# # Understanding Kill
APLawrence.com -  Resources for Unix and Linux Systems, Bloggers and the self-employed

2003/12/11 kill

I've removed advertising from most of this site and will eventually clean up the few pages where it remains.

While not terribly expensive to maintain, this does cost me something. If I don't get enough donations to cover that expense, I will be shutting the site down in early 2020.

If you found something useful today, please consider a small donation.



Some material is very old and may be incorrect today

© November 2003 Tony Lawrence
One of the first things people using a Unix machine learn is how to kill processes. Unfortunately, they usually learn the wrong thing, and use "kill -9" indiscriminately.

A better way to kill processes is to send a hangup (kill -1) or a sigterm (kill -15). Any process has the option of catching such signals and doing any final housekeeping tasks before dieing. The process can also choose to ignore you, so you may ultimately need a -9 anyway, but at least you gave it a chance to die gracefully.

The problem is that "-9" is an absolute kill - any process that is the target of that signal has to die immediately: no chance to clean up anything it might be doing, write out its file buffers or anything else: just die, right now. Unlike a "-15" or a "-1", the process itself never even knows this was done - it's the kernel that does the killing.

By the way, the "receives" doesn't happen in other cases, too. A process that is sleeping down in a driver doesn't get signals until the kernel wakes it up. That's why some processes seem unkillable:


But see TASK_KILLABLE, something new in Linux.

Kill isn't always about killing. Many processes uses signals ("signal" would have been a better name for "kill") to alert them to needed actions. "kill -1" often causes a process to re-read its configuration files, for example.

Many shells allow a "kill -0" to check if a process still exists. This can be useful where "wait" isn't what you want or where your shell didn't start the process you want to monitor. You can use that to kill off a process that might hang:

# spin off something that might hang:
myprog &
THAT=$!
sleep 300
kill -0 $THAT && echo "still running"
 

Linux "killall" is a useful program. You can "killall httpd" to kill of all running httpd processes. But watch out: some systems (SCO) have a similarly named command that has much more drastic consequences: see SCO killall.


If you found something useful today, please consider a small donation.



Got something to add? Send me email.





(OLDER)    <- More Stuff -> (NEWER)    (NEWEST)   

Printer Friendly Version

->
-> Understanding Kill


Inexpensive and informative Apple related e-books:

Take Control of Upgrading to El Capitan

Digital Sharing Crash Course

Take control of Apple TV, Second Edition

Take Control of Numbers

iOS 8: A Take Control Crash Course





More Articles by © Tony Lawrence





Printer Friendly Version

Have you tried Searching this site?

This is a Unix/Linux resource website. It contains technical articles about Unix, Linux and general computing related subjects, opinion, news, help files, how-to's, tutorials and more.

Contact us


Printer Friendly Version





The idea that Bill Gates has appeared like a knight in shining armour to lead all customers out of a mire of technological chaos neatly ignores the fact that it was he, by peddling second rate technology, led them into it in the first place, and continues to do so today. (Douglas Adams)




Linux posts

Troubleshooting posts


This post tagged:

Basics

Shell

Unix Kill



Unix/Linux Consultants

Skills Tests

Unix/Linux Book Reviews

My Unix/Linux Troubleshooting Book

This site runs on Linode