WEBVTT 00:01.350 --> 00:06.990 The kill command is used to kill processes, and this allows us to terminate programs that need killing. 00:06.990 --> 00:09.390 And this is some kind of pausing or termination. 00:09.390 --> 00:10.980 So here's an example here. 00:10.980 --> 00:16.890 And these are actually these are all the kill commands you can use with here, kill here. 00:17.100 --> 00:21.090 This is this is the list of commands we will use and we will discuss here. 00:21.090 --> 00:28.800 And let's actually clear and kill or actually, firstly, let's start X logo that our example with and 00:28.800 --> 00:29.520 background. 00:29.520 --> 00:34.410 So here this is our process ID and here it's actually always on top. 00:34.410 --> 00:37.350 So we first launch the x log in the background. 00:37.350 --> 00:42.570 Now let's kill this kill 71 608. 00:43.960 --> 00:46.180 Right here and be terminated. 00:47.430 --> 00:52.170 So the shell prints the job spec and the process ID of the background process. 00:53.000 --> 00:53.770 Right here. 00:53.780 --> 00:58.370 Next, we use the kill command and specify the process ID of the process we want to terminate. 00:58.400 --> 01:02.200 We could have also specified the process of job splits. 01:02.210 --> 01:05.180 For example, present one here. 01:06.780 --> 01:08.010 Always on top. 01:08.310 --> 01:08.940 Always on. 01:10.240 --> 01:10.510 Bob. 01:10.510 --> 01:17.830 And here we will write kill, kill Person one And as you can see, it will also terminate the X logo. 01:18.850 --> 01:21.960 So we will write at present one instead of the previous. 01:22.360 --> 01:26.440 So while this is all very straightforward, there is a more to it than that. 01:26.440 --> 01:31.930 So the kill command doesn't exactly kill processes, rather it sends them the signals. 01:31.960 --> 01:36.220 Signals are one of the several ways that the operating system communicates with programs. 01:36.220 --> 01:41.440 We already seen signals in action with the use of the control C and control Z. 01:41.740 --> 01:47.830 And when the terminal receives one of these keystrokes, it sends a signal to a program in the foreground. 01:48.130 --> 01:50.770 In the case of the control C right here. 01:51.930 --> 01:57.360 A signal called interrupt is sent and you will see you will learn all of this here. 01:57.360 --> 02:03.230 So with the control signal called interrupt is sent. 02:03.240 --> 02:04.950 So with control. 02:05.550 --> 02:09.360 With control Z here, control Z here. 02:09.570 --> 02:15.480 A signal called TFTP terminal stop is sent. 02:16.280 --> 02:21.410 So programs internal listen for the signals and may act upon them as they are received. 02:21.560 --> 02:28.010 The fact that program can listen and act upon signals allows a program to do things such as save work 02:28.010 --> 02:32.610 in progress when it sends a termination signal. 02:32.630 --> 02:37.040 So let's actually send the signals to process with the kill command here. 02:37.040 --> 02:38.260 So clear here. 02:38.270 --> 02:44.600 So the kill command is used to send signals in the signals to programs, Right? 02:44.870 --> 02:50.740 So it's most common syntax is going to in kill here we give it first the right kill. 02:50.750 --> 02:52.190 This is our command. 02:52.190 --> 02:54.830 And we will also specify some parameters here. 02:54.830 --> 02:57.590 So in this case, we're going to enter the signal type. 02:57.590 --> 03:02.090 We want to send signal single no here. 03:02.090 --> 03:04.880 So it's actually right here. 03:04.880 --> 03:06.770 Signal signal type. 03:06.770 --> 03:08.750 We want to send signal. 03:09.570 --> 03:13.380 And we will also enter the process ID. 03:14.680 --> 03:15.190 Right. 03:15.190 --> 03:24.880 So if no signal is specified on the command line, the if nothing if no signal specified in the command 03:24.880 --> 03:28.480 line, the termination terminate. 03:29.680 --> 03:30.070 Her. 03:35.890 --> 03:40.000 The terminate signal is sent by default. 03:40.270 --> 03:40.660 Right. 03:40.690 --> 03:42.510 The kill command is most often used. 03:42.670 --> 03:44.420 Send the signals here. 03:44.440 --> 03:47.820 I write all of these signals right down here. 03:47.830 --> 03:51.280 Most used and popular kill commands, which is whoop. 