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Editing: README.md

Command Line Interface for Zstandard library
============================================

Command Line Interface (CLI) can be created using the `make` command without any additional parameters.
There are however other Makefile targets that create different variations of CLI:
- `zstd` : default CLI supporting gzip-like arguments; includes dictionary builder, benchmark, and supports decompression of legacy zstd formats
- `zstd_nolegacy` : Same as `zstd` but without support for legacy zstd formats
- `zstd-small` : CLI optimized for minimal size; no dictionary builder, no benchmark, and no support for legacy zstd formats
- `zstd-compress` : version of CLI which can only compress into zstd format
- `zstd-decompress` : version of CLI which can only decompress zstd format

### Compilation variables
`zstd` scope can be altered by modifying the following `make` variables :

- __HAVE_THREAD__ : multithreading is automatically enabled when `pthread` is detected.
  It's possible to disable multithread support, by setting `HAVE_THREAD=0`.
  Example : `make zstd HAVE_THREAD=0`
  It's also possible to force multithread support, using `HAVE_THREAD=1`.
  In which case, linking stage will fail if neither `pthread` nor `windows.h` library can be found.
  This is useful to ensure this feature is not silently disabled.

- __ZSTD_LEGACY_SUPPORT__ : `zstd` can decompress files compressed by older versions of `zstd`.
  Starting v0.8.0, all versions of `zstd` produce frames compliant with the [specification](../doc/zstd_compression_format.md), and are therefore compatible.
  But older versions (< v0.8.0) produced different, incompatible, frames.
  By default, `zstd` supports decoding legacy formats >= v0.4.0 (`ZSTD_LEGACY_SUPPORT=4`).
  This can be altered by modifying this compilation variable.
  `ZSTD_LEGACY_SUPPORT=1` means "support all formats >= v0.1.0".
  `ZSTD_LEGACY_SUPPORT=2` means "support all formats >= v0.2.0", and so on.
  `ZSTD_LEGACY_SUPPORT=0` means _DO NOT_ support any legacy format.
  if `ZSTD_LEGACY_SUPPORT >= 8`, it's the same as `0`, since there is no legacy format after `7`.
  Note : `zstd` only supports decoding older formats, and cannot generate any legacy format.

- __HAVE_ZLIB__ : `zstd` can compress and decompress files in `.gz` format.
  This is ordered through command `--format=gzip`.
  Alternatively, symlinks named `gzip` or `gunzip` will mimic intended behavior.
  `.gz` support is automatically enabled when `zlib` library is detected at build time.
  It's possible to disable `.gz` support, by setting `HAVE_ZLIB=0`.
  Example : `make zstd HAVE_ZLIB=0`
  It's also possible to force compilation with zlib support, using `HAVE_ZLIB=1`.
  In which case, linking stage will fail if `zlib` library cannot be found.
  This is useful to prevent silent feature disabling.

- __HAVE_LZMA__ : `zstd` can compress and decompress files in `.xz` and `.lzma` formats.
  This is ordered through commands `--format=xz` and `--format=lzma` respectively.
  Alternatively, symlinks named `xz`, `unxz`, `lzma`, or `unlzma` will mimic intended behavior.
  `.xz` and `.lzma` support is automatically enabled when `lzma` library is detected at build time.
  It's possible to disable `.xz` and `.lzma` support, by setting `HAVE_LZMA=0`.
  Example : `make zstd HAVE_LZMA=0`
  It's also possible to force compilation with lzma support, using `HAVE_LZMA=1`.
  In which case, linking stage will fail if `lzma` library cannot be found.
  This is useful to prevent silent feature disabling.

- __HAVE_LZ4__ : `zstd` can compress and decompress files in `.lz4` formats.
  This is ordered through commands `--format=lz4`.
  Alternatively, symlinks named `lz4`, or `unlz4` will mimic intended behavior.
  `.lz4` support is automatically enabled when `lz4` library is detected at build time.
  It's possible to disable `.lz4` support, by setting `HAVE_LZ4=0` .
  Example : `make zstd HAVE_LZ4=0`
  It's also possible to force compilation with lz4 support, using `HAVE_LZ4=1`.
  In which case, linking stage will fail if `lz4` library cannot be found.
  This is useful to prevent silent feature disabling.

