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Why is 'while loop' not executing

Posted on 2011-02-17
Medium Priority
Last Modified: 2012-06-27

I'm writing a perl script where I read a conf file. The conf file is a csv file that has two columns of data. I want to read the first column and match to the host the script is running on and iterate through it's matches. It's clearer when you see the format of the csv file that is like this:


Basically it's a simplified routing table. The first column contains a source host and the second column is the destination it's connecting to.

If the script happens to be on hostA, the loop will find the "to hosts" and print them. Well, really I dont want it to print, I will do another task with the output but for testing purposes, so I know it's working I want to print it. The correct output would be:

The 'to-hosts' for this machine are "host1"
The 'to-hosts' for this machine are "host2"
The 'to-hosts' for this machine are "host3"

But it's not executing the loop. I'm a super newbie to perl, in fact I've never a perl script before so it's learn as I go so I'm not sure why the loop is not executing. Can someone take a look at the code and see what is wrong with it?


open (F, $file) || die ("Could not open $file");

while ($line = <F>)
        ($field1,$field2) = split ',', $line;
        if ($field1 eq $host){
                print "The 'to-hosts' for this machine are $field2\n";
close (F);

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Question by:akatsuki27
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Accepted Solution

Fairlight2cx earned 2000 total points
ID: 34921721
I strongly suspect your loop is running.  I think you're getting no output because of the values of $field1 and $host.  Nowhere in the code snippet you included is $host defined.

My first step towards debugging would be to just insert a print("Inside while().\n"); statement first thing inside the while() loop's block.  I suspect you'll see one per line read.  If that's true, then the conditional is your issue.

Assuming that, I'd then swap out the "Inside while()." with "field1: $field1\nhost: $host\n" in that extra print statement I had you add, and move that line down to right under the split statement, but before the conditional.

Author Comment

ID: 34921936
I figured it out. My conditional was failing because the two sides weren't equal in any of the tests. There were some quotes in one and not the other.

I added the line: $field1 =~ s/\"//g;

That solved it.

Btw Fairlight, I defined $host in the top of the script. But now that I have your attention, could I put that output into an array? Most likely yes. But do I need to do that outside of the while loop?

Assisted Solution

Fairlight2cx earned 2000 total points
ID: 34923702
You can put it into an array inside the loop.  Just push() the values onto an array:

LVL 37

Expert Comment

by:Harisha M G
ID: 34923994
Check what is the host name you are getting by printing it:
use Sys::Hostname;
$file = $ARGV[0];
$host = hostname;

open (F, $file) || die ("Could not open $file");

while ($line = <F>)
        ($field1,$field2) = split ',', $line;
        print "$field1, $field2, $host";
        if ($field1 eq $host)
                print "The 'to-hosts' for this machine are $field2\n";
close (F);

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