oasis-root

xz.1 (77549B)

'\" t
.\" SPDX-License-Identifier: 0BSD
.\"
.\" Authors: Lasse Collin
.\"          Jia Tan
.\"
.TH XZ 1 "2025-03-08" "Tukaani" "XZ Utils"
.
.SH NAME
xz, unxz, xzcat, lzma, unlzma, lzcat \- Compress or decompress .xz and .lzma files
.
.SH SYNOPSIS
.B xz
.RI [ option... ]
.RI [ file... ]
.
.SH COMMAND ALIASES
.B unxz
is equivalent to
.BR "xz \-\-decompress" .
.br
.B xzcat
is equivalent to
.BR "xz \-\-decompress \-\-stdout" .
.br
.B lzma
is equivalent to
.BR "xz \-\-format=lzma" .
.br
.B unlzma
is equivalent to
.BR "xz \-\-format=lzma \-\-decompress" .
.br
.B lzcat
is equivalent to
.BR "xz \-\-format=lzma \-\-decompress \-\-stdout" .
.PP
When writing scripts that need to decompress files,
it is recommended to always use the name
.B xz
with appropriate arguments
.RB ( "xz \-d"
or
.BR "xz \-dc" )
instead of the names
.B unxz
and
.BR xzcat .
.
.SH DESCRIPTION
.B xz
is a general-purpose data compression tool with
command line syntax similar to
.BR gzip (1)
and
.BR bzip2 (1).
The native file format is the
.B .xz
format, but the legacy
.B .lzma
format used by LZMA Utils and
raw compressed streams with no container format headers
are also supported.
In addition, decompression of the
.B .lz
format used by
.B lzip
is supported.
.PP
.B xz
compresses or decompresses each
.I file
according to the selected operation mode.
If no
.I files
are given or
.I file
is
.BR \- ,
.B xz
reads from standard input and writes the processed data
to standard output.
.B xz
will refuse (display an error and skip the
.IR file )
to write compressed data to standard output if it is a terminal.
Similarly,
.B xz
will refuse to read compressed data
from standard input if it is a terminal.
.PP
Unless
.B \-\-stdout
is specified,
.I files
other than
.B \-
are written to a new file whose name is derived from the source
.I file
name:
.IP \(bu 3
When compressing, the suffix of the target file format
.RB ( .xz
or
.BR .lzma )
is appended to the source filename to get the target filename.
.IP \(bu 3
When decompressing, the
.BR .xz ,
.BR .lzma ,
or
.B .lz
suffix is removed from the filename to get the target filename.
.B xz
also recognizes the suffixes
.B .txz
and
.BR .tlz ,
and replaces them with the
.B .tar
suffix.
.PP
If the target file already exists, an error is displayed and the
.I file
is skipped.
.PP
Unless writing to standard output,
.B xz
will display a warning and skip the
.I file
if any of the following applies:
.IP \(bu 3
.I File
is not a regular file.
Symbolic links are not followed,
and thus they are not considered to be regular files.
.IP \(bu 3
.I File
has more than one hard link.
.IP \(bu 3
.I File
has setuid, setgid, or sticky bit set.
.IP \(bu 3
The operation mode is set to compress and the
.I file
already has a suffix of the target file format
.RB ( .xz
or
.B .txz
when compressing to the
.B .xz
format, and
.B .lzma
or
.B .tlz
when compressing to the
.B .lzma
format).
.IP \(bu 3
The operation mode is set to decompress and the
.I file
doesn't have a suffix of any of the supported file formats
.RB ( .xz ,
.BR .txz ,
.BR .lzma ,
.BR .tlz ,
or
.BR .lz ).
.PP
After successfully compressing or decompressing the
.IR file ,
.B xz
copies the owner, group, permissions, access time,
and modification time from the source
.I file
to the target file.
If copying the group fails, the permissions are modified
so that the target file doesn't become accessible to users
who didn't have permission to access the source
.IR file .
.B xz
doesn't support copying other metadata like access control lists
or extended attributes yet.
.PP
Once the target file has been successfully closed, the source
.I file
is removed unless
.B \-\-keep
was specified.
The source
.I file
is never removed if the output is written to standard output
or if an error occurs.
.PP
Sending
.B SIGINFO
or
.B SIGUSR1
to the
.B xz
process makes it print progress information to standard error.
This has only limited use since when standard error
is a terminal, using
.B \-\-verbose
will display an automatically updating progress indicator.
.
.SS "Memory usage"
The memory usage of
.B xz
varies from a few hundred kilobytes to several gigabytes
depending on the compression settings.
The settings used when compressing a file determine
the memory requirements of the decompressor.
Typically the decompressor needs 5\ % to 20\ % of
the amount of memory that the compressor needed when
creating the file.
For example, decompressing a file created with
.B xz \-9
currently requires 65\ MiB of memory.