- __ZSTD_NOBENCH__ : `zstd` cli will be compiled without its integrated benchmark module.
  This can be useful to produce smaller binaries.
  In this case, the corresponding unit can also be excluded from compilation target.

- __ZSTD_NODICT__ : `zstd` cli will be compiled without support for the integrated dictionary builder.
  This can be useful to produce smaller binaries.
  In this case, the corresponding unit can also be excluded from compilation target.

- __ZSTD_NOCOMPRESS__ : `zstd` cli will be compiled without support for compression.
  The resulting binary will only be able to decompress files.
  This can be useful to produce smaller binaries.
  A corresponding `Makefile` target using this ability is `zstd-decompress`.

- __ZSTD_NODECOMPRESS__ : `zstd` cli will be compiled without support for decompression.
  The resulting binary will only be able to compress files.
  This can be useful to produce smaller binaries.
  A corresponding `Makefile` target using this ability is `zstd-compress`.

- __BACKTRACE__ : `zstd` can display a stack backtrace when execution
  generates a runtime exception. By default, this feature may be
  degraded/disabled on some platforms unless additional compiler directives are
  applied. When triaging a runtime issue, enabling this feature can provide
  more context to determine the location of the fault.
  Example : `make zstd BACKTRACE=1`

### Aggregation of parameters
CLI supports aggregation of parameters i.e. `-b1`, `-e18`, and `-i1` can be joined into `-b1e18i1`.

### Symlink shortcuts
It's possible to invoke `zstd` through a symlink.
When the name of the symlink has a specific value, it triggers an associated behavior.
- `zstdmt` : compress using all cores available on local system.
- `zcat` : will decompress and output target file using any of the supported formats. `gzcat` and `zstdcat` are also equivalent.
- `gzip` : if zlib support is enabled, will mimic `gzip` by compressing file using `.gz` format, removing source file by default (use `--keep` to preserve). If zlib is not supported, triggers an error.
- `xz` : if lzma support is enabled, will mimic `xz` by compressing file using `.xz` format, removing source file by default (use `--keep` to preserve). If xz is not supported, triggers an error.
- `lzma` : if lzma support is enabled, will mimic `lzma` by compressing file using `.lzma` format, removing source file by default (use `--keep` to preserve). If lzma is not supported, triggers an error.
- `lz4` : if lz4 support is enabled, will mimic `lz4` by compressing file using `.lz4` format. If lz4 is not supported, triggers an error.
- `unzstd` and `unlz4` will decompress any of the supported format.
- `ungz`, `unxz` and `unlzma` will do the same, and will also remove source file by default (use `--keep` to preserve).

### Dictionary builder in Command Line Interface
Zstd offers a training mode, which can be used to tune the algorithm for a selected
type of data, by providing it with a few samples. The result of the training is stored
in a file selected with the `-o` option (default name is `dictionary`),
which can be loaded before compression and decompression.

Using a dictionary, the compression ratio achievable on small data improves dramatically.
These compression gains are achieved while simultaneously providing faster compression and decompression speeds.
Dictionary work if there is some correlation in a family of small data (there is no universal dictionary).
Hence, deploying one dictionary per type of data will provide the greater benefits.
Dictionary gains are mostly effective in the first few KB. Then, the compression algorithm
will rely more and more on previously decoded content to compress the rest of the file.

Usage of the dictionary builder and created dictionaries with CLI:

1. Create the dictionary : `zstd --train PathToTrainingSet/* -o dictionaryName`
2. Compress with the dictionary: `zstd FILE -D dictionaryName`
3. Decompress with the dictionary: `zstd --decompress FILE.zst -D dictionaryName`

### Benchmark in Command Line Interface
CLI includes in-memory compression benchmark module for zstd.
The benchmark is conducted using given filenames. The files are read into memory and joined together.
It makes benchmark more precise as it eliminates I/O overhead.
Multiple filenames can be supplied, as multiple parameters, with wildcards,
or names of directories can be used as parameters with `-r` option.

The benchmark measures ratio, compressed size, compression and decompression speed.
One can select compression levels starting from `-b` and ending with `-e`.
The `-i` parameter selects minimal time used for each of tested levels.

