Bash script for nagios, that monitors bandwidth usage on remote host.
# usage ./bw_watch bw_warning bw_critical pkt_warning pkt_critical
It takes momentary 5 secs average bandwidth from “vnstat”, and returns value for nagios.

bc and vnstat packages at host side are necessary.
bc package should be in default centos repository, vnstat and nrpe You can find in EPEL repo.

Script itself, that You have to put to nagios plugins directory. In my case it was: /usr/lib64/nagios/plugins/

Script bw_watch.sh has to be in server that has to be checked.
Download link

#!/bin/bash
# script created by nsc
# usage ./bw_watch bw_warning bw_critical pkt_warning pkt_critical
# bw usage is in kbits/s

if [[ -z $1 ]] || [[ -z $2 ]] || [[ -z $3 ]] || [[ -z $4 ]]
then
	echo "VARIABLES ARE NOT SET!!!"
	echo "usage $0 bw_warning bw_critical pkt_warning pkt_critical"
	echo "bw usage is in kbits/s"
	exit 2
fi

bw_warn=$1
bw_crit=$2
packets_warn=$3
packets_crit=$4

bw_output=$(vnstat -tr 5 -s )
rx_value=$(echo $bw_output | grep -o "rx [[:digit:]]*\.*[[:digit:]]* .bit/s" | cut -f2 -d' ' )
rx_unit=$(echo $bw_output | grep -o "rx [[:digit:]]*\.*[[:digit:]]* .bit/s" | cut -f3 -d' ' )
rx_packets=$(echo $bw_output | grep -o "rx [[:digit:]]*\.*[[:digit:]]* .bit/s [[:digit:]]* packets/s" | cut -f4 -d' ' )
tx_value=$(echo $bw_output | grep -o "tx [[:digit:]]*\.*[[:digit:]]* .bit/s" | cut -f2 -d' ' )
tx_unit=$(echo $bw_output | grep -o "tx [[:digit:]]*\.*[[:digit:]]* .bit/s" | cut -f3 -d' ' )
tx_packets=$(echo $bw_output | grep -o "tx [[:digit:]]*\.*[[:digit:]]* .bit/s [[:digit:]]* packets/s" | cut -f4 -d' ' )

#convert rx to kbits/s
if [ $rx_unit == "Mbit/s" ]
then rx_value=`echo "$rx_value * 1024" | bc`
fi

#convert tx to kbits/s
if [ $tx_unit == "Mbit/s" ]
then tx_value=`echo "$tx_value * 1024" | bc`
fi

#convert to integer
rx_value=${rx_value/.*}
tx_value=${tx_value/.*}


if [ $bw_crit -lt $rx_value ] || [ $bw_crit -lt $tx_value ] || [ $packets_crit -lt $rx_packets ] || [ $packets_crit -lt $tx_packets ]
then
	echo "CRITICAL: RX/TX: $rx_value/$tx_value kbits/s. PKT: RX/TX: $rx_packets/$tx_packets"
	exit 2
elif [ $bw_warn -lt $rx_value ] || [ $bw_warn -lt $tx_value ] || [ $packets_warn -lt $rx_packets ] || [ $packets_warn -lt $tx_packets ]
then
	echo "WARNING: RX/TX: $rx_value/$tx_value kbits/s. PKT: RX/TX: $rx_packets/$tx_packets"
	exit 1
else
	echo "OK: RX/TX: $rx_value/$tx_value kbits/s. PKT: RX/TX: $rx_packets/$tx_packets"
	exit 0
fi

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On host, that has to be checked, you have to add to nrpe.conf (usually /etc/nagios/nrpe.cfg)

command[check_bandwidth]=/usr/lib64/nagios/plugins/bw_watch.sh 100 500 100 500

restart nrpe

service nrpe restart

nagios command definition at monitoring server (usually commands.cfg):

define command
        command_name    check_bandwidth
        command_line    $USER1$/check_nrpe -H $HOSTADDRESS$ -c check_bandwidth -t60 
}

define service at monitoring server (where monitored host configuration is located):
Parameters are: bandwidth_warning, bandwidth_critical, packets_warning, packets_critical
Bandwidth is metered in kbits/s.

define service{
        use                             generic-service
        host_name                       server.hostname.tld
        service_description             Bandwidth
        check_command                   check_bandwidth
        }

And reload nagios configuration

service nagios reload

Insipration source: nagios bandwidth plugin