# Gentle Guide to `rmlint`

Welcome to the Tutorial of `rmlint`.

We use a few terms that might not be obvious to you at first, so we gonna explain them to you here.

Original: | In a group of duplicate files, one file is said to be the original file. It might not, strictly speaking, be the original from which the copies where created, but is a convenient terminology for selecting which files to keep and which to delete.
Duplicate: | A file that matches the original. Note that depending on rmlint settings, “match” may mean an exact match or just that the files have matching hash values.

## Beginner Examples

Let’s just dive in into some examples:

    $ rmlint

This simply scans your current working directory for lint and reports them in your terminal. Note that nothing will be removed (even if it prints `rm`).

Despite its name, `rmlint` just finds suspicious files, but never modifies the filesystem itself [*]. Instead it gives you detailed reports in different formats to get rid of them yourself. These reports are called outputs. By default a shellscript will be written to `rmlint.sh` that contains readily prepared shell commands to remove duplicates and other finds,

[*] | You could say it should be named `findlint`.

So for the above example the full process, if you want to actually delete the lint that was found, would be:

    $ rmlint some/path
    # (wait for rmlint to finish running)
    $ gedit rmlint.sh
    # (or any editor you prefer... review the content of rmlint.sh to
    #  check what it plans to delete; make any edits as necessary)
    $ ./rmlint.sh
    # (the rmlint.sh script will ask for confirmation, then delete the
    #  appropriate lint, then delete itself)

On larger runs, it might be more preferable to show a progressbar instead of a long list of files. You can do this easily with the `-g` switch:

    $ rmlint -g

It will look like this:

## Filtering input files

What if we do not want to check all files as dupes? `rmlint` has a good repertoire of options to select only certain files. We won’t cover all options, but will get you started with a few useful ones. Note if you want a more do-it-yourself approach to file filtering, you can also use external tools to feed `rmlint's stdin`:

    $ find pics/ -iname '*.png' | rmlint -
    $ find pics/ -iname '*.png' -print0 | rmlint -0 # (also handles filenames with newline characters)

### Limit files by size using `--size`

    # only check files between 20 MB and 1 Gigabyte:
    $ rmlint --size 20M-1G
    # short form (-s) works just as well:
    $ rmlint -s 20M-1G
    # only check files bigger than 4 kB:
    $ rmlint -s 4K
    # only check files smaller than 1234 bytes:
    $ rmlint -s 0-1234
    # Find empty files and handle them as duplicates:
    $ rmlint -T df --size 0-1

Valid units include:

K,M,G,T,P for powers of 1000
KB, MB, GB etc for powers of 1024

If no units are given, `rmlint` will assume bytes.

### Limit duplicate matching according to basename

By default, `rmlint` compares file contents, regardless of file name. So if afile.jpg has the same content as bfile.txt (which is unlikely!), then `rmlint` will find and report this as a duplicate. You can speed things up a little bit by telling rmlint not to try to match files unless they have the same or similar file names. The three options here are:

`-b` (`--match-basename`)
`-e` (`--match-extension`)
`-i` (`--match-without-extension`) .

Examples:

    # Find all duplicate files with the same basename:
    $ rmlint -b some_dir/
    ls some_dir/one/hello.c
    rm some_dir/two/hello.c
    # Find all duplicate files that have the same extension:
    $ rmlint -e some_dir/
    ls some_dir/hello.c
    rm some_dir/hello_copy.c
    # Find all duplicate files that have the same basename:
    # minus the extension
    $ rmlint -e some_dir/
    ls some_dir/hello.c
    rm some_dir/hello.bak

### Limit files by their modification time

This is a useful feature if you want to investigate only files newer than a certain date or if you want to progressively update the results, i.e. when you run `rmlint` in a script that watches a directory for duplicates.

