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great simple plugin for Unix/linux. Satisfied my requirements. Just had one issue. Logic was wrong for calculating used memory. Fixed it as follows-

used=$(echo "(($total-$free) / $total)*100" | bc -l)

was -
used=$(echo "($free / $total)*100" | bc -l)

I used it with NRPE for remote check. Below steps-
1. Copy the downloaded script. Make change mentioned above. Name it check_memory.sh
Copy, check_memory.sh, to /usr/lib/nagios/plugins on remote host

2. Add a file check_memory.cfg on remote host with content as-

command[check_memory]=/usr/lib/nagios/plugins/check_memory.sh -w 85 -c 90

3. restart nrpe service
service nagios-nrpe-server restart

4. On nagios server, add below to /etc/nagios3/conf.d/services.cfg

#service for checking remote host memory usage
define service {
service_description check Memory Usage
host_name remote-host-name-from-hosts.cfg
check_command check_nrpe!check_memory
use default-service
}

5. reload nagios configuration

/etc/init.d/nagios reload