Still, it is possible to have
.B .xz
files that require several gigabytes of memory to decompress.
.PP
Especially users of older systems may find
the possibility of very large memory usage annoying.
To prevent uncomfortable surprises,
.B xz
has a built-in memory usage limiter, which is disabled by default.
While some operating systems provide ways to limit
the memory usage of processes, relying on it
wasn't deemed to be flexible enough (for example, using
.BR ulimit (1)
to limit virtual memory tends to cripple
.BR mmap (2)).
.PP
The memory usage limiter can be enabled with
the command line option \fB\-\-memlimit=\fIlimit\fR.
Often it is more convenient to enable the limiter
by default by setting the environment variable
.\" TRANSLATORS: Don't translate the uppercase XZ_DEFAULTS.
.\" It's a name of an environment variable.
.BR XZ_DEFAULTS ,
for example,
.BR XZ_DEFAULTS=\-\-memlimit=150MiB .
It is possible to set the limits separately
for compression and decompression by using
.BI \-\-memlimit\-compress= limit
and \fB\-\-memlimit\-decompress=\fIlimit\fR.
Using these two options outside
.B XZ_DEFAULTS
is rarely useful because a single run of
.B xz
cannot do both compression and decompression and
.BI \-\-memlimit= limit
(or
.B \-M
.IR limit )
is shorter to type on the command line.
.PP
If the specified memory usage limit is exceeded when decompressing,
.B xz
will display an error and decompressing the file will fail.
If the limit is exceeded when compressing,
.B xz
will try to scale the settings down so that the limit
is no longer exceeded (except when using
.B \-\-format=raw
or
.BR \-\-no\-adjust ).
This way the operation won't fail unless the limit is very small.
The scaling of the settings is done in steps that don't
match the compression level presets, for example, if the limit is
only slightly less than the amount required for
.BR "xz \-9" ,
the settings will be scaled down only a little,
not all the way down to
.BR "xz \-8" .
.
.SS "Concatenation and padding with .xz files"
It is possible to concatenate
.B .xz
files as is.
.B xz
will decompress such files as if they were a single
.B .xz
file.
.PP
It is possible to insert padding between the concatenated parts
or after the last part.
The padding must consist of null bytes and the size
of the padding must be a multiple of four bytes.
This can be useful, for example, if the
.B .xz
file is stored on a medium that measures file sizes
in 512-byte blocks.
.PP
Concatenation and padding are not allowed with
.B .lzma
files or raw streams.
.
.SH OPTIONS
.
.SS "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.
.TP
.B KiB
Multiply the integer by 1,024 (2^10).
.BR Ki ,
.BR k ,
.BR kB ,
.BR K ,
and
.B KB
are accepted as synonyms for
.BR KiB .
.TP
.B MiB
Multiply the integer by 1,048,576 (2^20).
.BR Mi ,
.BR m ,
.BR M ,
and
.B MB
are accepted as synonyms for
.BR MiB .
.TP
.B GiB
Multiply the integer by 1,073,741,824 (2^30).
.BR Gi ,
.BR g ,
.BR G ,
and
.B GB
are accepted as synonyms for
.BR GiB .
.PP
The special value
.B max
can be used to indicate the maximum integer value
supported by the option.
.
.SS "Operation mode"
If multiple operation mode options are given,
the last one takes effect.
.TP
.BR \-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,
.B unxz
implies
.BR \-\-decompress ).
.IP ""
.\" The DESCRIPTION section already says this but it's good to repeat it
.\" here because the default behavior is a bit dangerous and new users
.\" in a hurry may skip reading the DESCRIPTION section.
After successful compression, the source file is removed
unless writing to standard output or
.B \-\-keep
was specified.
.TP
.BR \-d ", " \-\-decompress ", " \-\-uncompress
Decompress.
.\" The DESCRIPTION section already says this but it's good to repeat it
.\" here because the default behavior is a bit dangerous and new users
.\" in a hurry may skip reading the DESCRIPTION section.
After successful decompression, the source file is removed
unless writing to standard output or
.B \-\-keep
was specified.
.TP
.BR \-t ", " \-\-test
Test the integrity of compressed
.IR files .
This option is equivalent to
.B "\-\-decompress \-\-stdout"
except that the decompressed data is discarded instead of being
written to standard output.
No files are created or removed.
.TP
.BR \-l ", " \-\-list
Print information about compressed
.IR files .
No uncompressed output is produced,
and no files are created or removed.
In list mode, the program cannot read
the compressed data from standard
input or from other unseekable sources.
.IP ""
The default listing shows basic information about
.IR files ,
one file per line.
To get more detailed information, use also the
.B \-\-verbose
option.
For even more information, use
.B \-\-verbose
twice, but note that this may be slow, because getting all the extra
information requires many seeks.
