I am trying to understand Larry's filename fixer. I saw it first in the cookbook and am now looking here to find help. I am new to Perl, but I think (I hope) I understand everything the script does. The only thing I don't get is how all the matching filenames end up in @ARGV. So if I call the rename script in a directory with three txt files like so:

rename 's/foo/bar/' *.txt

@ARGV would be an Array with four entries like this right?

after the shift that removes the 's/foo/bar/' I am then left with the @ARGV that contains just the file names. The script then loops through all of them and works it's magic, so far so good.

What I do not understand is how the script determines which files to put into @ARGV. At which point is "*.txt" being evaluated? Or is it the shell that tells the script which files in the directory match the pattern *.txt?

This is probably a daft question but any help is much appreciated, as I said I am very new to this.

In any UNIX-like shell, when you specify a filename glob (i.e. a filename with wildcards) on the shell commandline, it is the shell, and not the program you are calling, which expands the glob.

So yes, when you call

rename.pl *.txt

the shell expands the *.txt to all matching files, and the Perl script finds the already expanded args in its @ARGV.

If you want the script to do the glob expansion, you'd have to enclose the argument in single quotes, i.e. call it like this:

somescript.pl '*.txt'

Then the Perl script finds exactly one arg in @ARGV, namely *.txt and you would have to find some way to do the expansion.

With Larry's script however, I typically feed it ALL the files by matching *; it will only act on those files which max the regexp given as the first argument anyway, all others are skipped. Of course that requires some care in constructing the regexp.

@ARGV would be an Array with four entries like this right?

Yes, except for the new lines, \n. Prove this to yourself with a test script:

use strict;
use warnings;

print "These are the ", scalar @ARGV, " arguments:\n";
my $i = 0;
for (@ARGV) {
    print "ARGV[$i] = $_\n";
    $i++;
}
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prints:

These are the 4 arguments:
ARGV[0] = s/foo/bar/
ARGV[1] = file1.txt
ARGV[2] = file2.txt
ARGV[3] = file3.txt
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Alternately, you can use Data::Dumper:

use strict;
use warnings;
use Data::Dumper;
print Dumper(\@ARGV);
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prints:

$VAR1 = [
          's/foo/bar/',
          'file1.txt',
          'file2.txt',
          'file3.txt'
        ];
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What I do not understand is how the script determines which files to put into @ARGV. At which point is "*.txt" being evaluated? Or is it the shell that tells the script which files in the directory match the pattern *.txt?

The shift built-in function removes element 0 from the @ARGV array, shifting the remaining elements [1:3] down to new positions [0:2]. Again, here is a simple test script:

use strict;
use warnings;

my $op = shift;
# same as: my $op = shift @ARGV;

print "These are the ", scalar @ARGV, " arguments:\n";
my $i = 0;
for (@ARGV) {
    print "ARGV[$i] = $_\n";
    $i++;
}
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prints:

These are the 3 arguments:
ARGV[0] = file1.txt
ARGV[1] = file2.txt
ARGV[2] = file3.txt
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Hi toolic,

thank you for your quick reply. I understand what shift does in this script. What I don't understand is how all txt files in the directory end up in @ARGV.

When I copy and paste your code, what I get is:

These are the 2 arguments:
ARGV[0] = s/\.txt\.mex//
ARGV[1] = *.txt
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And this also makes sense to me, because how would the script be able to know which files in the directory match '*.txt'? Doesn't there have to be another process that matches '*.txt' against all files in the directory first?

Arian