Linux script to monitor remote port and launch script if not successful

Question

RHEL 7.1 is the OS this will be used on.

I have two servers which are identical (A and B). Server B needs to monitor a port on Server A and if it's down for 30 seconds, launch a script. I read netcat was replaced with ncat on RHEL 7 so this is what I have so far:

#!/bin/bash
Server=10.0.0.1
Port=123
ncat $Server $Port &> /dev/null; echo $?

If the port is up, the output is 0. If the port is down, the output is 1. I'm just not sure on how to do the next part which would be "if down for 30 seconds, then launch x script"

Any help would be appreciated. Thanks in advance.

Answer 1

If you really want to script this rather than using a dedicated tool like Pacemaker as @CharlesDuffy suggested, then you could do something like this:

• Run an infinite loop
• Check the port
  • If up, save the timestamp
  • Otherwise check the difference from the last saved timestamp
    • If more time passed then threshold, then run the script
• Sleep a bit

For example:

#!/bin/bash

server=10.0.0.1
port=123
seconds=30

seen=$(date +%s)
while :; do
    now=$(date +%s)
    if ncat $server $port &> /dev/null; then
        seen=$now
    else
        if ((now - seen > seconds)); then
            run-script && exit
        fi
    fi
    sleep 1
done