Module 70 kill | Illustrated Unix System V/Bsd

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Module 70
kill

DESCRIPTION

The external and internal kill command is used to send signals to processes currently executing on the system. Its name is derived from the fact that most signals will cause a process to die, thus it kills the process. Normally kill is used in conjunction with the ps command, system s, or the PID or ? variable.

COMMAND FORMAT

Following is the general format of the kill command.

 kill [ -signal ] PID ...      kill -signal -PGID ...      kill -l

BSD (Berkeley)

	`kill [ -signal ] PID ...`
	`kill -l`

C shell & Korn shell
The C shell and Korn shell have additional formats for the internal kill command if job control is supported on the system.
	kill -l
	kill [ -signal ] [ PID... ]
	kill [ -signal ] [ %n... ]

Options

The following options may be used to control how kill functions.

- signal

Specifies the signal sent to the given PIDs. You can use numbers or the corresponding names . Refer to the following table.

C shell & Korn shell

-l	Displays (lists) the signal numbers and names.
%n	Specifies a job number to kill.

	If no signal is specified, signal 15 is used. Signal 15 is the signal for software termination, which causes the program to perform internal cleanup, if it is written to do so, and then exit.

Arguments

The following arguments may be passed to the kill command.

PID ...	One or more Process IDs you wish to send a signal.
PGID ...	One or more Group Process IDs you wish to send a signal. All processes with the PGID group process ID are sent the signal.
	If the specified PID is zero, then the signal is sent to your login shell and all processes started by it. For example,

 kill 0

sends a software terminate signal to all of your processes, even your shell. The shell catches and ignores the signal. But most other commands are terminated .

FURTHER DISCUSSION

The kill command is useful for killing runaway or hung background processes. A runaway process is often caused by an infinite loop in a program. In any case the kill command can be used to remove a process from the system. A hung process may be caused by a disconnected device, such as a terminal, or printer that has hung. If a background process is waiting for input that will never arrive , you will need to kill the process.

NOTE:
Because of the way UNIX handles process signaling, it is possible to have a process hang and not be able to kill it. This is caused by the program not being able to process its signal queue. Thus when you send a signal to the process to terminate it, the process never actually receives the signal. In these situations the system administrator may have to reboot the system to remove the process.

The following table lists the signals you can send to a process. Use the -l option to list out available signals and their corresponding names on your system. The symbolic names and numbers are not the same on all systems. The symbolic names in parenthesis are aliases.

Signal Number	Symbolic Name	Description


1	HUP	Hang-up. The phone line disconnected.
2	INT	Interrupt. Pressed the Delete key.
3	QUIT	Quit. Pressed the Ctrl-\ key.
4	ILL	Illegal instruction
5	TRAP	Trace trap
6	IOT(ABRT)	IOT instruction
7	EMT	EMT instruction
8	FPE	Floating point exception
9	KILL	Kill process. Cannot be trapped
10	BUS	Bus error
11	SEGV	Segmentation violation
12	SYS	Bad argument to system call
13	PIPE	Write on a pipe with no one to read it
14	ALRM	Alarm clock
15	TERM	Software termination signal (default signal sent from kill)
16	USR1	User-defined signal 1
17	USR2	User-defined signal 2
18	CLD(CHLD)	Death of a child
19	PWR	Power fail restart
20	WINCH	Window or mouse signal
21	URG	Urgent socket condition
22	POLL(IO)	Selectable event pending (asynchronous i/o)
23	STOP	Stop process
24	TSTP	Stop process requested from tty (keyboard)
25	CONT	Continue after a stop
26	TTIN	Background process requested tty (keyboard) input; stops process
27	TTOU	Background process requested tty (terminal) output; stops process
28	VTALRM	Virtual timer alarm
29	PROF	Profiling timer alarm
30	XCPU	Exceeded cpu limit
31	XFSZ	Exceeded file size limit

NOTE:
For a list of signals handled by your system use the kill -l command. If your system does not support the -l option, then try using the command grep SIG /usr/include/*/signal.h. Remember to strip off the SIG prefix.

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