The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group


kill - terminate or signal processes


kill -s signal_name pid...

kill -l

[XSI] [Option Start] kill [-signal_name] pid...

[-signal_number] pid... [Option End]


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

For each pid operand, the kill utility shall perform actions equivalent to the kill() function defined in the System Interfaces volume of POSIX.1-2017 called with the following arguments:


The kill utility shall conform to XBD Utility Syntax Guidelines , [XSI] [Option Start]  except that in the last two SYNOPSIS forms, the - signal_number and - signal_name options are usually more than a single character. [Option End]

The following options shall be supported:

(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 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.1-2017) corresponding to that signal shall be written. Otherwise, the results are unspecified.
-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.
[XSI] [Option Start]
Equivalent to -s signal_name. [Option End]
[XSI] [Option Start]
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 undefined.


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. [Option End]


The following operands shall be supported:

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.1-2017. 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 XBD 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 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:

Provide a default value for the internationalization variables that are unset or null. (See XBD Internationalization Variables for the precedence of internationalization variables used to determine the values of locale categories.)
If set to a non-empty string value, override the values of all the other internationalization variables.
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 arguments).
Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard error.
[XSI] [Option Start] Determine the location of message catalogs for the processing of LC_MESSAGES. [Option End]




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 <newline>.

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:

At least one matching process was found for each pid operand, and the specified signal was successfully processed for at least one matching process.
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.1-2017 and this volume of POSIX.1-2017 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.1-2017 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 required.

To avoid an ambiguity of an initial negative number argument specifying either a signal number or a process group, POSIX.1-2017 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.




Shell Command Language, ps, wait

XBD Job Control Job ID, Environment Variables, Utility Syntax Guidelines, <signal.h>

XSH kill


First released in Issue 2.

Issue 6

The obsolescent versions of the SYNOPSIS are turned into non-obsolescent features of the XSI option, corresponding to a similar change in the trap special built-in.

Issue 7

SD5-XCU-ERN-97 is applied, updating the SYNOPSIS.

End of informative text.


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