The manual way is using `-N` (`--newer-than=<timestamp>`):

    # Use a Unix-UTC Timestamp (seconds since epoch)
    $ rmlint -N 1414755960

    # Find all files newer than file.png
    $ rmlint -N $(stat --print %Y file.png)

    # Alternatively use an ISO8601 formatted Timestamp
    $ rmlint -N 2014-09-08T00:12:32+0200

If you are periodically checking the same directory tree for duplicates, you can get a substantial speedup by creating an automatic timestamp file each time you run rmlint. To do this, use command line options: `-n` (`--newer-than-stamp`) and `-O stamp:stamp.file` (we’ll come to outputs in a minute): Here’s an example for incrementally scanning your home folder:

    # First run of rmlint:
    $ rmlint /home/foobar -O stamp:/home/foobar/.rmlint.stamp
    ls /home/foobar/a.file
    rm /home/foobar/b.file

    # Second run, no changes:
    $ rmlint /home/foobar -n /home/foobar/.rmlint.stamp
    <nothing>

    # Second run, new file copied:
    $ cp /home/foobar/a.file /home/foobar/c.file
    $ rmlint /home/foobar -n /home/foobar/.rmlint.stamp
    ls /home/foobar/a.file
    rm /home/foobar/b.file
    rm /home/foobar/c.file

Note that `-n` updates the timestamp file each time it is run.

## Outputs & Formatters

`rmlint` is capable of creating reports in several output formats, to either your screen or to a file. If you run it with the default options you already see two of those output formatters on your screen, namely `pretty` and `summary`.

Extra output formats can be added via either the `-O` (`--add-output`) or `-o` (`--output`) switch. The only difference is the `-o` clears all the default outputs while `-O` just adds to the defaults.

Note

If you just came here to learn how to print a nice progressbar: Just use the `-g` (`--progress`) option:

    $ rmlint -g /usr

Here’s an example:

    $ rmlint -o json:stderr

Here you would get this output printed on `stderr`:

    [{
      "description": "rmlint json-dump of lint files",
      "cwd": "/home/user/",
      "args": "rmlint -o json:stderr"
    },
    {
      "type": "duplicate_file",
      "path": "/home/user/test/b/one",
      "size": 2,
      "inode": 2492950,
      "disk_id": 64771,
      "progress": 100,
      "is_original": true,
      "mtime": 1414587002
    },
    ... snip ...
    {
      "aborted": false,
      "total_files": 145,
      "ignored_files": 9,
      "ignored_folders": 4,
      "duplicates": 11,
      "duplicate_sets": 2,
      "total_lint_size": 38
    }]

You probably noticed the colon in the commandline above. Everything before it is the name of the output-format, everything behind is the path where the output should land. Instead of a path you can also use `stdout` and `stderr`, as we did above or just omit the colon which will print everything to `stdout`.

Some formatters can be customised using the `-c` (`--config`) command. Here’s the list of currently available formatters and their config options:

json:

Outputs all finds as a json document. The document is a list of dictionaries, where the first and last element is the header and the footer respectively, everything between are data-dictionaries. This format was chosen to allow application to parse the output in realtime while `rmlint` is still running.

The header contains information about the program invocation, while the footer contains statistics about the program-run. Every data element has a type which identifies its lint type (you can lookup all types here_).

Config values:

- use_header=[true|false]: Print the header with metadata.
- use_footer=[true|false]: Print the footer with statistics.
- oneline=[true|false]: Print one json document per line.

sh:

Outputs a shell script defines a command function for each lint type, which it then calls for each file of each type. The script can be executed (it is already `chmod +x`’d by `rmlint`). By default it will ask you if you really want to proceed. If you do not want that confirmation prompt you can pass the `-d`. Additionally it will delete itself after running, unless you pass the `-x` switch to the `sh` script.

It is enabled by default and writes to `rmlint.sh`.

