packages icon



 ZSTD(1)                         zstd 1.5.6                          ZSTD(1)
 User Commands                                                 User Commands

                                 March 2024



 NAME
      zstd - zstd, zstdmt, unzstd, zstdcat - Compress or decompress .zst
      files

 SYNOPSIS
      zstd [OPTIONS] [-|INPUT-FILE] [-o OUTPUT-FILE]

      zstdmt is equivalent to zstd -T0

      unzstd is equivalent to zstd -d

      zstdcat is equivalent to zstd -dcf

 DESCRIPTION
      zstd is a fast lossless compression algorithm and data compression
      tool, with command line syntax similar to gzip(1) and xz(1). It is
      based on the LZ77 family, with further FSE & huff0 entropy stages.
      zstd offers highly configurable compression speed, from fast modes at
      > 200 MB/s per core, to strong modes with excellent compression
      ratios. It also features a very fast decoder, with speeds > 500 MB/s
      per core, which remains roughly stable at all compression settings.

      zstd command line syntax is generally similar to gzip, but features
      the following few differences:

      +   Source files are preserved by default. It's possible to remove
          them automatically by using the --rm command.

      +   When compressing a single file, zstd displays progress
          notifications and result summary by default. Use -q to turn them
          off.

      +   zstd displays a short help page when command line is an error. Use
          -q to turn it off.

      +   zstd does not accept input from console, though it does accept
          stdin when it's not the console.

      +   zstd does not store the input's filename or attributes, only its
          contents.



      zstd processes each file according to the selected operation mode. If
      no files are given or file is -, zstd reads from standard input and
      writes the processed data to standard output. zstd will refuse to
      write compressed data to standard output if it is a terminal: it will
      display an error message and skip the file. Similarly, zstd will
      refuse to read compressed data from standard input if it is a



                                    - 1 -           Formatted:  May 26, 2024






 ZSTD(1)                         zstd 1.5.6                          ZSTD(1)
 User Commands                                                 User Commands

                                 March 2024



      terminal.

      Unless --stdout or -o is specified, files are written to a new file
      whose name is derived from the source file name:

      +   When compressing, the suffix .zst is appended to the source
          filename to get the target filename.

      +   When decompressing, the .zst suffix is removed from the source
          filename to get the target filename



    Concatenation with .zst Files
      It is possible to concatenate multiple .zst files. zstd will
      decompress such agglomerated file as if it was a single .zst file.

 OPTIONS
    Integer Suffixes and Special Values
      In most places where an integer argument is expected, an optional
      suffix is supported to easily indicate large integers. There must be
      no space between the integer and the suffix.

      KiB  Multiply the integer by 1,024 (2^10). Ki, K, and KB are accepted
           as synonyms for KiB.

      MiB  Multiply the integer by 1,048,576 (2^20). Mi, M, and MB are
           accepted as synonyms for MiB.

    Operation Mode
      If multiple operation mode options are given, the last one takes
      effect.

      -z, --compress
           Compress. This is the default operation mode when no operation
           mode option is specified and no other operation mode is implied
           from the command name (for example, unzstd implies --decompress).

      -d, --decompress, --uncompress
           Decompress.

      -t, --test
           Test the integrity of compressed files. This option is equivalent
           to --decompress --stdout > /dev/null, decompressed data is
           discarded and checksummed for errors. No files are created or
           removed.

      -b#  Benchmark file(s) using compression level #. See BENCHMARK below
           for a description of this operation.



                                    - 2 -           Formatted:  May 26, 2024






 ZSTD(1)                         zstd 1.5.6                          ZSTD(1)
 User Commands                                                 User Commands

                                 March 2024



      --train FILES
           Use FILES as a training set to create a dictionary. The training
           set should contain a lot of small files (> 100). See DICTIONARY
           BUILDER below for a description of this operation.

      -l, --list
           Display information related to a zstd compressed file, such as
           size, ratio, and checksum. Some of these fields may not be
           available. This command's output can be augmented with the -v
           modifier.