The width of verbose output exceeds
80 characters, so piping the output to, for example,
.B "less\ \-S"
may be convenient if the terminal isn't wide enough.
.IP ""
The exact output may vary between
.B xz
versions and different locales.
For machine-readable output,
.B \-\-robot \-\-list
should be used.
.
.SS "Operation modifiers"
.TP
.BR \-k ", " \-\-keep
Don't delete the input files.
.IP ""
Since
.B xz
5.2.6,
this option also makes
.B xz
compress or decompress even if the input is
a symbolic link to a regular file,
has more than one hard link,
or has the setuid, setgid, or sticky bit set.
The setuid, setgid, and sticky bits are not copied
to the target file.
In earlier versions this was only done with
.BR \-\-force .
.TP
.BR \-f ", " \-\-force
This option has several effects:
.RS
.IP \(bu 3
If the target file already exists,
delete it before compressing or decompressing.
.IP \(bu 3
Compress or decompress even if the input is
a symbolic link to a regular file,
has more than one hard link,
or has the setuid, setgid, or sticky bit set.
The setuid, setgid, and sticky bits are not copied
to the target file.
.IP \(bu 3
When used with
.B \-\-decompress
.B \-\-stdout
and
.B xz
cannot recognize the type of the source file,
copy the source file as is to standard output.
This allows
.B xzcat
.B \-\-force
to be used like
.BR cat (1)
for files that have not been compressed with
.BR xz .
Note that in future,
.B xz
might support new compressed file formats, which may make
.B xz
decompress more types of files instead of copying them as is to
standard output.
.BI \-\-format= format
can be used to restrict
.B xz
to decompress only a single file format.
.RE
.TP
.BR \-c ", " \-\-stdout ", " \-\-to\-stdout
Write the compressed or decompressed data to
standard output instead of a file.
This implies
.BR \-\-keep .
.TP
.B \-\-single\-stream
Decompress only the first
.B .xz
stream, and
silently ignore possible remaining input data following the stream.
Normally such trailing garbage makes
.B xz
display an error.
.IP ""
.B xz
never decompresses more than one stream from
.B .lzma
files or raw streams, but this option still makes
.B xz
ignore the possible trailing data after the
.B .lzma
file or raw stream.
.IP ""
This option has no effect if the operation mode is not
.B \-\-decompress
or
.BR \-\-test .
.IP ""
Since
.B xz
5.7.1alpha,
.B \-\-single\-stream
implies
.BR \-\-keep .
.TP
.B \-\-no\-sparse
Disable creation of sparse files.
By default, if decompressing into a regular file,
.B xz
tries to make the file sparse if the decompressed data contains
long sequences of binary zeros.
It also works when writing to standard output
as long as standard output is connected to a regular file
and certain additional conditions are met to make it safe.
Creating sparse files may save disk space and speed up
the decompression by reducing the amount of disk I/O.
.TP
\fB\-S\fR \fI.suf\fR, \fB\-\-suffix=\fI.suf
When compressing, use
.I .suf
as the suffix for the target file instead of
.B .xz
or
.BR .lzma .
If not writing to standard output and
the source file already has the suffix
.IR .suf ,
a warning is displayed and the file is skipped.
.IP ""
When decompressing, recognize files with the suffix
.I .suf
in addition to files with the
.BR .xz ,
.BR .txz ,
.BR .lzma ,
.BR .tlz ,
or
.B .lz
suffix.
If the source file has the suffix
.IR .suf ,
the suffix is removed to get the target filename.
.IP ""
When compressing or decompressing raw streams
.RB ( \-\-format=raw ),
the suffix must always be specified unless
writing to standard output,
because there is no default suffix for raw streams.
.TP
\fB\-\-files\fR[\fB=\fIfile\fR]
Read the filenames to process from
.IR file ;
if
.I file
is omitted, filenames are read from standard input.
Filenames must be terminated with the newline character.
A dash
.RB ( \- )
is taken as a regular filename; it doesn't mean standard input.
If filenames are given also as command line arguments, they are
processed before the filenames read from
.IR file .
.TP
\fB\-\-files0\fR[\fB=\fIfile\fR]
This is identical to \fB\-\-files\fR[\fB=\fIfile\fR] except
that each filename must be terminated with the null character.
.
.SS "Basic file format and compression options"
.TP
\fB\-F\fR \fIformat\fR, \fB\-\-format=\fIformat
Specify the file
.I format
to compress or decompress:
.RS
.TP
.\" TRANSLATORS: Don't translate bold string B<auto>.
.B auto
This is the default.
When compressing,
.B auto
is equivalent to
.BR xz .
When decompressing,
the format of the input file is automatically detected.
Note that raw streams (created with
.BR \-\-format=raw )
cannot be auto-detected.
.TP
.B xz
Compress to the
.B .xz
file format, or accept only
.B .xz
files when decompressing.
