前提：把check_mem.sh放到/usr/local/nagios/libexec下，并赋予 户执行权限就可以 

chmod 755 check_mem.sh

一，监控本地（监控端本身）内存使用：
#修改commands配置
  
vim /usr/local/nagios/etc/objects/commands.cfg 
#增加以下内容 
define command{ 
                command_name        check_mem 
                command_line        $USER1$/check_mem.sh -w $ARG1$ -c $ARG2$ 
                }
#修改localhost.cfg
vim /usr/local/nagios/etc/objects/localhost.cfg 
#增加以下内容 
define service{ 
                use                                                         local-service 
                host_name                                             localhost 
                service_description                         memory 
                check_command                                     check_mem!20!10! 
                }
二，监控客户端内存使用情况：
 1,#修改客户端配置文件 
    vim /usr/local/nagios/etc/nrpe.cfg 

        #增加以下内容    
    command[check_mem]=/usr/local/nagios/libexec/check_mem.sh -w 10 -c 5 
            
  2,#修改主控制端相关配置文件 
     vim /usr/local/nagios/etc/service/liguxk.cfg    #主控端定义的被控端的配置文件 
     #增加以下内容 
                  define service{ 
                  use                                                         generic-service 
                  host_name                                             liguxk 
                  service_description                         Memory 
                  check_command                                     check_nrpe!check_mem 
                  }

chechk_mem.sh:

  
#script to check real memory usage
# L.Gill 02/05/06 - V.1.0
# ------------------------------------------
# ########  Script Modifications  ##########
# ------------------------------------------
# Who         When           What
# ---    ----      ----
# LGill         17/05/06  "$percent" lt 1% fix - sed edits dc result beggining with "."
#
#
#!/bin/bash
USAGE="`basename $0` [-w|--warning]<percent free> [-c|--critical]<percent free>"
THRESHOLD_USAGE="WARNING threshold must be greater than CRITICAL: `basename $0` $*"
calc=/tmp/memcalc
percent_free=/tmp/mempercent
critical=""
warning=""
STATE_OK=0
STATE_WARNING=1
STATE_CRITICAL=2
STATE_UNKNOWN=3
# print usage
if [[ $# -lt 4 ]]
then
        echo ""
        echo "Wrong Syntax: `basename $0` $*"
        echo ""
        echo "Usage: $USAGE"
        echo ""
        exit 0
fi
# read input
while [[ $# -gt 0 ]]
  do
        case "$1" in
               -w|--warning)
               shift
               warning=$1
        ;;
               -c|--critical)
               shift
               critical=$1
        ;;
        esac
        shift
  done

# verify input
if [[ $warning -eq $critical || $warning -lt $critical ]]
then
        echo ""
        echo "$THRESHOLD_USAGE"
        echo ""
        echo "Usage: $USAGE"
        echo ""
        exit 0
fi
# Total memory available
total=`free -m | head -2 |tail -1 |gawk '{print $2}'`
# Total memory used
used=`free -m | head -2 |tail -1 |gawk '{print $3}'`
# Calc total minus used
free=`free -m | head -2 |tail -1 |gawk '{print $2-$3}'`
# normal values
#echo "$total"MB total
#echo "$used"MB used
#echo "$free"MB free
# make it into % percent free = ((free mem / total mem) * 100)
echo "5" > $calc # decimal accuracy
echo "k" >> $calc # commit
echo "100" >> $calc # multiply
echo "$free" >> $calc # division integer
echo "$total" >> $calc # division integer
echo "/" >> $calc # division sign
echo "*" >> $calc # multiplication sign
echo "p" >> $calc # print
percent=`/usr/bin/dc $calc|/bin/sed 's/^\./0./'|/usr/bin/tr "." " "|/usr/bin/gawk {'print $1'}`
#percent1=`/usr/bin/dc $calc`
#echo "$percent1"
if [[ "$percent" -le  $critical ]]
        then
                echo "CRITICAL - $free MB ($percent%) Free Memory"
                exit 2
fi
if [[ "$percent" -le  $warning ]]
        then
                echo "WARNING - $free MB ($percent%) Free Memory"
                exit 1
fi
if [[ "$percent" -gt  $warning ]]
        then
                echo "OK - $free MB ($percent%) Free Memory"
                exit 0
fi
另一种方法：
由于LINUX使用内存的机制，监控内存没多大必要，还是记录下来
在被监控机/usr/local/nagios/etc/nrpe.cfg里添加
command[check_mem]=/usr/local/nagios/libexec/check_mem -w 150 -c 200
把check_mem(见附件)放到/usr/local/nagios/libexec/下，并赋予权限
chmod +x /usr/local/nagios/libexec/check_mem
chown nagios.nagios /usr/local/nagios/libexec/check_mem
监控端/usr/local/nagios/etc/objects/commands.cfg添加
define command{
        command_name    check_mem
        command_line    $USER1$/check_mem -w $ARG1$ -c $ARG2$
        }
/usr/local/nagios/etc/servers/252.cfg添加
define service{
        use                             generic-service         ; Name of service template to use
        host_name                       252
        service_description             memory
        check_command                   check_nrpe!check_mem!110,80!150,100
       }
重启nagios服务
 

说实话nagios监控内存没什么意思，你还要分析数据什么的不如直接利用cacti图形那样比较清晰，cacti的内存监控 是自己带的功能比较好实现的！