chmod syntax… be careful!

Ben Hutchings has noted a quirk in the chmod syntax: If you use the “chmod o-x” syntax, but omit the “o” (Others) (or the “u” (User) or “g” (Group)), it will default to “a” (All).

So “chmod -x foo” becomes “chmod a-x foo“; similarly, “chmod +x foo” becomes “chmod a+x foo“

Bash History

Interesting list of 15 examples of bash history syntax.

Most people probably know #5:
# !ps
ps aux | grep yp

I didn’t know #13, that looks really useful for arcane purposes:
# cp ~/longname.txt /really/a/very/long/path/long-filename.txt
# ls -l !cp:2
ls -l /really/a/very/long/path/long-filename.txt

Some useful Linux / *nix admin words of experience

Entitled “Lazy Linux: 10 essential tricks for admins“, this is a pretty good set of essential commands for *nix administrators.

If I have to suggest anything for in-depth study, it’s “Trick 6″: “Remote VNC session through an SSH tunnel”, because you can actually do just about anything through an SSH tunnel. If you’re faced with you -> firewall -> internet -> remotemachine -> internet -> restoftheworld, you can do whatever you like with “restof theworld” if you can ssh to “remotemachine”, whatever the local firewall thinks. And you can probably ssh over port 443 through the firewall. Hmm; maybe I should write this up properly at some point. I’m sure it’s well documented on the web if you look for it….

Bash Quiz

Network Theory have a Bash Quiz!

Out of the ten questions, one I’m not immediately sure of without checking; another I only happened to come across earlier today, and one (echo "\'")I got wrong

So, 7/10 really; 8/10 by good fortune of coming across the definition earlier today

Book

A serious publisher has contacted me about writing a serious book about Linux shell programming.

It is all really very serious. I’m not used to being serious, as you can probably tell from the fact that I have now used the word “serious” four times in this three-sentence post.

I am rather keen to write a book on the subject, not because I’m vain, or desperate for money, but because the stuff I have seen out there in dead-tree format has been of rather low quality. Also because of all the emails I’ve received over the years, they have all been positive, and none has said anything along the lines of “I didn’t need any of that because I bought Book[X]“, or indeed any book. People have emailed me, asking for advice as to what book to buy, and I have been unable to recommend any book that I have seen.

So:

What would you like to see in your ideal book about UNIX / Linux shell scripting, be it Bourne, Bash, ksh, tcsh, zsh, whatever?

Please don’t be timid; if you want to know how to work out how many nose-flutes can be fitted into the area of a Boeing 757, you won’t be anything like as strange as some of the correspondants I’ve had over the years, so please, tell me what is bugging you, what has bugged you, or even what you think might be likely to bug you in days / months / years to come.

I’m likely to answer any specific questions here and now, whether or not they end up in the book, but anything you’d like to see in a book, too… post that here, and I’ll have a stab at it.

Also, I would of course be interested to know if you have found any useful books on or around the subject, and what they did particularly well.

Steve

Happy First Birthday!

This blog has now been running for a year; the first post was Hello World on 17th Jan 2007.

I hadn’t realised it had been going for so long; in that time, I’ve made 41 posts, so I haven’t quite managed to make one post per week I have been a bit slack lately, for which I do apologise. New Years Resolution: I must make more posts here!

In the meantime, my main site, steve-parker.org, has celebrated its seventh birthday, having been born in June 2000 – looking forward to making the 8th birthday celebrations this June!

Ordering items

There are lots of small little quirks to the *nix shells; this is just one of them.

If you want to list the files in a directory, then ls will list them all for you, in alphabetical order.

If you want to list them by size, you can use ls -S; by timestamp: ls -t, and so on.

But ls is a particular utility. What happens when we do this:


for myfile in *
do
  echo "My file is called $myfile"
done

We get an alphabetically sorted list (see man ascii for the actual detail; they’re sorted by ASCII value, so numbers first, then uppercase letters, then lowercase letters).

This can be a pain, but it can also be quite useful. If you’ve got a bunch of files:

1.install.txt
2.setup.txt
3.use.txt
4.uninstall.txt

Then you can play with them in order, just by using the asterisk:

for i in *
do
  echo "File $i" >> all.txt
  cat $i >> all.txt
done

And it will sort them into order for you (“1″ comes before “2″ in ASCII, and so on…)

Or you could just do this:

more * > all.txt

Because more will prefix each file with its name in a header, if there is more than one file to process.