.TP
.BR lzma ", " alone
Compress to the legacy
.B .lzma
file format, or accept only
.B .lzma
files when decompressing.
The alternative name
.B alone
is provided for backwards compatibility with LZMA Utils.
.TP
.B lzip
Accept only
.B .lz
files when decompressing.
Compression is not supported.
.IP ""
The
.B .lz
format version 0 and the unextended version 1 are supported.
Version 0 files were produced by
.B lzip
1.3 and older.
Such files aren't common but may be found from file archives
as a few source packages were released in this format.
People might have old personal files in this format too.
Decompression support for the format version 0 was removed in
.B lzip
1.18.
.IP ""
.B lzip
1.4 and later create files in the format version 1.
The sync flush marker extension to the format version 1 was added in
.B lzip
1.6.
This extension is rarely used and isn't supported by
.B xz
(diagnosed as corrupt input).
.TP
.B raw
Compress or uncompress a raw stream (no headers).
This is meant for advanced users only.
To decode raw streams, you need use
.B \-\-format=raw
and explicitly specify the filter chain,
which normally would have been stored in the container headers.
.RE
.TP
\fB\-C\fR \fIcheck\fR, \fB\-\-check=\fIcheck
Specify the type of the integrity check.
The check is calculated from the uncompressed data and
stored in the
.B .xz
file.
This option has an effect only when compressing into the
.B .xz
format; the
.B .lzma
format doesn't support integrity checks.
The integrity check (if any) is verified when the
.B .xz
file is decompressed.
.IP ""
Supported
.I check
types:
.RS
.TP
.\" TRANSLATORS: Don't translate the bold strings B<none>, B<crc32>,
.\" B<crc64>, and B<sha256>. The command line option --check accepts
.\" only the untranslated strings.
.B none
Don't calculate an integrity check at all.
This is usually a bad idea.
This can be useful when integrity of the data is verified
by other means anyway.
.TP
.B crc32
Calculate CRC32 using the polynomial from IEEE-802.3 (Ethernet).
.TP
.B crc64
Calculate CRC64 using the polynomial from ECMA-182.
This is the default, since it is slightly better than CRC32
at detecting damaged files and the speed difference is negligible.
.TP
.B sha256
Calculate SHA-256.
This is somewhat slower than CRC32 and CRC64.
.RE
.IP ""
Integrity of the
.B .xz
headers is always verified with CRC32.
It is not possible to change or disable it.
.TP
.B \-\-ignore\-check
Don't verify the integrity check of the compressed data when decompressing.
The CRC32 values in the
.B .xz
headers will still be verified normally.
.IP ""
.B "Do not use this option unless you know what you are doing."
Possible reasons to use this option:
.RS
.IP \(bu 3
Trying to recover data from a corrupt .xz file.
.IP \(bu 3
Speeding up decompression.
This matters mostly with SHA-256 or
with files that have compressed extremely well.
It's recommended to not use this option for this purpose
unless the file integrity is verified externally in some other way.
.RE
.TP
.BR \-0 " ... " \-9
Select a compression preset level.
The default is
.BR \-6 .
If multiple preset levels are specified,
the last one takes effect.
If a custom filter chain was already specified, setting
a compression preset level clears the custom filter chain.
.IP ""
The differences between the presets are more significant than with
.BR gzip (1)
and
.BR bzip2 (1).
The selected compression settings determine
the memory requirements of the decompressor,
thus using a too high preset level might make it painful
to decompress the file on an old system with little RAM.
Specifically,
.B "it's not a good idea to blindly use \-9 for everything"
like it often is with
.BR gzip (1)
and
.BR bzip2 (1).
.RS
.TP
.BR "\-0" " ... " "\-3"
These are somewhat fast presets.
.B \-0
is sometimes faster than
.B "gzip \-9"
while compressing much better.
The higher ones often have speed comparable to
.BR bzip2 (1)
with comparable or better compression ratio,
although the results
depend a lot on the type of data being compressed.
.TP
.BR "\-4" " ... " "\-6"
Good to very good compression while keeping
decompressor memory usage reasonable even for old systems.
.B \-6
is the default, which is usually a good choice
for distributing files that need to be decompressible
even on systems with only 16\ MiB RAM.
.RB ( \-5e
or
.B \-6e
may be worth considering too.
See
.BR \-\-extreme .)
.TP
.B "\-7 ... \-9"
These are like
.B \-6
but with higher compressor and decompressor memory requirements.
These are useful only when compressing files bigger than
8\ MiB, 16\ MiB, and 32\ MiB, respectively.
.RE
.IP ""
On the same hardware, the decompression speed is approximately
a constant number of bytes of compressed data per second.
In other words, the better the compression,
the faster the decompression will usually be.
This also means that the amount of uncompressed output
produced per second can vary a lot.
.IP ""
The following table summarises the features of the presets:
.RS
.RS
.PP
.TS
tab(;);
c c c c c
n n n n n.
