eval, Read environment variable and expand using eval

tart has asked for the wisdom of the Perl Monks concerning the following question:

I am working on a project where I have to read list of
environment variables from configuration file and
execute perl “eval” to expand the environment variable
and write it to new file.
I have a condition,
(defined $ENV{HOST} ? $ENV{HOST} : 'SQLHOST')
if HOST is not defined garb a string.
So I tried following making environment variable HOST undefined which work,

my $val;
my $stmt = q{$val = (defined $ENV{HOST} ? $ENV{HOST} : 'sqlhost')};

eval $stmt;

if ($@)
{
    print $@, "\n";
    }
    else
    {
        print $val, "\n";
    }
[download]

OUTPUT= "sqlhost"

But I am suppose to read the configuration file so I tried this,

my ($r, $stmt);

while ($stmt = <DATA>)
{
    eval $stmt;

    print "$stmt\n";

    if ($@)
    {
        print $@, "\n";
    }
    else
    {
        print $r, "\n";
    }
}

__DATA__
$r = (defined $ENV{HOST} ? $ENV{HOST} : 'sqlhost')
[download]

OUTPUT= "$r = (defined $ENV{HOST} ? $ENV{HOST} : 'sqlhost')"

it didn;t work can any one suggest me better solution?
Thanks

Comment on eval, Read environment variable and expand using eval Select or Download Code

Replies are listed 'Best First'.
Re: eval, Read environment variable and expand using eval by toolic (Bishop) on Feb 19, 2010 at 02:45 UTC
it didn;t work Please elaborate. It seems to work for me when I run your 2nd code example. Here is the output I get: `$r = (defined $ENV{HOST} ? $ENV{HOST} : 'sqlhost') sqlhost` [download] What do you expect the output to be? First, you print the statement verbatim, then you print the eval'd statement. I do not have the HOST variable set.	[reply] [d/l]
Re^2: eval, Read environment variable and expand using eval by tart (Beadle) on Feb 19, 2010 at 04:34 UTC
There must be something which I don't know, In second code the output I am getting is, $r = (defined $ENV{INFORMIXSQLHOSTS} ? $ENV{INFORMIXSQLHOSTS} : 'sqlhost') But I am expecting, sqlhost which I am getting from first code. Cheers, Thanks	[reply]
Re^3: eval, Read environment variable and expand using eval by ikegami (Patriarch) on Feb 19, 2010 at 07:03 UTC
You should be getting that since you're printing `$stmt`. You should also be getting the value of `$ENV{HOST}` or `sqlhost`.	[reply] [d/l] [select]
Re^4: eval, Read environment variable and expand using eval by cdarke (Prior) on Feb 19, 2010 at 09:27 UTC
Re^3: eval, Read environment variable and expand using eval by bot403 (Beadle) on Feb 19, 2010 at 14:58 UTC
There is a subtle yet important difference between defined and exists on hash and array elements. Defined will autovivify your hash element and make its value undefined. Perhaps you meant this? `$r = (exists $ENV{INFORMIXSQLHOSTS}) ? $ENV{INFORMIXSQLHOSTS} : 'sqlho +st')` [download] If you want to check every possiblity this would be the best. However I usually just do the exists() check. `$r = ((exists $ENV{INFORMIXSQLHOSTS} && defined($ENV{INFORMIXSQLHOSTS} +) && length($ENV{INFORMIXSQLHOSTS}) ) ? $ENV{INFORMIXSQLHOSTS} : 's +qlhost'));` [download]	[reply] [d/l] [select]
Re^4: eval, Read environment variable and expand using eval by Anonymous Monk on Feb 19, 2010 at 15:04 UTC
Re^5: eval, Read environment variable and expand using eval by bot403 (Beadle) on Feb 19, 2010 at 17:08 UTC
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Re: eval, Read environment variable and expand using eval by rovf (Priest) on Feb 19, 2010 at 10:06 UTC
I have to read list of environment variables from configuration file and execute perl “eval” to expand the environment variable I don't understand why you need `eval` to get the value of an environment variable. If you have the name of the variable stored in `$envvarname`, you get the value with `$ENV{$envvarname}`. Maybe it would help if you can show an example of your configuration file. -- Ronald Fischer <ynnor@mm.st>	[reply] [d/l] [select]
Re: eval, Read environment variable and expand using eval by AnomalousMonk (Archbishop) on Feb 19, 2010 at 13:03 UTC
`while ($stmt = <DATA>) { eval $stmt; ... }` [download] Yikes! Surely, the two most important rules of using `eval` on data are: Never `eval` a string of data unless the data are absolutely trusted; Never absolutely trust data. Per rovf's reply, isn't there another way to parse your config file, or to structure your config file so it may be more easily parsed? Update: Changed some emphases.	[reply] [d/l] [select]
Re^2: eval, Read environment variable and expand using eval by Anonymous Monk on Feb 19, 2010 at 13:48 UTC
Is the data produced by your source code editor, or that of your collegues, to be absolutely trusted? How about the data you download from CPAN and process with your perl executable?	[reply]
Re^3: eval, Read environment variable and expand using eval by Anonymous Monk on Feb 19, 2010 at 14:09 UTC
Lame strawman, put it in module, write tests, verify ...	[reply]
Re^4: eval, Read environment variable and expand using eval by Anonymous Monk on Feb 19, 2010 at 14:38 UTC
Re^5: eval, Read environment variable and expand using eval by Anonymous Monk on Feb 19, 2010 at 15:05 UTC
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