Blog Posts

Fun with Find

I've had occasion to need to grab a specific set of files from a large directory — most recently, I needed to grab some specific access logs from our Apache logfiles at work.

Enter find.

I needed to get all files newer than a specific date, and with the pattern 'sitename-access_log.timestamp.gz'. I then needed to tar up these files and grab them for processing. So, here's what I did:

  • The -newer filename tells find to locate files newer than filename.
  • The -regex flag tells find to locate files matching the regular expression. The regex that find uses is a little strange, however, and didn't follow many conventions I know; for one thing, it's assumed that the pattern you write will match against the entire string, and not just a portion of it. What I ended up using was -regex '.*access_log.*gz', and that worked.
  • The -printf flag tells find to format the printing. This is useful when using the output of find in another program. For instance, tar likes a list of filenames… so I used -printf "%p ", which separated each filename with a space.

I then backticked my full find statement and used it as the final argument to a tar command; voila! instant tar file with the files I need!

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conditional use in perl

I've been struggling with how to use modules at runtime instead of compile time (I even wrote about this once before). I finally figured it out:

my $module = "ROX::Filer";
eval "use $module";
die "couldn't load module : $!n" if ($@);

Now I just need to figure out how to create objects from dynamic module names…!

Update: Creating objects from dynamic names is as easy as dynamically loading the module at run-time:

my $obj = $module->new();

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Where's that module?

One continual pain for me with perl is when I need to try to find the location of a specific module on my filesystem so that I can examine it myself; I end up first having to find out what my @INC path is, then having to dig through it until I find the module. Fortunately, I'm not the only one; somebody posted a solution to this problem on Perl Monks:

Updated: The original listing presented didn't work! The following one, garnered from a comment to the original PM post, does, and is what I'm now using.

#!/usr/bin/perl -w
use strict;

use File::Spec::Functions qw/catfile/;

my @loaded = grep {
    eval "require $_";
    !$@ ? 1 : ($@ =~ s/(@INC contains: Q@INCE)//, warn ("Failed loading $_: $@"), 0);
} @ARGV;

my @pm = map catfile(split '::') . (/.pmz/ ? '' : '.pm'), @loaded;

print "@INC{@pm}n";


=head1 NAME

whichpm - lists real paths of specified modules


  editor `whichpm Bar`


Analogous to the UN*X command which.


Just place it in your $PATH and let 'er rip!

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I was reading a thread on the cgiapp mailing list today from several of the core developers about developing a book on CGI::Application. In it, several mentioned that it might/should center around CGI::App and a handful of oft-used modules. One of those modules is Class::DBI.

I took a gander at Class::DBI over at CPAN, and it looks absolutely amazing, and at the same time perhaps too abstract. Basically, you create a number of packages and/or packages, one for each table you'll be using in your application, and one to establish your basic connection. Then, each package creates an object instance of the connection, and defines a number of properties: the name of the table, the columns you'll be using, and then the relations it has to other tables ( has_a( col_name => 'Package::Name'); has_many( col_name => 'Package::Name'); might_have(col_name => 'Package::Name'); ) etc.

Then you use the module/packages you need in your script, and you can then use object-oriented notation to do things like insert rows, update rows, search a table, select rows, etc. And it looks fairly natural.

I like the idea of data abstraction like this. I see a couple issues, however:

  1. I don't like the idea of one package per table; that gets so abstract as to make development come to a stand-still, especially during initial development. However, once development is sufficiently advanced, I could see doing this, particularly for large projects; it could vastly simplify many regular DBI calls.
  2. I like using SQL. If I need to debug why something isn't working when I interact with the database, I want to have absolute control over the language. Abstracting the SQL means I don't have that fine-grained control that helps me debug.

So, for now, I'll stick with straight DBI…. but this is an interesting avenue to explore.

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Ctrl-S and Ctrl-Q in *nix systems

I just ran into this not long ago, and wish I'd discovered it years ago. Basically, Ctrl-S suspends a process, while Ctrl-Q resumes it. This is useful when in g/vim or screen and you manage to lock up your application because you accidently hit Ctrl-S when reaching for another key combo.

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use autouse ... or not

Due to my cursory reading in the Perl Cookbook, 2nd Edition, earlier this week, I've been investigating the use autouse pragma, to see if it will indeed solve my issue of wanting to use different modules based on the current situation. Unfortunately, I cannot find any documentation on it in perldoc.

I remember seeing something about wrapping this stuff into a BEGIN block, but that would require knowing certain information immediately, and I might need the code to work through some steps before getting there.

Fortunately, this node just appeared on Perl Monks today, and I got to see other ways of doing it:

  • The if module lets you do something like use if $type eq 'x', "Some::Module"; However, $type must be known at compile time (i.e., it's based on system info or on @ARGV); this probably wouldn't work in a web-based application.
  • Use require and import instead: if $type wq 'ex') { require Some::Module; Some::Module->import if Some::Module->can("import"); } If your module doesn't export anything, you can even omit the call to import.
  • Use an eval: if ($type eq 'x') { eval "use Some::Module"; } This gets around the import problem, but could possibly run into other compile time issues.