    Operation Modifiers
      +   -#: selects # compression level [1-19] (default: 3). Higher
          compression levels generally produce higher compression ratio at
          the expense of speed and memory. A rough rule of thumb is that
          compression speed is expected to be divided by 2 every 2 levels.
          Technically, each level is mapped to a set of advanced parameters
          (that can also be modified individually, see below). Because the
          compressor's behavior highly depends on the content to compress,
          there's no guarantee of a smooth progression from one level to
          another.

      +   --ultra: unlocks high compression levels 20+ (maximum 22), using a
          lot more memory. Note that decompression will also require more
          memory when using these levels.

      +   --fast[=#]: switch to ultra-fast compression levels. If =# is not
          present, it defaults to 1. The higher the value, the faster the
          compression speed, at the cost of some compression ratio. This
          setting overwrites compression level if one was set previously.
          Similarly, if a compression level is set after --fast, it
          overrides it.

      +   -T#, --threads=#: Compress using # working threads (default: 1).
          If # is 0, attempt to detect and use the number of physical CPU
          cores. In all cases, the nb of threads is capped to
          ZSTDMT_NBWORKERS_MAX, which is either 64 in 32-bit mode, or 256
          for 64-bit environments. This modifier does nothing if zstd is
          compiled without multithread support.

      +   --single-thread: Use a single thread for both I/O and compression.
          As compression is serialized with I/O, this can be slightly
          slower. Single-thread mode features significantly lower memory
          usage, which can be useful for systems with limited amount of
          memory, such as 32-bit systems.

          Note 1: this mode is the only available one when multithread
          support is disabled.




                                    - 3 -           Formatted:  May 26, 2024






 ZSTD(1)                         zstd 1.5.6                          ZSTD(1)
 User Commands                                                 User Commands

                                 March 2024



          Note 2: this mode is different from -T1, which spawns 1
          compression thread in parallel with I/O. Final compressed result
          is also slightly different from -T1.

      +   --auto-threads={physical,logical} (default: physical): When using
          a default amount of threads via -T0, choose the default based on
          the number of detected physical or logical cores.

      +   --adapt[=min=#,max=#]: zstd will dynamically adapt compression
          level to perceived I/O conditions. Compression level adaptation
          can be observed live by using command -v. Adaptation can be
          constrained between supplied min and max levels. The feature works
          when combined with multi-threading and --long mode. It does not
          work with --single-thread. It sets window size to 8 MiB by default
          (can be changed manually, see wlog). Due to the chaotic nature of
          dynamic adaptation, compressed result is not reproducible.

          Note: at the time of this writing, --adapt can remain stuck at low
          speed when combined with multiple worker threads (>=2).

      +   --long[=#]: enables long distance matching with # windowLog, if #
          is not present it defaults to 27. This increases the window size
          (windowLog) and memory usage for both the compressor and
          decompressor. This setting is designed to improve the compression
          ratio for files with long matches at a large distance.

          Note: If windowLog is set to larger than 27, --long=windowLog or
          --memory=windowSize needs to be passed to the decompressor.

      +   -D DICT: use DICT as Dictionary to compress or decompress FILE(s)

      +   --patch-from FILE: Specify the file to be used as a reference
          point for zstd's diff engine. This is effectively dictionary
          compression with some convenient parameter selection, namely that
          windowSize > srcSize.

          Note: cannot use both this and -D together.

          Note: --long mode will be automatically activated if chainLog <
          fileLog (fileLog being the windowLog required to cover the whole
          file). You can also manually force it.

          Note: for all levels, you can use --patch-from in --single-thread
          mode to improve compression ratio at the cost of speed.

          Note: for level 19, you can get increased compression ratio at the
          cost of speed by specifying --zstd=targetLength= to be something
          large (i.e. 4096), and by setting a large --zstd=chainLog=.




