on all disk drives, and returns critical if any of the drives are above a threshold:

#!/usr/bin/env ruby
require 'nagi'

Nagi do
  name 'check_df'
  version '0.1'
  prefix 'DF'
  argument :percentage
  collect :all

  check do |args|
      output = execute 'df'
    rescue StandardError => e
      critical "df failed: #{e.message}", true
    output.lines.each do |line|
      if line =~ /^.*?(\d+)%\s*(.*)$/
        if $1.to_i > args[:percentage].to_i
          critical "#{$2} #{$1}% used"
          ok "#{$2} #{$1}% used"

When run, it will output something like this:

$ ./check_df.rb 90
DF OK: / 80% used, /Volumes/Media 57% used

$ ./check_df.rb 70
DF CRITICAL: / 80% used, /Volumes/Media 57% used


Plugin info

These describe the program, and are usually given first, if necessary.

  • name name: the program name.
  • version version: the program version.
  • prefix prefix: a prefix for the check output.
  • collect type: collect statuses and continue the check, rather than returning. type can be either :severe or :all - :severe will return the last, most severe status, and :all will use the most severe status but join all the status messages together into one.
  • fallback status, message: a default status to return if the check doesn't explicitly return one. status can be :ok, :warning, :critical, or :unknown.

Command-line arguments

Command-line arguments can be specified, and will be parsed automatically and passed to the check method as a hash, with keys given by the argument name.

  • argument name: a mandatory, positional argument.
  • switch name, args: an optional switch. args are passed directory to the standard Ruby OptionParser class - see its documentation for details.

Command-line arguments for -h/--help and -V/--version are added and handled automatically.


The check is given as a code block to the check method, and is passed the parsed command-line arguments as a hash. It should use one of the methods ok, warning, critical, or unknown to return a status. If no status is given, or the block raises an unhandled exception, an Unknown status will be returned.

The collect setting (see above) can be used to collect statuses and continue the check, rather than returning the first status encountered. However, if true is passed as the second parameter of the status method, it will return the status regardless of the collect setting.

  • check block: the code block the the check. Parsed command-line arguments are passed as a hash.
  • ok message, force=false: returns an OK status.
  • warning message, force=false: returns a Warning status.
  • critical message, force=false: returns a Critical status.
  • unknown message, force=false: returns an Unknown status.
  • execute command: executes a shell command, and returns any output (both stdout and stderr). If the command exits with a non-zero status, it will throw an exception. The shell is set to use the pipefail option, so non-zero exit statuses in pipelines are detected as well.


This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.