Page 198 - DCAP602_NETWORK_OPERATING_SYSTEMS_I

Network Operating Systems-I

notes Suppose that you have a program that becomes unresponsive, how do you get rid of it? You use
the kill command, of course. Let’s try this out on xload. First you need to identify the process
you want to kill. You can use either jobs or ps to do this. If you use jobs you will get back a job
number. With ps, you are given a process id (PID). We will do it both ways:
$ xload &
[1] 1292
$ jobs
[1]+ Running xload &
$ kill %1
$ xload &
[2] 1293
[1] Terminated xload
$ ps

PID TTY TIME CMD
1280 pts/5 00:00:00 bash
1293 pts/5 00:00:00 xload
1294 pts/5 00:00:00 ps
$ kill 1293
[2]+ Terminated xload
$
While the kill command is used to “kill” processes, its real purpose is to send signals to processes.
Most of the time the signal is intended to tell the process to go away, but there is more to it than
that. Programs (if they are properly written) listen for signals from the operating system and
respond to them, most often to allow some graceful method of terminating.

Example:

A text editor might listen for any signal that indicates that the user is logging off or that the
computer is shutting down. When it receives this signal, it saves the work in progress before it
exits. The kill command can send a variety of signals to processes. Typing:
kill -l
will give you a list of the signals it supports. Most are rather obscure, but several are useful to
know:

signal # name Description
1 SIGHUP Hang up signal. Programs can listen for this signal and act (or not act) upon it.
15 SIGTERM Termination signal. This signal is given to processes to terminate them. Again,
programs can process this signal and act upon it. You can also issue this signal
directly by typing control-c in the terminal window where the program is
running. This is the default signal sent by the kill command if no signal is
specified.
9 SIGKILL Kill signal. This signal causes the immediate termination of the process by the
Linux kernel. Programs cannot listen for this signal.

192 LoveLy professionaL university