I have found a way to logout any user by using the command line. By executing the command pkill -KILL -u <username>, I can now log myself out from the session I have entered.
My question is that why there is no description about -KILL switch or option available neither in man pkill nor in pkill --help.
I am using Ubuntu Mate 16.04. Thanck you very much in advance.


The -KILL argument is telling pkill which signal to send to all of the processes identified by the other arguments. So you are sending SIGKILL which is a very very unclean way do do what you appear to be trying to do. You probably want to send SIGHUP (with -HUP) or SIGTERM (with -TERM or omitting it, as SIGTERM is the default signal for kill/pkill).

To simplify, when a process receives SIGTERM from the kernel, it is being told "you need to go away now. Clean up after yourself and self-terminate". Most well-designed programs will upon receiving this signal promptly and sanely close themselves.

When a process reveives SIGHUP, it is being told "Hey, whoever you were talking to just hung up". While this was historically used for serial connections which were prone to unexpected disconnections, this is often used as a signal to have a shell's job control to spin down all pending processes (or ofttimes for a daemon to reinitialize itself or re-parse its configuration file).

When a process receives SIGKILL, the kernel (metaphorically) walks up behind it and shoots it in the back of the head. On one hand: no fuss, no muss. On the other hand: there may be a huge mess to clean up.

A KILLed process has no chance to clean up any open temporary files; any unflushed cached data are likely to be lost, and so forth. Don't send SIGKILL unless you really have to.

  • 2
    Note that SIGTERM is not handled by commands by default. The stdio buffers for instance are not flushed by default upon SIGTERM. The commands will have to actively install a handler on SIGTERM for that. On GNU/Linux systems see ps -eo caught,pid,args | gawk -v t="$(kill -l TERM)" 'and(strtonum("0x"$1), 2**(t-1))' for a list of processes that do have a handler on SIGTERM. Jan 31 '18 at 16:31
  • note kill and pkill will send SIGTERM by default. try pkill foo before sending pkill -KILL foo.
    – quixotic
    Jan 31 '18 at 16:51
  • While it is true that SIGTERM is the default, I prefer to explicitly state which signal I wish to send when using KILL as a self-imposed sanity check. It also makes editing the command line / script to send non-TERM signals easier on later iterations if needed.
    – DopeGhoti
    Jan 31 '18 at 17:15
  • 1
    If the goal is logging out users, then really a SIGHUP should be sent before sending either of those.
    – JdeBP
    Jan 31 '18 at 17:40

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