How to terminate unused or idle SSH sessions

Let's assume that when you are working on a server via ssh, your session connection is disconnected due to network, power, or local PC restart.

You may or may not log into the server again to continue working, but you will always be left with the ssh session that was not closed before.

How to close an inactive ssh session? First use the w command to identify the inactive or idle ssh session, then use the pstree command to get the PID of the idle session, and finally use the kill command to close the session.

Recommended reading: Mosh (Mobile Shell) - The best SSH remote connection alternative

Log in to the system and use the w command to check how many users are currently logged in. Once you have identified your session connection you can note other inactive or idle ssh sessions to close.
In my current example, I can see two users logged in, one of which is the ssh session where I am currently executing the w command and the other is the previous idle session.

# w
10:36:39 up 26 days, 20:29, 2 users, load average: 0.00, 0.02, 0.00
USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT
root pts/0 219.91.219.14 10:34 28.00s 0.00s 0.00s -bash
root pts/2 219.91.219.14 10:36 0.00s 0.00s 0.00s w

In order to close an idle ssh session, we need the PID of the parent process of the idle session process. We can execute the pstree command to view a tree diagram including all processes to obtain the pid of the parent process.
You will get output similar to the example below. The output of the pstree command will be much more than this, and I have deleted a lot of irrelevant content for better understanding.

# pstree -p
init(1)-+-abrtd(2131)
|-acpid(1958)
|-httpd(32413)-+-httpd(32442)
|
|-mingetty(2198)
|-mysqld_safe(24298)---mysqld(24376)-+-{mysqld}(24378)
|
|-php(32456)-+-php(32457)
|
|-sshd(2023)-+-sshd(10132)---bash(10136)
| `-sshd(10199)---bash(10208)---pstree(10226)
|-udevd(774)-+-udevd(2191)
`-udevd(27282)

From the output above, you can see a tree diagram of sshd processes and branches. The main process of sshd is sshd(2023), and the other two branches are sshd(10132) and sshd(10199).

Same as I said at the beginning of the article, one of them is my new session connection sshd(10199) which shows the pstree command I am executing, so the idle session is another process as sshd(10132).

Recommended reading: How to access a Secure Shell (SSH) server through a standard web browser
Recommended reading: PSSH - Executing commands in parallel on multiple Linux servers

We have got all the information about the idle session. So, let us use the kill command to close the idle session. Please make sure you replace the PID below with the idle session PID on your server.

# kill -9 10132

(LCTT Translation: Here is another tool pkill. Use pkill -t pts/0 -kill to close the session. It is available under debian 8. Some versions seem to need to change the location of -kill)

Use the w command again to check whether the idle session has been closed. That's right, only my current session is still there, so the idle session has been closed.

# w
10:40:18 up 26 days, 20:33, 1 user, load average: 0.11, 0.04, 0.01
USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT
root pts/2 219.91.219.14 10:36 0.00s 0.00s 0.00s w

Recommended reading: rtop - a useful tool to monitor remote servers through SSH

Recommended reading: DSH - Execute commands on multiple Linux servers simultaneously

Use the pstree command again to confirm. Yes, only my own ssh session is still there.

# pstree -p
init(1)-+-abrtd(2131)
|-acpid(1958)
|
|-httpd(32413)-+-httpd(32442)
|
|-mingetty(2198)
|-mysqld_safe(24298)---mysqld(24376)-+-{mysqld}(24378)
|
|-php(32456)-+-php(32457)
|
|-sshd(2023)---sshd(10199)---bash(10208)---pstree(10431)
|-udevd(774)-+-udevd(2191)
`-udevd(27282)

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