How To Kill Stuff On Linux

You are happily working on your linux box when all of a sudden something freezes and becomes unresponsive. What do you do? Let’s talk about killing today!

img © Rossy ℗∑ñℊ⊍ïñ found @ evilpenguins.tribe.net

Everything really depends on situation. If it’s just a single application that decided to stop responding you can simply kill it. Now, you may think that killing is a bit harsh punishment for not talking to you – and it is. Best practice is to wait a little bit, and the app may un-freeze itself eventually. Chances are that some blocking process seized up the focus, and rendered GUI unusable for a little while. Then again, applications sometimes just freeze or get themselves into a runaway loop. And then there is not much you can do but send them SIGTERM or SIGKILL signal using kill or killall. The former takes the PID of the process as an argument, while the latter takes a name of the binnary and additionally tries to kill the whole process tree which is usually (but not always) a good idea. Note that both commands send SIGTERM by default which is essentially equivalent to nicey asking them to drop dead. If you use -9 you no longer ask – you shiv them in the heart with a rusty spork and watch them bleed to death on your new carpet. And by that I mean data loss, and other unexpected side effects so use it carefully. RTFM for all the other fun signals you can send to your processes.

But how do I kill something if I don’t even know whats it’s pid or the name of the binary?

Good question, voice in my head. You always bring up these interesting things. There are three ways to deal with this problem. First one is to STFU, stop being such a luser and use ps, and top to figure out which application is acting up and then -9 it like a man. But if you can’t handle that, there are two other remaining alternatives for the whussies: pkill, and xkill.

I mentioned pkill before, so I’m not going to repeat myself. All you need to know is that pkill is a smart fellow. He went to ninja school and is sneaky enough to figure out that when you tell it to kill firefox, you really mean firefox-bin. By that logic Kill and killall are kinda like mob hitmen – effective, but dumb as bricks. You have to tell them exactly who to hit, and how.

Xkill is kill’s graphically inclined cousin who specializes in killing windowed applications. When you call him, he appears as a stylish (or tacky, depending on your taste) black skull and bones cursor. Kinda like an inverse Jolly Roger. If pkill is a ninja, then xkill is a pirate of the bunch. Think Jack Sparrow – can be quite destructive if aimed well, but tends to blow shit up if you underestimate him (ie. click in the wrong place). Point him at your frozen window, hit LMB and he will do the rest. Note that if you change your mind you can cancel it by clicking anywhere with RMB.

What if my whole X seems to be locked up?

Easy – swith to a TTY using Ctrl+Alt+F1 (or any function key from F2 to F6 for that matter)! Then use top/htop or whatever strokes your fancy to figure out which app is hugging all the CPU and eating all the memory. Kill it and chances are that all the other software will be given a breath of fresh air and will resume normal function. You can switch back to X with Ctrl+Alt+F7.

If that doesn’t work, and you don’t mind loosing some data, you can give X the good old 3 finger salute. And no, it’s not that one. The old Ctrl+Alt+Del salute remains a fond memory of Win-DOS 95 users whose machines had those awesome reset buttons on the case, that helped them to recover from frequent BSOD’s. Reset buttons are a thing of the past, and so is the Ctrl+Alt+Del which acts as a key shortcut to bring up task management console, or logout dialog on most modern systems. Let’s face it – pressing this key combination no longer feels like saying “Fuck You OS, eat shit and DIEEE!”.

The 3 finger salute I’m talking about is Ctrl+Alt+Backspace – and it’s the key combination that will kill your current X session. And it will give you as much gratification, and therapeutic frustration relief as Ctrl+Alt+Del in the good old times but without actually hosing the filesystem or making the system unstable. Think of it as a forced logout combined with a restart of the X server. 98% of the time this will do the trick and let you resume work quickly and easily.

What if my system is frozen solid?

There is still a way to restart it gracefully using the magic SysRq key combo. Simply hit Alt+PrintScrn (which is how you get SysRq key) and then type in R E I S U B with… Your nose… Or other part of your anatomy. This will trigger a nice reboot that will not corrupt any of your data, destroy the file system or wreck any other crazy damage. In fact each of these keys issues a special SysRq command:

R – switch the keyboard mode
E – send all processes the SIGTERM singal
I – sends all processes the SIGKILL signal
S – Sync filesystem
U – Unmount all devices
B – Reboot immediately

Theoretically you could just do SysRq+B to do an old school, Win 95 style Alt+Ctrl+Del reboot, but it’s just as bad as hitting that power button. Your best bet is to run the whole sequence to ensure that the system shuts down in the way it would normally shut down when you issue a reboot command.

And it’s almost guaranteed to work even on a completely locked up system. That is, if it’s enabled on your system. If you are not sure, do a really quick test right now:

cat /proc/sys/kernel/sysrq

If you see a 1, you are in good shape. If you see a zero, you can enable the SysRq keys by doing:

sudo echo 1 > /proc/sys/kernel/sysrq

Better do it now before you somehow hose your system and need to use it. It’s kinda hard to issue commands when nothing responds to you.

How the hell do you expect me to remember this REISUB thing?

Mnemonics my friend. That’s how you do it. The popular one circulating out there is Rising Elephants Is So Utterly Boring. If you are more of a visual person, I have another one for you. Observe:

Get it? It’s REI Ayanami from Neon Genesis Evangelion and a Subway SUB. It’s easier to remember if you are a fan of the show. Just think about Rei eating a sub and you are good to go. :)

So there you go – plenty of ways to kill, maim and slaughter your applications in many ways. And best of all – most of these are relatively safe, and won’t break shit. At least for the most part. Remember kids – kill responsibly.

[tags]kill, killall, pkill, xkill, reisub, sysrq, rei ayanami, subway, killing, linux, unix, process[/tags]