Preset;DictSize;CompCPU;CompMem;DecMem
\-0;256 KiB;0;3 MiB;1 MiB
\-1;1 MiB;1;9 MiB;2 MiB
\-2;2 MiB;2;17 MiB;3 MiB
\-3;4 MiB;3;32 MiB;5 MiB
\-4;4 MiB;4;48 MiB;5 MiB
\-5;8 MiB;5;94 MiB;9 MiB
\-6;8 MiB;6;94 MiB;9 MiB
\-7;16 MiB;6;186 MiB;17 MiB
\-8;32 MiB;6;370 MiB;33 MiB
\-9;64 MiB;6;674 MiB;65 MiB
.TE
.RE
.RE
.IP ""
Column descriptions:
.RS
.IP \(bu 3
DictSize is the LZMA2 dictionary size.
It is waste of memory to use a dictionary bigger than
the size of the uncompressed file.
This is why it is good to avoid using the presets
.BR \-7 " ... " \-9
when there's no real need for them.
At
.B \-6
and lower, the amount of memory wasted is
usually low enough to not matter.
.IP \(bu 3
CompCPU is a simplified representation of the LZMA2 settings
that affect compression speed.
The dictionary size affects speed too,
so while CompCPU is the same for levels
.BR \-6 " ... " \-9 ,
higher levels still tend to be a little slower.
To get even slower and thus possibly better compression, see
.BR \-\-extreme .
.IP \(bu 3
CompMem contains the compressor memory requirements
in the single-threaded mode.
It may vary slightly between
.B xz
versions.
.IP \(bu 3
DecMem contains the decompressor memory requirements.
That is, the compression settings determine
the memory requirements of the decompressor.
The exact decompressor memory usage is slightly more than
the LZMA2 dictionary size, but the values in the table
have been rounded up to the next full MiB.
.RE
.IP ""
Memory requirements of the multi-threaded mode are
significantly higher than that of the single-threaded mode.
With the default value of
.BR \-\-block\-size ,
each thread needs 3*3*DictSize plus CompMem or DecMem.
For example, four threads with preset
.B \-6
needs 660\(en670\ MiB of memory.
.TP
.BR \-e ", " \-\-extreme
Use a slower variant of the selected compression preset level
.RB ( \-0 " ... " \-9 )
to hopefully get a little bit better compression ratio,
but with bad luck this can also make it worse.
Decompressor memory usage is not affected,
but compressor memory usage increases a little at preset levels
.BR \-0 " ... " \-3 .
.IP ""
Since there are two presets with dictionary sizes
4\ MiB and 8\ MiB, the presets
.B \-3e
and
.B \-5e
use slightly faster settings (lower CompCPU) than
.B \-4e
and
.BR \-6e ,
respectively.
That way no two presets are identical.
.RS
.RS
.PP
.TS
tab(;);
c c c c c
n n n n n.
Preset;DictSize;CompCPU;CompMem;DecMem
\-0e;256 KiB;8;4 MiB;1 MiB
\-1e;1 MiB;8;13 MiB;2 MiB
\-2e;2 MiB;8;25 MiB;3 MiB
\-3e;4 MiB;7;48 MiB;5 MiB
\-4e;4 MiB;8;48 MiB;5 MiB
\-5e;8 MiB;7;94 MiB;9 MiB
\-6e;8 MiB;8;94 MiB;9 MiB
\-7e;16 MiB;8;186 MiB;17 MiB
\-8e;32 MiB;8;370 MiB;33 MiB
\-9e;64 MiB;8;674 MiB;65 MiB
.TE
.RE
.RE
.IP ""
For example, there are a total of four presets that use
8\ MiB dictionary, whose order from the fastest to the slowest is
.BR \-5 ,
.BR \-6 ,
.BR \-5e ,
and
.BR \-6e .
.TP
.B \-\-fast
.PD 0
.TP
.B \-\-best
.PD
These are somewhat misleading aliases for
.B \-0
and
.BR \-9 ,
respectively.
These are provided only for backwards compatibility
with LZMA Utils.
Avoid using these options.
.TP
.BI \-\-block\-size= size
When compressing to the
.B .xz
format, split the input data into blocks of
.I size
bytes.
The blocks are compressed independently from each other,
which helps with multi-threading and
makes limited random-access decompression possible.
This option is typically used to override the default
block size in multi-threaded mode,
but this option can be used in single-threaded mode too.
.IP ""
In multi-threaded mode about three times
.I size
bytes will be allocated in each thread for buffering input and output.
The default
.I size
is three times the LZMA2 dictionary size or 1 MiB,
whichever is more.
Typically a good value is 2\(en4 times
the size of the LZMA2 dictionary or at least 1 MiB.
Using
.I size
less than the LZMA2 dictionary size is waste of RAM
because then the LZMA2 dictionary buffer will never get fully used.
