unlz4 (1) - Linux Manuals

unlz4: lz4, unlz4, lz4cat - Compress or decompress .lz4 files


lz4 - lz4, unlz4, lz4cat - Compress or decompress .lz4 files



unlz4 is equivalent to lz4 -d

lz4cat is equivalent to lz4 -dcfm

When writing scripts that need to decompress files, it is recommended to always use the name lz4 with appropriate arguments (lz4 -d or lz4 -dc) instead of the names unlz4 and lz4cat.


lz4 is an extremely fast lossless compression algorithm, based on byte-aligned LZ77 family of compression scheme. lz4 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 .lz4 format.

Difference between lz4 and gzip

lz4 supports a command line syntax similar but not identical to gzip(1). Differences are :
lz4 compresses a single file by default (see -m for multiple files)
lz4 file1 file2 means : compress file1 into file2
lz4 file.lz4 will default to decompression (use -z to force compression)
lz4 preserves original files
lz4 shows real-time notification statistics during compression or decompression of a single file (use -q to silence them)
When no destination is specified, result is sent on implicit output, which depends on stdout status. When stdout is Not the console, it becomes the implicit output. Otherwise, if stdout is the console, the implicit output is filename.lz4.
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. -c ensures that output will be stdout. Conversely, providing a destination name, or using -m ensures that the output will be either the specified name, or filename.lz4 respectively.

Default behaviors can be modified by opt-in commands, detailed below.

lz4 -m makes it possible to provide multiple input filenames, which will be compressed into files using suffix .lz4. Progress notifications become disabled by default (use -v to enable them). This mode has a behavior which more closely mimics gzip command line, with the main remaining difference being that source files are preserved by default.
Similarly, lz4 -m -d can decompress multiple *.lz4 files.
It's possible to opt-in to erase source files on successful compression or decompression, using --rm command.
Consequently, lz4 -m --rm behaves the same as gzip.

Concatenation of .lz4 files

It is possible to concatenate .lz4 files as is. lz4 will decompress such files as if they were a single .lz4 file. For example:
lz4 file1  > foo.lz4
lz4 file2 >> foo.lz4

Then lz4cat foo.lz4 is equivalent to cat file1 file2.


Short commands concatenation

In some cases, some options can be expressed using short command -x or long command --long-word. Short commands can be concatenated together. For example, -d -c is equivalent to -dc. Long commands cannot be concatenated. They must be clearly separated by a space.

Multiple commands

When multiple contradictory commands are issued on a same command line, only the latest one will be applied.

Operation mode

-z --compress
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, unlz4 implies --decompress), nor from the input file name (for example, a file extension .lz4 implies --decompress by default). -z can also be used to force compression of an already compressed .lz4 file.
-d --decompress --uncompress
Decompress. --decompress is also the default operation when the input filename has an .lz4 extension.
-t --test
Test the integrity of compressed .lz4 files. The decompressed data is discarded. No files are created nor removed.
Benchmark mode, using # compression level.
List information about .lz4 files. note : current implementation is limited to single-frame .lz4 files.

Operation modifiers

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 =# is not present, it defaults to 1. This setting overrides compression level if one was set previously. Similarly, if a compression level is set after --fast, it overrides it.
Set highest compression level. Same as -12.
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).
-D dictionaryName
Compress, decompress or benchmark using dictionary dictionaryName. 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.
-f --[no-]force
This option has several effects:
If the target file already exists, overwrite it without prompting.
When used with --decompress and lz4 cannot recognize the type of the source file, copy the source file as is to standard output. This allows lz4cat --force to be used like cat (1) for files that have not been compressed with lz4.
-c --stdout --to-stdout
Force write to standard output, even if it is the console.
-m --multiple
Multiple input files. Compressed file names will be appended a .lz4 suffix. This mode also reduces notification level. Can also be used to list multiple files. lz4 -m has a behavior equivalent to gzip -k (it preserves source files by default).
operate recursively on directories. This mode also sets -m (multiple input files).
Block size [4-7](default : 7)
-B4= 64KB ; -B5= 256KB ; -B6= 1MB ; -B7= 4MB
Produce independent blocks (default)
Blocks depend on predecessors (improves compression ratio, more noticeable on small blocks)
Select frame checksum (default:enabled)
Header includes original size (default:not present)
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.
Sparse mode support (default:enabled on file, disabled on stdout)
Use Legacy format (typically for Linux Kernel compression)
Note : -l is not compatible with -m (--multiple) nor -r

Other options

-v --verbose
Verbose mode
-q --quiet
Suppress warnings and real-time statistics; specify twice to suppress errors too
-h -H --help
Display help/long help and exit
-V --version
Display Version number and exit
-k --keep
Preserve source files (default behavior)
Delete source files on successful compression or decompression
Treat all subsequent arguments as files

Benchmark mode

Benchmark file(s), using # compression level
Benchmark multiple compression levels, from b# to e# (included)
Minimum evaluation time in seconds [1-9] (default : 3)


Report bugs at: https://github.com/lz4/lz4/issues


Yann Collet