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 starting some program and pressing CTRL+C during execution. Most programs will stop, you could lose 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:

:) [[email protected] ~]$ 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