In multi-threaded mode,
the sizes of the blocks are stored in the block headers.
This size information is required for multi-threaded decompression.
.IP ""
In single-threaded mode no block splitting is done by default.
Setting this option doesn't affect memory usage.
No size information is stored in block headers,
thus files created in single-threaded mode
won't be identical to files created in multi-threaded mode.
The lack of size information also means that
.B xz
won't be able decompress the files in multi-threaded mode.
.TP
.BI \-\-block\-list= items
When compressing to the
.B .xz
format, start a new block with an optional custom filter chain after
the given intervals of uncompressed data.
.IP ""
The
.I items
are a comma-separated list.
Each item consists of an optional filter chain number
between 0 and 9 followed by a colon
.RB ( : )
and a required size of uncompressed data.
Omitting an item (two or more consecutive commas) is a
shorthand to use the size and filters of the previous item.
.IP ""
If the input file is bigger than the sum of
the sizes in
.IR items ,
the last item is repeated until the end of the file.
A special value of
.B 0
may be used as the last size to indicate that
the rest of the file should be encoded as a single block.
.IP ""
An alternative filter chain for each block can be
specified in combination with the
.BI \-\-filters1= filters
\&...\&
.BI \-\-filters9= filters
options.
These options define filter chains with an identifier
between 1\(en9.
Filter chain 0 can be used to refer to the default filter chain,
which is the same as not specifying a filter chain.
The filter chain identifier can be used before the uncompressed
size, followed by a colon
.RB ( : ).
For example, if one specifies
.B \-\-block\-list=1:2MiB,3:2MiB,2:4MiB,,2MiB,0:4MiB
then blocks will be created using:
.RS
.IP \(bu 3
The filter chain specified by
.B \-\-filters1
and 2 MiB input
.IP \(bu 3
The filter chain specified by
.B \-\-filters3
and 2 MiB input
.IP \(bu 3
The filter chain specified by
.B \-\-filters2
and 4 MiB input
.IP \(bu 3
The filter chain specified by
.B \-\-filters2
and 4 MiB input
.IP \(bu 3
The default filter chain and 2 MiB input
.IP \(bu 3
The default filter chain and 4 MiB input for every block until
end of input.
.RE
.IP ""
If one specifies a size that exceeds the encoder's block size
(either the default value in threaded mode or
the value specified with \fB\-\-block\-size=\fIsize\fR),
the encoder will create additional blocks while
keeping the boundaries specified in
.IR items .
For example, if one specifies
.B \-\-block\-size=10MiB
.B \-\-block\-list=5MiB,10MiB,8MiB,12MiB,24MiB
and the input file is 80 MiB,
one will get 11 blocks:
5, 10, 8, 10, 2, 10, 10, 4, 10, 10, and 1 MiB.
.IP ""
In multi-threaded mode the sizes of the blocks
are stored in the block headers.
This isn't done in single-threaded mode,
so the encoded output won't be
identical to that of the multi-threaded mode.
.TP
.BI \-\-flush\-timeout= timeout
When compressing, if more than
.I timeout
milliseconds (a positive integer) has passed since the previous flush and
reading more input would block,
all the pending input data is flushed from the encoder and
made available in the output stream.
This can be useful if
.B xz
is used to compress data that is streamed over a network.
Small
.I timeout
values make the data available at the receiving end
with a small delay, but large
.I timeout
values give better compression ratio.
.IP ""
This feature is disabled by default.
If this option is specified more than once, the last one takes effect.
The special
.I timeout
value of
.B 0
can be used to explicitly disable this feature.
.IP ""
This feature is not available on non-POSIX systems.
.IP ""
.\" FIXME
.B "This feature is still experimental."
Currently
.B xz
is unsuitable for decompressing the stream in real time due to how
.B xz
does buffering.
.TP
.B \-\-no\-sync
Do not synchronize the target file and its directory
to the storage device before removing the source file.
This can improve performance if compressing or decompressing
many small files.
However, if the system crashes soon after the deletion,
it is possible that the target file was not written
to the storage device but the delete operation was.
In that case neither the original source file
nor the target file is available.
.IP ""
This option has an effect only when
.B xz
is going to remove the source file.
In other cases synchronization is never done.
.IP ""
The synchronization and
.B \-\-no\-sync
were added in
.B xz
5.7.1alpha.
.TP
.BI \-\-memlimit\-compress= limit
Set a memory usage limit for compression.
If this option is specified multiple times,
the last one takes effect.
.IP ""
If the compression settings exceed the
.IR limit ,
.B xz
will attempt to adjust the settings downwards so that
the limit is no longer exceeded and display a notice that
automatic adjustment was done.
The adjustments are done in this order:
reducing the number of threads,
switching to single-threaded mode
if even one thread in multi-threaded mode exceeds the
.IR limit ,
and finally reducing the LZMA2 dictionary size.
