Same as pyr's, but with Nagios-correct exit codes.

check_repli.rb

#!/usr/bin/env ruby

require 'mysql'
require 'yaml'
require 'optparse'

def parse argv
    env = {:path => 'check_repli.yml'}
    env[:path] = ENV['CHECK_REPLI_CONFIG'] if ENV.include? 'CHECK_REPLI_CONFIG'

    OptionParser.new do |opts|
        opts.banner = "Usage: check_db_repli -h host"

        opts.on("-?", "--help", "Show this message") do
            puts opts
            exit 3
        end
        opts.on("-h", "--host host", String, "host to connect to") do |host|
            env[:host] = host
        end
        opts.on("-f", "--config path", String, "configuration file") do |path|
            env[:path] = path
        end

        begin
            argv = ["-?"] if argv.empty?
            opts.parse!(argv)

            unless env.include? :host # need host to continue
                STDERR.puts opts
                exit 3
            end
            
        rescue OptionParser::ParseError => e
            STDERR.puts e.message, "\n", opts
            exit 3
        end
    end
    env
end

def getconfig env
    config = YAML.load_file env[:path]
    raise "need configuration for #{env[:host]}" unless config.include? env[:host]
    config[env[:host]]
end

def check_sql config, env
    dbh = Mysql.connect(config['host'], config['user'], config['password'], config['database'])
    res = dbh.query("show slave status")
    vals = res.fetch_hash
    fail_count = %w(Slave_IO_Running Slave_SQL_Running).select{|k| vals[k] !~ /yes/i}.length 
    if fail_count > 0
        puts "CRIT: SQL replication not running on: #{config['host']}: #{vals['Last_SQL_Error']}"
        exit 2
    end
end

env = parse ARGV
config = getconfig env
check_sql config, env

# all good so far
puts "OK: SQL replication running"
exit 0