A fast and powerful cross-platform duplicate file finder https://www.jdupes.com/
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Jody Bruchon a8b0812a7c hashdb: don't overwrite existing DB, use a temp file instead
Now the database won't be destroyed on write errors
2023-11-10 11:04:05 -05:00
example_scripts Add delete_but_exclude example scripts 2021-05-13 17:16:58 -04:00
icon Add transparent PNG for jdupes icon 2023-06-24 12:46:05 -04:00
testdir Fix numeric sort when zeroes aren't followed by a digit 2021-03-24 10:07:11 -04:00
.gitignore Use libjodycode.lib/.dll on Windows; add non-working MSVC builder 2023-10-12 13:17:34 -04:00
CHANGES.txt v1.27.3 release 2023-08-26 21:12:34 -04:00
INSTALL.txt Missed a GitHub link 2023-10-05 13:21:42 -04:00
ISSUES.txt Add ISSUES.txt: list of issue titles from defunct GitHub repo 2023-10-18 18:32:13 -04:00
LICENSE.txt Add .txt extensions 2023-06-16 19:57:09 -04:00
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README.stupid_dupes Update copyright years 2023-01-04 22:05:10 -05:00
act_dedupefiles.c Remove all Docker and GitHub stuff; make Codeberg the new dev site 2023-10-05 13:15:18 -04:00
act_dedupefiles.h Add timer-based progress updates on Windows (MinGW) 2023-06-09 22:19:41 -04:00
act_deletefiles.c Add safeguards to interactive deletion input 2023-10-18 18:51:31 -04:00
act_deletefiles.h Add NO_DELETE to disable file deletion code at build time 2022-12-01 13:52:02 -05:00
act_linkfiles.c Fix several compile issues 2023-10-09 00:06:31 -04:00
act_linkfiles.h Add timer-based progress updates on Windows (MinGW) 2023-06-09 22:19:41 -04:00
act_printjson.c Replace PATH_MAX with PATHBUF_SIZE 2023-10-12 12:42:43 -04:00
act_printjson.h Add timer-based progress updates on Windows (MinGW) 2023-06-09 22:19:41 -04:00
act_printmatches.c Unify "No duplicates found." string and support across actions 2023-06-10 18:20:36 -04:00
act_printmatches.h Remove more code when hard link support is disabled 2023-06-23 02:07:10 -04:00
act_summarize.c Unify "No duplicates found." string and support across actions 2023-06-10 18:20:36 -04:00
act_summarize.h Remove MIT+GPL dual licensing in some code; update copyright info 2016-12-31 17:29:07 -05:00
args.c Fix several compile issues 2023-10-09 00:06:31 -04:00
args.h Add timer-based progress updates on Windows (MinGW) 2023-06-09 22:19:41 -04:00
build_msvc.bat Use libjodycode.lib/.dll on Windows; add non-working MSVC builder 2023-10-12 13:17:34 -04:00
checks.c Fix several compile issues 2023-10-09 00:06:31 -04:00
checks.h Move checks out to their own compilation unit; fix header includes 2023-05-20 23:04:13 -04:00
chroot_build.sh Make chroot multi-package builds on Linux produce better names 2022-09-03 14:22:44 -04:00
compare_jdupes.sh compare_jdupes: handle when no original jdupes can be found 2023-10-14 18:23:11 -04:00
dumpflags.c Integrate hashdb into most of jdupes 2023-08-22 10:46:09 -04:00
dumpflags.h Move debug flag dumping to its own compilation unit 2023-06-22 18:21:52 -04:00
extfilter.c Add NO_HELPTEXT build option 2023-07-06 18:54:57 -04:00
extfilter.h Move checks out to their own compilation unit; fix header includes 2023-05-20 23:04:13 -04:00
