Lesson 4	Reading from file streams
Objective	Read from a file stream.

Reading from File Streams using Perl

Reading file streams is also quite simple in Perl. In fact, Perl has a special case of the while() loop just for reading streams.
This example reads the same file that the previous example wrote and creates a Web page that shows all the URLs that have referred people to your site:

Open file in read mode

#!/usr/bin/perl
$logdir = "/home/billw/var/logs";
$logfile = "test-referer.log";
$reflog = "$logdir/$logfile";

$crlf = "\x0d\x0a";
print "content-type: text/html$crlf";
print "$crlf";

open(REFLOG, "<$reflog") or die "cannot open $reflog!\n";
while(<REFLOG>) { 
 chomp; 
 print qq(<a href="../module5/$_">$_</a><br>\n) unless $refhash{$_};
 $refhash{$_} += 1; 
}
close REFLOG;

This program opens the same file, but this time in read mode. Then it uses the special-case version of the while() loop, which automatically assigns the next line from the file to the special $_ variable.
The special case of the while() loop is probably the most common construct that you will use with streams, so let us take a closer look at it.
To help you understand this construct, take a look at the following two lines of Perl. They are functionally identical:

while(<REFLOG>) { print }               # print the whole stream
while($line = <REFLOG>) { print $line } # print the whole stream

In Perl, the special variable $_ is the default argument to a number of functions, including print() and chomp(), which you will use a lot when working with streams. This special case of the while() loop assigns the value of the current line from the stream to the $_ variable.
This makes it possible to write an entire cat program like this:

#!/usr/bin/perl
while(<>) { print }

<> is a special file stream, which defaults to the standard input stream, or each of the files referenced on the command line.
Or, if you want to print only the lines with matched angle brackets:

#!/usr/bin/perl
while(<>) { print if /\<.*\>/ }

Reading From File Streams - Exercise

Are you starting to see how convenient this is? These same techniques work with all the different types of streams that Perl offers, including pipes.
Click the Exercise link below to write a simple Perl script that opens a file and prints to the screen.
Reading From File Streams - Exercise