Friday, 5 February 2010

Raising elephants on a ThinkPad T42 and an EeePC 701

A magic key

If you look at your computer keyboard, you probably have a key labelled SysRq somewhere, either on its own or as a secondary function of another key such as PrtSc. You probably never use it but if you run Linux, it's a magic key that enables you to perform low level commands even when the computer is completely stuck. A particularly useful combination of such commands uses the mnemonic Raising Elephants Is So Utterly Boring.

Finding SysRq

That's the theory. In practice, it can be difficult to find out what exact key combination triggers SysRq on your particular computer and whether you need to press SysRq and the command key at the same time or whether you can press SysRq first, release and then press the command key. To find out, switch to a text virtual terminal, by pressing Ctrl+Alt+F1. You will then be presented with a full screen black console with a login prompt. Don't worry, you can switch back to your normal screen by pressing Ctrl+Alt+F7 at any time. Log in with your user name and password. For this to work, we first need to check that support for SysRq is compiled in your kernel. To do this, enter the following command:

$ cat /boot/config-`uname -r` | grep CONFIG_MAGIC_SYSRQ
CONFIG_MAGIC_SYSRQ=y

If the result is as shown above, SysRq is compiled in your kernel. Next, we need to make sure it's enabled:

$ cat /proc/sys/kernel/sysrq
1

If it is enabled, you should get 1 as an answer. If you get 0, it's not enabled, if you get anything else, it's partially enabled. The full list of values can be found in the sysrq.c documentation. If it all enabled, it should be possible to try it out by sending a command directly to the SysRq trigger:

$ sudo su -
$ echo h > /proc/sysrq-trigger

You need to be root to do this, hence the sudo su - to start with. When you do this, you should see the SysRq held text printed out to the console. After that, revert to your own user immediately to avoid any risk of doing something bad to your computer: root is very powerful and it's therefore very easy to do something bad by accident. So type this command to revert to your own user:

$ exit

Now it's a case of trying all possible key combination with the h command to find out what your SysRq key is. If you have a full SysRq key, typing Alt+SysRq+h should display the same help message as above. If that's the case, you're done. Otherwise try any number of combinations of the keys Shift, Ctrl, Alt, AltGr or Fn with one of the keys SysRq, PrtSc, Insert, Delete, ScrLk and key h until you find what combination produces the help message. Once you've found it, log out by typing exit and revert to your standard virtual terminal by pressing Ctrl+Alt+F7

SysRq on a ThinkPad T42 and an EeePC 701

On my IBM ThinkPad T42, the SysRq key combination is AltGr+PrtSc so to get the help text, I would do AltGr+PrtSc+h. On my EeePC 701, the combination is Fn+Alt+Del, which means I have to press four keys at the same time to get the SysRq help text: Fn+Alt+Del+h! Luckily it's a small keyboard. On both machines, I have to press all 3 or 4 keys at the same time.

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