How I can check for exact amount of space (MB or GB) in Nagios using check_nt command instead of percentage used value?

check_nt!USEDDISKSPACE returns the both of size and percentage of disk usage. But thresholds are percentage.

If you want to receive alert based on size, you can write a wrapper shell script for check_nt command, e.g check_disk_by_size:

#!/bin/bash

FREESPACE=`/usr/local/nagios/libexec/check_nt -H $2 -p 12489 -s pa$$word \
-v USEDDISKSPACE -l $4 | awk -F"- " '{ print $4 }' | awk -F "|" '{ print $1 }'`

SIZE=`echo $FREESPACE | awk '{ print $2 }'`
UNIT=`echo $FREESPACE | awk '{ print $3 }'`

if [ $UNIT == "Gb" ]; then
    SIZE=`echo $SIZE \* 1024 | bc`
fi

if [ `echo "$SIZE >= $6" | bc` -eq 1 ]; then
    echo "$4:\_Drive_Space OK - $FREESPACE"
    exit 0
elif [ `echo "$SIZE < $6" | bc` -eq 1 -a `echo "$SIZE > $8" | bc` -eq 1 ]; then
    echo "$4:\_Drive_Space WARNING - $FREESPACE"
    exit 1
elif [ `echo "$SIZE <= $8" | bc` -eq 1 ]; then
    echo "$4:\_Drive_Space CRITICAL - $FREESPACE"
    exit 2
fi

Testing:

$ check_disk_by_size.sh -H 192.168.6.31 -l c -w 10240 -c 5120
c:\_Drive_Space OK - free 13.01 Gb (36%)

$ check_disk_by_size.sh -H 192.168.6.31 -l c -w 14240 -c 5120
c:\_Drive_Space WARNING - free 13.01 Gb (36%)

$ check_disk_by_size.sh -H 192.168.6.31 -l c -w 16240 -c 15120
c:\_Drive_Space CRITICAL - free 13.01 Gb (36%)

You can add it to Nagios like this:

"check_disk_by_size.sh".

# 'check_disk_by_size' command definition
define command{
       command_name    check_disk_by_size
       command_line    $USER1$/check_disk_by_size.sh -H $HOSTADDRESS$ $ARG1$ $ARG2$ $ARG3$
       }

define service{
        use                     generic-service
        host_name               fileserver1
        service_description     Drive Space fileserver1: L:
        check_command           check_disk_by_size!-l L -w 4096 -c 2048
        notifications_enabled   1
        }

I have modified the code to adapt new BASH syntax and more robust check.

#!/bin/bash

usage() { echo "$0 check_nt -H host [-p port] [-w warning] [-c critical] [-l params] [-t timeout]" 1>&2; exit 1; }

while getopts ":H:p:l:w:c:" opt; do
    case "${opt}" in
        H ) HOST_NAME=$OPTARG;;
        p ) PORT=$OPTARG;;
        l ) DISC=$OPTARG;;
        w ) WARN_THRESHOLD=$OPTARG;;
        c ) CRITICAL_THRESHOLD=$OPTARG;;
#        t ) TIME_OUT=$OPTARG;;
        \?) echo "Invalid option: -$OPTARG" >&2; exit 1 ;;
        : ) echo "Option -$OPTARG requires an argument." >&2; exit 1;;
    esac
done
shift $((OPTIND-1))

if [[ -z "$HOST_NAME" ]] || [[ -z "$PORT" ]] || [[ -z "$DISC" ]] || [[ -z "$WARN_THRESHOLD" ]] || [[ -z "$CRITICAL_THRESHOLD" ]] ; then
    usage
fi

FREESPACE=`/usr/local/nagios/libexec/check_nt -H $HOST_NAME -p $PORT \
-v USEDDISKSPACE -l $DISC | awk -F"- " '{ print $4 }' | awk -F "|" '{ print $1 }'`