Example output:

    $ rmlint -o sh:stdout
    #!/bin/sh
    # This file was autowritten by rmlint
    # rmlint was executed from: /home/user/
    # You command line was: ./rmlint -o sh:rmlint.sh

    # ... snip ...

    echo  '/home/user/test/b/one' # original
    remove_cmd '/home/user/test/b/file' # duplicate
    remove_cmd '/home/user/test/a/two' # duplicate
    remove_cmd '/home/user/test/a/file' # duplicate

    if [ -z $DO_REMOVE ]
    then
      rm -f 'rmlint.sh';
    fi

Config values:

- clone: reflink-capable filesystems only. Try to clone both files with the FIDEDUPERANGE `ioctl(3p)` (or BTRFS_IOC_FILE_EXTENT_SAME on older kernels). This will free up duplicate extents. Needs at least kernel 4.2.
- reflink: Try to reflink the duplicate file to the original. See also `--reflink` in `man 1 cp`. Fails if the filesystem does not support it.
- hardlink: Replace the duplicate file with a hardlink to the original file. Fails if both files are not on the same partition.
- symlink: Tries to replace the duplicate file with a symbolic link to the original. Never fails.
- remove: Remove the file using `rm -rf`. (`-r` for duplicate dirs). Never fails.
- usercmd: Use the provided user defined command (`-c sh:cmd='*user command*'`). Use “$1” within ‘user command’ to refer to the duplicate file and (optionally) “$2” to refer to the original.

Example (predefined config):

    $ rmlint -o sh:stdout -o sh:rmlint.sh -c sh:symlink
    ...
    echo  '/home/user/test/b/one' # original
    cp_symlink '/home/user/test/b/file' '/home/user/test/b/one' # duplicate
    $ ./rmlint.sh -d
    Keeping: /home/user/test/b/one
    Symlinking to original: /home/user/test/b/file

Example (custom command):

The following example uses the trash-put command from the trash-cli utility to move duplicate files to trash:

    $ rmlint -o sh -c sh:cmd='echo "Trashing $1" && trash-put "$1"'

py:

Outputs a python script and a JSON file. The json file is the same as that produced by the json formatter. The JSON file is written to `.rmlint.json`, executing the python script will find it there. The default python script produced by rmlint does pretty much the same thing as the shell script described above (although not reflinking or hardlinking or symlinking at the moment). You can customise the python script for just about any use case (Python is a simple and extremely powerful programming language).

Example:

    $ rmlint -o py:remover.py
    $ ./remover.py --dry-run    # Needs Python3
    Deleting twins of /home/user/sub2/a
    Handling (duplicate_file): /home/user/sub1/a
    Handling (duplicate_file): /home/user/a

    Deleting twins of /home/user/sub2/b
    Handling (duplicate_file): /home/user/sub1/b

csv:

Outputs a csv formatted dump of all lint files. Handy for all the spreadsheet-jockeys out there! It looks like this:

    $ rmlint -o csv -D
    type,path,size,checksum
    emptydir,"/home/user/tree2/b",0,00000000000000000000000000000000
    duplicate_dir,"/home/user/test/b",4,f8772f6fda08bbc826543334663d6f13
    duplicate_dir,"/home/user/test/a",4,f8772f6fda08bbc826543334663d6f13
    duplicate_dir,"/home/user/tree/b",8,62202a79add28a72209b41b6c8f43400
    duplicate_dir,"/home/user/tree/a",8,62202a79add28a72209b41b6c8f43400
    duplicate_dir,"/home/user/tree2/a",4,311095bc5669453990cd205b647a1a00

Config values:

- use_header=[true|false]: Print the column name headers.

stamp:

Outputs a timestamp of the time `rmlint` was run.

Config values:

- iso8601=[true|false]: Write an ISO8601 formatted timestamps or seconds since epoch?

pretty: | Pretty-prints the found files in a colorful output (intended to be printed on stdout or stderr). This is enabled by default.
summary: | Sums up the run in a few lines with some statistics. This enabled by default too.

progressbar:

Prints a progressbar during the run of `rmlint`. This is recommended for large runs where the `pretty` formatter would print thousands of lines.

