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16.6. Preprocessing Input

You'd like your programs to work on
files with funny formats, such as compressed files or remote web
documents specified with a URL, but your program only knows how to
access regular text in local files.

16.6.2. Solution

Take advantage of Perl's easy pipe handling by changing your input
files' names to pipes before opening them.

To autoprocess gzipped or compressed files by decompressing them with
gzip, use:

@ARGV = map { /\.(gz|Z)$/ ? "gzip -dc $_ |" : $_ } @ARGV;
while (<>) {
# .......
}

To fetch URLs before processing them, use the
GET program from LWP (see Chapter 20):

@ARGV = map { m#^\w+://# ? "GET $_ |" : $_ } @ARGV;
while (<>) {
# .......
}

You might prefer to fetch just the text, of course, not the HTML.
That just means using a different command, perhaps lynx
-dump.

16.6.3. Discussion

As shown in
Recipe 16.1, Perl's built-in
open function is magical: you don't have to do
anything special to get Perl to open a pipe instead of a file.
(That's why it's sometimes called magic open
and, when applied to implicit ARGV processing,
magic ARGV.) If it looks like a pipe, Perl will
open it like a pipe. We take advantage of this by rewriting certain
filenames to include a decompression or other preprocessing stage.
For example, the file "09tails.gz" becomes
"gzcat -dc
09tails.gz|".

This technique has further applications. Suppose you wanted to read
/etc/passwd if the machine isn't using NIS, and
the output of ypcat passwd if it is. You'd use
the output of the domainname program to decide
if you're running NIS, and then set the filename to open to be either
"</etc/passwd" or "ypcat
passwd|":

$pwdinfo = `domainname` =~ /^(\(none\))?$/
? '</etc/passwd'
: 'ypcat passwd |';
open(PWD, $pwdinfo) or die "can't open $pwdinfo: $!";

The wonderful thing is that even if you didn't think to build such
processing into your program, Perl already did it for you. Imagine a
snippet of code like this:

print "File, please? ";
chomp($file = <>);
open (FH, $file) or die "can't open $file: $!";

The user can enter a regular filename—or something like
"webget http://www.perl.com
|" instead—and your program would suddenly
be reading from the output of some webget
program. They could even enter -, a lone minus sign, which, when
opened for reading, interpolates standard input instead.

This also comes in handy with the automatic ARGV
processing we saw in Recipe 7.14.

16.6.4. See Also

Recipe 7.14; Recipe 16.4

16.5. Filtering Your Own Output		16.7. Reading STDERR from a Program

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