Kill (Unix)
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- The correct title of this article is kill. The initial letter is shown capitalized due to technical restrictions.
In Unix and Unix-like operating systems, kill is a command used to send simple messages to processes running on the system. By default, the message sent is the "termination" signal, which requests that the process exit. But kill is something of a misnomer; the signal sent may have nothing to do with process killing. The kill command is a wrapper around the kill() system call, which sends signals to processes or process groups on the system, referenced by their numeric process IDs (PIDs) or process group IDs (PGIDs). kill is always provided as a standalone utility, but most shells have built-in kill commands that may slightly differ from it.
There are many different signals that can be sent (see signal for a full list), although the signals that users are generally most interested in are SIGTERM and SIGKILL. The default signal sent is SIGTERM. Programs that handle this signal can do useful cleanup operations (such as saving configuration information to a file) before quitting. However, many programs do not implement a special handler for this signal, and so a default signal handler is called instead. Other times, even a process that has a special handler has gone awry in a way that prevents it from properly handling the signal.
All signals except for SIGKILL and SIGSTOP can be "intercepted" by the process, meaning that a special function can be called when the program receives those signals. However, SIGKILL and SIGSTOP are only seen by the host system's kernel, providing reliable ways of controlling the execution of processes. SIGKILL kills the process, and SIGSTOP pauses it until a SIGCONT is received.
Unix provides security mechanisms to prevent unauthorized users from killing other processes. Essentially, for a process to send a signal to another, the owner of the signaling process must be the same as the owner of the receiving process or be the superuser.
The available signals all have different names, and are mapped to certain numbers. It is important to note that the specific mapping between numbers and signals can vary between Unix implementations. SIGTERM is often numbered 15 while SIGKILL is often numbered 9.
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[edit] Examples
A process can be sent a SIGTERM signal in three ways (the process ID is '1234' in this case):
kill 1234kill -TERM 1234kill -15 1234
The process can be sent a SIGKILL signal in two ways:
kill -KILL 1234kill -9 1234
Other useful signals include HUP, TRAP, INT and ALRM. A SIGINT signal can be generated very simply by pressing CTRL+C in most Unix shells. It is also common for CTRL+Z to be mapped to SIGTSTP, and for CTRL+\ (backslash) to be mapped to SIGQUIT, which can force a program to do a core dump.
[edit] killall
A related program is killall. On some variations of Unix, such as Solaris, this utility is automatically invoked when the system is going through a shutdown. It behaves much like the kill command above, but instead of sending a signal to an individual process, the signal is sent to all processes on the system. However, on others such as IRIX, Linux, and FreeBSD, an argument is supplied specifying the name of the process (or processes) to kill. For instance, to kill a process such as an instance of the XMMS music player invoked by xmms, the user would run the command killall xmms. This would kill all processes named xmms.
[edit] pkill
pkill signals processes based on name and other attributes. It was introduced in Solaris 7 and has since been ported to Linux and other UNIX variants.
[edit] Windows
Windows uses the command taskkill to terminate processes.
kill is also supported in Windows through Cygwin or Microsoft Windows Services for UNIX (WSU)
[edit] See also
[edit] External links
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