Learning Perl, Fourth Edition

Answers to Chapter 11 Exercises

1. Here’s one way to do it:

       foreach my $file (@ARGV) {
         my $attribs = &attributes($file);
         print "'$file' $attribs.\n";
       }
       sub attributes {
         # report the attributes of a given file
         my $file = shift @_;
         return "does not exist" unless -e $file;
         my @attrib;
         push @attrib, "readable" if -r $file;
         push @attrib, "writable" if -w $file;
         push @attrib, "executable" if -x $file;
         return "exists" unless @attrib;
         'is ' . join " and ", @attrib;  # return value
       }

   In this one, it’s convenient to use a subroutine. The main loop prints one line of attributes for each file, perhaps telling us that 'cereal-killer' is executable or that 'sasquatch' does not exist.

   The subroutine tells us the attributes of the given filename. If the file doesn’t exist, there’s no need for the other tests, so we test for that first. If there’s no file, we’ll return early.

   If the file does exist, we’ll build a list of attributes. (Give yourself extra credit points if you used the special _ filehandle instead of $file on these tests to keep from calling the system separately for each new attribute.) It would be easy to add additional tests like the three we show here. But what happens if none of the attributes is true? Well, if we can’t say anything else, at least we can say that the file exists, so we do. The unless clause uses the fact that @attrib will be true (in a Boolean context, which is a special case of a scalar context) if it has any elements.

   If we have some attributes, we’ll join them with ...