One simple way to do this is to have the user put the data after the
__DATA__ in the script and have the script read it in:
#!/usr/bin/perl -w
use strict;
my %config = configure();
foreach my $opt (keys %config) {
print "$opt : $config{$opt}\n";
}
sub configure {
my %cfg;
while(<DATA>){
next if /^\s*#/ or /^\s*$/;
chomp;
my($option, $value) = split /\s*=\s*/;
$cfg{$option} = $value;
}
return %cfg;
}
__DATA__
# This is the configuration section. Syntax is:
# OPTION_NAME = VALUE
# set user name:
USER_NAME = nobody
# set email address
EMAIL = nobody@nowhere.com
# debugging level (1,2,3)
DEBUG = 2
The pitfalls of this approach are twofold: first, only one user of the
script may configure it (each user must run their own copy of the
program with their own configuration details), and second, we are
responsible for parsing the configuration info.
We can get around the second pitfall by asking the user to use Perl's
hash syntax and set their configuration info in the hash at the top of
the script:
#!/usr/bin/perl -w
use strict;
my %config = (
# set user name:
USER_NAME => 'nobody',
# set email address:
EMAIL => 'nobody@nowhere.com',
# set debugging level (1,2,3):
DEBUG => 2,
);
However, this is still set within the script and we want to allow for
different users of the same program. To do this we use a separate
config file for each user (stored in the users home directory with a
special name). We can choose to use the same form as the __DATA__
example above and write our own parser, or we can specify a format that
Perl can understand. One trick for having Perl do the work is to
realize that Perl's require() function (used to load in modules or
other chunks of Perl code) has a return value of the last expression
evaluated in the file. This means we can use an anonymous hash as our
configuration format and have it be the only thing in the file -- for
example, a config file could look like:
{
# set user name:
USER_NAME => 'nobody',
# set email address:
EMAIL => 'nobody@nowhere.com',
# set debugging level (1,2,3):
DEBUG => 2,
}
As far as Perl is concerned, that is an anonymous hash and if this file
is stored as '.config' in the users HOME directory (where the
environment variable $HOME points to), we can simply require() it and
grab the return value:
#!/usr/bin/perl -w
use strict;
my $config = require "$ENV{HOME}/.config";
foreach my $opt (keys %$config) {
print "$opt : $config->{$opt}\n";
}
Now each user can have their own config file and we avoid having to
parse it by using a format (an anonymous hash) that Perl understands.
Utilizing the return value of the require() function can be useful in
other situations as well.
Next Week: Tying Variables
