Which command is used to determine which program is listening on which open port?

The command that is primarily used to determine which program is listening on which open port is 'ss'. This command is part of the iproute2 package and is considered a modern replacement for the older netstat command. It provides detailed information about sockets, including TCP and UDP connections, and reveals which processes are associated with specific ports.

Using 'ss' allows users to quickly display socket connection information in a more efficient manner compared to older utilities. When executed with specific flags, 'ss' can show listening ports along with the corresponding processes, making it a valuable tool for system administrators for diagnosing network issues and managing network services.

While 'lsof' can also list open files and the processes that opened them, it may not be as optimized specifically for socket information as 'ss'. The 'netstat' command, though still used by some, has largely been replaced by 'ss' in many modern Linux distributions due to its more efficient processing. The 'fuser' command can show which processes are using files or sockets, but it is not directly aimed at providing the listening status of ports in the same comprehensive way that 'ss' does.