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=head1 TITLE
Angle brackets should not be used for file globbing.
=head1 VERSION
Maintainer: Jon Ericson <[EMAIL PROTECTED]>
Date: 4 August 2000
Version: 1
Mailing List: [EMAIL PROTECTED]
Number: 34
=head1 ABSTRACT
Angle brackets have two separate meanings in Perl - file globbing and
line input from a filehandle. This means that perl attempts to Do What
I Mean and often misinterprets me. Since file globbing can be
accomplished with the glob function and since file input is the more
common use of angle brackets, they should not be used for file globbing.
=head1 DESCRIPTION
=head2 Background
>From perlop/"I/O Operators":
If angle brackets contain is a simple scalar variable (e.g.,
<$foo>), then that variable contains the name of the
filehandle to input from, or its typeglob, or a reference to the
same. For example:
$fh = \*STDIN;
$line = <$fh>;
If what's within the angle brackets is neither a filehandle nor a simple
scalar variable containing a filehandle name, typeglob, or typeglob
reference, it is interpreted as a filename pattern to be globbed, and
either a list of filenames or the next filename in the list is returned,
depending on context. This distinction is determined on syntactic
grounds alone. That means C<< <$x> >> is always a readline() from
an indirect handle, but C<< <$hash{key}> >> is always a glob().
That's because $x is a simple scalar variable, but C<$hash{key}> is
not--it's a hash element.
One level of double-quote interpretation is done first, but you can't
say C<< <$foo> >> because that's an indirect filehandle as explained
in the previous paragraph. (In older versions of Perl, programmers
would insert curly brackets to force interpretation as a filename glob:
C<< <${foo}> >>. These days, it's considered cleaner to call the
internal function directly as C<glob($foo)>, which is probably the right
way to have done it in the first place.) For example:
while (<*.c>) {
chmod 0644, $_;
}
is roughly equivalent to:
open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
while (<FOO>) {
chop;
chmod 0644, $_;
}
except that the globbing is actually done internally using the standard
C<File::Glob> extension. Of course, the shortest way to do the above
is:
chmod 0644, <*.c>;
A (file)glob evaluates its (embedded) argument only when it is
starting a new list. All values must be read before it will start
over. In list context, this isn't important because you automatically
get them all anyway. However, in scalar context the operator returns
the next value each time it's called, or C<undef> when the list has
run out. As with filehandle reads, an automatic C<defined> is
generated when the glob occurs in the test part of a C<while>,
because legal glob returns (e.g. a file called F<0>) would otherwise
terminate the loop. Again, C<undef> is returned only once. So if
you're expecting a single value from a glob, it is much better to
say
($file) = <blurch*>;
than
$file = <blurch*>;
because the latter will alternate between returning a filename and
returning false.
It you're trying to do variable interpolation, it's definitely better
to use the glob() function, because the older notation can cause people
to become confused with the indirect filehandle notation.
@files = glob("$dir/*.[ch]");
@files = glob($files[$i]);
=head2 Areas of confusion
=over
=item Complex filehandle references
my %filesystem;
my $filename = '/etc/shells';
open $filesystem{$filename}, $filename
or die "can't open $filename: $!";
print <$filesystem{$filename}>;
__END__
GLOB{0xa042284}
=item Simple file glob variables
my $fileglob = '/etc/sh*';
print <$fileglob>;
__END__
[nothing printed]
=item Why don't these have the same output?
$ perl -e 'print </etc/sh*>'
/etc/shells
$ perl -e 'print <>' /etc/sh*
/bin/sh
/bin/bash
=back
=head2 The glob function exists
Now that we have a function interface to file globbing, why do we need
the obscure operator interface?
=head2 The readline meaning is more common
Angle brackets are a shortcut for either glob() or readline(). If we
are going to eliminate one meaning, it should be the less common
meaning.
=head1 IMPLEMENTATION
Remove the file-globbing behavior of the angle brackets.
=head1 REFERENCES
perlop/"I/O Operators", perlfunc/glob, perlfunc/readline