filehash.c Use libjodycode jc_fopen() throughout code 2023-09-13 19:58:53 -04:00
filehash.h filehash.h: fix header declaration 2023-07-12 19:30:07 -04:00
filestat.c Fix several compile issues 2023-10-09 00:06:31 -04:00
filestat.h Move stat()-related functions to their own compilation unit 2023-05-14 11:18:16 -04:00
generate_packages.sh Fix numerous memory access errors; define v2 file format 2023-08-26 20:57:26 -04:00
hashdb.c hashdb: don't overwrite existing DB, use a temp file instead 2023-11-10 11:04:05 -05:00
hashdb.h Add hashdb_util, start work on hash database utility code 2023-09-13 19:14:26 -04:00
hashdb_util.c Use new libjodycode v4 API and checks 2023-10-04 22:31:51 -04:00
helptext.c Rename NO_JODY_SORT to NO_NUMSORT 2023-10-04 22:38:09 -04:00
helptext.h Fix helptext.h header barriers 2023-05-19 22:30:13 -04:00
interrupt.c Fix: -d with no dirs scanned causes internal error 2023-06-22 11:36:24 -04:00
interrupt.h interrupt: rename sighandler() to catch_interrupt() 2023-06-10 17:52:46 -04:00
jdupes.1 Remove GitHub references as much as possible 2023-08-25 02:09:51 -04:00
jdupes.c Update jdupes.c information 2023-10-25 18:14:40 -04:00
jdupes.h Replace PATH_MAX with PATHBUF_SIZE 2023-10-12 12:42:43 -04:00
libjodycode_check.c Use new libjodycode v4 API and checks 2023-10-04 22:31:51 -04:00
libjodycode_check.h Use new libjodycode v4 API and checks 2023-10-04 22:31:51 -04:00
likely_unlikely.h Add guards to likely_unlikely.h 2023-06-19 15:38:58 -04:00
linux-dedupe-static.h Fix linux static header IOCTL decimal literal (#212) 2022-09-12 14:42:50 -04:00
loaddir.c Switch loaddir to use new libjodycode dir functions entirely 2023-10-11 19:00:52 -04:00
loaddir.h Consolidate jc_slash_convert() calls 2023-09-20 18:53:14 -04:00
match.c Use libjodycode.lib/.dll on Windows; add non-working MSVC builder 2023-10-12 13:17:34 -04:00
match.h Move file open code for match confirmation into match.c 2023-08-28 22:01:12 -04:00
progress.c Fix CTRL-C not showing a message 2023-06-11 21:53:58 -04:00
progress.h Add timer-based progress updates on Windows (MinGW) 2023-06-09 22:19:41 -04:00
remove_hashdb_dead_entries.sh Improvements to hashdb cleaning script 2023-09-03 13:34:10 -04:00
sort.c Rename NO_JODY_SORT to NO_NUMSORT 2023-10-04 22:38:09 -04:00
sort.h Move sorting code into its own compilation unit 2023-05-19 22:54:11 -04:00
stupid_dupes.sh Remove GitHub references as much as possible 2023-08-25 02:09:51 -04:00
test.sh Add dummy test.sh so automated builds succeed 2023-06-19 13:20:24 -04:00
travcheck.c Don't bring libjodycode.h in via jdupes.h; declare independently 2023-05-19 22:35:24 -04:00
travcheck.h Rename all "travdone" things to "travcheck" 2023-04-14 13:01:39 -04:00
tune_winres.sh Add Windows XP build support; tune winres manifest XML too 2020-08-07 10:56:22 -04:00
version.h v1.27.3 release 2023-08-26 21:12:34 -04:00
winres.manifest.xml v1.27.3 release 2023-08-26 21:12:34 -04:00
winres.rc v1.27.3 release 2023-08-26 21:12:34 -04:00
winres_xp.rc v1.27.3 release 2023-08-26 21:12:34 -04:00
xxhash.c Re-add jody_hash as a build-time option and use it for LOW_MEMORY 2022-12-01 15:07:20 -05:00
xxhash.h xxhash: strip out 32-bit functions (we only use XXH64) 2022-12-01 14:28:52 -05:00