03:51.430 --> 03:52.810 This is a high gap. 03:53.050 --> 03:53.920 Now. 03:54.570 --> 03:55.500 This hoop here. 03:55.530 --> 04:01.050 This is a vestige of a good old days when terminals were attached to remote computers with phone lines 04:01.050 --> 04:01.950 and modems. 04:02.280 --> 04:10.380 So the signal is used to indicate the two programs that the controlling terminal has hung up. 04:10.470 --> 04:15.510 So the effect of this signal can be demonstrated by closing a terminal session and the foreground program 04:15.510 --> 04:18.990 running on the terminal will send the signal and it will terminate. 04:19.020 --> 04:25.350 The signal is also used by many Daemon programs to cause a reinitialization. 04:25.740 --> 04:28.100 And this means what? 04:28.100 --> 04:31.320 The daemon is set to send the signal. 04:31.320 --> 04:37.830 It will restart and reread its configuration file and the patch web server is an example of a daemon 04:38.220 --> 04:42.990 that uses the hop signal hangup signal in this way. 04:42.990 --> 04:45.200 And we also have the interrupt. 04:45.210 --> 04:49.230 So this performs the same function as the control C send from the terminal. 04:49.230 --> 04:52.710 It will usually terminate a program in an interrupt. 04:52.710 --> 05:00.750 And we also have quit actually, we will discuss quit Segway and whinge in the end of today's lecture. 05:00.750 --> 05:03.390 But firstly, we will discuss kill here right now. 05:03.390 --> 05:06.630 So the kill this this signal is special, right? 05:06.630 --> 05:12.240 So whereas programs may choose to handle signals sent to them in a different ways, including ignoring 05:12.240 --> 05:16.530 them altogether, so the kill signal is never actually ignoring them altogether. 05:17.130 --> 05:22.260 Uh, the kill is never actually sent to the target program, right? 05:22.260 --> 05:28.470 The rather than kernel here, kernel immediately terminates the program or process. 05:28.470 --> 05:35.820 So when a process is terminated in this manner, it is given no opportunity to clean up after itself 05:35.820 --> 05:36.870 or save its work. 05:36.870 --> 05:45.210 For this reason, the kill signal should be used only as a last resort when the other termination signals 05:45.210 --> 05:45.470 fails. 05:45.480 --> 05:53.820 As you can see, I have noted here that context directly with the kernel to kill this process rather 05:53.820 --> 05:56.620 than asking that please kill right. 05:56.620 --> 05:59.350 It will contact directly with the kernel. 05:59.530 --> 06:04.480 And we also have so we will discuss segue in end of this lecture. 06:04.480 --> 06:06.190 And we also have term. 06:06.490 --> 06:11.080 So this is the terminate and this is the default signal sent by the kill command. 06:11.080 --> 06:17.020 So if a program is still alive enough to receive signals, it will terminate. 06:17.020 --> 06:21.190 And we also have Continuum continue right here. 06:21.190 --> 06:26.590 So this continuum, this will restore a process after a stop or stop signal. 06:26.590 --> 06:33.190 And this is signal is sent by the uh, BG and FG command that we used in previous lectures. 06:33.190 --> 06:41.500 And we also have stop this, stop this signal causes a process to pause without terminating it. 06:41.500 --> 06:49.420 Like this is like the kill signal signal, but it is not sent to the target process and thus it cannot 06:49.420 --> 06:51.280 be ignored. 06:51.490 --> 06:54.370 And we also have terminal stop. 06:54.610 --> 06:58.540 This is the signal sent by the terminal when the control is pressed. 06:58.540 --> 07:05.140 Unlike the stop signal, the stop signal is received by the program, but the program may choose to 07:05.140 --> 07:05.680 ignore it. 07:05.680 --> 07:06.190 Right. 07:06.190 --> 07:11.410 So now you are asking the program, Please, uh, please terminate my process. 07:11.410 --> 07:13.580 But it the program can also choose. 07:13.600 --> 07:15.070 Ignore your. 07:15.850 --> 07:16.510 Option. 07:16.750 --> 07:19.120 And here we will. 07:19.120 --> 07:27.580 Now, let's actually we don't always here now let's actually start our X logo with background here and. 07:28.470 --> 07:28.730 Oops. 07:30.030 --> 07:31.140 Always on top. 07:35.120 --> 07:44.390 So here, as you can see, we will use kill minus one and we will enter our So minus actually it's parameter 07:44.390 --> 07:46.490 like we will enter with our. 07:47.670 --> 07:49.140 And these are these are the. 07:50.870 --> 07:54.110 So these are the signal IDs and these are the signal names. 