                                    - 4 -           Formatted:  May 26, 2024






 ZSTD(1)                         zstd 1.5.6                          ZSTD(1)
 User Commands                                                 User Commands

                                 March 2024



      +   --rsyncable: zstd will periodically synchronize the compression
          state to make the compressed file more rsync-friendly. There is a
          negligible impact to compression ratio, and a potential impact to
          compression speed, perceptible at higher speeds, for example when
          combining --rsyncable with many parallel worker threads. This
          feature does not work with --single-thread. You probably don't
          want to use it with long range mode, since it will decrease the
          effectiveness of the synchronization points, but your mileage may
          vary.

      +   -C, --[no-]check: add integrity check computed from uncompressed
          data (default: enabled)

      +   --[no-]content-size: enable / disable whether or not the original
          size of the file is placed in the header of the compressed file.
          The default option is --content-size (meaning that the original
          size will be placed in the header).

      +   --no-dictID: do not store dictionary ID within frame header
          (dictionary compression). The decoder will have to rely on
          implicit knowledge about which dictionary to use, it won't be able
          to check if it's correct.

      +   -M#, --memory=#: Set a memory usage limit. By default, zstd uses
          128 MiB for decompression as the maximum amount of memory the
          decompressor is allowed to use, but you can override this manually
          if need be in either direction (i.e. you can increase or decrease
          it).

          This is also used during compression when using with
          --patch-from=. In this case, this parameter overrides that maximum
          size allowed for a dictionary. (128 MiB).

          Additionally, this can be used to limit memory for dictionary
          training. This parameter overrides the default limit of 2 GiB.
          zstd will load training samples up to the memory limit and ignore
          the rest.

      +   --stream-size=#: Sets the pledged source size of input coming from
          a stream. This value must be exact, as it will be included in the
          produced frame header. Incorrect stream sizes will cause an error.
          This information will be used to better optimize compression
          parameters, resulting in better and potentially faster
          compression, especially for smaller source sizes.

      +   --size-hint=#: When handling input from a stream, zstd must guess
          how large the source size will be when optimizing compression
          parameters. If the stream size is relatively small, this guess may
          be a poor one, resulting in a higher compression ratio than



                                    - 5 -           Formatted:  May 26, 2024






 ZSTD(1)                         zstd 1.5.6                          ZSTD(1)
 User Commands                                                 User Commands

                                 March 2024



          expected. This feature allows for controlling the guess when
          needed. Exact guesses result in better compression ratios.
          Overestimates result in slightly degraded compression ratios,
          while underestimates may result in significant degradation.

      +   --target-compressed-block-size=#: Attempt to produce compressed
          blocks of approximately this size. This will split larger blocks
          in order to approach this target. This feature is notably useful
          for improved latency, when the receiver can leverage receiving
          early incomplete data. This parameter defines a loose target:
          compressed blocks will target this size "on average", but
          individual blocks can still be larger or smaller. Enabling this
          feature can decrease compression speed by up to ~10% at level 1.
          Higher levels will see smaller relative speed regression, becoming
          invisible at higher settings.

      +   -f, --force: disable input and output checks. Allows overwriting
          existing files, input from console, output to stdout, operating on
          links, block devices, etc. During decompression and when the
          output destination is stdout, pass-through unrecognized formats
          as-is.

      +   -c, --stdout: write to standard output (even if it is the
          console); keep original files (disable --rm).

      +   -o FILE: save result into FILE. Note that this operation is in
          conflict with -c. If both operations are present on the command
          line, the last expressed one wins.

      +   --[no-]sparse: enable / disable sparse FS support, to make files
          with many zeroes smaller on disk. Creating sparse files may save
          disk space and speed up decompression by reducing the amount of
          disk I/O. default: enabled when output is into a file, and
          disabled when output is stdout. This setting overrides default and
          can force sparse mode over stdout.

      +   --[no-]pass-through enable / disable passing through uncompressed
          files as-is. During decompression when pass-through is enabled,
          unrecognized formats will be copied as-is from the input to the
          output. By default, pass-through will occur when the output
          destination is stdout and the force (-f) option is set.

