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From: =?UTF-8?Q?Josef_M=C3=B6llers?= <josef@invalid.invalid>
Newsgroups: comp.lang.c
Subject: Re: program to remove duplicates
Date: Tue, 1 Oct 2024 16:34:47 +0200
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On 21.09.24 20:53, fir wrote:
> 
> 
> i think if to write a simple comandline program
> that remove duplicates in a given folder
[...]

I have had the same problem. My solution was to use extended file 
attributes and some file checksum, eg sha512sum, also, I wrote this in 
PERL (see code below). Using the file attributes, I can re-run the 
program after a while without having to re-calculate the checksums.
So, this solution only works for filesystems that have extended file 
attributes, but you could also use some simple database (sqlite3?) to 
map checksums to pathnames.

What I did was to walk through the directory tree and check if the file 
being considered already has a checksum in an extended attribute. If 
not. I'll calculate the checksum and store that in the extended 
attribute. Also, I store the pathname in a hash (remember, this is 
PERL), key is the checksum.
If there is a collision (checksum already in the hash), I remove the new 
file (and link the new filename to the old file). One could be paranoid 
and do a byte-by-byte file comparison then.

If I needed to do this in a C program, I'd probably use a GList to store 
the hash, but otherwise the code logic would be the same.

HTH,

Josef

#! /usr/bin/perl

use warnings;
use strict;
use File::ExtAttr ':all';       # In case of problems, maybe insert "use 
Scalar:Utils;" in /usr/lib/x86_64-linux-gnu/perl5/5.22/File/ExtAttr.pm
use Digest::SHA;
use File::Find;
use Getopt::Std;

# OPTIONS:
# s: force symlink
# n: don't do the actula removing/linking
# v: be more verbose
# h: print short help
my %opt = (
     s => undef,
     n => undef,
     v => undef,
     h => undef,
);
getopts('hnsv', \%opt);

if ($opt{h}) {
     print STDERR "usage: lndup [-snvh] [dirname..]\n";
     print STDERR "\t-s: use symlink rather than hard link\n";
     print STDERR "\t-n: don't remove/link, just show what would be done\n";
     print STDERR "\t-v: be more verbose (show pathname and SHA512 sum\n";
     print STDERR "\t-h: show this text\n";
     exit(0);
}

my %file;

if (@ARGV == 0) {
     find({ wanted => \&lndup, no_chdir => 1 }, '.');
} else {
     find({ wanted => \&lndup, no_chdir => 1 }, @ARGV);
}

# NAME: lndup
# PURPOSE: To handle a single file
# ARGUMENTS: None, pathname is taken from $File::Find::name
# RETURNS: Nothing
# NOTE: The SHA512 sum of a file is calculated.
#       IF a file with the same sum was already found earlier, AND
#       iF both files are NOT the same (hard link) AND
#       iF both files reside on the same disk
#       THEN the second occurrence is removed and
#           replaced by a link to the first occurrence
sub lndup {
     my $pathname = $File::Find::name;

     return if ! -f $pathname;
     if (-s $pathname) {
         my $sha512sum = getfattr($pathname, 'SHA512');
         if (!defined $sha512sum) {
             my $ctx = Digest::SHA->new(512);
             $ctx->addfile($pathname);
             $sha512sum = $ctx->hexdigest;
             print STDERR "$pathname $sha512sum\n" if $opt{v};
             setfattr($pathname, "SHA512", $sha512sum);
         } elsif ($opt{v}) {
             print STDERR "Using sha512sum from attributes\n";
         }

         if (exists $file{$sha512sum}) {
             if (!same_file($pathname, $file{$sha512sum})) {
                 my $links1 = (stat($pathname))[3];
                 my $links2 = (stat($file{$sha512sum}))[3];
                 # If one of them is a symbolic link, make sure it's 
$pathname
                 if (is_symlink($file{$sha512sum})) {
                     print STDERR "Swapping $pathname and 
$file{$sha512sum}\n" if $opt{v};
                     swap($file{$sha512sum}, $pathname);
                 }
                 # If $pathname has more links than $file{$sha512sum},
                 # exchange the two names.
                 # This ensures that $file{$sha512sum} has the most links.
                 elsif ($links1 > $links2) {
                     print STDERR "Swapping $pathname and 
$file{$sha512sum}\n" if $opt{v};
                     swap($file{$sha512sum}, $pathname);
                 }

                 print "rm \"$pathname\"; ln \"$file{$sha512sum}\" 
\"$pathname\"\n";
                 if (! $opt{n}) {
                     my $same_disk = same_disk($pathname, 
$file{$sha512sum});
                     if (unlink($pathname)) {
                         if (! $same_disk || $opt{s}) {
                             symlink($file{$sha512sum}, $pathname) || 
print STDERR "Failed to symlink($file{$sha512sum}, $pathname): $!\n";
                         } else {
                             link($file{$sha512sum}, $pathname) || print 
STDERR "Failed to link($file{$sha512sum}, $pathname): $!\n";
                         }
                     } else {
                         print STDERR "Failed to unlink $pathname: $!\n";
                     }
                 }
                 # print "Removing $pathname\n";
                 # unlink $pathname or warn "$0: Cannot remove $_: $!\n";

             }
         } else {
             $file{$sha512sum} = $pathname;
         }
     }
}

# NAME: same_disk
# PURPOSE: To check if two files are on the same disk
# ARGUMENTS: pn1, pn2: pathnames of files
# RETURNS: true if files are on the same disk, else false
# NOTE: The check is made by comparing the device numbers of the
#       filesystems of the two files.
sub same_disk {
     my ($pn1, $pn2) = @_;

     my @s1 = stat($pn1);
     my @s2 = stat($pn2);

     return $s1[0] == $s2[0];
}

# NAME: same_file
# PURPOSE: To check if two files are the same
# ARGUMENTS: pn1, pn2: pathnames of files
# RETURNS: true if files are the same, else false
# NOTE: files are the same if device number AND inode number
#       are identical
sub same_file {
     my ($pn1, $pn2) = @_;

     my @s1 = stat($pn1);
     my @s2 = stat($pn2);

     return ($s1[0] == $s2[0]) && ($s1[1] == $s2[1]);
}

sub is_symlink {
     my ($path) = @_;

     return -l $path;
}

sub swap {
     my $tmp;
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