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PeaZip is a free cross-platform file archiver and hashing utility that provides an unified portable GUI for many Open Source technologies like 7-Zip, FreeArc, PAQ, UPX...
Create 7Z, ARC, BZ2, GZ, *PAQ, PEA, QUAD/BALZ, TAR, UPX, WIM, XZ, ZIP files
Open and extract over 180 archive types: ACE, ARJ, CAB, DMG, ISO, LHA, RAR, UDF, ZIPX files and more...
Features of PeaZip includes extract, create and convert multiple archives at once, create self-extracting archives, split/join files, strong encryption with two factor authentication, encrypted password manager, secure deletion, find duplicate files, calculate hashes, export job definition as script.

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FAQ / FILE MANAGER > DETECT IDENTICAL FILES
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How to find duplicate files



DETECT REDUNDANT FILES WITH IDENTICAL CHECKSUM / HASH VALUE

find duplicate files



Data deduplication, to identify and (possibly) remove duplicate content, is important to reduce disk occupation without loss of information (the data being removed exists in other copies), in order to keep under control the size of backup - possibly speeding up the process and sparing space on backup media supports - and to reduce the final size of archives. Some compressors pushes the principle further and integrate mechanisms to identify / remove duplicate data blocks in order to improve compression ratio.


Search for duplicate files


When browsing a filesystem the file browser can show file checksum / hash value on demand in last column, allowing to identify binary identical files which have same checksum/hash value.
Clicking the name of the function (in context menu, "File tools" group) will display hash or checksum value for all (or selected) files.
Clicking "Find duplicates" will display size and hash or checksum value only for duplicate files - same binary identical content featured in two or more distinct files - and report the number of non-unique files identified.

In both cases, sorting for CRC column allows to group all files (in same folder, or same search filter) with identical hash or checksum.

The verification function used to deduplicate files can be set in main application's menu: Organize, Browser, Checksum/hash), a wide selection of algorithms can be selected, ranging from simple checksum functions as Adler32, CRC family (CRC16/24/32, and CRC64) to hash functions like eDonkey/eMule, MD4, MD5, and cryptographically strong hash as Ripemd160, SHA-1 and SHA-2 (SHA224/256/384, and SHA512), and Whirlpool512.

When browsing an archive this on demand verification is not available, but some archive types provides the same integrity-checking information, saving for each archived object the pre-computed checksum or hash value depending on the archive format, and on the archival settings employed - i.e. CRC32 in ZIP archives - allowing to sort archive content by CRC column to group identical files and find out duplicates.


Find similar images


When browsing a filesystem, PeaZip can display image thumbnails to help deduplication: in context menu, organize, check show picture thumbnails, or select a file browser's preset style showing thumbnails.
While checksum/hash based inspection allows to search for exactly identical files (and images), thumbnails allows the user to visually detect similar images (i.e. same picture or graphic saved in different formats, or with different color depth or compression settings, or scaled to different sizes), to help in deciding if the (pseudo) duplication is acceptable, and what copy (or version) to keep or delete.
As role of thumb for deleting extra versions, the best quality image (larger resolution, lower compression or possibly lossless format as RAW, BMP, TIFF, PNG) should be kept, discarding lower quality copies: once lost, information/quality cannot be recreated.


Verify multiple checksum and hash values at once


Check files utility in "File tools" submenu (context menu) allows to compare multiple hash and checksum algorithms of multiple files at once.
Employing multiple functions, and relying on cryptographically strong hash algorithms as Ripemd, SHA-2, Whirlpool, can identify even malicious attempt of forging identical-looking files, detecting differences that would go undetected to weaker algorithms, subject to easier found collisions.


Byte-to-byte comparison (alternative deduplication method)


Compare files utility in "File tools" submenu performs byte to byte comparison between two files; unlike checksum/hash method it is not subject of collisions under any circumstance, and can find out and report exactly what the different bytes are - so it not only tells if two files are not identical, but also what changes were made to content between the two versions.

Read more: checksumvalidate data integrity, and hash functionsfind hash value definitions on Wikipedia.


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