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kill — terminate or signal processes


kill −s signal_name pid...

kill −l [exit_status]

kill [signal_name] pid...

kill [signal_number] pid...


The kill utility shall send a signal to the process or processes specified by each pid

For each pid operand, the kill utility shall perform actions equivalent to the kill()
function defined in the System Interfaces volume of POSIX.1‐2008 called with the following

* The value of the pid operand shall be used as the pid argument.

* The sig argument is the value specified by the −s option, signal_number option, or
the signal_name option, or by SIGTERM, if none of these options is specified.


The kill utility shall conform to the Base Definitions volume of POSIX.1‐2008, Section
12.2, Utility Syntax Guidelines, except that in the last two SYNOPSIS forms, the
signal_number and signal_name options are usually more than a single character.

The following options shall be supported:

−l (The letter ell.) Write all values of signal_name supported by the
implementation, if no operand is given. If an exit_status operand is given and
it is a value of the '?' shell special parameter (see Section 2.5.2, Special
Parameters and wait) corresponding to a process that was terminated by a signal,
the signal_name corresponding to the signal that terminated the process shall be
written. If an exit_status operand is given and it is the unsigned decimal
integer value of a signal number, the signal_name (the symbolic constant name
without the SIG prefix defined in the Base Definitions volume of POSIX.1‐2008)
corresponding to that signal shall be written. Otherwise, the results are

−s signal_name
Specify the signal to send, using one of the symbolic names defined in the
<signal.h> header. Values of signal_name shall be recognized in a case-
independent fashion, without the SIG prefix. In addition, the symbolic name 0
shall be recognized, representing the signal value zero. The corresponding
signal shall be sent instead of SIGTERM.

Equivalent to −s signal_name.

Specify a non-negative decimal integer, signal_number, representing the signal
to be used instead of SIGTERM, as the sig argument in the effective call to
kill(). The correspondence between integer values and the sig value used is
shown in the following list.

The effects of specifying any signal_number other than those listed below are

0 0








If the first argument is a negative integer, it shall be interpreted as a
signal_number option, not as a negative pid operand specifying a process group.


The following operands shall be supported:

pid One of the following:

1. A decimal integer specifying a process or process group to be signaled. The
process or processes selected by positive, negative, and zero values of the
pid operand shall be as described for the kill() function. If process number
0 is specified, all processes in the current process group shall be
signaled. For the effects of negative pid numbers, see the kill() function
defined in the System Interfaces volume of POSIX.1‐2008. If the first pid
operand is negative, it should be preceded by "−−" to keep it from being
interpreted as an option.

2. A job control job ID (see the Base Definitions volume of POSIX.1‐2008,
Section 3.204, Job Control Job ID) that identifies a background process
group to be signaled. The job control job ID notation is applicable only for
invocations of kill in the current shell execution environment; see Section
2.12, Shell Execution Environment.

A decimal integer specifying a signal number or the exit status of a process
terminated by a signal.


Not used.




The following environment variables shall affect the execution of kill:

LANG Provide a default value for the internationalization variables that are unset or
null. (See the Base Definitions volume of POSIX.1‐2008, Section 8.2,
Internationalization Variables for the precedence of internationalization
variables used to determine the values of locale categories.)

LC_ALL If set to a non-empty string value, override the values of all the other
internationalization variables.

LC_CTYPE Determine the locale for the interpretation of sequences of bytes of text data
as characters (for example, single-byte as opposed to multi-byte characters in

Determine the locale that should be used to affect the format and contents of
diagnostic messages written to standard error.

NLSPATH Determine the location of message catalogs for the processing of LC_MESSAGES.




When the −l option is not specified, the standard output shall not be used.

When the −l option is specified, the symbolic name of each signal shall be written in the
following format:

"%s%c", <signal_name>, <separator>

where the <signal_name> is in uppercase, without the SIG prefix, and the <separator> shall
be either a <newline> or a <space>. For the last signal written, <separator> shall be a

When both the −l option and exit_status operand are specified, the symbolic name of the
corresponding signal shall be written in the following format:

"%s\n", <signal_name>


The standard error shall be used only for diagnostic messages.






The following exit values shall be returned:

0 At least one matching process was found for each pid operand, and the specified
signal was successfully processed for at least one matching process.

>0 An error occurred.



The following sections are informative.


Process numbers can be found by using ps.

The job control job ID notation is not required to work as expected when kill is operating
in its own utility execution environment. In either of the following examples:

nohup kill %1 &
system("kill %1");

the kill operates in a different environment and does not share the shell's understanding
of job numbers.


Any of the commands:

kill −9 100 −165
kill −s kill 100 −165
kill −s KILL 100 −165

sends the SIGKILL signal to the process whose process ID is 100 and to all processes whose
process group ID is 165, assuming the sending process has permission to send that signal
to the specified processes, and that they exist.

The System Interfaces volume of POSIX.1‐2008 and this volume of POSIX.1‐2008 do not
require specific signal numbers for any signal_names. Even the signal_number option
provides symbolic (although numeric) names for signals. If a process is terminated by a
signal, its exit status indicates the signal that killed it, but the exact values are not
specified. The kill −l option, however, can be used to map decimal signal numbers and exit
status values into the name of a signal. The following example reports the status of a
terminated job:

if [ $stat −eq 0 ]
echo job completed successfully.
elif [ $stat −gt 128 ]
echo job terminated by signal SIG$(kill −l $stat).
echo job terminated with error code $stat.

To send the default signal to a process group (say 123), an application should use a
command similar to one of the following:

kill −TERM −123
kill −− −123


The −l option originated from the C shell, and is also implemented in the KornShell. The C
shell output can consist of multiple output lines because the signal names do not always
fit on a single line on some terminal screens. The KornShell output also included the
implementation-defined signal numbers and was considered by the standard developers to be
too difficult for scripts to parse conveniently. The specified output format is intended
not only to accommodate the historical C shell output, but also to permit an entirely
vertical or entirely horizontal listing on systems for which this is appropriate.

An early proposal invented the name SIGNULL as a signal_name for signal 0 (used by the
System Interfaces volume of POSIX.1‐2008 to test for the existence of a process without
sending it a signal). Since the signal_name 0 can be used in this case unambiguously,
SIGNULL has been removed.

An early proposal also required symbolic signal_names to be recognized with or without the
SIG prefix. Historical versions of kill have not written the SIG prefix for the −l option
and have not recognized the SIG prefix on signal_names. Since neither applications
portability nor ease-of-use would be improved by requiring this extension, it is no longer

To avoid an ambiguity of an initial negative number argument specifying either a signal
number or a process group, POSIX.1‐2008 mandates that it is always considered the former
by implementations that support the XSI option. It also requires that conforming
applications always use the "−−" options terminator argument when specifying a process
group, unless an option is also specified.

The −s option was added in response to international interest in providing some form of
kill that meets the Utility Syntax Guidelines.

The job control job ID notation is not required to work as expected when kill is operating
in its own utility execution environment. In either of the following examples:

nohup kill %1 &
system("kill %1");

the kill operates in a different environment and does not understand how the shell has
managed its job numbers.



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