Script to check open ports in Linux

In this brief article I'd like to share a short script I recently wrote to check for port status for different ports in Linux. This script may prove useful as a pre-check or post-check after a maintenance activity or you can also put the script in cron if you need to monitor the port number corresponding to a service at regular intervals and relying on conventional monitoring tools is not an option.

Here is the script:

[root@still ~]# cat port_check.bash
#!/bin/bash

##Add a file containing a list of port numbers to chek##

PORT_LIST="/root/plist"

while read PNUM
do

netstat -tulpn | grep -w ":${PNUM}" > /dev/null

if [ $? -eq 0 ]
 then
  echo "port number ${PNUM} is listening on `hostname`"
else
 echo "port number ${PNUM} is not listening on `hostname`"
fi

done < ${PORT_LIST}

To test it I've created a file ptest with some port numbers to test the script.

[root@still ~]# cat /root/plist

80

22

30

Let's run the script:

[root@still ~]# ./port_check.bash

port number 80 is listening on still

port number 22 is listening on still

port number 30 is not listening on still

This is more of an arbritrary setup. You can add logic to send an email to you if any of the ports are not found to be in listening state