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9.4. Recognizing Two Names for the Same File
9.4.1. Problem
You want to determine whether two
filenames in a list correspond to the same file on disk (because of
hard and soft links, two filenames can refer to a single file). You
might do this to make sure that you don't change a file you've
already worked with.
9.4.2. Solution
Maintain a hash, keyed by the device and inode number of the files
you've seen. The values are the names of the files:
%seen = ( );
sub do_my_thing {
my $filename = shift;
my ($dev, $ino) = stat $filename;
unless ($seen{$dev, $ino}++) {
# do something with $filename because we haven't
# seen it before
}
}
9.4.3. Discussion
A key in %seen is made by combining the device
number ($dev) and inode number
($ino) of each file. Files that are the same will
have the same device and inode numbers, so they will have the same
key.If you want to maintain a list of all files of the same name, instead
of counting the number of times seen, save the name of the file in an
anonymous array.
foreach $filename (@files) {
($dev, $ino) = stat $filename;
push( @{ $seen{$dev,$ino} }, $filename);
}
foreach $devino (sort keys %seen) {
($dev, $ino) = split(/$;/o, $devino);
if (@{$seen{$devino}} > 1) {
# @{$seen{$devino}} is a list of filenames for the same file
}
}The
$; variable contains the separator string using
the old multidimensional associative array emulation syntax,
$hash{$x,$y,$z}. It's still a one-dimensional
hash, but it has composite keys. The key is really
join($; =>
$x, $y, $z).
The split separates them again. Although you'd
normally just use a real multilevel hash directly, here there's no
need, and it's cheaper not to.
9.4.4. See Also
The $; ($SUBSEP) variable in
perlvar(1), and in the "Special Variables"
section of Chapter 28 of Programming Perl; the
stat function in perlfunc(1)
and in Chapter 29 of Programming Perl; Chapter 5
