StackOverflow is an amazing site for coding questions. It was created by Joel Spolsky of joelonsoftware.com, Jeff Atwood of codinghorror.com, and some other incredibly smart guys who truly care about user experience. I have been a total fan of SO since it went mainstream and it's now a borderline addiction (you can see my StackOverflow badge on the right sidebar).

The Story

So, one day someone at StackOverflow started a "Hidden features of" post about a famous language (I don't feel like finding out which one was first exactly), and it turned out to be so popular that other posts in the same series started popping up.

Such questions were quickly turned into community wikis, for the purposes of harvesting and organizing information coming from the best developers on the planet and voted by users of the site. There are literally hundreds of answers, sorted by votes.

Hidden Features Of

Programming Languages

Hidden features of ActionScript3 / Flex

Hidden features of ASP.NET

Hidden features of x86 assembly

Hidden features of C

Hidden features of C++

Hidden features of C#

Hidden features of ColdFusion

Hidden features of D

Hidden features of Delphi

Hidden features of Erlang

Hidden features of F#

Hidden features of Flash development, Flash language (AS2/3), and Flash IDE

Hidden features of Java

Hidden features of JavaScript

Hidden features of Haskell

Hidden features of Lisp

Hidden features of Lua

Hidden features of Objective-C

Hidden features of Perl

Hidden features of PHP

Hidden features of Python

Hidden features of R

Hidden features of Ruby

Hidden features of Ruby on Rails

Hidden features of Scala

Hidden features of Silverlight

Hidden features and Dark Corners of STL?

Hidden features of TCL/TK

Hidden features of VB.Net

Hidden features of VBA

Databases

Hidden features of MySQL

Hidden features of Oracle

Hidden features of PL/SQL

Hidden features of PostgreSQL

Hidden features of SQL Server

Mobile

Hidden features of Android development

Other

Hidden features of Bash – also see my bash cheatsheet.

Hidden features of CSS

Hidden features of Django

Hidden features of Eclipse

Hidden features of Emacs

Hidden features of Grails

Hidden features of Greasemonkey

Hidden features of HTML

Hidden features of HTTP

Hidden features of jQuery

Hidden features of mod_rewrite

Hidden features of Qt

Hidden features of RegEx

Hidden features of Spring framework

Hidden features of VHDL

Hidden features of Visual Studio (2005-2008)

Hidden features of Windows.Forms

Hidden features of WPF and XAML

Hidden features of Xpath+Xslt

I will try to maintain this list, adding new languages that join the series as I find them. Now go learn something new!

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Let me briefly discuss some of the other methods first.

Limited

Using glob() (or <>, TODO verify) you can find files in a single directory, using only the limited shell wildcard support. For example,

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my @files = glob("tmp*");

Boring

File::Find is the de facto standard for searching in Perl.

This method finds files that end in .pl in "." and "../SomeDir", following symlinks:

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#!/usr/bin/perl -w
 
use File::Find;
use Data::Dumper;
use File::Basename;
my @directories_to_search = (".", "../SomeDir");
my @file_list = ();
 
find(
  { wanted =>
    sub {
      if ( basename($File::Find::name) =~ /\.pl$/i )
      {
        push @file_list, $File::Find::name;
      }
    },
    follow => 1
  },
  @directories_to_search
);
print Dumper @file_list;

It works fine, except it's horribly ugly and boring. Let's have a look at something more fun.

The Fun And Elegant Way

File::Find::Rule. Just have a look at this beauty.

Just like above, find all .pl files in "." and "../SomeDir", following symlinks:

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print Dumper (File::Find::Rule->name("*.pl")->file->extras({ follow => 1 })->
in(".", "../SomeDir"));

Same as above, except bypass .svn directories (shaves off a ton of time with a lot of directories):

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print Dumper (File::Find::Rule->not(File::Find::Rule->directory->name('.svn')->
prune->discard)->name("*.pl")->file->extras({ follow => 1 })->in(".", "../SomeDir"));

Find all .log files that are older than 24 hours in "."

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my $epoch_time_1_day_ago = time() - 60*60*24;
print Dumper (File::Find::Rule->file->name("*.log")->
mtime("<$epoch_time_1_day_ago")->in('.'));

Be sure to read the File::Find::Rule perldoc for more options and remember: have fun with your code!

Thanks to Perlbuzz and Andy Lester for pointing me to this library a few months ago.

• [Perl] How To Get The Path Of An Included Library (.pm), Regardless Of Current Directory
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While writing a 1093985th Perl script the other day I was facing the following dilemma:

• Let’s say there is a local library, called TheUberLib.pm. It is so uber that most of my scripts, located all over the machine, include it.
• Now, let’s also say that there’s an even more uberly important binary called run_me_now_or_you_will_die but the only way to find it is by using a relative path to the aforementioned TheUberLib.pm, for example ../bin (RELATIVE TO TheUberLib.pm).
• I don’t want to hardcode the path to run_me_now_or_you_will_die because it can be different on multiple machines and the code wouldn’t be robust enough – all I know is that the path is relative to an included library.

So how does one get the path to an included TheUberLib.pm (in my case to deduce the path to run_me_now_or_you_will_die), regardless of the script location and the current directory?

I started looking at all possible ways of getting this information.

• At first I looked at $0. $0 returns the path to the Perl script itself (LibraryPath/lib_path.pl) and is completely useless here.
• The FindBin module that I sometimes use was also of no help as it also deals with the caller script path.

Solution

After digging around for a bit, this is the method I came up with that did exactly what I wanted:

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use File::Basename;
my $path_to_uber_lib = dirname($INC{'TheUberLib.pm'});

and the clean way to get the final path to run_me_now_or_you_will_die is:

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my $relative_path_to_bin_dir = "../the_path_we_want/run_me_now_or_you_will_die";
my $path_to_deadly_bin = Cwd::realpath("$path_to_uber_lib/$relative_path_to_bin_dir");
print "The path to deadly bin is: $path_to_deadly_bin\n";

which prints /tmp/LibraryPath/some_path1/the_path_we_want/run_me_now_or_you_will_die.

Here is the full program with all directories that you can check out from SVN:

svn co http://beerpla.net/svn/public/Perl/LibraryPath/

or just browse the code here: http://beerpla.net/svn/public/Perl/LibraryPath/

Explanation

The %INC hash contains all included libraries, with library names as keys and full paths as values.

The dirname() function is part of File::Basename and returns just the directory part of the path.

Finally, Cwd::realpath() is a function that resolves relative and symbolic links to canonical absolute ones.

And there you have it.

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The best solution I could find to this problem to date was posted at http://www.perlmonks.org/?node_id=19119 and involved opening a piped filehandle. It worked quite well but always felt like a hack (which it was). Having used the new Perl 5.10 for a few months, I was shocked today to find this new variable that I've been dreaming about for years:

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${^CHILD_ERROR_NATIVE}

This variable gives the native status returned by the last pipe close, backtick command, successful call to wait() or waitpid(), or from the system() operator. See perlrun for details. (Contributed by Gisle Aas.)

http://search.cpan.org/dist/perl-5.10.0/pod/perl5100delta.pod#New_internal_variables

I've just tested it and it works as described. Finally!.. what else can I say?

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