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

I've a question regarding the usage of backquotes. Here is the code: 
$good = `grep -i $vals[0] $file | awk '{ print $2 }'`

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You can see I'm trying to get specific output with an awk statement. It doesn't seem to like the quoted statement. I've tried inserting backslashes with no luck. I am forced to put the awk statement in a file and call it like so: 
$good = `grep -i $vals[0] $file | awk -f myprint`

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What am I missing? Is there a better way. Thanks.

2004-05-05 Edit by jdporter: Changed title from 'backquotes'

Replies are listed 'Best First'.
Re: Unwanted interpolation in backquoted string
by Roger (Parson) on May 04, 2004 at 06:41 UTC
    You should escape the "$2" with backslashes otherwise it gets interpolated by perl.

    A better way? Umm, that depends on the point of view. To me, a better way is a perl only way to minimize dependency on external programs and platform independent as well. So instead of using external programs, I would do it in perl.

    use IO::File;

...

my $good = getvalue($file, $vals[0]);

...

sub getvalue
{
  my ($file, $val) = @_;
  my $val;

  my $f = new IO::File $file, "r" or die "Can not open file";
  while (my $line = <$f>) {
    if ($line =~ /$val/) {
       return (split /,/, $line)[1]; # return 2nd field
    }
  }
  return undef;
}

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[d/l]
      Your "pure-perl" alternative is not exactly equivalent to the OP. First, by default "awk" splits on white-space, just like the default behavior of "split" (so you shouldn't be splitting on commas). Second, if "$file" contains multiple lines that match $vals[0], the OP's backtick version will return a string with multiple lines, whereas your code will only return at most a single line. (Oh, and why do you have "my $val" a second time?) To replicate the "grep | awk" pipeline properly, the sub would actually be simpler: 
      sub getvalue
{ 
   my ($file, $val) = @_;

   my $f = new IO::File $file, "r" or die "Can not open $file: $!";
   my $matched = '';

   while (<$f>)
   {
      $matched .= (split)[1] . "\n" if (/$val/);
   }
   return $matched;
}

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[d/l]
[select]
        Or even simpler. 
        $good = do {
    local @ARGV = ($file);
    join '', map { /$vals[0]/i ? (split)[1]."\n" : () } <>;
};

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[d/l]
        I was just giving an idea on how this could be done, not on the absolute correctness of the code though.

[reply]
      Ahhh. Of course. Thanks much.
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Re: Unwanted interpolation in backquoted string
by sgifford (Prior) on May 04, 2004 at 06:44 UTC

    The $2 is being interpreted by perl. Put a backslash before it and you should be good to go.

    Turning on warnings is what gave me the hint to tell me why this wasn't working.

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[d/l]
Re: Unwanted interpolation in backquoted string
by coec (Chaplain) on May 04, 2004 at 06:46 UTC
    Something like 
    open(FILE, $file) or die "$!";
my @file = <FILE>;
close(FILE);
foreach (@file) {
   if (m/$val[0]/) {
      $tmp = (split /\s+/,$_)[1];
      print $tmp . "\n";
   }
}

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    but the actual problem with the back quotes is that $2 should be escaped. (s/$2/\$2/) The reason is that you want awk to interpret $2, not Perl.

    CC

    Note that this is completely untested...

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[d/l]
      I think you mean s/\$2/\\\$2/ :)
[reply]
        Doh!
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