07:54.110 --> 07:59.450 You can also call, call kill with the signal IDs and signal names. 07:59.660 --> 08:00.050 Same. 08:00.050 --> 08:06.170 But in this case, now I will call it with one signal ID, So which is hang up. 08:06.230 --> 08:06.920 Right. 08:06.920 --> 08:10.970 So now 75006. 08:11.730 --> 08:16.980 And here, as you can see here, hang up signal is called, as we have written here. 08:16.980 --> 08:23.280 So in this example, we start the logo program in the background and then send the Q signal with the 08:23.520 --> 08:24.710 hangup signal with a kill. 08:24.720 --> 08:31.200 So the program terminates and the shell indicates that the background process has received a hangup 08:31.200 --> 08:31.860 signal. 08:32.940 --> 08:33.300 Right. 08:33.300 --> 08:38.080 So we may need to press enter a couple of times before the message appears. 08:38.100 --> 08:43.960 Note that the signal may be specified either by a number or by name, as I said earlier. 08:43.980 --> 08:47.600 So including the name prefixed with the SIG here. 08:47.610 --> 08:51.810 So let's actually write our X logger again. 08:51.810 --> 08:55.170 Always on top and here. 08:56.050 --> 08:57.710 We can repeat this process, right? 08:57.730 --> 09:00.950 Kill it here, which is interrupt. 09:00.970 --> 09:02.860 Let's actually see what interrupt is. 09:02.890 --> 09:04.000 Interrupt. 09:04.000 --> 09:04.390 Interrupt. 09:04.390 --> 09:05.420 The same as the control here. 09:05.470 --> 09:05.920 Right. 09:05.920 --> 09:08.280 So interrupt 75. 09:08.290 --> 09:14.830 We will enter our process ID 757075708. 09:15.260 --> 09:18.090 You know, as you can see here, interrupt signal is sent. 09:19.020 --> 09:23.970 And let's start another example here and always on top here. 09:23.970 --> 09:26.310 Now we will send a sig integer. 09:26.520 --> 09:27.840 Sig sg interrupt. 09:29.530 --> 09:32.050 Here, which is you will learn here, right? 09:32.530 --> 09:33.970 So now we will. 09:34.990 --> 09:35.420 Heil. 09:36.250 --> 09:36.810 Sieg. 09:37.060 --> 09:39.700 Interrupt, and we will enter. 09:39.700 --> 09:42.970 Seven, five, nine, six, five. 09:43.180 --> 09:45.850 And here we also set an interrupt. 09:45.850 --> 09:46.960 And let's see. 09:46.990 --> 09:48.730 Sieg interrupt here right now. 09:48.730 --> 09:49.180 Right. 09:49.180 --> 09:52.990 So repeat this example and try other signals as well. 09:52.990 --> 10:00.280 And remember, we can also use the job space specs in place of process IDs and processes is like files, 10:00.280 --> 10:00.610 right? 10:01.060 --> 10:08.770 They have owners and you must be owner of the process or the superuser to send it signals with kill. 10:09.010 --> 10:16.060 In addition, the list of the list of the signals covered so far, which are most often used with kill. 10:16.060 --> 10:22.780 And there are other signals that I introduced in the start of this lecture that I told you that we will 10:22.780 --> 10:27.490 also discuss about quit CV and whinge. 10:28.880 --> 10:31.460 So the quit is basically quit here. 10:31.460 --> 10:33.800 And the segue V here, this is interesting. 10:33.800 --> 10:37.490 This is a segmentation violation and the signal is sent. 10:37.490 --> 10:41.270 If a program makes illegal use of a memory. 10:41.270 --> 10:49.550 That is if I if if program tried to write somewhere, it wasn't allowed to write and vincere will. 10:49.550 --> 10:51.380 Lastly, this is a Windows change. 10:51.380 --> 10:55.580 This is the signal sent by the system when a window changes size. 10:55.580 --> 11:03.620 So some programs such as top here, as you can see here and less will respond to the signal by redrawing 11:03.620 --> 11:09.350 themselves to fit the new window dimensions here, as you can see here. 11:12.800 --> 11:16.610 As you can see here, nothing is breaking when I. 11:17.420 --> 11:18.920 Change the window dimensions here. 11:18.920 --> 11:20.870 It will look just fine. 11:22.160 --> 11:28.490 And for the if you are curious about the more skilled commands, you can also display a complete list 11:28.490 --> 11:32.450 of the kill commands with this command right here.