jdupes is a program for identifying and taking actions upon duplicate files such as deleting, hard linking, symlinking, and block-level deduplication (also known as "dedupe" or "reflink"). It is faster than most other duplicate scanners. It prioritizes data safety over performance while also giving expert users access to advanced (and sometimes dangerous) features.

Please consider financially supporting continued development of jdupes using the links on my home page (Ko-fi, PayPal, SubscribeStar, etc.):


Why use jdupes instead of the original fdupes or other duplicate finders?

The biggest reason is raw speed. In testing on various data sets, jdupes is over 7 times faster than fdupes-1.51 on average.

jdupes provides a native Windows port. Most duplicate scanners built on Linux and other UNIX-like systems do not compile for Windows out-of-the-box and even if they do, they don't support Unicode and other Windows-specific quirks and features.

jdupes is generally stable. All releases of jdupes are compared against a known working reference versions of fdupes or jdupes to be certain that output does not change. You get the benefits of an aggressive development process without putting your data at increased risk.

Code in jdupes is written with data loss avoidance as the highest priority. If a choice must be made between being aggressive or careful, the careful way is always chosen.

jdupes includes features that are not always found elsewhere. Examples of such features include block-level data deduplication and control over which file is kept when a match set is automatically deleted. jdupes is not afraid of dropping features of low value; a prime example is the -1 switch which outputs all matches in a set on one line, a feature which was found to be useless in real-world tests and therefore thrown out.

While jdupes maintains some degree of compatibility with fdupes from which it was originally derived, there is no guarantee that it will continue to maintain such compatibility in the future. However, compatibility will be retained between minor versions, i.e. jdupes-1.6 and jdupes-1.6.1 should not have any significant differences in results with identical command lines.

If the program eats your dog or sets fire to your lawn, the authors cannot be held responsible. If you notice a bug, please report it.

What jdupes is not: a similar (but not identical) file finding tool

Please note that jdupes ONLY works on 100% exact matches. It does not have any sort of "similarity" matching, nor does it know anything about any specific file formats such as images or sounds. Something as simple as a change in embedded metadata such as the ID3 tags in an MP3 file or the EXIF information in a JPEG image will not change the sound or image presented to the user when opened, but technically it makes the file no longer identical to the original.

Plenty of excellent tools already exist to "fuzzy match" specific file types using knowledge of their file formats to help. There are no plans to add this type of matching to jdupes.

There are some match options available in jdupes that enable dangerous file matching based on partial or likely but not 100% certain matching. These are considered expert options for special situations and are clearly and loudly documented as being dangerous. The -Q and -T options are notable examples, and the extreme danger of the -T option is safeguarded by a requirement to specify it twice so it can't be used accidentally.

How can I do stuff with jdupes that isn't supported by fdupes?

The standard output format of jdupes is extremely simple. Match sets are presented with one file path per line, and match sets are separated by a blank line. This is easy to process with fairly simple shell scripts. You can find example shell scripts in the "example scripts" directory in the jdupes source code. The main example script, "example.sh", is easy to modify to take basic actions on each file in a match set. These scripts are used by piping the standard jdupes output to them:

jdupes dir1 dir2 dir3 | example.sh scriptparameters


Usage: jdupes [options] DIRECTORY...

Duplicate file sets will be printed by default unless a different action option is specified (delete, summarize, link, dedupe, etc.)

