Using Perl with Linux

Although originally designed as a data-extraction and report-generation language, Perl appeals to many Linux system administrators because it can be used to create utilities that fill a gap between the capabilities of shell scripts and compiled C programs. Another advantage of Perl over other UNIX tools is that it can process and extract data from binary files, whereas sed and awk cannot.

Note

In Perl, "there is more than one way to do it." This is the unofficial motto of Perl, and it comes up so often that it is usually abbreviated as TIMTOWTDI.

You can use Perl at your shell's command line to execute one-line Perl programs, but most often the programs (usually ending in .pl) are run as a command. These programs generally work on any computer platform because Perl has been ported to just about every operating system. Perl is available by default when you install Ubuntu, and you will find its package files on the DVD included with this book.

Perl programs are used to support a number of Ubuntu services, such as system logging. For example, the logwatch.pl program is run every morning at 4:20 a.m. by the crond (scheduling) daemon on your system. Other Ubuntu services supported by Perl include the following:

• Amanda for local and network backups

• Fax spooling with the faxrunqd program

• Printing supported by Perl document filtering programs

• Hardware sensor monitoring setup using the sensors-detect Perl program

Perl Versions

As of this writing, the current production version of Perl is 5.8.8 (which is Perl version 5 point 8, patch level 8).

You can download the code from http://www.perl.com and build it yourself from source. You will occasionally find updated versions in APT format for Ubuntu, which can be installed by updating your system. See Chapter 7, "Managing Software," to see how to quickly get a list of available updates for Ubuntu.

You can determine what version of Perl you installed by typing perl -v at a shell prompt. If you are installing the latest Ubuntu distribution, you should have the latest version of Perl.

A Simple Perl Program

This section introduces a very simple sample Perl program to get you started using Perl. Although trivial for experienced Perl hackers, a short example is necessary for new users who want to learn more about Perl.

To introduce you to the absolute basics of Perl programming, Listing 27.1 illustrates a simple Perl program that prints a short message.

Listing 27.1. A Simple Perl Program

#!/usr/bin/perl print "Look at all the camels!\n"; 

Type that in and save it to a file called trivial.pl. Then make the file executable using the chmod command (see the following sidebar) and run it at the command prompt.

Command-Line Error

If you get the message bash: trivial.pl: command not found or bash: ./trivial.pl: Permission denied, you have either typed the command line incorrectly or forgotten to make TRivial.pl executable (with the chmod command):

$ chmod +x trivial.pl

You can force the command to execute in the current directory as follows:

$ ./trivial.pl

Or you can use Perl to run the program like this:

$ perl trivial.pl

The sample program in the listing is a two-line Perl program. Typing in the program and running it (using Perl or making the program executable) shows how to create your first Perl program, a process duplicated by Linux users around the world every day!

Note

#! is often pronounced she-bang, which is short for sharp (the musicians name for the # character), and bang, which is another name for the exclamation point. This notation is also used in shell scripts. See Chapter 15, "Automating Tasks," for more information about writing shell scripts.

The #! line is technically not part of the Perl code at all. The # character indicates that the rest of the screen line is a comment. The comment is a message to the shell, telling it where it should go to find the executable to run this program. The interpreter ignores the comment line.

Exceptions to this practice include when the # character is in a quoted string and when it is being used as the delimiter in a regular expression. Comments are useful to document your scripts, like this:

#!/usr/bin/perl # a simple example to print a greeting print "hello there\n"; 

A block of code, such as what might appear inside a loop or a branch of a conditional statement, is indicated with curly braces ({}). For example, here is an infinite loop:

#!/usr/bin/perl # a block of code to print a greeting forever while (1) {         print "hello there\n"; }; 

Perl statements are terminated with a semicolon. A Perl statement can extend over several actual screen lines because Perl is not concerned about whitespace.

The second line of the simple program prints the text enclosed in quotation marks. \n is the escape sequence for a newline character.

Tip

Using the perldoc and man commands is an easy way to get more information about the version of Perl installed on your system. To learn how to use the perldoc command, enter the following:

$ perldoc perldoc

To get introductory information on Perl, you can use either of these commands:

$ perldoc perl $ man perl 

For an overview or table of contents of Perl's documentation, use the perldoc command like this:

$ perldoc perltoc

The documentation is extensive and well organized. Perl includes a number of standard Linux manual pages as brief guides to its capabilities, but perhaps the best way to learn more about Perl is to read its perlfunc document, which lists all the available Perl functions and their usage. You can view this document by using the perldoc script and typing perldoc perlfunc at the command line. You can also find this document online at http://www.cpan.org/doc/manual/html/pod/perlfunc.html.