> Not recommended in combination with `pretty`

Config values:

- update_interval=number: Number of milliseconds to wait between updates. Higher values use less resources.

fdupes:

A formatter that behaves similar to fdupes(1) - another duplicate finder. This is mostly indented for compatibility (e.g. scripts that relied on that format). Duplicate set of files are printed as block, each separated by a newline. Original files are highlighted in green (this is an addition). During scanning a progressbar and summary are printed, followed by the fdupes output. The first two are printed to `stderr`, while the parseable lines will be printed to `stdout`.

Consider using the far more powerful `json` output for scripting purposes, unless you already have a script that expects fdupes output.

## Paranoia mode

Let’s face it, why should you trust `rmlint`?

Technically it only computes a hash of your file which might, by its nature, collide with the hash of a totally different file. If we assume a perfect hash function (i.e. one that distributes its hash values perfectly even over all possible values), the probability of having a hash-collision is \(\frac{1}{2^{160}}\) for the default 160-bit hash. Of course hash functions are not totally random, so the collision probability is slightly higher. Due to the “birthday paradox”, collision starts to become a real risk if you have more than about \(2^{80}\) files of the same size.

If you’re wary, you might want to make a bit more paranoid than the default. By default the `blake2b` (previously `sha1` was the default) hash algorithm is used, which we consider a good trade-off of speed and accuracy. `rmlint`’s paranoia level can be adjusted using the `-p` (`--paranoid`) switch.

Here’s what they do in detail:

- `-p` is equivalent to `--algorithm=paranoid`
- `-P` is equivalent to `--algorithm=highway256`
- `-PP` is equivalent to `--algorithm=metro256`
- `-PPP` is equivalent to `--algorithm=metro`

As you see, it just enables a certain duplicate detection algorithm to either use a stronger hash function or to do a byte-by-byte comparison. While this might sound slow it’s often only a few seconds slower than the default behaviour.

There is a bunch of other hash functions you can lookup in the manpage. We recommend never to use anything worse than the default.

Note

Even with the default options, the probability of a false positive doesn’t really start to get significant until you have around 1,000,000,000,000,000,000,000,000 different files all of the same file size. Bugs in `rmlint` are sadly (or happily?) more likely than hash collisions. See http://preshing.com/20110504/hash-collision-probabilities/ for discussion.

## Original detection / selection

As mentioned before, `rmlint` divides a group of dupes in one original and one or more duplicates of that one. While the chosen original might not be the one that was there first, you generally want to select one file to keep from each duplicate set.

By default, if you specify multiple paths in the rmlint command, the files in the first-named paths are treated as more “original” than the later named paths. If there are two files in the same path, then the older one will be treated as the original. If they have the same modification time then it’s just a matter of chance which one is selected as the original.

The way `rmlint` chooses the original can be customised by the `-S` (`--rank-by`) option.

Here’s an example:

    # Normal run:
    $ rmlint
    ls c
    rm a
    rm b

    # Use alphabetically first one as original
    $ rmlint -S a
    ls a
    rm b
    rm c

Alphabetically first makes sense in the case of backup files, ie a.txt.bak comes after a.txt.

Here’s a table of letters you can supply to the `-S` option:

m | keep lowest mtime (oldest) | M | keep highest mtime (newest)
a | keep first alphabetically | A | keep last alphabetically
p | keep first named path | P | keep last named path
d | keep path with lowest depth | D | keep path with highest depth
l | keep path with shortest basename | L | keep path with longest basename
r | keep paths matching regex | R | keep path not matching regex
x | keep basenames matching regex | X | keep basenames not matching regex
h | keep file with lowest hardlink count | H | keep file with highest hardlink count
o | keep file with lowest number of hardlinks outside of the paths traversed by `rmlint`. | O | keep file with highest number of hardlinks outside of the paths traversed by `rmlint`.

The default setting is `-S pOma`. Multiple sort criteria can be specified, eg `-S mpa` will sort first by mtime, then (if tied), based on which path you specified first in the rmlint command, then finally based on alphabetical order of file name. Note that “original directory” criteria (see below) take precedence over any `-S` options.

