How can I test if a port is available on Linux?

On Linux, tools like telnet, nc, ncat, and nmap can be used to test the availability of ports.

1. Test the availability of a port using the telnet command:
   telnet IP address port number
   For example: telnet 192.168.1.1 80
   If the port is available, a message indicating successful connection will be displayed; if the port is not available, a message indicating connection failure or inability to connect will be displayed.
2. Test the availability of a port using the nc command:
   nc -z -v IP address port number
   For example: nc -z -v 192.168.1.1 80
   If the port is available, it will display a message saying “Connection to IP address port number [tcp/] succeeded!”; if the port is unavailable, it will display a message saying “Connection to IP address port number [tcp/] failed: Connection refused”.
3. Test the availability of a port using the ncat command:
   ncat -v IP address port number
   For example: ncat -v 192.168.1.1 80
   If the port is available, you will see the message “Ncat: Connected to IP address: port number”; if the port is not available, you will see the message “Ncat: Connection refused”.
4. To test if a port is available using the nmap command:
   nmap -p port number IP address
   For example: nmap -p 80 192.168.1.1
   If the port is available, it will display a message showing the port number/tcp open; if the port is not available, it will display a message showing the port number/tcp closed.

Here are several commonly used methods that you can choose from to test if a port is available.