The lsof command, meaning list open files, is a command-line utility in the Linux system to display information about files that are opened by processes1. The lsof command can take various options and arguments to filter and format the output. One of the options that can be used to identify the PID of a process which opened a TCP port is the -i option, which selects the listing of files whose Internet address matches the specified address. The address can be specified as a port number, a host name, or a combination of both. For example, to list the processes that are listening on TCP port 80, one can run:

lsof -i TCP:80

The output shows the command name, the PID, the user name, the file descriptor, the type, the device, the size/off, the node, and the name for each process. The name column shows the local and remote addresses and port numbers for the TCP connection. For example, the output may look like:

COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME httpd 1234 root 4u IPv4 12345 0t0 TCP *:80 (LISTEN) httpd 2345 www-data 4u IPv4 12345 0t0 TCP *:80 (LISTEN) httpd 3456 www-data 4u IPv4 23456 0t0 TCP 192.168.1.10:80->192.168.1.20:1234 (ESTABLISHED)

This shows that the httpd command, which is the Apache web server, is listening on TCP port 80 with the PID 1234 and 2345, and has an established connection with the remote address 192.168.1.20 and port 1234 with the PID 3456. To kill the process by PID, one can use the kill command with the -SIGTERM option, which sends a termination signal to the process. For example, to kill the process with the PID 3456, one can run:

kill -SIGTERM 3456

The other options are not correct because:

ptrace: This is not a command, but a system call that allows a process to trace and control the execution of another process. It is used by debuggers and other tools that need to monitor and manipulate the behavior of other processes2. It does not display the PID of a process which opened a TCP port.

strace: This is a command that traces the system calls and signals of a process. It can be used to diagnose, debug, and monitor the interaction between a process and the kernel3. It does not display the PID of a process which opened a TCP port.

debug: This is not a command, but a general term that refers to the process of finding and fixing errors in a program or system. There are various tools and methods that can be used for debugging, such as debuggers, loggers, profilers, etc4. It does not display the PID of a process which opened a TCP port.

nessus: This is a command that runs the Nessus vulnerability scanner, which is a tool that scans a network or a system for security flaws and potential attacks5. It does not display the PID of a process which opened a TCP port. References:

https://www.howtogeek.com/28609/how-can-i-tell-what-is-listening-on-a-tcpip-port-in-windows/

https://bing.com/search?q=identify+PID+of+process+that+opened+a+TCP+port