Intro To 'kill' Command In Linux

2024-01-31 - By Robert Elder

I use the 'kill' command to kill a running process:

kill 66042

Example Use Case Of 'kill' Command

Here, I have a short video of my favourite cat:

When I click on the video file, it will open in VLC media player, except for when it doesn't. For some reason, the VLC media player process will sometimes get stuck running in the background. When this happens, it refuses to open anymore video files.

I can use the 'ps' command to find the process ID of the stuck VLC media player process, and then use the 'kill' command to force the process to end:

ps -ef | grep vlc

robert     77746    2999  2 16:29 ?        00:00:01 /usr/bin/vlc --started-from-file /cool-videos/favourite-cat.mp4
robert     77917   65790  0 16:30 pts/1    00:00:00 grep --color=auto vlc

kill -s SIGKILL 77746

Now, I can watch the video of my cat again.

Listing Signal Types

There are many different types of signals that you may want to send to a process. The most common ones are 'SIGTERM' or 'SIGKILL'. You can use the '-l' flag to see a list of different signals that you can send to a process:

kill -l

 1) SIGHUP	 2) SIGINT	 3) SIGQUIT	 4) SIGILL	 5) SIGTRAP
 6) SIGABRT	 7) SIGBUS	 8) SIGFPE	 9) SIGKILL	10) SIGUSR1
11) SIGSEGV	12) SIGUSR2	13) SIGPIPE	14) SIGALRM	15) SIGTERM
16) SIGSTKFLT	17) SIGCHLD	18) SIGCONT	19) SIGSTOP	20) SIGTSTP
21) SIGTTIN	22) SIGTTOU	23) SIGURG	24) SIGXCPU	25) SIGXFSZ
26) SIGVTALRM	27) SIGPROF	28) SIGWINCH	29) SIGIO	30) SIGPWR
31) SIGSYS	34) SIGRTMIN	35) SIGRTMIN+1	36) SIGRTMIN+2	37) SIGRTMIN+3
38) SIGRTMIN+4	39) SIGRTMIN+5	40) SIGRTMIN+6	41) SIGRTMIN+7	42) SIGRTMIN+8
43) SIGRTMIN+9	44) SIGRTMIN+10	45) SIGRTMIN+11	46) SIGRTMIN+12	47) SIGRTMIN+13
48) SIGRTMIN+14	49) SIGRTMIN+15	50) SIGRTMAX-14	51) SIGRTMAX-13	52) SIGRTMAX-12
53) SIGRTMAX-11	54) SIGRTMAX-10	55) SIGRTMAX-9	56) SIGRTMAX-8	57) SIGRTMAX-7
58) SIGRTMAX-6	59) SIGRTMAX-5	60) SIGRTMAX-4	61) SIGRTMAX-3	62) SIGRTMAX-2
63) SIGRTMAX-1	64) SIGRTMAX

SIGTERM Vs. SIGKILL

Here, I've written a busy loop program that catches the SIGTERM signal:

#include <signal.h>
#include <stdio.h>

static volatile int isRunning = 1;

void sighupHandler(int dummy) {
        isRunning = 0;
}

int main(){
        signal(SIGTERM, sighupHandler);
        while(isRunning) { }
        printf("SIGTERM caught, exiting gracefully...\n");
        return 0;
}

If I compile and run this program

gcc sigterm.c -o sigterm
./sigterm

and then send the process a SIGTERM signal:

ps -ef | grep sigterm

robert     67710   67389 99 16:02 pts/2    00:00:13 ./sigterm
robert     67737   67459  0 16:02 pts/3    00:00:00 grep --color=auto sigterm

kill -s SIGTERM 67710

the signal handler in my program will catch the SIGTERM signal and exit the program gracefully:

SIGTERM caught, exiting gracefully...

However, if I run the program again:

./sigterm

but send a SIGKILL signal instead:

ps -ef | grep sigterm

robert     67767   67389 97 16:02 pts/2    00:00:10 ./sigterm
robert     67771   67459  0 16:02 pts/3    00:00:00 grep --color=auto sigterm

kill -s SIGKILL 67767

it will not catch the signal and exit the program abruptly without being given a chance to shut down:

Killed

And that's why the 'kill' command is my favourite Linux command.

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