Perl For Loop

The Perl for loop is used to loop through a block of code until a specified condition is met. The for loop statement contains three sections followed by a block of code. Below is an example.

A Simple For Loop Example

for (my $number = 1; $number <= 10; $number++) {
  print "$number ";
}

The first section initializes a variable

my $number = 1;

Then, the loop condition is provided. The loop will run as long as this is true.

$number <= 10;

Finally, the last section is executed at the end of each loop iteration. In our example's case, $number increments by one.

$number++

Executing this loop results in:

1 2 3 4 5 6 7 8 9 10

For Loop and Arrays

Here is an example of how the for statement can be used to loop through an array.

my @languages = ("Perl", "Python", "C", "Fortran");
my $size = @languages;
for (my $i = 0; $i <= $size; $i++) {
  print "$languages[$i]\n";
}

Perl Search and Replace

Perl can be used to easily parse through files and perform a search and replace. For instance, the following command replaces all occurrences of 'old' with 'new' in myfile.txt after backing up the original as myfile.txt-OLD:

perl -pi-OLD -e 's/old/new/g' myfile.txt

Here is a description of the switches used according to Perl's help (perl --help):

• -p assume loop like -n but print line also, like sed
• -i[extension] edit <> files in place (makes backup if extension supplied)
• -e program one line of program (several -e's allowed, omit programfile)

If you want to get creative, use Perl along with the Linux find command. With the next command, I replace 'old' with 'new' in all files that end in .html:

find /start/path -type f -name '*.html' -exec perl -pi-OLD -e 's/old/new/g' {} \;

Search and Replace Piped Output

You can also search and replace piped results. In this case, I display /etc/shadow and replace password hashes with 'HIDDEN':

sudo cat /etc/shadow | perl -p -e 's/(:)[^:]*/$1HIDDEN/'

Using Perl LWP::UserAgent to View Server Headers

Unless configured to do otherwise, most HTTP and FTP servers will supply you with identifying information in the form of a server header. Using Perl's LWP::UserAgent, you can connect to a server and display the header information.

First, you'll need LWP::UserAgent. You can install this using Perl's CPAN repositories:

sudo perl -MCPAN -e 'install LWP::UserAgent'

After LWP::UserAgent is installed, you can use Perl to connect to the server and download the information. Here is an example script which takes one argument and displays the information:

#!/bin/env perl
 
use warnings;           # keep us warned
use strict;             # keep us honest
use LWP::UserAgent;     # for web requests
 
my $url;                # init url var
my $serverheader;       # init server header var
 
# require one argument
if ($#ARGV != 0) {
 
    printf("Usage: %s <URL>\n", $0);
    exit(1);
 
} else { $url = $ARGV[0]; }
 
# build connection properties
my $ua = LWP::UserAgent->new();
$ua->timeout(10);
$ua->agent('Mozilla/5.0');
 
# connect and get
my $response = $ua->get($url);
 
# if we can connect...
if ($response->is_success) {
 
    # grab server header
    $serverheader = $response->server;
 
    # if server header exists...
    if (defined($serverheader)) {
 
        # print server header
        printf("%s\n", $response->server);
 
    # else, print a message
    } else { printf("No server header available.\n"); }
 
# else, print connection status
} else { printf("%s\n", $response->status_line); }
 
exit(0);

Assuming that the code is saved as getinfo.pl and executable (chmod +x), you can use it to fetch interesting information from various HTTP and FTP servers.

bash$ ./getinfo.pl http://www.apache.org
Apache/2.3.15-dev (Unix) mod_ssl/2.3.15-dev OpenSSL/1.0.0c
 
bash$ ./getinfo.pl ftp://apache.mirrors.pair.com
apache.mirrors.pair.com NcFTPd Server (licensed copy)


Of course, some sites do not publish HTTP headers (but most do).

bash$ ./getinfo.pl http://www.facebook.com
No server header available.

Finally, the code will show you if there is an error:

bash$ ./getinfo.pl http://badurl
500 Can't connect to badurl:80 (Bad hostname 'badurl')