.IP ""
When compressing with
.B \-\-format=raw
or if
.B \-\-no\-adjust
has been specified,
only the number of threads may be reduced
since it can be done without affecting the compressed output.
.IP ""
If the
.I limit
cannot be met even with the adjustments described above,
an error is displayed and
.B xz
will exit with exit status 1.
.IP ""
The
.I limit
can be specified in multiple ways:
.RS
.IP \(bu 3
The
.I limit
can be an absolute value in bytes.
Using an integer suffix like
.B MiB
can be useful.
Example:
.B "\-\-memlimit\-compress=80MiB"
.IP \(bu 3
The
.I limit
can be specified as a percentage of total physical memory (RAM).
This can be useful especially when setting the
.B XZ_DEFAULTS
environment variable in a shell initialization script
that is shared between different computers.
That way the limit is automatically bigger
on systems with more memory.
Example:
.B "\-\-memlimit\-compress=70%"
.IP \(bu 3
The
.I limit
can be reset back to its default value by setting it to
.BR 0 .
This is currently equivalent to setting the
.I limit
to
.B max
(no memory usage limit).
.RE
.IP ""
For 32-bit
.B xz
there is a special case: if the
.I limit
would be over
.BR "4020\ MiB" ,
the
.I limit
is set to
.BR "4020\ MiB" .
On MIPS32
.B "2000\ MiB"
is used instead.
(The values
.B 0
and
.B max
aren't affected by this.
A similar feature doesn't exist for decompression.)
This can be helpful when a 32-bit executable has access
to 4\ GiB address space (2 GiB on MIPS32)
while hopefully doing no harm in other situations.
.IP ""
See also the section
.BR "Memory usage" .
.TP
.BI \-\-memlimit\-decompress= limit
Set a memory usage limit for decompression.
This also affects the
.B \-\-list
mode.
If the operation is not possible without exceeding the
.IR limit ,
.B xz
will display an error and decompressing the file will fail.
See
.BI \-\-memlimit\-compress= limit
for possible ways to specify the
.IR limit .
.TP
.BI \-\-memlimit\-mt\-decompress= limit
Set a memory usage limit for multi-threaded decompression.
This can only affect the number of threads;
this will never make
.B xz
refuse to decompress a file.
If
.I limit
is too low to allow any multi-threading, the
.I limit
is ignored and
.B xz
will continue in single-threaded mode.
Note that if also
.B \-\-memlimit\-decompress
is used,
it will always apply to both single-threaded and multi-threaded modes,
and so the effective
.I limit
for multi-threading will never be higher than the limit set with
.BR \-\-memlimit\-decompress .
.IP ""
In contrast to the other memory usage limit options,
.BI \-\-memlimit\-mt\-decompress= limit
has a system-specific default
.IR limit .
.B "xz \-\-info\-memory"
can be used to see the current value.
.IP ""
This option and its default value exist
because without any limit the threaded decompressor
could end up allocating an insane amount of memory with some input files.
If the default
.I limit
is too low on your system,
feel free to increase the
.I limit
but never set it to a value larger than the amount of usable RAM
as with appropriate input files
.B xz
will attempt to use that amount of memory
even with a low number of threads.
Running out of memory or swapping
will not improve decompression performance.
.IP ""
See
.BI \-\-memlimit\-compress= limit
for possible ways to specify the
.IR limit .
Setting
.I limit
to
.B 0
resets the
.I limit
to the default system-specific value.
.TP
\fB\-M\fR \fIlimit\fR, \fB\-\-memlimit=\fIlimit\fR, \fB\-\-memory=\fIlimit
This is equivalent to specifying
.BI \-\-memlimit\-compress= limit
.BI \-\-memlimit-decompress= limit
\fB\-\-memlimit\-mt\-decompress=\fIlimit\fR.
.TP
.B \-\-no\-adjust
Display an error and exit if the memory usage limit cannot be
met without adjusting settings that affect the compressed output.
That is, this prevents
.B xz
from switching the encoder from multi-threaded mode to single-threaded mode
and from reducing the LZMA2 dictionary size.
Even when this option is used the number of threads may be reduced
to meet the memory usage limit as that won't affect the compressed output.
.IP ""
Automatic adjusting is always disabled when creating raw streams
.RB ( \-\-format=raw ).
.TP
\fB\-T\fR \fIthreads\fR, \fB\-\-threads=\fIthreads
Specify the number of worker threads to use.
Setting
.I threads
to a special value
.B 0
makes
.B xz
use up to as many threads as the processor(s) on the system support.
The actual number of threads can be fewer than
.I threads
if the input file is not big enough
for threading with the given settings or
if using more threads would exceed the memory usage limit.
.IP ""
The single-threaded and multi-threaded compressors produce different output.