 -@ --loud              output annoying low-level debug info while running
 -0 --print-null        output nulls instead of CR/LF (like 'find -print0')
 -1 --one-file-system   do not match files on different filesystems/devices
 -A --no-hidden         exclude hidden files from consideration
 -B --dedupe            do a copy-on-write (reflink/clone) deduplication
 -C --chunk-size=#      override I/O chunk size in KiB (min 4, max 262144)
 -d --delete            prompt user for files to preserve and delete all
                        others; important: under particular circumstances,
                        data may be lost when using this option together
                        with -s or --symlinks, or when specifying a
                        particular directory more than once; refer to the
                        documentation for additional information
 -D --debug             output debug statistics after completion
 -e --error-on-dupe     exit on any duplicate found with status code 255
 -f --omit-first        omit the first file in each set of matches
 -h --help              display this help message
 -H --hard-links        treat any linked files as duplicate files. Normally
                        linked files are treated as non-duplicates for safety
 -i --reverse           reverse (invert) the match sort order
 -I --isolate           files in the same specified directory won't match
 -j --json              produce JSON (machine-readable) output
 -l --link-soft         make relative symlinks for duplicates w/o prompting
 -L --link-hard         hard link all duplicate files without prompting
                        Windows allows a maximum of 1023 hard links per file
 -m --summarize         summarize dupe information
 -M --print-summarize   will print matches and --summarize at the end
 -N --no-prompt         together with --delete, preserve the first file in
                        each set of duplicates and delete the rest without
                        prompting the user
 -o --order=BY          select sort order for output, linking and deleting:
                        by mtime (BY=time) or filename (BY=name, the default)
 -O --param-order       sort output files in order of command line parameter
                        Parameter order is more important than selected -o sort
                        which applies should several files share the same
parameter order
 -p --permissions       don't consider files with different owner/group or
                        permission bits as duplicates
 -P --print=type        print extra info (partial, early, fullhash)
 -q --quiet             hide progress indicator
 -Q --quick             skip byte-by-byte duplicate verification. WARNING:
                        this may delete non-duplicates! Read the manual first!
 -r --recurse           for every directory, process its subdirectories too
 -R --recurse:          for each directory given after this option follow
                        subdirectories encountered within (note the ':' at
                        the end of the option, manpage for more details)
 -s --symlinks          follow symlinks
 -S --size              show size of duplicate files
 -t --no-change-check   disable security check for file changes (aka TOCTTOU)
 -T --partial-only      match based on partial hashes only. WARNING:
                        EXTREMELY DANGEROUS paired with destructive actions!
                        -T must be specified twice to work. Read the manual!
 -u --print-unique      print only a list of unique (non-matched) files
 -U --no-trav-check     disable double-traversal safety check (BE VERY CAREFUL)
                        This fixes a Google Drive File Stream recursion issue
 -v --version           display jdupes version and license information
 -X --ext-filter=x:y    filter files based on specified criteria
                        Use '-X help' for detailed extfilter help
 -y --hash-db=file      use a hash database text file to speed up repeat runs
                        Passing '-y .' will expand to  '-y jdupes_hashdb.txt'
 -z --zero-match        consider zero-length files to be duplicates
 -Z --soft-abort        If the user aborts (i.e. CTRL-C) act on matches so far
                        You can send SIGUSR1 to the program to toggle this

Detailed help for jdupes -X/--extfilter options
General format: jdupes -X filter[:value][size_suffix]

noext:ext1[,ext2,...]           Exclude files with certain extension(s)

onlyext:ext1[,ext2,...]         Only include files with certain extension(s)

size[+-=]:size[suffix]          Only Include files matching size criteria
                                Size specs: + larger, - smaller, = equal to
                                Specs can be mixed, i.e. size+=:100k will
                                only include files 100KiB or more in size.

nostr:text_string               Exclude all paths containing the string
onlystr:text_string             Only allow paths containing the string
                                HINT: you can use these for directories:
                                -X nostr:/dir_x/  or  -X onlystr:/dir_x/
newer:datetime                  Only include files newer than specified date
older:datetime                  Only include files older than specified date
                                Date/time format: "YYYY-MM-DD HH:MM:SS"
                                Time is optional (remember to escape spaces!)

Some filters take no value or multiple values. Filters that can take
a numeric option generally support the size multipliers K/M/G/T/P/E
with or without an added iB or B. Multipliers are binary-style unless
the -B suffix is used, which will use decimal multipliers. For example,
16k or 16kib = 16384; 16kb = 16000. Multipliers are case-insensitive.

Filters have cumulative effects: jdupes -X size+:99 -X size-:101 will
cause only files of exactly 100 bytes in size to be included.

Extension matching is case-insensitive.
Path substring matching is case-sensitive.

The -U/--no-trav-check option disables the double-traversal protection. In the VAST MAJORITY of circumstances, this SHOULD NOT BE DONE, as it protects against several dangerous user errors, including specifying the same files or directories twice causing them to match themselves and potentially be lost or irreversibly damaged, or a symbolic link to a directory making an endless loop of recursion that will cause the program to hang indefinitely. This option was added because Google Drive File Stream presents directories in the virtual hard drive used by GDFS with identical device:inode pairs despite the directories actually being different. This triggers double-traversal prevention against every directory, effectively blocking all recursion. Disabling this check will reduce safety, but will allow duplicate scanning inside Google Drive File Stream drives. This also results in a very minor speed boost during recursion, but the boost is unlikely to be noticeable.

