Provided by: zstd_1.3.1+dfsg-1~ubuntu0.16.04.1_amd64 
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, with fast modes
at > 200 MB/s per code, and strong modes nearing lzma compression ratios. It also features
a very fast decoder, with speeds > 500 MB/s per core.
zstd command line syntax is generally similar to gzip, but features the following
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 does not accept input from console, but it properly accepts stdin when it´s not
the console.
• zstd displays a short help page when command line is an error. Use -q to turn it off.
zstd compresses or decompresses 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 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 .zst files as is. zstd will decompress such files as if they
were 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 except that the decompressed data is discarded instead of being written to
standard output. No files are created or removed.
-b# Benchmark file(s) using compression level #
--train FILEs
Use FILEs as a training set to create a dictionary. The training set should contain
a lot of small files (> 100).
-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 can be augmented
with the -v modifier.
Operation modifiers
-# # compression level [1-19] (default: 3)
--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.
-T#, --threads=#
Compress using # 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_NBTHREADS_MAX==256. This modifier does nothing if zstd is compiled without
multithread support.
-D file
use file as Dictionary to compress or decompress FILE(s)
--nodictID
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.
-o file
save result into file (only possible with a single INPUT-FILE)
-f, --force
overwrite output without prompting, and (de)compress symbolic links
-c, --stdout
force write to standard output, even if it is the console
--[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.
--rm remove source file(s) after successful compression or decompression
-k, --keep
keep source file(s) after successful compression or decompression. This is the
default behavior.
-r operate recursively on dictionaries
-h/-H, --help
display help/long help and exit
-V, --version
display version number and exit. Advanced : -vV also displays supported formats.
-vvV also displays POSIX support.
-v verbose mode
-q, --quiet
suppress warnings, interactivity, and notifications. specify twice to suppress
errors too.
-C, --[no-]check
add integrity check computed from uncompressed data (default : enabled)
-- All arguments after -- are treated as files
DICTIONARY BUILDER
zstd offers dictionary compression, useful for very small files and messages. It´s
possible to train zstd with some samples, the result of which is saved into a file called
a dictionary. Then during compression and decompression, reference the same dictionary. It
will improve compression ratio of small files. Typical gains range from 10% (at 64KB) to
x5 better (at <1KB).
--train FILEs
Use FILEs as training set to create a dictionary. The training set should contain a
lot of small files (> 100), and weight typically 100x the target dictionary size
(for example, 10 MB for a 100 KB dictionary).
Supports multithreading if zstd is compiled with threading support. Additional
parameters can be specified with --train-cover. The legacy dictionary builder can
be accessed with --train-legacy. Equivalent to --train-cover=d=8,steps=4.
-o file
Dictionary saved into file (default name: dictionary).
--maxdict=#
Limit dictionary to specified size (default: 112640).
--dictID=#
A dictionary ID is a locally unique ID that a decoder can use to verify it is using
the right dictionary. By default, zstd will create a 4-bytes random number ID. It´s
possible to give a precise number instead. 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. However, it´s up to
the dictionary manager to not assign twice the same ID to 2 different dictionaries.
--train-cover[=k#,d=#,steps=#]
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. Requires that d <= k.
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]. Supports multithreading if zstd is
compiled with threading support.
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
--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 possible maximum size. --train-legacy=s=#
is also accepted.
Examples:
zstd --train-legacy FILEs
zstd --train-legacy=selectivity=8 FILEs
BENCHMARK
-b# benchmark file(s) using compression level #
-e# benchmark file(s) using multiple compression levels, from -b# to -e# (inclusive)
-i# minimum evaluation time, in seconds (default : 3s), benchmark mode only
-B#, --block-size=#
cut file(s) into independent blocks of size # (default: no block)
--priority=rt
set process priority to real-time
ADVANCED COMPRESSION OPTIONS
--zstd[=options]:
zstd provides 22 predefined compression levels. The selected or default predefined
compression level can be changed with advanced compression 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 8 strategies numbered from 1 to 8, from faster to stronger: 1=ZSTD_fast,
2=ZSTD_dfast, 3=ZSTD_greedy, 4=ZSTD_lazy, 5=ZSTD_lazy2, 6=ZSTD_btlazy2,
7=ZSTD_btopt, 8=ZSTD_btultra.
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 27 (128 MiB).
hashLog=hlog, hlog=hlog
Specify the maximum number of bits for a hash table.
Bigger hash tables cause less collisions which usually makes compression faster,
but requires more memory during compression.
The minimum hlog is 6 (64 B) and the maximum is 26 (128 MiB).
chainLog=clog, clog=clog
Specify the maximum number of bits for a hash chain or a binary tree.
Higher numbers of bits increases the chance to find a match which usually improves
compression ratio. It also slows down compression speed and increases memory
requirements for compression. This option is ignored for the ZSTD_fast strategy.
The minimum clog is 6 (64 B) and the maximum is 28 (256 MiB).
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 26.
searchLength=slen, slen=slen
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 slen is 3 and the maximum is 7.
targetLen=tlen, tlen=tlen
Specify the minimum match length that causes a match finder to stop searching for
better matches.
A larger minimum match length usually improves compression ratio but decreases
compression speed. This option is only used with strategies ZSTD_btopt and
ZSTD_btultra.
The minimum tlen is 4 and the maximum is 999.
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. 0 means "no overlap", hence
completely independent jobs. 9 means "full overlap", meaning up to windowSize is
reloaded from previous job. Reducing ovlog by 1 reduces the amount of reload by a
factor 2. Default ovlog is 6, which means "reload windowSize / 8". Exception : the
maximum compression level (22) has a default ovlog of 9.
-B#:
Select the size of each compression job. This parameter is available only when
multi-threading is enabled. Default value is 4 * windowSize, which means it varies
depending on compression level. -B# makes it possible to select a custom value. Note that
job size must respect a minimum value which is enforced transparently. This minimum is
either 1 MB, or overlapSize, whichever is largest.
Example
The following parameters sets advanced compression options to those of predefined level 19
for files bigger than 256 KB:
--zstd=windowLog=23,chainLog=23,hashLog=22,searchLog=6,searchLength=3,targetLength=48,strategy=6
BUGS
Report bugs at: https://github.com/facebook/zstd/issues
AUTHOR
Yann Collet