Section (1) kill

Linux manual pages Section 1  


kill — terminate a process


kill [ signal | −s signal | −p ] [ −q value ] [−a] [−−] pid | name...

kill −l [number] | −L


The command kill sends the specified signal to the specified processes or process groups.

If no signal is specified, the TERM signal is sent. The default action for this signal is to terminate the process. This signal should be used in preference to the KILL signal (number 9), since a process may install a handler for the TERM signal in order to perform clean-up steps before terminating in an orderly fashion. If a process does not terminate after a TERM signal has been sent, then the KILL signal may be used; be aware that the latter signal cannot be caught, and so does not give the target process the opportunity to perform any clean-up before terminating.

Most modern shells have a builtin kill command, with a usage rather similar to that of the command described here. The −−all, −−pid, and −−queue options, and the possibility to specify processes by command name, are local extensions.

If signal is 0, then no actual signal is sent, but error checking is still performed.


The list of processes to be signaled can be a mixture of names and PIDs.


Each pid can be one of four things:


where n is larger than 0. The process with PID n is signaled.


All processes in the current process group are signaled.


All processes with a PID larger than 1 are signaled.


where n is larger than 1. All processes in process group n are signaled. When an argument of the form _zsingle_quotesz_-n_zsingle_quotesz_ is given, and it is meant to denote a process group, either a signal must be specified first, or the argument must be preceded by a _zsingle_quotesz_--_zsingle_quotesz_ option, otherwise it will be taken as the signal to send.


All processes invoked using this name will be signaled.


−s, −−signal signal

The signal to send. It may be given as a name or a number.

−l, −−list [number]

Print a list of signal names, or convert the given signal number to a name. The signals can be found in /usr/include/linux/signal.h

−L, −−table

Similar to −l, but it will print signal names and their corresponding numbers.

−a, −−all

Do not restrict the command-name-to-PID conversion to processes with the same UID as the present process.

−p, −−pid

Only print the process ID (PID) of the named processes, do not send any signals.


Print PID(s) that will be signaled with kill along with the signal.

−q, −−queue value

Use sigqueue(3) rather than kill(2). The value argument is an integer that is sent along with the signal. If the receiving process has installed a handler for this signal using the SA_SIGINFO flag to sigaction(2), then it can obtain this data via the si_sigval field of the siginfo_t structure.


Although it is possible to specify the TID (thread ID, see gettid(2)) of one of the threads in a multithreaded process as the argument of kill, the signal is nevertheless directed to the process (i.e., the entire thread group). In other words, it is not possible to send a signal to an explicitly selected thread in a multithreaded process. The signal will be delivered to an arbitrarily selected thread in the target process that is not blocking the signal. For more details, see signal(7) and the description of CLONE_THREAD in clone(2).


kill has the following return codes:






partial success (when more than one process specified)


bash(1), tcsh(1), sigaction(2), kill(2), sigqueue(3), signal(7)


Salvatore Valente

Karel Zak

The original version was taken from BSD 4.4.


The kill command is part of the util-linux package and is available from Linux Kernel Archive

  Copyright 1994 Salvatore Valente (
Copyright 1992 Rickard E. Faith (
May be distributed under the GNU General Public License

Section (2) kill

Linux manual pages Section 2  


kill — send signal to a process


#include <sys/types.h>
#include <signal.h>
int kill( pid_t pid,
  int sig);
[Note] Note
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):


The kill() system call can be used to send any signal to any process group or process.

If pid is positive, then signal sig is sent to the process with the ID specified by pid.

If pid equals 0, then sig is sent to every process in the process group of the calling process.

If pid equals −1, then sig is sent to every process for which the calling process has permission to send signals, except for process 1 (init), but see below.

If pid is less than −1, then sig is sent to every process in the process group whose ID is −pid.

If sig is 0, then no signal is sent, but existence and permission checks are still performed; this can be used to check for the existence of a process ID or process group ID that the caller is permitted to signal.

For a process to have permission to send a signal, it must either be privileged (under Linux: have the CAP_KILL capability in the user namespace of the target process), or the real or effective user ID of the sending process must equal the real or saved set-user-ID of the target process. In the case of SIGCONT, it suffices when the sending and receiving processes belong to the same session. (Historically, the rules were different; see NOTES.)


On success (at least one signal was sent), zero is returned. On error, −1 is returned, and errno is set appropriately.



An invalid signal was specified.


The calling process does not have permission to send the signal to any of the target processes.


The target process or process group does not exist. Note that an existing process might be a zombie, a process that has terminated execution, but has not yet been wait(2)ed for.


POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.


The only signals that can be sent to process ID 1, the init process, are those for which init has explicitly installed signal handlers. This is done to assure the system is not brought down accidentally.

POSIX.1 requires that kill(−1,sig) send sig to all processes that the calling process may send signals to, except possibly for some implementation-defined system processes. Linux allows a process to signal itself, but on Linux the call kill(−1,sig) does not signal the calling process.

POSIX.1 requires that if a process sends a signal to itself, and the sending thread does not have the signal blocked, and no other thread has it unblocked or is waiting for it in sigwait(3), at least one unblocked signal must be delivered to the sending thread before the kill() returns.

Linux notes

Across different kernel versions, Linux has enforced different rules for the permissions required for an unprivileged process to send a signal to another process. In kernels 1.0 to 1.2.2, a signal could be sent if the effective user ID of the sender matched effective user ID of the target, or the real user ID of the sender matched the real user ID of the target. From kernel 1.2.3 until 1.3.77, a signal could be sent if the effective user ID of the sender matched either the real or effective user ID of the target. The current rules, which conform to POSIX.1, were adopted in kernel 1.3.78.


In 2.6 kernels up to and including 2.6.7, there was a bug that meant that when sending signals to a process group, kill() failed with the error EPERM if the caller did not have permission to send the signal to any (rather than all) of the members of the process group. Notwithstanding this error return, the signal was still delivered to all of the processes for which the caller had permission to signal.


kill(1), _exit(2), pidfd_send_signal(2), signal(2), tkill(2), exit(3), killpg(3), sigqueue(3), capabilities(7), credentials(7), signal(7)


This page is part of release 5.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at−pages/.

  Copyright (c) 1992 Drew Eckhardt (, March 28, 1992

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Modified by Michael Haardt <>
Modified by Thomas Koenig <>
Modified 1993-07-23 by Rik Faith <>
Modified 1993-07-25 by Rik Faith <>
Modified 1995-11-01 by Michael Haardt
Modified 1996-04-14 by Andries Brouwer <>
 [added some polishing contributed by Mike Battersby <>]
Modified 1996-07-21 by Andries Brouwer <>
Modified 1997-01-17 by Andries Brouwer <>
Modified 2001-12-18 by Andries Brouwer <>
Modified 2002-07-24 by Michael Kerrisk <>
Added note on historical rules enforced when an unprivileged process
sends a signal.
Modified 2004-06-16 by Michael Kerrisk <>
    Added note on CAP_KILL
Modified 2004-06-24 by aeb
Modified, 2004-11-30, after idea from