The -t/--no-change-check option disables file change checks during/after scanning. This opens a security vulnerability that is called a TOCTTOU (time of check to time of use) vulnerability. The program normally runs checks immediately before scanning or taking action upon a file to see if the file has changed in some way since it was last checked. With this option enabled, the program will not run any of these checks, making the algorithm slightly faster, but also increasing the risk that the program scans a file, the file is changed after the scan, and the program still acts like the file was in its previous state. This is particularly dangerous when considering actions such as linking and deleting. In the most extreme case, a file could be deleted during scanning but match other files prior to that deletion; if the file is the first in the list of duplicates and auto-delete is used, all of the remaining matched files will be deleted as well. This option was added due to user reports of some filesystems (particularly network filesystems) changing the reported file information inappropriately, rendering the entire program unusable on such filesystems.

The -n/--no-empty option was removed for safety. Matching zero-length files as duplicates now requires explicit use of the -z/--zero-match option instead.

Duplicate files are listed together in groups with each file displayed on a separate line. The groups are then separated from each other by blank lines.

The -s/--symlinks option will treat symlinked files as regular files, but direct symlinks will be treated as if they are hard linked files and the -H/--hard-links option will apply to them in the same manner.

When using -d or --delete, care should be taken to insure against accidental data loss. While no information will be immediately lost, using this option together with -s or --symlink can lead to confusing information being presented to the user when prompted for files to preserve. Specifically, a user could accidentally preserve a symlink while deleting the file it points to. A similar problem arises when specifying a particular directory more than once. All files within that directory will be listed as their own duplicates, leading to data loss should a user preserve a file without its "duplicate" (the file itself!)

Using -1 or --one-file-system prevents matches that cross filesystems, but a more relaxed form of this option may be added that allows cross-matching for all filesystems that each parameter is present on.

-Z or --soft-abort used to be --hard-abort in jdupes prior to v1.5 and had the opposite behavior. Defaulting to taking action on abort is probably not what most users would expect. The decision to invert rather than reassign to a different option was made because this feature was still fairly new at the time of the change.

On non-Windows platforms that support SIGUSR1, you can toggle the state of the -Z option by sending a SIGUSR1 to the program. This is handy if you want to abort jdupes, didn't specify -Z, and changed your mind and don't want to lose all the work that was done so far. Just do 'killall -USR1 jdupes' and you will be able to abort with -Z. This works in reverse: if you want to prevent a -Z from happening, a SIGUSR1 will toggle it back off. That's a lot less useful because you can just stop and kill the program to get the same effect, but it's there if you want it for some reason. Sending the signal twice while the program is stopped will behave as if it was only sent once, as per normal POSIX signal behavior.

The -O or --param-order option allows the user greater control over what appears in the first position of a match set, specifically for keeping the -N option from deleting all but one file in a set in a seemingly random way. All directories specified on the command line will be used as the sorting order of result sets first, followed by the sorting algorithm set by the -o or --order option. This means that the order of all match pairs for a single directory specification will retain the old sorting behavior even if this option is specified.

When used together with options -s or --symlink, a user could accidentally preserve a symlink while deleting the file it points to.

The -Q or --quick option only reads each file once, hashes it, and performs comparisons based solely on the hashes. There is a small but significant risk of a hash collision which is the purpose of the failsafe byte-for-byte comparison that this option explicitly bypasses. Do not use it on ANY data set for which any amount of data loss is unacceptable. You have been warned!

