Perl to gnu less redirection problem

gnidoc · 08-10-2008, 09:50 AM

This is a small perl script I wrote to check mirror lists (files filled with one URL per line). It prints "[FAIL] hxxp://..." when an URL can't be retrieved and just prints the url when everything goes fine. The code is pretty straightforward and it works well until I am trying to redirect its output.

There is one thing I can't figure out. As far as I know the program called 'less' prints its input on the screen providing a nice interface for examining the contents of another program's output.

less works fine in situations like:

Code:

perl -e 'print "test"' | less
ls | less
cat /etc/passwd | less

The problem is that using my script 'less' only gets the lines sent to STDERR.

linkchk.pl

Code:

#!/usr/bin/perl
#
# Usage: ./linkchk.pl filename_containing_urls
# or cat file | ./linkchk.pl

use LWP;
$ua=LWP::UserAgent->new;
$ua->timeout(3);
$req=HTTP::Request->new;
$req->method("HEAD");

# create and urllist file
# try ./linkchk.pl urllist | less
print STDOUT "Why can't ./linkchk.pl urllist|less see this?\n";

while (<>)
{
        chomp($line=$_);
        $req->uri($line);
        $res=$ua->request($req);
        if ($res->is_success)
        {
          print STDOUT "$line\n";
        }
        else
        {
          print STDERR "[FAIL] $line\n";
        }
}

any clues?

(Fedora9, perl 5.10.0)

ne pas · 08-10-2008, 12:33 PM

First thought: STDOUT output is line buffered on default. Set autoflush to a non-zero value, right after your Shebang (#!)

Code:

#!/usr/bin/perl
$| = 1;

# or more common

$|++;

# also possible

use IO::Handle;

STDOUT->autoflush(1);

gnidoc · 08-10-2008, 04:01 PM

Both does the trick, thanks for your help!