When writing scripts that need to decompress files, it is recommended to always use the name \fBlz4\fR with appropriate arguments (\fBlz4 \-d\fR or \fBlz4 \-dc\fR) instead of the names \fBunlz4\fR and \fBlz4cat\fR\.
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.SH"DESCRIPTION"
\fBlz4\fR is an extremely fast lossless compression algorithm, based on \fBbyte\-aligned LZ77\fR family of compression scheme\.\fBlz4\fR offers compression speeds of 400 MB/s per core, linearly scalable with multi\-core CPUs\. It features an extremely fast decoder, with speed in multiple GB/s per core, typically reaching RAM speed limit on multi\-core systems\. The native file format is the \fB\.lz4\fR format\.
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.SS"Difference between lz4 and gzip"
\fBlz4\fR supports a command line syntax similar \fIbut not identical\fR to \fBgzip(1)\fR\. Differences are :
When no destination is specified, result is sent on implicit output, which depends on \fBstdout\fR status\. When \fBstdout\fR\fIis Not the console\fR, it becomes the implicit output\. Otherwise, if \fBstdout\fR is the console, the implicit output is \fBfilename\.lz4\fR\.
It is considered bad practice to rely on implicit output in scripts\. because the script\'s environment may change\. Always use explicit output in scripts\.\fB\-c\fR ensures that output will be \fBstdout\fR\. Conversely, providing a destination name, or using \fB\-m\fR ensures that the output will be either the specified name, or \fBfilename\.lz4\fR respectively\.
\fBlz4 \-m\fR makes it possible to provide multiple input filenames, which will be compressed into files using suffix \fB\.lz4\fR\. Progress notifications become disabled by default (use \fB\-v\fR to enable them)\. This mode has a behavior which more closely mimics \fBgzip\fR command line, with the main remaining difference being that source files are preserved by default\.
In some cases, some options can be expressed using short command \fB\-x\fR or long command \fB\-\-long\-word\fR\. Short commands can be concatenated together\. For example, \fB\-d \-c\fR is equivalent to \fB\-dc\fR\. Long commands cannot be concatenated\. They must be clearly separated by a space\.
Compress\. This is the default operation mode when no operation mode option is specified, no other operation mode is implied from the command name (for example, \fBunlz4\fR implies \fB\-\-decompress\fR), nor from the input file name (for example, a file extension \fB\.lz4\fR implies \fB\-\-decompress\fR by default)\.\fB\-z\fR can also be used to force compression of an already compressed \fB\.lz4\fR file\.
Compression level, with # being any value from 1 to 12\. Higher values trade compression speed for compression ratio\. Values above 12 are considered the same as 12\. Recommended values are 1 for fast compression (default), and 9 for high compression\. Speed/compression trade\-off will vary depending on data to compress\. Decompression speed remains fast at all settings\.
Switch to ultra\-fast compression levels\. The higher the value, the faster the compression speed, at the cost of some compression ratio\. If \fB=#\fR is not present, it defaults to \fB1\fR\. This setting overrides compression level if one was set previously\. Similarly, if a compression level is set after \fB\-\-fast\fR, it overrides it\.
Generate compressed data optimized for decompression speed\. Compressed data will be larger as a consequence (typically by ~0\.5%), while decompression speed will be improved by 5\-20%, depending on use cases\. This option only works in combination with very high compression levels (>=10)\.
Compress, decompress or benchmark using dictionary \fIdictionaryName\fR\. Compression and decompression must use the same dictionary to be compatible\. Using a different dictionary during decompression will either abort due to decompression error, or generate a checksum error\.
If the target file already exists, overwrite it without prompting\.
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.IP
When used with \fB\-\-decompress\fR and \fBlz4\fR cannot recognize the type of the source file, copy the source file as is to standard output\. This allows \fBlz4cat \-\-force\fR to be used like \fBcat (1)\fR for files that have not been compressed with \fBlz4\fR\.
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.TP
\fB\-c\fR\fB\-\-stdout\fR\fB\-\-to\-stdout\fR
Force write to standard output, even if it is the console\.
Multiple input files\. Compressed file names will be appended a \fB\.lz4\fR suffix\. This mode also reduces notification level\. Can also be used to list multiple files\.\fBlz4 \-m\fR has a behavior equivalent to \fBgzip \-k\fR (it preserves source files by default)\.
Header includes original size (default:not present)
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.br
Note : this option can only be activated when the original size can be determined, hence for a file\. It won\'t work with unknown source size, such as stdin or pipe\.