      +   --rm: remove source file(s) after successful compression or
          decompression. This command is silently ignored if output is
          stdout. If used in combination with -o, triggers a confirmation
          prompt (which can be silenced with -f), as this is a destructive
          operation.

      +   -k, --keep: keep source file(s) after successful compression or



                                    - 6 -           Formatted:  May 26, 2024






 ZSTD(1)                         zstd 1.5.6                          ZSTD(1)
 User Commands                                                 User Commands

                                 March 2024



          decompression. This is the default behavior.

      +   -r: operate recursively on directories. It selects all files in
          the named directory and all its subdirectories. This can be useful
          both to reduce command line typing, and to circumvent shell
          expansion limitations, when there are a lot of files and naming
          breaks the maximum size of a command line.

      +   --filelist FILE read a list of files to process as content from
          FILE. Format is compatible with ls output, with one file per line.

      +   --output-dir-flat DIR: resulting files are stored into target DIR
          directory, instead of same directory as origin file. Be aware that
          this command can introduce name collision issues, if multiple
          files, from different directories, end up having the same name.
          Collision resolution ensures first file with a given name will be
          present in DIR, while in combination with -f, the last file will
          be present instead.

      +   --output-dir-mirror DIR: similar to --output-dir-flat, the output
          files are stored underneath target DIR directory, but this option
          will replicate input directory hierarchy into output DIR.

          If input directory contains "..", the files in this directory will
          be ignored. If input directory is an absolute directory (i.e.
          "/var/tmp/abc"), it will be stored into the
          "output-dir/var/tmp/abc". If there are multiple input files or
          directories, name collision resolution will follow the same rules
          as --output-dir-flat.

      +   --format=FORMAT: compress and decompress in other formats. If
          compiled with support, zstd can compress to or decompress from
          other compression algorithm formats. Possibly available options
          are zstd, gzip, xz, lzma, and lz4. If no such format is provided,
          zstd is the default.

      +   -h/-H, --help: display help/long help and exit

      +   -V, --version: display version number and immediately exit. note
          that, since it exits, flags specified after -V are effectively
          ignored. Advanced: -vV also displays supported formats. -vvV also
          displays POSIX support. -qV will only display the version number,
          suitable for machine reading.

      +   -v, --verbose: verbose mode, display more information

      +   -q, --quiet: suppress warnings, interactivity, and notifications.
          specify twice to suppress errors too.




                                    - 7 -           Formatted:  May 26, 2024






 ZSTD(1)                         zstd 1.5.6                          ZSTD(1)
 User Commands                                                 User Commands

                                 March 2024



      +   --no-progress: do not display the progress bar, but keep all other
          messages.

      +   --show-default-cparams: shows the default compression parameters
          that will be used for a particular input file, based on the
          provided compression level and the input size. If the provided
          file is not a regular file (e.g. a pipe), this flag will output
          the parameters used for inputs of unknown size.

      +   --exclude-compressed: only compress files that are not already
          compressed.

      +   --: All arguments after -- are treated as files



    gzip Operation Modifiers
      When invoked via a gzip symlink, zstd will support further options
      that intend to mimic the gzip behavior:

      -n, --no-name
           do not store the original filename and timestamps when
           compressing a file. This is the default behavior and hence a
           no-op.

      --best
           alias to the option -9.

    Environment Variables
      Employing environment variables to set parameters has security
      implications. Therefore, this avenue is intentionally limited. Only
      ZSTD_CLEVEL and ZSTD_NBTHREADS are currently supported. They set the
      default compression level and number of threads to use during
      compression, respectively.

      ZSTD_CLEVEL can be used to set the level between 1 and 19 (the
      "normal" range). If the value of ZSTD_CLEVEL is not a valid integer,
      it will be ignored with a warning message. ZSTD_CLEVEL just replaces
      the default compression level (3).

