Learning to Love Linux - Module 4 - Perl

MODULE 4: Scripting with Perl

Introduction

Perl is a very powerfull scripting language, that is used in many occasions on any Linux system. It is usefull on the command line, it is usefull in cgi on a webserver. Overall it is a very powerful language, and very easy to use.

References
There are a number of great web pages available for you to use.
Steps to creating and running a Perl script
- 1. Create a file, `script.pl`
- 2. Write your perl script
- 3. Make the file executable `chmod +x script.pl`
- 4. Run the file `./script.pl` or `perl script.pl`

Things to Learn and Things to Practice

How to use basic Perl scripting to perform simple tasks
How to create a Perl script file and execute it
How to create for and while loops

Example:
for ( $i = 0; $i < 10; $i++ ) {  print $i, "\n"; };

AND

$i = 0;
while($i < 10) {
 print $i, "\n";
 $i = $i + 1;
}

How to do conditional checking, if statements

Example: Write a program that prints out the prime numbers less than 10000

How to read and write to and from a file

Example: Modify the previous for and while loops to write to a file instead

How to read command line arguments

Example: Modify the prime number script to take in an upper bound from the command line

How to use Perl regular expressions

Example: find every instance of your username in a listing of `lastlog`

Show & Tell

We are going to write a little RSS reader. RSS is a XML format that stores mostly current headlines on a news server. When this file is downloaded and parsed you can get up to date information from that website, or news source. It can also be used for other information, but you will mostly find it on news sites, and other sites that have frequent updates. In order to create this reader, there are certain steps you will need to take, I have listed them here. All these steps should be able to run depending only on the steps below them. So I suggest you do one at a time.
Some example RSS files you can use are these:
- http://www.slashdot.org/index.rss
- http://fxfeeds.mozilla.org/rss20.xml
The program will be run like this: ./rss.pl [URL to RSS feed]
- 1. Execute a shell command to download the RSS file, and store it in a temporary location. (hint: you can use wget)
- 2. Open a filehandle to the file saved in the temporary file.
- 3. Iterate through all lines in the file. (hint: use a while loop)
- 4. For every line that you go through, use a regular expression to find a <title></title> flag, and print the information between them, on a new line every time. (hint: use the (.*) operator)
- 5. Close the filehandle.
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