The -T or --partial-only option produces results based on a hash of the first block of file data in each file, ignoring everything else in the file. Partial hash checks have always been an important exclusion step in the jdupes algorithm, usually hashing the first 4096 bytes of data and allowing files that are different at the start to be rejected early. In certain scenarios it may be a useful heuristic for a user to see that a set of files has the same size and the same starting data, even if the remaining data does not match; one example of this would be comparing files with data blocks that are damaged or missing such as an incomplete file transfer or checking a data recovery against known-good copies to see what damaged data can be deleted in favor of restoring the known-good copy. This option is meant to be used with informational actions and can result in EXTREME DATA LOSS if used with options that delete files, create hard links, or perform other destructive actions on data based on the matching output. Because of the potential for massive data destruction, this option MUST BE SPECIFIED TWICE to take effect and will error out if it is only specified once.

The -I/--isolate option attempts to block matches that are contained in the same specified directory parameter on the command line. Due to the underlying nature of the jdupes algorithm, a lot of matches will be blocked by this option that probably should not be. This code could use improvement.

The -C/--chunk-size option overrides the size of the I/O "chunk" used for all file operations. Larger numbers will increase the amount of data read at once from each file and may improve performance when scanning lots of files that are larger than the default chunk size by reducing "thrashing" of the hard disk heads. Smaller numbers may increase algorithm speed depending on the characteristics of your CPU but will usually increase I/O and system call overhead as well. The number also directly affects memory usage: I/O chunk size is used for at least three allocations in the program, so using a chunk size of 16777216 (16 MiB) will require 48 MiB of RAM. The default is usually between 32768 and 65536 which results in the fastest raw speed of the algorithm and generally good all-around performance. Feel free to experiment with the number on your data set and report your experiences (preferably with benchmarks and info on your data set.)

Using -P/--print will cause the program to print extra information that may be useful but will pollute the output in a way that makes scripted handling difficult. Its current purpose is to reveal more information about the file matching process by printing match pairs that pass certain steps of the process prior to full file comparison. This can be useful if you have two files that are passing early checks but failing after full checks.

The -y/--hash-db feature creates and maintains a text file with a list of file paths, hashes, and other metadata that enables jdupes to "remember" file data across runs. Specifying a period '.' as the database file name will use a name of "jdupes_hashdb.txt" instead; this alias makes it easy to use the hash database feature without typing a descriptive name each time. THIS FEATURE IS CURRENTLY UNDER DEVELOPMENT AND HAS MANY QUIRKS. USE IT AT YOUR OWN RISK. In particular, one of the biggest problems with this feature is that it stores every path exactly as specified on the command line; if any paths are passed into jdupes on a subsequent run with a different prefix then they will not be recognized and they will be treated as totally different files. For example, running jdupes -y . foo/ is not the same as jdupes -y . ./foo nor the same as (from a sibling directory) jdupes -y ../foo. You must run jdupes from the same working directory and with the same path specifications to take advantage of the hash database feature. When used correctly, a fully populated hash database can reduce subsequent runs with hundreds of thousands of files that normally take a very long time to run down to the directory scanning time plus a couple of seconds. If the directory data is already in the OS disk cache, this can make subsequent runs with over 100K files finish in under one second.

Hard and soft (symbolic) linking status symbols and behavior

A set of arrows are used in file linking to show what action was taken on each link candidate. These arrows are as follows:

----> File was hard linked to the first file in the duplicate chain

-@@-> File was symlinked to the first file in the chain

-##-> File was cloned from the first file in the chain

-==-> Already a hard link to the first file in the chain

-//-> File linking failed due to an error during the linking process

If your data set has linked files and you do not use -H to always consider them as duplicates, you may still see linked files appear together in match sets. This is caused by a separate file that matches with linked files independently and is the correct behavior. See notes below on the "triangle problem" in jdupes for technical details.

Microsoft Windows platform-specific notes

Windows has a hard limit of 1024 hard links per file. There is no way to change this. The documentation for CreateHardLink() states: "The maximum number of hard links that can be created with this function is 1023 per file. If more than 1023 links are created for a file, an error results." (The number is actually 1024, but they're ignoring the first file.)

The current jdupes algorithm's "triangle problem"

Pairs of files are excluded individually based on how the two files compare. For example, if --hard-links is not specified then two files which are hard linked will not match one another for duplicate scanning purposes. The problem with only examining files in pairs is that certain circumstances will lead to the exclusion being overridden.

