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The 3 most important "kill" signals on the Linux/UNIX command line

Most Linux or UNIX users know that there is a kill(1) command to stop processes, but what are the options, what do they mean?

These options are called signals, which can be expressed in either numbers or words. Some known once are "-1" or "-HUP". Also well known is "-9" (aka "-KILL")

  • -1 or -HUP - This argument makes kill send the "Hang Up" signal to processes. This probably originates from the modem/dial-in era. Processes have to be programmed to actually listen to this process and do something with it. Most daemons are programmed to re-read their configuration when they receive such a signal. Anyway; this is very likely the safest kill signal there is, it should not obstruct anything.
  • -2 or -SIGINT - This is the same as stating some program and pressing CTRL+C during execution. Most programs will stop, you could loose data.
  • -9 or -KILL - The kernel will let go of the process without informing the process of it. An unclean kill like this could result in data loss. This is the "hardest", "roughest" and most unsafe kill signal available, and should only be used to stop something that seems unstoppable.
  • -15 or -TERM - Tell the process to stop whatever it's doing, and end itself. When you don't specify any signal, this signal is used. It should be fairly safe to perform, but better start with a "-1" or "-HUP".

To figure out how a program should respond to kill signals, check the man(1) page of the process you are trying to kill. For example the process init(8) has a man-page that explains:

SIGHUP
            Has the same effect as telinit q.
...
       Q or q tell init to re-examine the /etc/inittab file.

By the way, there are many signals:

:) [robertdb@node1 ~]$ 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     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

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References Red Hat Certified Architect By Robert de Bock Robert de Bock
Curriculum Vitae By Fred Clausen +31 6 14 39 58 72
By Nelson Manning robert@meinit.nl