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).
Practical Task Scheduling Deployment
July 20, 2016 12:00 pm CDT
One of the best things about the UNIX environment (aside from being stable and efficient) is the vast array of software tools available to help you do your job. Traditionally, a UNIX tool does only one thing, but does that one thing very well. For example, grep is very easy to use and can search vast amounts of data quickly. The find tool can find a particular file or files based on all kinds of criteria. It's pretty easy to string these tools together to build even more powerful tools, such as a tool that finds all of the .log files in the /home directory and searches each one for a particular entry. This erector-set mentality allows UNIX system administrators to seem to always have the right tool for the job.
Cron traditionally has been considered another such a tool for job scheduling, but is it enough? This webinar considers that very question. The first part builds on a previous Geek Guide, Beyond Cron, and briefly describes how to know when it might be time to consider upgrading your job scheduling infrastructure. The second part presents an actual planning and implementation framework.
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With all the industry talk about the benefits of Linux on Power and all the performance advantages offered by its open architecture, you may be considering a move in that direction. If you are thinking about analytics, big data and cloud computing, you would be right to evaluate Power. The idea of using commodity x86 hardware and replacing it every three years is an outdated cost model. It doesn’t consider the total cost of ownership, and it doesn’t consider the advantage of real processing power, high-availability and multithreading like a demon.
This ebook takes a look at some of the practical applications of the Linux on Power platform and ways you might bring all the performance power of this open architecture to bear for your organization. There are no smoke and mirrors here—just hard, cold, empirical evidence provided by independent sources. I also consider some innovative ways Linux on Power will be used in the future.Get the Guide