      ZSTD_NBTHREADS can be used to set the number of threads zstd will
      attempt to use during compression. If the value of ZSTD_NBTHREADS is
      not a valid unsigned integer, it will be ignored with a warning
      message. ZSTD_NBTHREADS has a default value of (1), and is capped at
      ZSTDMT_NBWORKERS_MAX==200. zstd must be compiled with multithread
      support for this variable to have any effect.

      They can both be overridden by corresponding command line arguments:
      -# for compression level and -T# for number of compression threads.



                                    - 8 -           Formatted:  May 26, 2024






 ZSTD(1)                         zstd 1.5.6                          ZSTD(1)
 User Commands                                                 User Commands

                                 March 2024



 ADVANCED COMPRESSION OPTIONS
      zstd provides 22 predefined regular compression levels plus the fast
      levels. A compression level is translated internally into multiple
      advanced parameters that control the behavior of the compressor (one
      can observe the result of this translation with
      --show-default-cparams). These advanced parameters can be overridden
      using advanced compression options.

    --zstd[=options]:
      The options are provided as a comma-separated list. You may specify
      only the options you want to change and the rest will be taken from
      the selected or default compression level. The list of available
      options:

      strategy=strat, strat=strat
           Specify a strategy used by a match finder.

           There are 9 strategies numbered from 1 to 9, from fastest to
           strongest: 1=ZSTD_fast, 2=ZSTD_dfast, 3=ZSTD_greedy, 4=ZSTD_lazy,
           5=ZSTD_lazy2, 6=ZSTD_btlazy2, 7=ZSTD_btopt, 8=ZSTD_btultra,
           9=ZSTD_btultra2.

      windowLog=wlog, wlog=wlog
           Specify the maximum number of bits for a match distance.

           The higher number of increases the chance to find a match which
           usually improves compression ratio. It also increases memory
           requirements for the compressor and decompressor. The minimum
           wlog is 10 (1 KiB) and the maximum is 30 (1 GiB) on 32-bit
           platforms and 31 (2 GiB) on 64-bit platforms.

           Note: If windowLog is set to larger than 27, --long=windowLog or
           --memory=windowSize needs to be passed to the decompressor.

      hashLog=hlog, hlog=hlog
           Specify the maximum number of bits for a hash table.

           Bigger hash tables cause fewer collisions which usually makes
           compression faster, but requires more memory during compression.

           The minimum hlog is 6 (64 entries / 256 B) and the maximum is 30
           (1B entries / 4 GiB).

      chainLog=clog, clog=clog
           Specify the maximum number of bits for the secondary search
           structure, whose form depends on the selected strategy.

           Higher numbers of bits increases the chance to find a match which
           usually improves compression ratio. It also slows down



                                    - 9 -           Formatted:  May 26, 2024






 ZSTD(1)                         zstd 1.5.6                          ZSTD(1)
 User Commands                                                 User Commands

                                 March 2024



           compression speed and increases memory requirements for
           compression. This option is ignored for the ZSTD_fast strategy,
           which only has the primary hash table.

           The minimum clog is 6 (64 entries / 256 B) and the maximum is 29
           (512M entries / 2 GiB) on 32-bit platforms and 30 (1B entries / 4
           GiB) on 64-bit platforms.

      searchLog=slog, slog=slog
           Specify the maximum number of searches in a hash chain or a
           binary tree using logarithmic scale.

           More searches increases the chance to find a match which usually
           increases compression ratio but decreases compression speed.

           The minimum slog is 1 and the maximum is 'windowLog' - 1.

      minMatch=mml, mml=mml
           Specify the minimum searched length of a match in a hash table.

           Larger search lengths usually decrease compression ratio but
           improve decompression speed.

           The minimum mml is 3 and the maximum is 7.

      targetLength=tlen, tlen=tlen
           The impact of this field vary depending on selected strategy.

           For ZSTD_btopt, ZSTD_btultra and ZSTD_btultra2, it specifies the
           minimum match length that causes match finder to stop searching.
           A larger targetLength usually improves compression ratio but
           decreases compression speed.

