Check if a remote machine is listening on some specific port, in a *nix system

When working in a distributed environment, I always use to set up some boxes to communicate together using some transport layer (TCP or UDP). So there has been always a need to check whether this or that port is open on some machine, and since ping command is nothing but an Echo Request on some IP address, I had to look around for a simple but efficient solution and finally crossed one.

Here I share the script that does the checking using the nc (netcat tool), so you have to make sure it is already installed on your machine.

How To:

Once you are sure to have netcat ready for use, create a bash script with underlines code (either using your favorite text editor or following command line instructions which I will be using right now):

• Create a bash script with name check_port.sh:

  touch check_port.sh

• Edit the script:

  vim check_port.sh

• Then copy/paste below lines in:

  #!/bin/bash
  # portcheck tool
  
  usage()
  {
      echo -e "\n USAGE: ./${0##*/} [host|ip] [port] \n"
      exit
  }
  
  check_port()
  {
      local host=${1}
      local port=${2}
      if nc -w 5 -z ${host-ip} ${port} 2>/dev/null
      then
      echo -e "\a\n => Port ${port} at ${host} is open"
      else
      echo -e "\a\n => Port ${port} at ${host} is closed"
      fi
  }
  
  [[ $# -ne 2 ]] && usage
  
  check_port ${1} ${2}

• Make the script executable:

  chmod +x check_port.sh

Usage:

The script is as straightforward as it can be and you can use it by just issuing the script name (in its directory path) followed by an IP address and a Port you want to check, e.g:

cd /path/to/check_port.sh
sh check_port.sh google.com 80
 => Port 80 at google.com.br is open

sh check_port.sh wisebrains.org 8080
 => Port 8080 at wisebrains.org is closed

Note: You can create a link to your script file under your /usr/bin directory so it will be available under your PATH env variable and you are free to use it as a command:

cd /usr/bin
sudo ln -s /path/to/check_port.sh portcheck

Now you can just use the portcheck command from any path:

portcheck google.com 80
 => Port 80 at google.com.br is open

Source: virtual-server Blog