Alphabetical sort will only use the basename of the file and ignore its case. One can have multiple criteria, e.g.: `-S am` will choose first alphabetically; if tied then by mtime. Note: original path criteria (specified using //) will always take first priority over -S options.

For more fine grained control, it is possible to give a regular expression to sort by. This can be useful when you know a common fact that identifies original paths (like a path component being `src` or a certain file ending).

To use the regular expression you simply enclose it in the criteria string by adding <REGULAR_EXPRESSION> after specifying r or x. Example: `-S 'r<.*\.bak$>'` makes all files that have a `.bak` suffix original files.

Warning: When using r or x, try to make your regex to be as specific as possible! Good practice includes adding a `$` anchor at the end of the regex.

Tips:

- l is useful for files like file.mp3 vs file.1.mp3 or file.mp3.bak.
- a can be used as last criteria to assert a defined order.
- o/O and h/H are only useful if there any hardlinks in the traversed path.
- o/O takes the number of hardlinks outside the traversed paths (and thereby minimizes/maximizes the overall number of hardlinks). h/H in contrast only takes the number of hardlinks inside of the traversed paths. When hardlinking files, one would like to link to the original file with the highest outer link count (O) in order to maximise the space cleanup. H does not maximise the space cleanup, it just selects the file with the highest total hardlink count. You usually want to specify O.
- pOma is the default since p ensures that first given paths rank as originals, O ensures that hardlinks are handled well, m ensures that the oldest file is the original and a simply ensures a defined ordering if no other criteria applies.

### Flagging original directories

Sometimes you have a specific path that only contains originals, or only contains backups. In this case you can flag directories on the commandline by using a special separator (//) between the duplicate and original paths. Every path after the // separator is considered to be “tagged” and will be treated as an original where possible. Tagging always takes precedence over the `-S` options above.

    $ rmlint a // b
    ls b/file
    rm a/file

If there are more than one tagged files in a duplicate group then the highest ranked (per `-S` options) will be kept. In order to never delete any tagged files, there is the `-k` (`--keep-all-tagged`) option. A slightly more esoteric option is `-m` (`--must-match-tagged`), which only looks for duplicates where there is an original in a tagged path.

Here’s a real world example using these features: You have a portable backup drive with some old backups on it. You have just backed up your home folder to a new backup drive. You want to reformat the old backup drive and use it for something else. But first you want to check that there is nothing on the old drive that you don’t have somewhere else. The old drive is mounted at /media/portable.

    # Find all files on /media/portable that can be safely deleted:
    $ rmlint --keep-all-tagged --must-match-tagged /media/portable // ~
    # check the shellscript looks ok:
    $ less ./rmlint.sh # or use gedit or any other viewer/editor
    # run the shellscript to delete the redundant backups
    $ ./rmlint.sh
    # run again (to delete empty dirs)
    $ rmlint -km /media/portable // ~
    $ ./rmlint.sh
    # see what files are left:
    $ tree /media/portable
    # recover any files that you want to save, then you can safely reformat the drive

In the case of nested mountpoints, it may sometimes makes sense to use the opposite variations, `-K` (`--keep-all-untagged`) and `-M` (`--must-match-untagged`).

## Finding duplicate directories

Note

`--merge-directories` is still an experimental option that is non-trivial to implement. Please double check the output and report any possible bugs.

As far as we know, `rmlint` is the only duplicate finder that can do this. Basically, all you have to do is to specify the `-D` (`--merge-directories`) option and `rmlint` will cache all duplicates until everything is found and then merge them into full duplicate directories (if any). All other files are printed normally.

This may sound simple after all, but there are some caveats you should know of.