### Usage of Command Line Interface
The full list of options can be obtained with `-h` or `-H` parameter:
```
Usage :
      zstd [args] [FILE(s)] [-o file]

FILE    : a filename
          with no FILE, or when FILE is - , read standard input
Arguments :
 -#     : # compression level (1-19, default: 3)
 -d     : decompression
 -D DICT: use DICT as Dictionary for compression or decompression
 -o file: result stored into `file` (only 1 output file)
 -f     : overwrite output without prompting, also (de)compress links
--rm    : remove source file(s) after successful de/compression
 -k     : preserve source file(s) (default)
 -h/-H  : display help/long help and exit

Advanced arguments :
 -V     : display Version number and exit
 -c     : force write to standard output, even if it is the console
 -v     : verbose mode; specify multiple times to increase verbosity
 -q     : suppress warnings; specify twice to suppress errors too
--no-progress : do not display the progress counter
 -r     : operate recursively on directories
--filelist FILE : read list of files to operate upon from FILE
--output-dir-flat DIR : processed files are stored into DIR
--output-dir-mirror DIR : processed files are stored into DIR respecting original directory structure
--[no-]check : during compression, add XXH64 integrity checksum to frame (default: enabled). If specified with -d, decompressor will ignore/validate checksums in compressed frame (default: validate).
--      : All arguments after "--" are treated as files

Advanced compression arguments :
--ultra : enable levels beyond 19, up to 22 (requires more memory)
--long[=#]: enable long distance matching with given window log (default: 27)
--fast[=#]: switch to very fast compression levels (default: 1)
--adapt : dynamically adapt compression level to I/O conditions
 -T#    : spawns # compression threads (default: 1, 0==# cores)
 -B#    : select size of each job (default: 0==automatic)
--single-thread : use a single thread for both I/O and compression (result slightly different than -T1)
--rsyncable : compress using a rsync-friendly method (-B sets block size)
--exclude-compressed: only compress files that are not already compressed
--stream-size=# : specify size of streaming input from `stdin`
--size-hint=# optimize compression parameters for streaming input of approximately this size
--target-compressed-block-size=# : generate compressed block of approximately targeted size
--no-dictID : don't write dictID into header (dictionary compression only)
--[no-]compress-literals : force (un)compressed literals
--format=zstd : compress files to the .zst format (default)
--format=gzip : compress files to the .gz format
--format=xz : compress files to the .xz format
--format=lzma : compress files to the .lzma format
--format=lz4 : compress files to the .lz4 format

Advanced decompression arguments :
 -l     : print information about zstd compressed files
--test  : test compressed file integrity
 -M#    : Set a memory usage limit for decompression
--[no-]sparse : sparse mode (default: disabled)

Dictionary builder :
--train ## : create a dictionary from a training set of files
--train-cover[=k=#,d=#,steps=#,split=#,shrink[=#]] : use the cover algorithm with optional args
--train-fastcover[=k=#,d=#,f=#,steps=#,split=#,accel=#,shrink[=#]] : use the fast cover algorithm with optional args
--train-legacy[=s=#] : use the legacy algorithm with selectivity (default: 9)
 -o DICT : DICT is dictionary name (default: dictionary)
--maxdict=# : limit dictionary to specified size (default: 112640)
--dictID=# : force dictionary ID to specified value (default: random)

Benchmark arguments :
 -b#    : benchmark file(s), using # compression level (default: 3)
 -e#    : test all compression levels successively from -b# to -e# (default: 1)
 -i#    : minimum evaluation time in seconds (default: 3s)
 -B#    : cut file into independent blocks of size # (default: no block)
 -S     : output one benchmark result per input file (default: consolidated result)
--priority=rt : set process priority to real-time
```

### Passing parameters through Environment Variables
There is no "generic" way to pass "any kind of parameter" to `zstd` in a pass-through manner.
Using environment variables for this purpose has security implications.
Therefore, this avenue is intentionally restricted and only supports `ZSTD_CLEVEL` and `ZSTD_NBTHREADS`.

`ZSTD_CLEVEL` can be used to modify the default compression level of `zstd`
(usually set to `3`) to another value between 1 and 19 (the "normal" range).

`ZSTD_NBTHREADS` can be used to specify a number of threads
that `zstd` will use for compression, which by default is `1`.
This functionality only exists when `zstd` is compiled with multithread support.
`0` means "use as many threads as detected cpu cores on local system".
The max # of threads is capped at: `ZSTDMT_NBWORKERS_MAX==200`.

