Now that we can open a file, we can then move on to reading the file one line at a time. We do this by replacing the STDIN filehandle in with our new filehandle, to get . Just as reads a single line from the keyboard, reads one line from a filehandle. This <...> construction is called the diamond operator, or readline operator:

Try It Out : Numbering Lines

We'll use the construct in conjunction with a while loop to go through each line in a file. So then, to print a file with line numbers added, you can say something like this:

#!/usr/bin/perl

# nl.plx

use warnings;

use strict;

open FILE, "nlexample.txt" or die $!;

my $lineno = 1;

while () {

   print $lineno++;

   print ": $_";

}

Now, create the file nlexample.txt with the following contents:

One day you're going to have to face

   A deep dark truthful mirror,

And it's gonna tell you things that I still

   Love you too much to say.

####### Elvis Costello, Spike, 1988 #######

This is what you should see when you run the program:

> perl nl.plx

1: One day you're going to have to face

2:      A deep dark truthful mirror,

3: And it's gonna tell you things that I still

4:      Love you too much to say.

5. ####### Elvis Costello, Spike, 1988 #######

>

How It Works

We begin by opening our file and making sure it was opened correctly:

open FILE, "nlexample.txt" or die $!;

Since we're expecting our line numbers to start at one, we'll initialize our counter:

my $lineno = 1;

Now we read each line from the file in turn, which we do with a little magic:

while () {

This syntax is actually equivalent to:

while (defined ($_ = )) {

That is, we read a line from a file and assign it to $_, and we see whether it is defined. If it is, we do whatever's in the loop. If not, we are probably at the end of the file so we need to come out of the loop. This gives us a nice, easy way of setting $_ to each line in turn.

As we have a new line, we print out its line number and advance the counter:

   print $lineno++;

Finally, we print out the line in question:

   print ": $_";

There's no need to add a newline since we didn't bother chomping the incoming line. Of course, using a statement modifier, we can make this even more concise:

open FILE, "nlexample.txt" or die $!;

my $lineno = 1;

print $lineno++, ": $_" while

But since we're going to want to expand the capabilities of our program -adding more operations to the body of the loop - we're probably better off with the original format.