Let’s create a tricky folder structure to demonstrate the feature:

    $ mkdir -p fake/one/two/ fake/one/two_copy fake/one_copy/two fake/one_copy/two_copy
    $ echo xxx > fake/one/two/file
    $ echo xxx > fake/one/two_copy/file
    $ echo xxx > fake/one_copy/two/file
    $ echo xxx > fake/one_copy/two_copy/file
    $ echo xxx > fake/file
    $ echo xxx > fake/another_file

Now go run `rmlint` on it like that:

    $ rmlint fake -D -S a
    # Duplicate Directorie(s):
        ls -la /home/sahib/rmlint/fake/one
        rm -rf /home/sahib/rmlint/fake/one_copy
        ls -la /home/sahib/rmlint/fake/one/two
        rm -rf /home/sahib/rmlint/fake/one/two_copy

    # Duplicate(s):
        ls /home/sahib/rmlint/fake/another_file
        rm /home/sahib/rmlint/fake/one/two/file
        rm /home/sahib/rmlint/fake/file

    ==> In total 6 files, whereof 5 are duplicates in 1 groups.
    ==> This equals 20 B of duplicates which could be removed.

As you can see it correctly recognized the copies as duplicate directories. Also, it did not stop at `fake/one` but also looked at what parts of this original directory could be possibly removed too.

Files that could not be merged into directories are printed separately. Note here, that the original is taken from a directory that was preserved. So exactly one copy of the `xxx`-content file stays on the filesystem in the end.

`rmlint` finds duplicate directories by counting all files in the directory tree and looking up if there’s an equal amount of duplicate and empty files. If yes, it checks if the hashes are equal using a Merkle Tree. If it matches, it continues on the parent directory.

Some file like hidden files will not be recognized as duplicates, but still added to the count. This will of course lead to unmerged directories. That’s why the `-D` option implies the `-r` (`--hidden`) and `-l` (`--hardlinked`) option in order to make this convenient.

A note to symbolic links: The default behaviour with –merge-directories is to not follow symbolic links, but to compare the link targets. If the target is the same, the link will be the same. This is a sane default for duplicate directories, since twin copies often are created by doing a backup of some files. In this case any symlinks in the backed-up data will still point to the same target. If you have symlinks that reference a file in each respective directory tree, consider using `-f`.

Warning

Do never ever modify the filesystem (especially deleting files) while running with the `-D` option. This can lead to mismatches in the file count of a directory, possibly causing dataloss. You have been warned!

Sometimes it might be nice to only search for duplicate directories, banning all the sole files from littering the screen. While this will not delete all files, it will give you a nice overview of what you copied where.

Since duplicate directories are just a lint type as every other, you can just pass it to `-T`: `-T "none +dd"` (or `-T "none +duplicatedirs"`). There’s also a preset of it to save you some typing: `-T minimaldirs`.

Warning

Also take note that `-D` will cause a higher memory footprint and might add a bit of processing time. This is due to the fact that all files need to be cached till the end and some other internal data structures need to be created.

## Replaying results

Often it is useful to just re-output the results you got from `rmlint`. That’s kind of annoying for large datasets, especially when you have big files. For this, `rmlint` features a special mode, where it re-outputs the result of previous runs. By default, `rmlint` will spit out a `.json` file (ususally called `rmlint.json`). When `--replay` is given, you can pass one or more of those `.json` files to the commandline as they would be normal directories. `rmlint` will then merge and re-output then. Note however, that no filesystem input/output is done.

The usage of the `--replay` feature is best understood by example:

    $ rmlint real-large-dir --progress
    # ... lots of output ...
    $ cp rmlint.json large.json  # Save json, so we don't overwrite it.
    $ rmlint --replay large.json real-large-dir
    # ... same output, just faster ...
    $ rmlint --replay large.json --size 2M-512M --sort-by sn real-large-dir
    # ... filter stuff; and rank by size and by size and groupsize ....
    $ rmlint --replay large.json real-large-dir/subdir
    # ... only show stuff in /subdir ...

Warning

Details may differ

The generated output might differ slightly in order and details. For example the total number of files in the replayed runs will be the total of entries in the json document, not the total number of traversed files.