This functionality can be useful when `zstd` CLI is invoked in a way that doesn't allow passing arguments.
One such scenario is `tar --zstd`.
As `ZSTD_CLEVEL` and `ZSTD_NBTHREADS` only replace the default compression level
and number of threads respectively, they can both be overridden by corresponding command line arguments:
`-#` for compression level and `-T#` for number of threads.

### Long distance matching mode
The long distance matching mode, enabled with `--long`, is designed to improve
the compression ratio for files with long matches at a large distance (up to the
maximum window size, `128 MiB`) while still maintaining compression speed.

Enabling this mode sets the window size to `128 MiB` and thus increases the memory
usage for both the compressor and decompressor. Performance in terms of speed is
dependent on long matches being found. Compression speed may degrade if few long
matches are found. Decompression speed usually improves when there are many long
distance matches.

Below are graphs comparing the compression speed, compression ratio, and
decompression speed with and without long distance matching on an ideal use
case: a tar of four versions of clang (versions `3.4.1`, `3.4.2`, `3.5.0`,
`3.5.1`) with a total size of `244889600 B`. This is an ideal use case as there
are many long distance matches within the maximum window size of `128 MiB` (each
version is less than `128 MiB`).

Compression Speed vs Ratio | Decompression Speed
---------------------------|---------------------
![Compression Speed vs Ratio](https://raw.githubusercontent.com/facebook/zstd/v1.3.3/doc/images/ldmCspeed.png "Compression Speed vs Ratio") | ![Decompression Speed](https://raw.githubusercontent.com/facebook/zstd/v1.3.3/doc/images/ldmDspeed.png "Decompression Speed")

| Method | Compression ratio | Compression speed | Decompression speed  |
|:-------|------------------:|-------------------------:|---------------------------:|
| `zstd -1`  | `5.065`    | `284.8 MB/s`  | `759.3 MB/s`  |
| `zstd -5`  | `5.826`    | `124.9 MB/s`  | `674.0 MB/s`  |
| `zstd -10` | `6.504`    | `29.5 MB/s`   | `771.3 MB/s`  |
| `zstd -1 --long` | `17.426` | `220.6 MB/s` | `1638.4 MB/s` |
| `zstd -5 --long` | `19.661` | `165.5 MB/s` | `1530.6 MB/s` |
| `zstd -10 --long`| `21.949` |  `75.6 MB/s` | `1632.6 MB/s` |

On this file, the compression ratio improves significantly with minimal impact
on compression speed, and the decompression speed doubles.

On the other extreme, compressing a file with few long distance matches (such as
the [Silesia compression corpus]) will likely lead to a deterioration in
compression speed (for lower levels) with minimal change in compression ratio.

The below table illustrates this on the [Silesia compression corpus].

[Silesia compression corpus]: http://sun.aei.polsl.pl/~sdeor/index.php?page=silesia

| Method | Compression ratio | Compression speed | Decompression speed  |
|:-------|------------------:|------------------:|---------------------:|
| `zstd -1`        | `2.878` | `231.7 MB/s`      | `594.4 MB/s`   |
| `zstd -1 --long` | `2.929` | `106.5 MB/s`      | `517.9 MB/s`   |
| `zstd -5`        | `3.274` | `77.1 MB/s`       | `464.2 MB/s`   |
| `zstd -5 --long` | `3.319` | `51.7 MB/s`       | `371.9 MB/s`   |
| `zstd -10`       | `3.523` | `16.4 MB/s`       | `489.2 MB/s`   |
| `zstd -10 --long`| `3.566` | `16.2 MB/s`       | `415.7 MB/s`   |

### zstdgrep

`zstdgrep` is a utility which makes it possible to `grep` directly a `.zst` compressed file.
It's used the same way as normal `grep`, for example :
`zstdgrep pattern file.zst`

`zstdgrep` is _not_ compatible with dictionary compression.

To search into a file compressed with a dictionary,
it's necessary to decompress it using `zstd` or `zstdcat`,
and then pipe the result to `grep`. For example  :
`zstdcat -D dictionary -qc -- file.zst | grep pattern`

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