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November 2019


/proc file system utilities


Miscellaneous system utilities for Linux


kill - terminate a process


kill [-signal|-s signal|-p] [-q value] [-a] [--timeout milliseconds signal] [--] 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.
pid Each pid can be one of four things:
n where n is larger than 0. The process with PID n is signaled.
0 All processes in the current process group are signaled.
-1 All processes with a PID larger than 1 are signaled.
-n where n is larger than 1. All processes in process group n are signaled. When an argument of the form ’-n’ 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 ’--’ option, otherwise it will be taken as the signal to send.
name 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.
--timeout milliseconds signal
Send a signal defined the usual way to a process. --timeout will make kill to wait for a period defined in milliseconds before sending follow-up signal to process. This feature is implemented by PID file-descriptor and guaranties that follow-up signals are sent to the same process or not sent if the process no more exist. Note that the operating system may re-use PIDs and implement the same feature in a shell by kill and sleep commands sequence may introduce a race. This option can be specified more than once than signals are sent sequentially in defined timeouts. The --timeout option can be combined with --queue option.
Example. Send signals QUIT, TERM and KILL in sequence and wait for 1000 milliseconds between the signals
kill --verbose --timeout 1000 TERM --timeout 1000 KILL --signal QUIT 12345


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:
0 success
1 failure
64 partial success (when more than one process specified)


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
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