So, basically, I already had the tools to do the job; just needed to examine the problem more.

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More CGI::App research... Try the manual!

So, I'm a bit of an idiot… it's been so long since I looked at CGI::App, and yet I felt I had such a grasp on it, that I overlooked the obvious step: look at the manual!

In particular, there's a whole series of methods that are used to tailor CGI:App to your particular needs, and these include cgiapp_init(), cgiapp_prerun(), and cgiapp_postrun().

  • cgiapp_init() is used to perform application specific initialization behaviour, and is called immediately before the setup() method. It can be used to load settings from elsewhere; if it were called only from a superclass from which other modules inherited, it would then provide common settings for all modules.
  • cgiapp_prerun() is called immediately before the selected run-mode. If it were called only by your superclass, you could perform items such as authorization or even form validation; this would then be standard for all your applications. (You can use the $self->prerun_mode('mode') call to to override the selected run-mode, for instance, thus allowing you to redirect to a different mode if a user isn't permitted there.)
  • cgiapp_postrun() is called after the run-mode has returned its output, but before http headers have been generated or anything sent to the web browser. Again, if defined in a superclass, it means that you could then place the run-mode output in a specific place within a larger template, and even call other routines to fill in other parts of the main template. You could even check to see if certain parameters were passed to the page, and change the type of output you send back (XML, PDF, image, etc.), allowing you to have a common query element that changes the output type (e.g., a 'print' parameter that returns a PDF or a stripped down template).

In addition, you could specify in the superclass that you're using CGI::Simple for the query object (using the cgiapp_get_query method), or you could rewrite the load_tmpl() method to use Template::Toolkit or some other templating system, etc.

Doesn't look so crazy anymore…

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CGI::Application Research

I've been wanting to redevelop my home website for some time using CGI::Application. The last time I rewrote it from PHP to perl, I developed something that was basically a subset of the things CGI::App does, and those things weren't done nearly as well.

The problem I've been running into has to do with having sidebar content, and wanting to run basically a variety of applications. I want to have a WikiWikiWeb, a photo gallery, some mail forms, and an article database/blog; CGI::App-based modules for each of these all exist. But I want them all to utilize the same sidebar content, as well — and that sidebar content may vary based on the user.

My interest got sparked by this node on Perl Monks. The author tells of an acquaintance who goes by the rule that a CGI::App should have 10-12 states at most; more than that, and you need to either break it apart or rethink your design. And all CGI::Apps inherit from a common superclass, so that they share the same DB connections, templates, etc.

So, I've been investigating this problem. One node on PM notes that his ISP uses CGI::App with hundreds of run modes spread across many applications; they created a module for session management and access control that calls use base CGI::Application; each aplication then calls use base Control, and they all automatically have that same session management and access, as well as CGI::Application.

Another node mentions the same thing, but gives a little more detail. That author writes a module per application, each inheriting from a super class:,,,, etc. You create an API for that super class, and each CGI::App utilizes that API to do its work.

This also seems to be the idea behind CheesePizza, a CGI::App-based framework for building applications. (All pizzas start out as cheese pizzas; you simply add ingredients.) The problem with that, though, is that I have to learn another framework on top of CGI::App, instead of intuiting my own.

But how do I write the superclass? Going back to the original node that sparked my interest, I found a later reply that described how you do this. The big key is that you override the print method — this allows you to customize the output, and from here you could call functions that create your sidebar blocks, and output the content of the CGI::App you just called in a main content area of your template.

Grist for the mill…

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One thing I've wondered about is the syntax of the robots.txt file, where it's placed, and how it's used. I've known that it is used to block spiders from accessing your site, but that's about it. I've had to look into it recently because we're offering free memberships at work, and we don't want them indexed by search engines. I've also wondered how we can exclude certain areas, such as where we collate our site statistics, from these engines.

As it turns out, it's really dead simple. Simply create a robots.txt file in your htmlroot, and the syntax is as follows:

User-agent: *
Disallow: /path/
Disallow: /path/to/file

The User-agent can specify specific agents or the wildcard; there are so many spiders out there, it's probably safest to simply disallow all of them. The Disallow line should have only one path or name, but you can have multiple Disallow lines, so you can exclude any number of paths or files.

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More SSH tips: Tunnelling

I wrote up a short tutorial today on the IT wiki about SSH tunneling. What I didn't know is that you can start a tunnel after you've already ssh'd to another machine. Basically, you:

  • Press Enter
  • Type ~C

and you're at an ssh> prompt. From there, you can issue the tunnel command of your choice: -R7111:localhost:22, for instance.

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