Also be careful when replaying on a modified filesystem. `rmlint` will ignore files with newer mtime than in the `.json` file for safety reason.

Warning

Not all options might work

Options that are related to traversing and hashing/reading have no effect. Those are:

- –followlinks
- –algorithm and –paranoid
- –clamp-low
- –hardlinked
- –write-unfinished
- all other caching options.

## Miscellaneous options

If you read so far, you know `rmlint` pretty well by now. Here’s just a list of options that are nice to know, but are not essential:

- Consecutive runs of `rmlint` can be speed up by using `--xattr`.

      $ rmlint large_dataset/ --xattr
      $ rmlint large_dataset/ --xattr

  Here, the second run should (or might) run a lot faster, since the first run saved all calculated checksums to the extended attributes of each processed file. But be sure to read the caveats stated in the manpage! Especially keep in mind that you need to have write access to the files for this to work.

- `-r` (`--hidden`): Include hidden files and directories. The default is to ignore these, to save you from destroying git repositories (or similar programs) that save their information in a `.git` directory where `rmlint` often finds duplicates.

  If you want to be safe you can do something like this:

      $ # find all files except everything under .git or .svn folders
      $ find . -type d | grep -v '\(.git\|.svn\)' -print0 | rmlint -0 --hidden

  But you would have checked the output anyways, wouldn’t you?

- If something ever goes wrong, it might help to increase the verbosity with `-v` (up to `-vvv`).

- Usually the commandline output is colored, but you can disable it explicitly with `-w` (`--with-color`). If stdout or stderr is not a terminal anyways, `rmlint` will disable colors itself.

- You can limit the traversal depth with `-d` (`--max-depth`):

      $ rmlint -d 0
      <finds everything in the same working directory>

- If you want to prevent `rmlint` from crossing mountpoints (e.g. scan a home directory, but no the HD mounted in there), you can use the `-x` (`--no-crossdev`) option.

- It is possible to tell `rmlint` that it should not scan the whole file. With `-q` (`--clamp-low`) / `-Q` (`--clamp-top`) it is possible to limit the range to a starting point (`-q`) and end point (`-Q`). The point where to start might be either given as percent value, factor (percent / 100) or as an absolute offset.

  If the file size is lower than the absolute offset, the file is simply ignored.

  This feature might prove useful if you want to examine files with a constant header. The constant header might be different, i.e. by a different ID, but the content might be still the same. In any case it is advisable to use this option with care.

  Example:

      # Start hashing at byte 100, but not more than 90% of the filesize.
      $ rmlint -q 100 -Q .9

## Scripting

Sometimes `rmlint` can’t help you with everything. While we get a lot of feature request to have some more comfort-options built-in, we have to decline most of them to make sure that `rmlint` does not become too bloated. But fret not, we have lots of possibilities to do some scripting. Let’s look at some common problem that can be easily solved with some other tools.

### Copying unique files

Imagine you have a folder from which you want to extract one copy of each file by content. This does not only mean unique files, but also original files - but not their duplicates. This sounds like a job for the `uniques` formatter (which outputs all unique paths, one by line) and `jq`. `jq` is a great little tool which makes it really easy to extract data from a `json` file:

    # This command produces two files:
    # - unique_files: Files that have a unique checksum in the directory.
    # - original_files: Files that have the "is_original" field set to true in the json output.
    # The '.[1:-1]' part is for filtering the header and footer part of the json response.
    $ rmlint t -o json -o uniques:unique_files |  jq -r '.[1:-1][] | select(.is_original) | .path' | sort > original_files
    # Now we only need to combine both files:
    $ cat unique_files original_files

### Filter by regular expressions

A very often requested feature is to provide something like `--include` or `--exclude` where you could pass a regular expression to include or exclude some files from traversing. But `rmlint` is not a clone of `find`. Instead you can use `find` just fine to pipe output into `rmlint`:

    # Deduplicate all png pictures that were not modified in the last 24 hours:
    $ find ~ -iname '*.png' ! -mtime 0 | rmlint -