Question

I ran across a very strange line of code in a legacy Perl application. The code here is part of a homegrown RSS reader that does some caching to prevent being blacklisted.

open(CAT, "/usr/bin/cat -v /tmp/cat-cache 2>&1|");

Does it seem likely that the original author ran the results through cat -v to strip out non-printing characters to deal with any number of character sets? Wouldn’t this make more sense using a regular expression in Perl itself? Also, I am most perplexed by the pipe on the end.

Answer 1

Functionally

that code would do something similar to this:

open my $fh, '<' , '/tmp/cat-cache' or Carp::croak("Cant open file $@ $! ");

sub lessquote {
    my $x  = shift;
    my $meta = shift; # meta means were repeating thise code for >128
    # Special Case for whitespace 
    if(( not defined $meta ) && ( $x  == 9 or $x == 10 ) ){
        return chr($x);
    }
    # Null and M-^@
    if(  $x  == 0 ){ 
        return "^@"; 
    }
    # ^A to ^Z as well as M-^A to M-^Z
    if( ( 0 <= $x ) && ( $x  <= 31 )){
        return "^" . chr( $x + ord('A') - 1 );
    }
    # Also M-^?
    if( $x == 127 ){ 
        return "^?";
    } 
    # Does the M- Family
    if( $x >= 128 && $x <= 255 ){ 
        return "M-" . lessquote( $x - 128 , 1); 
    }
    return chr( $x );
}

while( my $line = <$fh> ){
   $line =~ s{(.)}{ lessquote( ord( $1 ) ) }eg;
}

Not identical, but similar.

NB: lessquote appears to match my ‘cat -v’ output.

But as you can see, doing the same thing is a bit less than trivial and not directly suited for a regular expression, but still, I don’t see why they shelled out to ‘cat’.

As far as their style goes

They are shelling out in a bad way, the code style is so 1990’s and it should be avoided.

open my $fh , '-|' , 'cat' , '-v' , '/tmp/cat-cache' or Carp::croak("Cant open file $@ $! ");

Syntax:

open my $FILEHANDLE , $OPENMODE, $FILENAME            || Carp::croak($ERRORMESSAGE)
open my $FILEHANDLE , $OPENMODE, $SHELLCOMMAND        || Carp::croak($ERRORMESSAGE) 
open my $FILEHANDLE , $OPENMODE, $SHELLPROGRAM, @ARGS || Carp::croak($ERRORMESSAGE)

Is the “preferred” notation these days for a multitude of reasons.
Of course, you wouldn’t ACTUALLY want to use cat, but I’ve left it in here for a clear example.