How to kill a Unix process

From http://www.ncl.ac.uk/iss/unix/unixhelp/kill.html and http://www.decf.berkeley.edu/help/unix/kill.html

If you are logged in to a Unix system and your session "hangs" or "freezes" (i.e. nothing happens for a long time and it does not respond to Ctrl/Q Ctrl/C or Ctrl/D), you can attempt to unfreeze it yourself by "killing" the process which has stopped your session.

You can also use Ctrl/Z to get out of a process in UNIX.

  • Login to the system where your session is frozen

  • Type the command:

    w usid

    where usid is your own username. This will produce a line of output for each login something like this: 

      9:53am  up 82 day(s), 15:58,  132 users,  load average: 6.50, 6.25, 5.70
    User     tty           login@  idle   JCPU   PCPU  what
    nabc     pts/160       9:12am                      -csh
    nabc     pts/166       9:53am                      w nabc

    Note the contents of the tty column for the frozen login -- in this example attached to pts/160.

  • Type the command

    ps -t tty

    where tty is the tty designation noted above. For example

    ps -t pts/160

    The output will be something like 

       PID TTY      TIME CMD
    20865 pts/160  0:00 elm
    18986 pts/160  0:00 csh

    Note (under PID) which of your processes issued the command which is now "hung".

  • Type the command

    kill -KILL xxxxx

    where xxxxx is the process number (PID) of the frozen process. For example

    kill -KILL 20865

Or you could just type ps -u , which will give you the pid of all the processes running/hang; and just kill them one by one whichever you like. The kill command is kill -9 .

A little explanation on that -9 thing and kill.

     The `kill' command

   Sometimes a process will ignore both of these keystroke-generated
   signals.  This will happen when a process has been `backgrounded'
   or `submitted', in which case you can use the `kill'  command  to
   abort  it.  It may also happen when your terminal is frozen in an
   unresponsive state, in which case it may  be  necessary  to  kill
   your  login process (i.e. log you out) just to return your termi-
   nal to a sensible state.

   The `kill' command has the general form "kill -N PID", where  you
   replace  `PID'  with  a process identification number and replace
   `N' with a `signal' number.  Usually, `N' will  be  `1'  and  the
   PID,  if  you  do not already know it, can be learned through the
   `ps' command as described below.  This command causes the speci-
   fied signal to be sent to the specified process.

   Signal numbers

   Signal number `1', a `hangup' signal, is recommended  because  it
   should kill the process and, if it is an editor, save the buffer.
   Signal number `9', a `kill' signal, is the surest way to  kill  a
   process  and  is  recommended only as a last resort since it will
   not save editor buffers.

   You may wish to note that the interrupt and quit (keystroke) sig-
   nals  mentioned  above  can be sent to a process by replacing `N'
   with `2' and `3', respectively.  There are 15  different  signals
   that  can be sent to a process via `kill'.  Each of them conveys
   slightly different information but generally causes death, whence
   the name of the command.

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