Let's say we have three files with identical contents:


and a/file1 is linked to a/file3. Here's how jdupes a/ sees them:

    Are 'a/file1' and 'a/file2' matches? Yes
    [point a/file1->duplicates to a/file2]

    Are 'a/file1' and 'a/file3' matches? No (hard linked already, `-H` off)

    Are 'a/file2' and 'a/file3' matches? Yes
    [point a/file2->duplicates to a/file3]

Now you have the following duplicate list:

a/file1->duplicates ==> a/file2->duplicates ==> a/file3

The solution is to split match sets into multiple sets, but doing this will also remove the guarantee that files will only ever appear in one match set and could result in data loss if handled improperly. In the future, options for "greedy" and "sparse" may be introduced to switch between allowing triangle matches to be in the same set vs. splitting sets after matching finishes without the "only ever appears once" guarantee.

Does jdupes meet the "Good Practice when Deleting Duplicates" by rmlint?

Yes. If you've not read this list of cautions, it is available at http://rmlint.readthedocs.io/en/latest/cautions.html

Here's a breakdown of how jdupes addresses each of the items listed.

"Backup your data"/"Measure twice, cut once"

These guidelines are for the user of duplicate scanning software, not the software itself. Back up your files regularly. Use jdupes to print a list of what is found as duplicated and check that list very carefully before automatically deleting the files.

"Beware of unusual filename characters"

The only character that poses a concern in jdupes is a newline \n and that is only a problem because the duplicate set printer uses them to separate file names. Actions taken by jdupes are not parsed like a command line, so spaces and other weird characters in names aren't a problem. Escaping the names properly if acting on the printed output is a problem for the user's shell script or other external program.

"Consider safe removal options"

This is also an exercise for the user.

"Traversal Robustness"

jdupes tracks each directory traversed by dev:inode pair to avoid adding the contents of the same directory twice. This prevents the user from being able to register all of their files twice by duplicating an entry on the command line. Symlinked directories are only followed if they weren't already followed earlier. Files are renamed to a temporary name before any linking is done and if the link operation fails they are renamed back to the original name.

"Collision Robustness"

jdupes uses xxHash for file data hashing. This hash is extremely fast with a low collision rate, but it still encounters collisions as any hash function will ("secure" or otherwise) due to the pigeonhole principle. This is why jdupes performs a full-file verification before declaring a match. It's slower than matching by hash only, but the pigeonhole principle puts all data sets larger than the hash at risk of collision, meaning a false duplicate detection and data loss. The slower completion time is not as important as data integrity. Checking for a match based on hashes alone is irresponsible, and using secure hashes like MD5 or the SHA families is orders of magnitude slower than xxHash while still suffering from the risk brought about by the pigeonholing. An example of this problem is as follows: if you have 365 days in a year and 366 people, the chance of having at least two birthdays on the same day is guaranteed; likewise, even though SHA512 is a 512-bit (64-byte) wide hash, there are guaranteed to be at least 256 pairs of data streams that causes a collision once any of the data streams being hashed for comparison is 65 bytes (520 bits) or larger.

"Unusual Characters Robustness"

jdupes does not protect the user from putting ASCII control characters in their file names; they will mangle the output if printed, but they can still be operated upon by the actions (delete, link, etc.) in jdupes.

"Seek Thrash Robustness"

jdupes uses an I/O chunk size that is optimized for reading as much as possible from disk at once to take advantage of high sequential read speeds in traditional rotating media drives while balancing against the significantly higher rate of CPU cache misses triggered by an excessively large I/O buffer size. Enlarging the I/O buffer further may allow for lots of large files to be read with less head seeking, but the CPU cache misses slow the algorithm down and memory usage increases to hold these large buffers. jdupes is benchmarked periodically to make sure that the chosen I/O chunk size is the best compromise for a wide variety of data sets.

"Memory Usage Robustness"

This is a very subjective concern considering that even a cell phone in someone's pocket has at least 1GB of RAM, however it still applies in the embedded device world where 32MB of RAM might be all that you can have. Even when processing a data set with over a million files, jdupes memory usage (tested on Linux x86-64 with -O3 optimization) doesn't exceed 2GB. A low memory mode can be chosen at compile time to reduce overall memory usage with a small performance penalty.

