rm -rf /

February 1, 2012

It is commonly said that Solaris 10 will not allow you to issue a rm -rf / command. Few get to try it, but it was being discussed in the office today, so I thought I’d try it on a virtual machine.

The classic rm -rf / gets the message “rm of / is not allowed” and a return code of 2. Some variations get the same, whilst others (such as cd /; rm -rf .) get no message, a return code of zero, but nothing happens. Here is a transcript:

$ ssh sunflare
Last login: Tue Jan 31 21:38:05 2012 from goldie
Oracle Corporation SunOS 5.10 Generic Patch January 2005
steve@sunflare:~$ su -
Oracle Corporation SunOS 5.10 Generic Patch January 2005
# bash
root@sunflare:/# df -h .

Filesystem             size   used  avail capacity  Mounted on
/dev/md/dsk/d10        9.6G   3.6G   5.9G    38%    /
root@sunflare:/# rm -rf /
rm of / is not allowed
root@sunflare:/# echo $?
root@sunflare:/# cd /usr
root@sunflare:/usr# rm -rf ../
rm of / is not allowed
root@sunflare:/usr# cd /tmp
root@sunflare:/tmp# rm -rf ../
rm of / is not allowed
root@sunflare:/tmp# cd /usr/sfw/bin
root@sunflare:/usr/sfw/bin# rm -rf ../../..
root@sunflare:/usr/sfw/bin# cd /
root@sunflare:/# rm -rf .
root@sunflare:/# echo $?
root@sunflare:/# pwd
root@sunflare:/# df -h .

/dev/md/dsk/d10        9.6G   3.6G   5.9G    38%    /
root@sunflare:/# cd /dev
root@sunflare:/dev# rm -rf ../tmp/../
rm of / is not allowed
root@sunflare:/dev# rm -rf ../tmp/..
root@sunflare:/dev# exit
steve@sunflare:~$ logout
Connection to sunflare closed.

Shell Scripting book – out now

September 24, 2011

In case you hadn’t heard via other channels, my 564-page book, Shell Scripting: Expert Recipes for Linux, Bash and more is on sale now.

Paper books:
Amazon USA
Amazon UK
Also available at Barnes & Noble, WH Smiths, Waterstones, Supermarkets (online, if not in-store), and as the saying goes, every good bookshop (and some others too).

Kindle: Kindle

iPad / etc: iTunes

Nook: Nook

There’s also a Facebook page at facebook.com/shellscript.

Arcade Games written in a Shell Script

July 13, 2011

Yes, it is perfectly possible. Do watch this YouTube Video

Space Game

Space Game

Space Invaders – written entirely in the Bash shell, in a 248-line shell script.

The shell is often seen as old hat, hard to use, green screens and no interaction. In my new book, Shell Scripting (http://facebook.com/shellscript), I attempt to throw a new perspective on this; the shell can be used in lots of creative and imaginative ways.

The book also covers creating CSV files to easily plot complex data in spreadsheets, creating HTML as well as parsing HTML documents; I plan to post more updates before its launch on 12th August 2011.

Shell Scripting by Steve Parker. ISBN 1118024486


You can get the code for the game here. The description and explanation is in the book, of course. But at least the code itself is available online for inspection.

Shell Scripting page on Facebook

July 11, 2011

Shell Scripting

Shell Scripting

My Shell Scripting book, due out on August 12th by Wrox, now has a page on Facebook: http://www.facebook.com/pages/Shell-Scripting/175263275869249. Feel free to “Like” it, and get the latest updates on the project.

I have the final pages to proofread this week, ready to go to the printers. It’s looking like 576 pages, a little bit over the target of 504 pages, but close enough.

I will update the Table of Contents at http://sgpit.com/book/ once the page count is finalised.

Fork Bomb!

June 3, 2011

A few days ago I had to deal with my first ever real-life fork-bombed server.

By logging in to the console, I was somehow able to get a shell (one process). Having got that shell, even though I was root, it was difficult to be able to spawn other processes. It turned out that this was because we had restricted the CPU count on the kernel command line (maxcpus=2) so that a dual processor, 16-core machine had only one eighth of its processing power available. The dynamic change to the nproc value does not take this into account, so this unprivileged user was able to fork-bomb the entire machine.

The first thing you might want to do in this situation is to run ps -eaf. That’s another process, and even as root, you don’t get to do it. Being Linux, you can see how many processes exist on the system by listing /proc:
cd /proc
echo *

Neither of these commands spawn a new shell, they are both shell builtin commands, so they will work. In this case, with over 69,000 processes, I killed the output before I got too bored. Since there are usually around 200 processes running, that was enough to tell me that something was wrong.

After many attempts, a ps command did work, and confirmed that a certain shell script was being run a lot of times. I couldn’t cat that file, and didn’t even have its full name (ps truncates output to match the terminal’s width). I had the PID, so /proc/$PID/fd gave the filename.

It’s not possible to cat the script to see what it’s doing, so more builtin commands are required.
$ while read f
> do
> echo $f
> done < /path/to/script.sh

This uses all builtin commands, and tells you what the script is. From there, you may have some insight into what it is doing, and how to stop it.

Shell Scripting Recipes – Expert Ingredients for Linux, Bash and more

May 29, 2011

Another update on the upcoming Shell Scripting Recipes book. The writing is complete, the editing formatting and reviewing is now almost totally complete. In the meantime, Amazon’s UK discount has dropped to £4.80; Amazon.com is still offering a whopping $19.76 (40%) off the pre-order price. The trend seems to be that the prices are going back upwards towards the list price as the launch date (12th August 2011) comes closer so you may save more by ordering a copy now.

The full chapter list is at http://sgpit.com/book/ along with links to other pages with more details about the book.

Update on Shell Scripting Recipes book

April 23, 2011

Wow, it’s been nearly two months since I last made a post about the upcoming book on shell scripting. I’m really sorry, I had intended to give much more real-time updates here. The book focusses on GNU/Linux and the Bash shell in particular, but it does cover the other environments too – Solaris, Bourne Shell, as well as mentions for ksh, zsh, *BSD and the rest of the Unix family.

In terms of page count, it is currently 89% finished. There is still the proof-reading to be done, and whatever delivery details the publishers need to deal with, so the availability date of some time in August is still on schedule. I notice that http://amzn.com/1118024486 is already offering a massive discount on the cover price; I have no idea what that is about, I’m trying not to take offence – they can’t have dismissed the book already as I have not quite finished writing it yet! So hopefully you can get a bargain while it’s cheap.

The subject matter has the potential to be quite boring if presented as a list of tedious system administration tasks, so I have tried to make it light and fun whenever I can; it’s still with Legal at the moment, but I hope to have a Space Invaders clone written entirely in the shell published in the book. People don’t tend to see the Shell as being capable of doing anything interactive at all, so it is nice to write a playable interactive game in the shell. The main problem in terms of playability is in working out how much to slow it down, and at what stage! Of course, being a shell script, you can tweak the starting value, the level at which it speeds up, and anything else about the gameplay. If the game doesn’t make it in to the book, I’ll post it here anyway, and will welcome your contributions on gameplay.

Other than games, I’ve got recipes for init scripts, conditional execution, translating scripts into other (human) languages, even writing CGI scripts in the shell. There is coverage of arrays, functions, libraries, process control, wildcards and filename expansion, pipes and pipelines, exec and redirection of input and output; this book aims to cover pretty much all that you need to know about shell scripting without being a tedious list of what the bash shell can do.

There is a status page at http://sgpit.com/book which also has order information; you can pre-order your copy from there.


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