Works great!

I modified the script, now there is also a -m or –metric flag, where you can select which format the warn and crit input and the echo output should be (KB, MB, GB or TB)
Here it is:

#!/bin/sh
#
# ## Plugin for Nagios to monitor directory size
# ## Written by Gerd Stammwitz (http://www.enbiz.de/)
# ##
# ## – 20040727 coded and tested for Solaris and Linux
# ## – 20041216 published on NagiosExchange
# ## – 20070710 modified by Jose Vicente Mondejar to add perfdata option
#
#
# ## You are free to use this script under the terms of the Gnu Public License.
# ## No guarantee – use at your own risc.
#
#
# Usage: ./check_dirsize -d -w -c
#
# ## Description:
#
# This plugin determines the size of a directory (including sub dirs)
# and compares it with the supplied thresholds.
# It might also be used to monitor a regular file instead of a directory.
#
# ## Output:
#
# The plugin prints the size of the directory in KB followed by “ok” or
# either “warning” or “critical” if the corresponing threshold is reached.
#
# Exit Codes
# 0 OK Directory size checked and everything is ok
# 1 Warning Directory size above “warning” threshold
# 2 Critical Directory size above “critical” threshold
# 3 Unknown Invalid command line arguments or could not determine directory size
#
# Example: check_dirsize -d . -w 1000 -c 1400
#
# 121 KB – ok (exit code 0)
# 1234 KB – warning (exit code 1)
# 1633 KB – critical (exit code 2)

# Paths to commands used in this script. These
# may have to be modified to match your system setup.

PATH=””

DU=”/usr/bin/du”
CUT=”/usr/bin/cut”
WC=”/usr/bin/wc”

PROGNAME=`/usr/bin/basename $0`
PROGPATH=`echo $0 | /bin/sed -e ‘s,[\/][^\/][^\/]*$,,’`
REVISION=”Revision 1.1″
AUTHOR=”(c) 2004,2007 Gerd Stammwitz (http://www.enbiz.de/)”

# Exit codes
STATE_OK=0
STATE_WARNING=1
STATE_CRITICAL=2
STATE_UNKNOWN=3
STATE_DEPENDENT=4

print_revision() {
echo “$REVISION $AUTHOR”
}

print_usage() {
echo “Usage: $PROGNAME -d|–dirname [-w|–warning ] [-c|–critical ] [-f|–perfdata] [-m|–metric ]”
echo “Usage: $PROGNAME -h|–help”
echo “Usage: $PROGNAME -V|–version”
echo “”
echo ” must be KB, MB, GB or TB”
}

print_help() {
print_revision $PROGNAME $REVISION
echo “”
echo “Directory size monitor plugin for Nagios”
echo “”
print_usage
echo “”
}

# Make sure the correct number of command line
# arguments have been supplied

if [ $# -lt 1 ]; then
print_usage
exit $STATE_UNKNOWN
fi

# Grab the command line arguments

thresh_warn=””
thresh_crit=””
metric=”KB”
perfdata=0
exitstatus=$STATE_WARNING #default
while test -n “$1”; do
case “$1” in
–help)
print_help
exit $STATE_OK
;;
-h)
print_help
exit $STATE_OK
;;
–version)
print_revision $PROGNAME $VERSION
exit $STATE_OK
;;
-V)
print_revision $PROGNAME $VERSION
exit $STATE_OK
;;
–dirname)
dirpath=$2
shift
;;
-d)
dirpath=$2
shift
;;
–warning)
thresh_warn=$2
shift
;;
-w)
thresh_warn=$2
shift
;;
–critical)
thresh_crit=$2
shift
;;
-c)
thresh_crit=$2
shift
;;
–metric)
metric=$2
shift
;;
-m)
metric=$2
shift
;;
-f)
perfdata=1
;;
-perfdata)
perfdata=1
;;
*)
echo “Unknown argument: $1″
print_usage
exit $STATE_UNKNOWN
;;
esac
shift
done

##### Get size of specified directory
error=””
duresult=`$DU -s $dirpath 2>&1` || error=”Error”
if [ ! “$error”=”” ]; then
errtext=`echo $duresult | $CUT -f3 -d”:”`
echo “$error:$errtext”
exit $STATE_UNKNOWN
fi

dirsize=`echo $duresult | $CUT -f1 -d” “`
result=”ok”
exitstatus=$STATE_OK

##### Compare with thresholds
calc_value=1
case $metric in
“MB”)
calc_value=$((dirsize / 1024))
thresh_warn=$((thresh_warn * 1024))
thresh_crit=$((thresh_crit * 1024))
;;
“GB”)
calc_value=$((dirsize / 1024 / 1024))
thresh_warn=$((thresh_warn * 1024 * 1024))
thresh_crit=$((thresh_crit * 1024 * 1024))
;;
“TB”)
calc_value=$((dirsize / 1024 / 1024 / 1024))
thresh_warn=$((thresh_warn * 1024 * 1024 * 1024))
thresh_crit=$((thresh_crit * 1024 * 1024 * 1024))
;;
“KB”)
calc_value=$dirsize
;;
*)
echo “Wrong Metric! Must be KB, MB, GB or TB!”
exit $STATE_UNKNOWN
;;
esac

if [ “$thresh_warn” != “” ]; then
if [ $dirsize -ge $thresh_warn ]; then
result=”warning”
exitstatus=$STATE_WARNING
fi
fi
if [ “$thresh_crit” != “” ]; then
if [ $dirsize -ge $thresh_crit ]; then
result=”critical”
exitstatus=$STATE_CRITICAL
fi
fi
if [ $perfdata -eq 1 ]; then
result=”$result|’size’=${calc_value};${thresh_warn};${thresh_crit}”
fi

echo “$calc_value $metric – $result”
exit $exitstatus