How does a duplicate scanner algorithm work?

The most naive way to look for files that are the same is to compare all files to all other files using a tool like cmp command on Linux/macOS/BSD or the fc command on Windows/DOS. This works but is extremely slow and wastes a lot of time. For every new file to compare, the number of comparisons increases exponentially (the formula is n(n-1)/2 for the discrete math nerds):

Files Compares
2 1
3 3
4 6
5 10
10 45
100 4950
1000 499500
5000 12497500
10000 49995000
14142 99991011

Let's say that every file is 1,000 bytes in size and you have 10,000 files for a total size of 10,000,000 bytes (about 9.53 MiB). Using this naive comparison approach means the actual amount of data to compare is around 47,679 MiB. You should be able to see how extreme this can get--especially with larger files.

A slightly smarter approach is to use file hashes as a substitute for the full file contents. A hash is a number based on the data fed into a hash function and the number is always the same when the same data is fed in. If the hash for two files is different then the contents of those files are guaranteed to be different; if the hash is the same then the data might be the same, though this is not guaranteed due to the birthday problem: the size of the number is much smaller than the size of the data it represents, so there will always be many different inputs that produce the same hash value. Files with matching hash values must still be compared just to be sure that they are 100% identical.

49,995,000 comparisons can be done much quicker when you're only comparing a single big number every time instead of thousands or millions of bytes. This makes a big difference in performance since the only files being compared are files that look likely to be identical.

Fast exclusion of non-duplicates is the main purpose of duplicate scanners.

jdupes uses a lot of fast exclusion techniques beyond this. A partial list of these in the order they're performed is as follows:

  1. Files that the user asks the program to exclude are skipped entirely
  2. Files with different sizes can't be identical, so they're not compared
  3. The first 4 KiB is hashed and compared which avoids reading full files
  4. Entire files are hashed and compared which avoids comparing data directly
  5. Finally, actual file data is compared to verify that they are duplicates

The vast majority of non-duplicate file pairs never make it past the partial (4 KiB) hashing step. This reduces the amount of data read from disk and time spent comparing things to the smallest amount possible.

v1.20.0 specific: most long options have changed and -n has been removed

Long options now have consistent hyphenation to separate the words used in the option names. Run jdupes -h to see the correct usage. Legacy options will remain in place until the next major or minor release (v2.0 or v1.21.0) for compatibility purposes. Users should change any scripts using the old options to use the new ones...or better yet, stop using long options in your scripts in the first place, because it's unnecessarily verbose and wasteful to do so.

v1.15+ specific: Why is the addition of single files not working?

If a file was added through recursion and also added explicitly, that file would end up matching itself. This issue can be seen in v1.14.1 or older versions that support single file addition using a command like this in the jdupes source code directory:

/usr/src/jdupes$ jdupes -rH testdir/isolate/1/ testdir/isolate/1/1.txt testdir/isolate/1/1.txt testdir/isolate/1/1.txt testdir/isolate/1/2.txt

Even worse, using the special dot directory will make it happen without the -H option, which is how I discovered this bug:

/usr/src/jdupes/testdir/isolate/1$ jdupes . 1.txt ./1.txt ./2.txt 1.txt

This works for any path with a single dot directory anywhere in the path, so it has a good deal of potential for data loss in some use cases. As such, the best option was to shove out a new minor release with this feature turned off until some additional checking can be done, e.g. by making sure the canonical paths aren't identical between any two files.

A future release will fix this safely.

Website and contact information

General program information, help, and tech info: https://www.jdupes.com/

Development, source code, and releases: https://codeberg.org/jbruchon/jdupes

Have a bug report or questions? contact Jody Bruchon jody@jodybruchon.com

jdupes is Copyright (C) 2015-2023 by Jody Bruchon jody@jodybruchon.com

Derived from the original 'fdupes' 1.51 (C) 1999-2014 by Adrian Lopez

The MIT License

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.