           For ZSTD_fast, it triggers ultra-fast mode when > 0. The value
           represents the amount of data skipped between match sampling.
           Impact is reversed: a larger targetLength increases compression
           speed but decreases compression ratio.

           For all other strategies, this field has no impact.

           The minimum tlen is 0 and the maximum is 128 KiB.

      overlapLog=ovlog, ovlog=ovlog
           Determine overlapSize, amount of data reloaded from previous job.
           This parameter is only available when multithreading is enabled.
           Reloading more data improves compression ratio, but decreases
           speed.

           The minimum ovlog is 0, and the maximum is 9. 1 means "no



                                   - 10 -           Formatted:  May 26, 2024






 ZSTD(1)                         zstd 1.5.6                          ZSTD(1)
 User Commands                                                 User Commands

                                 March 2024



           overlap", hence completely independent jobs. 9 means "full
           overlap", meaning up to windowSize is reloaded from previous job.
           Reducing ovlog by 1 reduces the reloaded amount by a factor 2.
           For example, 8 means "windowSize/2", and 6 means "windowSize/8".
           Value 0 is special and means "default": ovlog is automatically
           determined by zstd. In which case, ovlog will range from 6 to 9,
           depending on selected strat.

      ldmHashLog=lhlog, lhlog=lhlog
           Specify the maximum size for a hash table used for long distance
           matching.

           This option is ignored unless long distance matching is enabled.

           Bigger hash tables usually improve compression ratio at the
           expense of more memory during compression and a decrease in
           compression speed.

           The minimum lhlog is 6 and the maximum is 30 (default: 20).

      ldmMinMatch=lmml, lmml=lmml
           Specify the minimum searched length of a match for long distance
           matching.

           This option is ignored unless long distance matching is enabled.

           Larger/very small values usually decrease compression ratio.

           The minimum lmml is 4 and the maximum is 4096 (default: 64).

      ldmBucketSizeLog=lblog, lblog=lblog
           Specify the size of each bucket for the hash table used for long
           distance matching.

           This option is ignored unless long distance matching is enabled.

           Larger bucket sizes improve collision resolution but decrease
           compression speed.

           The minimum lblog is 1 and the maximum is 8 (default: 3).

      ldmHashRateLog=lhrlog, lhrlog=lhrlog
           Specify the frequency of inserting entries into the long distance
           matching hash table.

           This option is ignored unless long distance matching is enabled.

           Larger values will improve compression speed. Deviating far from
           the default value will likely result in a decrease in compression



                                   - 11 -           Formatted:  May 26, 2024






 ZSTD(1)                         zstd 1.5.6                          ZSTD(1)
 User Commands                                                 User Commands

                                 March 2024



           ratio.

           The default value is wlog - lhlog.

    Example
      The following parameters sets advanced compression options to
      something similar to predefined level 19 for files bigger than 256 KB:

      --zstd=wlog=23,clog=23,hlog=22,slog=6,mml=3,tlen=48,strat=6

    -B#:
      Specify the size of each compression job. This parameter is only
      available when multi-threading is enabled. Each compression job is run
      in parallel, so this value indirectly impacts the nb of active
      threads. Default job size varies depending on compression level
      (generally 4 * windowSize). -B# makes it possible to manually select a
      custom size. Note that job size must respect a minimum value which is
      enforced transparently. This minimum is either 512 KB, or overlapSize,
      whichever is largest. Different job sizes will lead to non-identical
      compressed frames.

 DICTIONARY BUILDER
      zstd offers dictionary compression, which greatly improves efficiency
      on small files and messages. It's possible to train zstd with a set of
      samples, the result of which is saved into a file called a dictionary.
      Then, during compression and decompression, reference the same
      dictionary, using command -D dictionaryFileName. Compression of small
      files similar to the sample set will be greatly improved.

