Kill: The Command to End All Commands
Suppose we had inadvertently started cat in the background; Ctrl-c would be ineffective, because the signal wouldn't get to the cat process. So we need to send it a signal some other way. With the command kill, you can send any signal to any process you own. The command's syntax is:
If no signal is specified on the kill command line, the terminate signal SIGTERM (default) is sent. This will normally terminate the process in question. If it fails to do so—that is, SIGTERM was caught or ignored—you can send the signal SIGKILL, which will always terminate the process.
Thus, we might do the following to terminate our runaway cat process commands. First determine the PID:
$ ps PID TT STAT TIME COMMAND 2037 p0 S 0:01 cat
Now, kill process 2037, which is the cat process:
kill 20371If cat had been written to catch SIGTERM, we would have to use a signal that cannot be caught or ignored.
kill -SIGKILL 20371In addition to killing errant processes, kill can be used to inform processes that the status of something has changed. For example, suppose you are writing a program and you wish to have it change its mode of operation on the occurrence of some external event. By coding what is called an “interrupt handler” in your program, you can have it catch any number of signals which have meaning to you. In particular, you might choose SIGUSR1 or SIGUSR2, which are non-specific. By sending your chosen signal, you can make your program aware of the change in circumstances, so it can proceed into its alternate mode of operation.
When you use kill, the desired signal is sent only to processes you own (that is, processes that you invoked). This prevents inadvertent termination of the wrong process. The exception is that the superuser (root) can use kill to send a signal to any process. Similarly, any process owned by root can send a signal to any other process.
An orderly shutdown of your system can occur in this way. While the kill command is not used at shutdown, the equivalent system call kill(2) is used to terminate everything. This guarantees that no files are left open and that all buffers are written to disk. For a description of kill(2), enter man 2 kill at the prompt.
A related command is killall, which takes the name of the process as an argument rather than the process ID (PID). (Some versions of kill can take process names too.) This is a convenient way to terminate all processes with the same name. If a path is used to identify the process to be signaled, only the processes executing that particular file are selected. In addition, you can ask to be consulted before killall kills a particular process, and you can receive confirmation that the signal was actually sent.
Although full details are listed in the man page, an example may be useful here. Suppose you have two programs that are different but have the same name—perhaps different release levels. In order to be different and have the same name, they must be stored in different directories. Assume they have the name sample_prog, but one is stored in /usr/a and the other in /usr/b. Entering ps gives the following output:
PID TTY STAT TIME COMMAND 123 pp0 S 0:03 /usr/a/sample_prog 124 pp1 R 0:02 /usr/b/sample_prog
The following commands perform different actions:
# To kill both processes killall sample_prog # To kill only process 123 killall /usr/a/sample_prog
In summary, the kill and killall commands can be useful tools to control the execution of processes on your Linux system. In combination with other tools described in previous “Take Command” columns, they will allow you to become true masters of a very powerful desktop appliance. For specific information on their very few options, and for a description of the signals they can invoke, read the relevant manual pages (enter man kill or man killall at the prompt).
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