Perl is a powerful scripting language that is commonly used for system administration, web development, and network programming. One of the key features of Perl is its ability to perform text manipulation. This includes the ability to print source line numbers, which can be extremely useful for debugging and troubleshooting.
To print source line numbers in Perl, we first need to understand how Perl reads and executes code. Perl reads code line by line, starting from the first line of the file. Each line is numbered sequentially, with the first line being line 1, the second line being line 2, and so on.
To print the line numbers, we can use the special variable $., also known as the "current line number" variable. This variable stores the current line number being read by Perl. By printing this variable, we can easily display the line number along with the corresponding code.
Let's take a look at an example. We have a Perl script named "example.pl" with the following code:
#!/usr/bin/perl
use strict;
use warnings;
print "Hello World!\n";
print "This is line 2.\n";
print "And this is line 3.\n";
When we run this script, we will get the following output:
Hello World!
This is line 2.
And this is line 3.
However, if we want to print the line numbers as well, we can modify the script to include the $., variable.
#!/usr/bin/perl
use strict;
use warnings;
print "1: Hello World!\n";
print "2: This is line 2.\n";
print "3: And this is line 3.\n";
Now when we run the script, we will get the following output:
1: Hello World!
2: This is line 2.
3: And this is line 3.
As you can see, by simply adding the line numbers before each line of code, we are able to easily identify which line is causing a problem or producing unexpected results. This is especially useful when dealing with longer scripts or troubleshooting complex issues.
It's important to note that the $., variable is global, meaning it will continue to increment even if we have multiple files being read by Perl. This can be useful when working with modules or including other files in our script.
In addition to using the $., variable, we can also use the built-in function "tell" to print the current line number. The "tell" function returns the current position in the file, which in the case of Perl, is the current line number.
Let's modify our example script to use the "tell" function instead:
#!/usr/bin/perl
use strict;
use warnings;
print "Current line number is: " . tell() . "\n";
print "Hello World!\n";
print "Current line number is: " . tell() . "\n";
print "This is line 2.\n";
print "Current line number is: " . tell() . "\n";
print "And this is line 3.\n";
print "Current line number is: " . tell() . "\n";
When we run this script, we will get the following output:
Current line number is: 0
Hello World!
Current line number is: 14
This is line 2.
Current line number is: 33
And this is line 3.
