kill(C)

kill -- terminate a process

Syntax

kill -s signame pid ...

kill -l [ exit_status ]

kill [ -signame ] pid ...

kill [ -signumber ] pid ...

Description

kill sends signals to processes. By default, it sends signal 15 (SIGTERM, software termination signal) to the processes identified by their process ID, pid. Other signals can be specified by symbolic name or number.

A signal normally kills processes that are not configured to catch or ignore it. For details of how to handle signals in the shell, see the description of the trap command on the sh(C) and ksh(C) manual pages. System programmers should refer to kill(S), sigaction(S), and signal(S).

To find the process ID of a given process, use ps(C). Additionally the shell reports the process ID of each asynchronous (background) process started with ``&'' (unless more than one process is started in a pipeline, in which case the number of the last process in the pipeline is reported).

kill recognizes the following options:

-s signame
-signame
-signumber: Specify a signal by name (signame) or number (signumber) (see ``Signals'' below).
-l: List all the signal names recognized by kill.
-l exit_status: If exit_status is the exit status of a previously executed command then kill lists the signal which terminated that process.

When determining the process or processes to send a signal to, kill interprets the value of pid as follows:

>1: Send the signal to the process whose process ID is pid.
0: Send the signal to all processes whose process group ID is equal to the process group ID of the sender (except for processes 0 and 1).
1: If the effective user ID of the sender is not root, send the signal to all processes (except processes 0 and 1) whose real user ID is equal to the effective user ID of the sender.
If the effective user ID of the sender is root, send the signal to all processes (except processes 0 and 1).
<1: If the process ID is negative but not 1, send the signal to all processes whose process group ID is equal to the absolute (positive) value of PID.

Signals

Some of the more useful values of signame are shown below (signumber is given in parentheses):

HUP (1): Hangup signal. Sent to processes when their controlling terminal hangs up the phone line, or their process group leader exits.
INT (2): Interrupt signal. If the process fails to trap the interrupt signal, it terminates; otherwise its behavior is program dependent.
QUIT (3): Quit signal. Causes the process to dump core. See core(FP) for details.
KILL (9): Kill signal. Always kills the receiving process since it cannot be ignored or caught.
TERM (15): Software termination signal. This is the default signal issued by kill.

Note that there are many other signals, most of which are generated by error conditions within the system; they are not relevant to most users. Signals are defined in /usr/include/sys/signal.h; strip off the SIG prefix before using them with kill. For further details, see sigaction(S) or signal(S).

It is advisable to kill a process with the least severe signal possible, and that child processes should be killed before parents. These practices help ensure an orderly closure of unwanted processes.

Korn shell version of kill

The version of kill built into the Korn shell accepts job IDs as arguments as well as process IDs if job control is enabled. See ksh(C) for details. Refer to jobs(C) for details of acceptable job ID formats.

Exit values

kill returns the following values:

0: successful completion: at least one matching process was found for each specified pid and the specified signal was successfully processed for at least one matching process
>0: an error occurred

Examples

To kill processes 21344 and 104:

kill 21344 104

To send signal 2 (SIGINT) to process 1091:

kill -s INT 1091

To determine the signal that terminated the process job:

   #!/bin/sh
   job
   CODE=$?
   if [ ${CODE} -eq 0 ]
   then
     echo "Job completed normally."
   elif [ ${CODE} -gt 128 ]
   then
     echo "Job terminated by SIG`kill -l ${CODE}`"
   else
     echo "Job terminated with error code ${CODE}."
   fi

Limitations

It is not always possible to kill a process. If the target process is sleeping with priority greater than or equal to 77, it ignores all signals (including KILL). See ps(C) for more information about process priorities.

Standards conformance

kill is conformant with:

ISO/IEC DIS 99452:1992, Information technology Portable Operating System Interface (POSIX) Part 2: Shell and Utilities (IEEE Std 1003.21992);
AT&T SVID Issue 2;
X/Open CAE Specification, Commands and Utilities, Issue 4, 1992.