Linux Shell Scripting Cookbook - Third Edition

Displaying the status of tcp sockets

A tcp socket connection is opened for every HTTP access, every SSH session, and so on. The -t option reports the status of TCP connections:

    $ ss -t
    ESTAB      0      0   192.168.1.44:740          192.168.1.2:nfs  
    ESTAB      0      0   192.168.1.44:35484        192.168.1.4:ssh

    CLOSE-WAIT 0      0   192.168.1.44:47135         23.217.139.9:http    

This example shows an NFS server connected at IP address 192.168.1.2 and an SSH connection to 192.168.1.4.

The CLOSE-WAIT socket status means that the FIN signal has been sent, but the socket has not been fully closed. A socket can remain in this state forever (or until you reboot). Terminating the process that owns the socket may free the socket, but that's not guaranteed.