      --train FILEs
           Use FILEs as training set to create a dictionary. The training
           set should ideally contain a lot of samples (> 100), and weight
           typically 100x the target dictionary size (for example, ~10 MB
           for a 100 KB dictionary). --train can be combined with -r to
           indicate a directory rather than listing all the files, which can
           be useful to circumvent shell expansion limits.

           Since dictionary compression is mostly effective for small files,
           the expectation is that the training set will only contain small
           files. In the case where some samples happen to be large, only
           the first 128 KiB of these samples will be used for training.

           --train supports multithreading if zstd is compiled with
           threading support (default). Additional advanced parameters can
           be specified with --train-fastcover. The legacy dictionary
           builder can be accessed with --train-legacy. The slower cover
           dictionary builder can be accessed with --train-cover. Default
           --train is equivalent to --train-fastcover=d=8,steps=4.




                                   - 12 -           Formatted:  May 26, 2024






 ZSTD(1)                         zstd 1.5.6                          ZSTD(1)
 User Commands                                                 User Commands

                                 March 2024



      -o FILE
           Dictionary saved into FILE (default name: dictionary).

      --maxdict=#
           Limit dictionary to specified size (default: 112640 bytes). As
           usual, quantities are expressed in bytes by default, and it's
           possible to employ suffixes (like KB or MB) to specify larger
           values.

      -#   Use # compression level during training (optional). Will generate
           statistics more tuned for selected compression level, resulting
           in a small compression ratio improvement for this level.

      -B#  Split input files into blocks of size # (default: no split)

      -M#, --memory=#
           Limit the amount of sample data loaded for training (default: 2
           GB). Note that the default (2 GB) is also the maximum. This
           parameter can be useful in situations where the training set size
           is not well controlled and could be potentially very large. Since
           speed of the training process is directly correlated to the size
           of the training sample set, a smaller sample set leads to faster
           training.

           In situations where the training set is larger than maximum
           memory, the CLI will randomly select samples among the available
           ones, up to the maximum allowed memory budget. This is meant to
           improve dictionary relevance by mitigating the potential impact
           of clustering, such as selecting only files from the beginning of
           a list sorted by modification date, or sorted by alphabetical
           order. The randomization process is deterministic, so training of
           the same list of files with the same parameters will lead to the
           creation of the same dictionary.

      --dictID=#
           A dictionary ID is a locally unique ID. The decoder will use this
           value to verify it is using the right dictionary. By default,
           zstd will create a 4-bytes random number ID. It's possible to
           provide an explicit number ID instead. It's up to the dictionary
           manager to not assign twice the same ID to 2 different
           dictionaries. Note that short numbers have an advantage: an ID <
           256 will only need 1 byte in the compressed frame header, and an
           ID < 65536 will only need 2 bytes. This compares favorably to 4
           bytes default.

           Note that RFC8878 reserves IDs less than 32768 and greater than
           or equal to 2^31, so they should not be used in public.

      --train-cover[=k#,d=#,steps=#,split=#,shrink[=#]]



                                   - 13 -           Formatted:  May 26, 2024






 ZSTD(1)                         zstd 1.5.6                          ZSTD(1)
 User Commands                                                 User Commands

                                 March 2024



           Select parameters for the default dictionary builder algorithm
           named cover. If d is not specified, then it tries d = 6 and d =
           8. If k is not specified, then it tries steps values in the range
           [50, 2000]. If steps is not specified, then the default value of
           40 is used. If split is not specified or split <= 0, then the
           default value of 100 is used. Requires that d <= k. If shrink
           flag is not used, then the default value for shrinkDict of 0 is
           used. If shrink is not specified, then the default value for
           shrinkDictMaxRegression of 1 is used.

