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Linux Command - kill

The Linux kill command is used to send signals to processes. The default signal sent is TERM.

You may or may not have more than one version of the kill command on your system. For example, one from the procps-ng package and one that is built into the shell.

Sending the TERM signal to a process will usually kill the process unless that process has logic to catch the signal.

Kill process with PID 123 ( send TERM signal ):



kill 123

Same but for four PIDS:



kill 123 543 2341 3453

Force kill a process ( -9 can’t be caught ):



kill -9  123

Note, when specifying signals by name, don’t include “SIG”:



kill -hup 150746
kill -term 150750
kill -kill 150753

Show a table of signals and signal names:



kill -L

Top 3 signals you should really know for normal sysadmin use:

This list of signal descriptions has been cobbled together from different sources. This is meant to be correct for Linux and may be different for other Unix based systems.

0   Nothing No signal sent, still does error checking, checks if proc is running, and often not listed in documentation
1 SIGHUP Terminate (“signal hang up”) means controlling terminal is closed, will cause some daemons will restart and re-read configs
2 SIGINT Terminate Interrupt, causes process to terminate, can be ignored or caught allowing for cleanup, etc. ctrl - c
3 SIGQUIT Terminate (core dump) Terminate process and create core dump ctrl + \
4 SIGILL Terminate (core dump) Sent to a process when it attempts to execute an illegal, malformed, unknown, or privileged instruction.
5 SIGTRAP Terminate (core dump) Sent to process when trap occurs - debugger wants to be informed of
6 SIGABRT Terminate (core dump) Tell process to terminate, usually sent by self using abort(), can be from other process
7 SIGBUS Terminate (core dump) Sent to process when it causes a bus error. example: incorrect memory access alignment or non-existent physical address
8 SIGFPE Terminate (core dump) Exceptional (but not necessarily erroneous) condition has been detected in the floating point or integer arithmetic hardware.
9 SIGKILL Terminate Kill, terminate immediately, can’t be caught or ignored, proc can’t cleanup, exception procs: zombie, blocked, init, uninterruptibly sleeping
10 SIGUSR1 Terminate User defined
11 SIGSEGV Terminate (core dump) Terminate (core dump) Invalid memory reference - segmentation fault - segmentation violation.
12 SIGUSR2 Terminate User defined
13 SIGPIPE Terminate Write on a pipe with no one to read it
14 SIGALRM Terminate Alarm clock
15 SIGTERM Terminate Request process termination, can be ignored or caught allowing for cleanup, etc.
16 SIGSTKFLT NA Coprocessor experiences a stack fault ( NOT ON LINUX )
17 SIGCHLD Ignore Sent when child process terminates
18 SIGCONT Continue Start process back up after it was stopped / paused
19 SIGSTOP Stop The SIGSTOP signal instructs the operating system to stop a process for later resumption. ( pause )
20 SIGTSTP Stop Terminal stop, causes process to suspend execution ctrl - z
21 SIGTTIN Stop Background process attempting read
22 SIGTTOU Stop Background process attempting write
23 SIGURG Ignore Out-of-band data is available at a socket
24 SIGXCPU Terminate (core dump) CPU time limit exceeded
25 SIGXFSZ Terminate (core dump) File size limit exceeded
26 SIGVTALRM Terminate Virtual timer expired
27 SIGPROF Terminate Profiling timer expired
28 SIGWINCH Ignore Terminal window size changed
29 SIGIO …. SIGIO is a signal that is sent when normal data, OOB data, error conditions, or just about anything happens on any type of socket.
30 SIGPWR …. sent on power failure
31 SIGSYS Terminate (core dump ) bad argument sent to system call, rare because of libraries
34 SIGRTMIN …. User defined real time signal
64 SIGRTMAX …. User defined real time signal

Listing the signals:



$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
$