Question

#tf.pl
#!/usr/local/bin/perl
use Util;
$file = shift;
$text = `cat $file`;
my @words = split_words ($text);
my @words = lc_words (@words);
my %count = count_hash(@words);
while (my ($w, $c) = each %count) {
    print "$w\t$c\n";
    $df{$w} = 1;
}

I came across that code on a website. On line 3 $file is given the name of the file that would have been supplied with the command line argument (correct me if I am wrong, I’m a Perl rookie)

On line 4 you see:

$text = `cat $file`;

I want to know what does this line do exactly? I know cat filename in shows you the content of the files in the terminal (again, correct me if wrong. Linux rookie too)

I asked this on IRC and someone said this was a bad way of doing something but I really want to know what this does more than knowing the better alternative of it

Answer 1

The backticks (so as to differentiate them from quotes ' or ") run whatever they contain as an external application, then the assignment captures the output to the variable $text. The contents (inside the backticks) can be any runnable process:

$fileList = `ls`
$userList = `who`

and so on.

It’s not usually a good idea since it ties the program to a specific operating system (those with a cat command in this case).

That may not be a problem but you should be aware that it reduces portability.

There are perfectly acceptable ways to already get the contents of a file into a variable in Perl, one that will work across all platforms (open, while <BLAH>, appending strings, and close), such as this sample program xyzzy.pl which reads itself in in two different ways (the first portable, the other not):

$sample1 = "";
open (INFILE, "xyzzy.pl") || die "Urk!";
while (<INFILE>) {
    $sample1 .= $_;
}
close (INFILE);

$sample2 = `cat xyzzy.pl`;

if ($sample1 ne $sample2) {
    print "Different\n";
    print "[$sample1]\n";
    print "[$sample2]\n";
} else {
    print "Same\n";
}

which outputs:

Same