           Selects segments of size k with highest score to put in the
           dictionary. The score of a segment is computed by the sum of the
           frequencies of all the subsegments of size d. Generally d should
           be in the range [6, 8], occasionally up to 16, but the algorithm
           will run faster with d <= 8. Good values for k vary widely based
           on the input data, but a safe range is [2 * d, 2000]. If split is
           100, all input samples are used for both training and testing to
           find optimal d and k to build dictionary. Supports multithreading
           if zstd is compiled with threading support. Having shrink enabled
           takes a truncated dictionary of minimum size and doubles in size
           until compression ratio of the truncated dictionary is at most
           shrinkDictMaxRegression% worse than the compression ratio of the
           largest dictionary.

           Examples:

           zstd --train-cover FILEs

           zstd --train-cover=k=50,d=8 FILEs

           zstd --train-cover=d=8,steps=500 FILEs

           zstd --train-cover=k=50 FILEs

           zstd --train-cover=k=50,split=60 FILEs

           zstd --train-cover=shrink FILEs

           zstd --train-cover=shrink=2 FILEs

      --train-fastcover[=k#,d=#,f=#,steps=#,split=#,accel=#]
           Same as cover but with extra parameters f and accel and different
           default value of split If split is not specified, then it tries
           split = 75. If f is not specified, then it tries f = 20. Requires
           that 0 < f < 32. If accel is not specified, then it tries accel =
           1. Requires that 0 < accel <= 10. Requires that d = 6 or d = 8.

           f is log of size of array that keeps track of frequency of
           subsegments of size d. The subsegment is hashed to an index in



                                   - 14 -           Formatted:  May 26, 2024






 ZSTD(1)                         zstd 1.5.6                          ZSTD(1)
 User Commands                                                 User Commands

                                 March 2024



           the range [0,2^f - 1]. It is possible that 2 different
           subsegments are hashed to the same index, and they are considered
           as the same subsegment when computing frequency. Using a higher f
           reduces collision but takes longer.

           Examples:

           zstd --train-fastcover FILEs

           zstd --train-fastcover=d=8,f=15,accel=2 FILEs

      --train-legacy[=selectivity=#]
           Use legacy dictionary builder algorithm with the given dictionary
           selectivity (default: 9). The smaller the selectivity value, the
           denser the dictionary, improving its efficiency but reducing its
           achievable maximum size. --train-legacy=s=# is also accepted.

           Examples:

           zstd --train-legacy FILEs

           zstd --train-legacy=selectivity=8 FILEs

 BENCHMARK
      The zstd CLI provides a benchmarking mode that can be used to easily
      find suitable compression parameters, or alternatively to benchmark a
      computer's performance. Note that the results are highly dependent on
      the content being compressed.

      -b#  benchmark file(s) using compression level #

      -e#  benchmark file(s) using multiple compression levels, from -b# to
           -e# (inclusive)

      -d   benchmark decompression speed only (requires providing an already
           zstd-compressed content)

      -i#  minimum evaluation time, in seconds (default: 3s), benchmark mode
           only

      -B#, --block-size=#
           cut file(s) into independent chunks of size # (default: no
           chunking)

      --priority=rt
           set process priority to real-time (Windows)

      Output Format: CompressionLevel#Filename: InputSize -> OutputSize
      (CompressionRatio), CompressionSpeed, DecompressionSpeed



                                   - 15 -           Formatted:  May 26, 2024






 ZSTD(1)                         zstd 1.5.6                          ZSTD(1)
 User Commands                                                 User Commands

                                 March 2024



      Methodology: For both compression and decompression speed, the entire
      input is compressed/decompressed in-memory to measure speed. A run
      lasts at least 1 sec, so when files are small, they are
      compressed/decompressed several times per run, in order to improve
      measurement accuracy.

 SEE ALSO
      zstdgrep(1), zstdless(1), gzip(1), xz(1)

      The zstandard format is specified in Y. Collet, "Zstandard Compression
      and the 'application/zstd' Media Type",
      https://www.ietf.org/rfc/rfc8878.txt, Internet RFC 8878 (February
      2021).

 BUGS
      Report bugs at: https://github.com/facebook/zstd/issues

 AUTHOR
      Yann Collet

































                                   - 16 -           Formatted:  May 26, 2024