Emptying deleted files

November 8, 2008

Mike Hommey has a nice article on clearing down deleted (but still-open) files:
http://glandium.org/blog/?p=211. He explains the problem clearly, and goes on to provide the solution, too. Well worth bookmarking, for that 2am emergency!

someone had to free some space on a 1GB filesystem, and thought a good idea would be to delete that 860MB log file that nobody cares about. Except that it didn’t really remove it, but he didn’t really check.

Later, the “filesystem full” problem came back at someone else, who came to ask me what files from a small list he could remove. But the files were pretty small, and that wouldn’t have freed enough space. That gave me the feeling that we probably were in this typical case I introduced this post with, which du -sk confirmed: 970MB used on the filesystem according to df, but only 110MB worth of data…

Mike has solutions to this for Solaris and Linux; lsof is also useful for this kind of thing, on systems which have it.


more (or less) vi

October 17, 2008

When using the more tool (this works with less, also, and less is generally more useful; you can scroll backwards and forwards with less), you can press “v” to start editing the file, in vi, at the line you are currently viewing.

This works on Linux and Solaris; please confirm on other OSes as appropriate, though I would assume that what works on more on Solaris, will work on more or less on just about any OS.

This makes using more or less far more powerful than before, and a new way into vi.


chmod syntax… be careful!

September 9, 2008

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

September 1, 2008

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

August 21, 2008

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

June 9, 2008

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

April 23, 2008

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


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