Perl : Optimizing pattern searches with Regexp::Assemble

When you perform a pattern matching with multiple “or” (|) clauses, such as /pattern1|pattern2|pattern3/, Perl regexp engine will try to match each of them one after the other in sequence, resulting in poor performance if you have a long list of “or” clauses.

In order to optimize such a pattern matching, you can use the Regexp::Assemble module.

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Linux : Taking control of Virtual Terminals (VT) from command line

When you use Linux in text mode (as opposed to with an X server), you readily have access to multiple Virtual Terminals (aka VT for short) by hitting one of your <Alt-Fn> keys (if you are running a X server, you’ll need to hit <Ctrl-Alt-Fn> simultaneously).

This lets you access one of the VTs which are initialized at boot time, but won’t let you create new ones even if your kernel configuration would allow more VTs. Furthermore, what if you want to deal with VTs from a script ?

This post covers the 3 commands which will let you control your VTs from the command line or from a script.

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Unix : the “script” command

The script command is a must for any unix sysadmin.

Once invoked, it will faithfully write anything you typed as well as any output generated in your terminal into a file of your choice (defaults to “typescript”).

This is great when you want to document everything you did on a specific server, for example.
spaghetti:~$ script
Script started, file is typescript

When launched, you don’t see anything, but everything displayed goes to a file as well as the terminal.

As usual see the man page for details (like appending instead of creating a new file, launching another command instead of your default shell, and so on)

sed : replacing a text in a file

To replace a text in a file, you can invoke sed as in the following example :

% cat file.txt | sed -e 's/text/replacement/g' > result.txt

This will change all the occurences of “text” to “replacement” in “file.txt” and output the result in “result.txt”

Note : As suggested by Matthias from adminlife in the comments, if you wanted to do “in place” text replacement (that is modify the file without a temporary file in between), you can do the following :

sed -i s/text/replacement/g file.txt

For more complicated text manipulations you might consider moving to Perl, but sometimes you don’t need the sledge-hammer 🙂

CFEngine – Installing on Debian GNU/Linux

In this post we’ll install CFEngine on a Debian system. Debian make is really simple to install any packages, so let’s follow the “standard” package installation procedure (I’ll assume that apt is correctly setup on your system ! If you have troubles with it, let me know, I’ll write a post on this topic).
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Keeping track of changes with cfengine and SubVersioN

Cfengine is a tool which purpose is to describe what is a healthy system and how to bring it back to normal when something fails.

I won’t go into an explanation about how cfengine works, because the project webpage already has a neat tutorial and complete reference. Instead of that, I’ll explain how I used cfengine to build a fool proof Linux firewall.

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