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: 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 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/

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; 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 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 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 which also has order information; you can pre-order your copy from there.

Shell Scripting Recipes

March 3, 2011

This is just a heads-up that my Shell Scripting Recipes book is due out in August 2011.

I hope to publish more details here as things progress; for now, it is well on the way, but it is not too late for readers to contact me ( if there is anything that you see as vital for a Shell Scripting Recipes book which was maybe missing from some other book you saw.
Shell Scripting Recipes by Steve Parker

Part I covers Language and Usage; all of the concepts of the Shell and how it works.
Part II is Recipes using System Tools. This covers the commands that are necessary for shell scripting, and includes quite a few surprising ways to use them.
Part III is Recipes using Shell Features. This is similar to Part II but it gives concrete uses for the theory presented in Part I.
Part IV is Recipes for Systems Administration. This provides (and explains) various recipes for real-world systems administration tasks of and beyond the ordinary.

I do intend to keep you appraised of progress; you can also follow my personal blog at for more detailed updates. The RSS feed for that blog is

lsof, fuser, nohup, disown, bg, fg, and jobs

February 4, 2011

Bit of a cheeky one here – what does anybody want to know about these topics?

There is a book in the pipeline, and I have lots to say about all these things, but am very interested to hear what you think is easy / hard / intuitive / arcane / stupid about these commands and the whole job control side of Unix/Linux and the different shells.

lsof is great, but almost only GNU/Linux; fuser is good, but restricted in how much it actually tells you – you have to go digging into PIDs to see what has to be KILLed or otherwise dealt with.

What, oh faithful few who may still be following this terribly intermittent blog, do you want to see on the subject of processes and job control in the *nix shell?

2010 in review

January 2, 2011

The stats helper monkeys at mulled over how this blog did in 2010, and here’s a high level summary of its overall blog health:

Healthy blog!

The Blog-Health-o-Meter™ reads Fresher than ever.

Crunchy numbers

Featured image

A helper monkey made this abstract painting, inspired by your stats.

About 3 million people visit the Taj Mahal every year. This blog was viewed about 26,000 times in 2010. If it were the Taj Mahal, it would take about 3 days for that many people to see it.

The busiest day of the year was November 9th with 142 views. The most popular post that day was Simple Maths in the Unix Shell.

Where did they come from?

The top referring sites in 2010 were,,,, and

Some visitors came searching, mostly for bash maths, suid bit, shell script timestamp, awk one liners, and bash field separator.

Attractions in 2010

These are the posts and pages that got the most views in 2010.


Simple Maths in the Unix Shell January 2007


Timestamps for Log Files March 2007


suid shell scripts – setting “the SUID bit” April 2007


IFS – Internal Field Separator September 2007


Calculating Averages March 2007