Check for the existence of empty parameters in CGI.pm

Question

To check if a request parameter has a value (http://example.com?my_param=value) you can use

query->param('my_param')

This seems to be flaky if the no_value form is used: http://example.com?my_param .

Is there a way to check simply the existence of parameters in CGI.pm ?

Please read the following [article](https://www.perl.com/article/perl-and-cgi/) and decide for yourself if you should use CGI module. — Polar Bear, May 12 '20 at 08:48
For a more humorous take on the same topic, see [this talk by Perl's release manager Sawyer X](https://youtu.be/jKOqtRMT85s). — simbabque, May 12 '20 at 08:55
@serv-inc I could do it - if you are willing to pay me it ;-) — Georg Mavridis, May 13 '20 at 10:56

score 2 · Answer 1 · answered May 12 '20 at 08:50

2

This seems to depend on the browser implementation. According to this Perlmonks thread from 2001, empty parameters might not be returned. But if they are, the key will be there, but contain an empty string q{}. If the argument wasn't present, it will be undef, so you can check with defined.

answered May 12 '20 at 08:50

simbabque

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This did not work : `defined query->param('my_param');` ist not true if the param exists (cgi.pm 4.44) . Still thanks – serv-inc May 13 '20 at 09:56
This doesn't work when it is the only parameter. Don't know if this is a bug or not. – Georg Mavridis May 14 '20 at 08:05
1

@GeorgMavridis you mean `http://example.org/foo?bar` where there is just one param but no value and no equals? – simbabque May 14 '20 at 08:08
Yes, there it doesn't. But in `http://example.org/foo?bar&baz` it does work. – Georg Mavridis May 14 '20 at 09:22

score 2 · Accepted Answer · answered May 13 '20 at 10:49

You can check whether the parameter is present with defined. the following code

#!/usr/bin/env perl
use strict;
use warnings;
use CGI;
my $cgi = CGI->new();
print $cgi->header;

my $defined = defined $cgi->param('tt');
my $val = $cgi->param('tt');
print <<END;
<!doctype html>
<html> HTML Goes Here 
<h1>tt: "$defined" "$val"</h1>
</html>
END

Will generate the output:

<!doctype html>
<html> HTML Goes Here 
<h1>tt: "1" ""</h1>
</html>

when the URI is http:///cgi-bin/t.pl?someother=something&tt

Unfortunately this doesn't work when it is the only parameter.

So to be sure, you have to test:

my $defined = defined $cgi->param('tt') || $ENV{QUERY_STRING} eq 'tt';

Check for the existence of empty parameters in CGI.pm

2 Answers2