Welcome to WebProNews Breaking eBusiness and Search News
Advertise | Newsletter | Sitemap | News Feeds News Feed 
 WebProNews Search Part of the iEntry network iEntry inc. 

Magic Sysrq

A.P. Lawrence
Expert Author
Published: 2005-05-10

WebProNews RSS Feed

The "Magic Sysrequest key" is Alt (left or right Alt key) and Sysrq (up there under Print Screen, next to F12 on most keyboards). To use it, you need to have it enabled in your kernel (CONFIG_MAGIC_SYSRQ).

It usually is; if you have a file called '/proc/sys/kernel/sysrq' you have this. To ENABLE the magic functions, you need a "1" in that file. If it has 0, Alt-SysRq just returns you to the previous console you were using.

You do NOT want this enabled on any machine that the public can walk up to. With this enabled, pressing Alt, SysRq and another key all at the same time can do things like immediately reboot the whole box - and all it takes is the keyboard, no login required. That's all three keys held down - don't release Alt-SysRq before pressing the other key (right, it's not supposed to be easy to do)

The capabilities should be documented in "man proc", but (at least on my system) are not. The "sysrq" is mentioned, but not documented. You can find documentation in /usr/src/linux-2.4.29/Documentation/sysrq.txt if you have source installed. This is from that file:

'r' - Turns off keyboard raw mode and sets it to XLATE.

'k' - Secure Access Key (SAK) Kills all programs on the current virtual console. NOTE: See important comments below in SAK section.

'b' - Will immediately reboot the system without syncing or unmounting your disks.

'o' - Will shut your system off (if configured and supported).

's' - Will attempt to sync all mounted filesystems.

'u' - Will attempt to remount all mounted filesystems read-only.

'p' - Will dump the current registers and flags to your console.

't' - Will dump a list of current tasks and their information to your console.

'm' - Will dump current memory info to your console.

'0'-'9' - Sets the console log level, controlling which kernel messages will be printed to your console. ('0', for example would make it so that only emergency messages like PANICs or OOPSes would make it to your console.)

'e' - Send a SIGTERM to all processes, except for init.

'i' - Send a SIGKILL to all processes, except for init.

'l' - Send a SIGKILL to all processes, INCLUDING init. (Your system will be non-functional after this.)

'h' - Will display help ( actually any other key than those listed above will display help. but 'h' is easy to remember :-)

The key is also self-documenting: doing Alt-Sysrq-h (or any other unrecognized key) prints out a HELP message that briefly reminds you of the above functions.

If you find that holding three keys is difficult, you do have some other choices. You can trigger it manually:

echo t > /proc/sysrq-trigger

Or, you can make the Alt-SysRq "sticky" by:

echo 1 > sysrq-sticky

With that done, you can hold Alt-SysRq, release it, and then leisurely press "t" or whatever key you wanted.

You could also change the SysRq itself in /proc/sys/kernel/sysrq-key (it's normally 84 - Alt-SysRq for Intel).

If you have this enabled, it can be useful in the case where the system has escaped your control and nothing else is working. The following sequence may be better than just hitting the power button:

Alt+SysRq+s - sync the disk
Alt+SysRq+e - try to nicely kill processes
(wait a little bit here)
Alt+SysRq+i - no more mister nice guy
Alt+SysRq+u - unmount disks
(wait a bit here, too)
Alt+SysRq+b - reboot
I can remember that with "So Everything Is Unusual - Boot!"

*Originally published at APLawrence.com

Receive Our Daily Email of Breaking eBusiness News

About the Author:
A.P. Lawrence provides SCO Unix and Linux consulting services http://www.pcunix.com

WebProNews RSS Feed

More Articles

Contact WebProNews


Targeted Information for Business
WebProNews is part of the iEntry network

Internet Business: Marketing: Small Business:
WebProNews MarketingNewz SmallBusinessNewz
WebProWorld AdvertisingDay PromoteNews
EcommNewz SalesNewz EntrepreneurNewz

Software: Search Engines: Web Design:
WebMasterFree Jayde B2B DesignNewz
NetworkingFiles SearchZA FlashNewz
SecurityConfig SearchNewz WebSiteNotes

Developer: IT Management: Security:
DevWebPro ITManagement SecurityProNews
DevNewz SysAdminNews SecurityConfig
TheDevWeb NetworkingFiles NetworkNewz

The iEntry Network consists of over 100 web publications reaching millions of Internet Professionals. Contact us to advertise.

 Advertise | Contact Us | Corporate | Newsletter | Sitemap | Submit an Article | News Feeds
 WebProNews is an iEntry, Inc. ® publication - $line) { echo $line ; } ?> All Rights Reserved
About WebProNews
WebProNews is the number one source for eBusiness News. Over 5 million eBusiness professionals read WebProNews and other iEntry business and tech publications.

WebProNews provides real-time coverage of internet business.

Free Email Newsletters:
WebProNews SearchNewz
WebProWorld DevWebPro
Marketing SecurityNews
Plus over 100 other newsletters!

Send me relevant info on products and services.

Ten most recent posts.

Featured Software

WebProNews in the News
View all recent mentions of WebProNews from around the world!

Recent Articles On ...
Google eBusiness
Yahoo Ask Jeeves
MSN Blogs
Search Engines Blogging
Affiliate Programs Marketing
eCommerce Advertising
eBay Sun Microsystems
AOL Adsense
Microsoft Adwords
Oracle IBM
Amazon Apple
SEO iPod
Adsense XBox
PR Adobe

iEntry.com WebProWorld RSS Feed WebProWorld Contact WebProNews Print Version Email a friend Bookmark us