SIZE=`echo $FREESPACE | awk '{ print $2 }'`
UNIT=`echo $FREESPACE | awk '{ print $3 }'`

if [[ $UNIT == "Gb" ]];then
    SIZE=`echo $SIZE \* 1024 | bc`
fi

if [[ `echo "$SIZE >= $WARN_THRESHOLD" | bc` -eq 1 ]];then
    echo "$DISC:\_Drive_Space OK - $FREESPACE"
    exit 0
elif [[ `echo "$SIZE  $CRITICAL_THRESHOLD" | bc` -eq 1 ]];then
    echo "$DISC:\_Drive_Space WARNING - $FREESPACE"
    exit 1
elif [[ `echo "$SIZE 

I have modified Gnought's script which is a modification of quanta's script even further to remove some code errors and modified the results to more-closely resemble the original output of check_nt which gives better data when analyzing trends over time as well as performance data being added back in.

#!/bin/bash
# Date: 2015-06-30
# Purpose: A wrapper script for check_nt to set threshold for exact space
#          free rather than just percentage.  Useful on VERY large drives.
# Example: check_disk_by_size.sh -H 192.168.0.1 -l c -w 10240 -c 5120

usage() { echo "$0 -H host [-s password] [-p port] [-w warning] [-c critical] [-l params]" 1>&2; exit 1; }

while getopts ":H:s:p:l:w:c:" opt; do
    case "${opt}" in
        H ) HOST_NAME=$OPTARG;;
        s ) PASSW0RD=$OPTARG;;
        p ) PORT=$OPTARG;;
        l ) DISC=$OPTARG;;
        w ) WARN_THRESHOLD=$OPTARG;;
        c ) CRITICAL_THRESHOLD=$OPTARG;;
        \?) echo "Invalid option: -$OPTARG" >&2; exit 1 ;;
        : ) echo "Option -$OPTARG requires an argument." >&2; exit 1;;
    esac
done
shift $((OPTIND-1))

if [[ -z "${HOST_NAME}" ]] || [[ -z "${PASSW0RD}" ]] || [[ -z "${PORT}" ]] || [[ -z "${DISC}" ]] || [[ -z "${WARN_THRESHOLD}" ]] || [[ -z "${CRITICAL_THRESHOLD}" ]] ; then
    usage
fi

CHECKRESULT=`/usr/local/nagios/libexec/check_nt -H ${HOST_NAME} -p ${PORT} -s ${PASSW0RD} -v USEDDISKSPACE -l ${DISC}`
PERFDATA=`echo ${CHECKRESULT} | awk -F"- " '{ print $4 }' | awk -F "|" '{ print $2 }'`
FREESPACE=`echo ${CHECKRESULT} | awk -F"- " '{ print $4 }' | awk -F "|" '{ print $1 }'`
USEDSPACE=`echo ${CHECKRESULT} | awk -F"- " '{ print $3 }'`
TOTALSPACE=`echo ${CHECKRESULT} | awk -F"- " '{ print $2 }'`

if [[ -z ${FREESPACE} ]]; then
    ## Command failed or server offline
    echo "ERROR ERROR: Command failed"
    exit 1
fi

SIZE=`echo $FREESPACE | awk '{ print $2 }'`
UNIT=`echo $FREESPACE | awk '{ print $3 }'`

if [[ ${UNIT} == "Gb" ]]; then
    SIZE=`echo ${SIZE} \* 1024 | bc`
fi

if [[ `echo "${SIZE} >= ${WARN_THRESHOLD}" | bc` -eq 1 ]]; then
    echo "${DISC}:\ OK - ${TOTALSPACE} - ${USEDSPACE} - ${FREESPACE} | ${PERFDATA}"
    exit 0
elif [[ `echo "${SIZE} < ${WARN_THRESHOLD}" | bc` -eq 1 && `echo "${SIZE} > ${CRITICAL_THRESHOLD}" | bc` -eq 1 ]]; then
    echo "${DISC}:\ WARNING - ${TOTALSPACE} - ${USEDSPACE} - ${FREESPACE} | ${PERFDATA}"
    exit 1
elif [[ `echo "${SIZE} <= ${CRITICAL_THRESHOLD}" | bc` -eq 1 ]]; then
    echo "${DISC}:\ CRITICAL - ${TOTALSPACE} - ${USEDSPACE} - ${FREESPACE} | ${PERFDATA}"
    exit 2
fi