Single-threaded compressor will give the smallest file size but
only the output from the multi-threaded compressor can be decompressed
using multiple threads.
Setting
.I threads
to
.B 1
will use the single-threaded mode.
Setting
.I threads
to any other value, including
.BR 0 ,
will use the multi-threaded compressor
even if the system supports only one hardware thread.
.RB ( xz
5.2.x
used single-threaded mode in this situation.)
.IP ""
To use multi-threaded mode with only one thread, set
.I threads
to
.BR +1 .
The
.B +
prefix has no effect with values other than
.BR 1 .
A memory usage limit can still make
.B xz
switch to single-threaded mode unless
.B \-\-no\-adjust
is used.
Support for the
.B +
prefix was added in
.B xz
5.4.0.
.IP ""
If an automatic number of threads has been requested and
no memory usage limit has been specified,
then a system-specific default soft limit will be used to possibly
limit the number of threads.
It is a soft limit in sense that it is ignored
if the number of threads becomes one,
thus a soft limit will never stop
.B xz
from compressing or decompressing.
This default soft limit will not make
.B xz
switch from multi-threaded mode to single-threaded mode.
The active limits can be seen with
.BR "xz \-\-info\-memory" .
.IP ""
Currently the only threading method is to split the input into
blocks and compress them independently from each other.
The default block size depends on the compression level and
can be overridden with the
.BI \-\-block\-size= size
option.
.IP ""
Threaded decompression only works on files that contain
multiple blocks with size information in block headers.
All large enough files compressed in multi-threaded mode
meet this condition,
but files compressed in single-threaded mode don't even if
.BI \-\-block\-size= size
has been used.
.IP ""
The default value for
.I threads
is
.BR 0 .
In
.B xz
5.4.x and older the default is
.BR 1 .
.
.SS "Custom compressor filter chains"
A custom filter chain allows specifying
the compression settings in detail instead of relying on
the settings associated to the presets.
When a custom filter chain is specified,
preset options
.RB ( \-0
\&...\&
.B \-9
and
.BR \-\-extreme )
earlier on the command line are forgotten.
If a preset option is specified
after one or more custom filter chain options,
the new preset takes effect and
the custom filter chain options specified earlier are forgotten.
.PP
A filter chain is comparable to piping on the command line.
When compressing, the uncompressed input goes to the first filter,
whose output goes to the next filter (if any).
The output of the last filter gets written to the compressed file.
The maximum number of filters in the chain is four,
but typically a filter chain has only one or two filters.
.PP
Many filters have limitations on where they can be
in the filter chain:
some filters can work only as the last filter in the chain,
some only as a non-last filter, and some work in any position
in the chain.
Depending on the filter, this limitation is either inherent to
the filter design or exists to prevent security issues.
.PP
A custom filter chain can be specified in two different ways.
The options
.BI \-\-filters= filters
and
.BI \-\-filters1= filters
\&...\&
.BI \-\-filters9= filters
allow specifying an entire filter chain in one option using the
liblzma filter string syntax.
Alternatively, a filter chain can be specified by using one or more
individual filter options in the order they are wanted in the filter chain.
That is, the order of the individual filter options is significant!
When decoding raw streams
.RB ( \-\-format=raw ),
the filter chain must be specified in the same order as
it was specified when compressing.
Any individual filter or preset options specified before the full
chain option
(\fB\-\-filters=\fIfilters\fR)
will be forgotten.
Individual filters specified after the full chain option will reset the
filter chain.
.PP
Both the full and individual filter options take filter-specific
.I options
as a comma-separated list.
Extra commas in
.I options
are ignored.
Every option has a default value, so
specify those you want to change.
.PP
To see the whole filter chain and
.IR options ,
use
.B "xz \-vv"
(that is, use
.B \-\-verbose
twice).
This works also for viewing the filter chain options used by presets.
.TP
.BI \-\-filters= filters
Specify the full filter chain or a preset in a single option.
Each filter can be separated by spaces or two dashes
.RB ( \-\- ).
.I filters
may need to be quoted on the shell command line so it is
parsed as a single option.
To denote
.IR options ,
use
.B :
or
.BR = .
A preset can be prefixed with a
.B \-
and followed with zero or more flags.
The only supported flag is
.B e
to apply the same options as
.BR \-\-extreme .
.TP
\fB\-\-filters1\fR=\fIfilters\fR ... \fB\-\-filters9\fR=\fIfilters
Specify up to nine additional filter chains that can be used with
.BR \-\-block\-list .
.IP ""
For example, when compressing an archive with executable files
followed by text files, the executable part could use a filter
chain with a BCJ filter and the text part only the LZMA2 filter.
.TP
.B \-\-filters-help
Display a help message describing how to specify presets and
custom filter chains in the
.B \-\-filters
and
.BI \-\-filters1= filters
\&...\&
.BI \-\-filters9= filters
options, and exit successfully.
.TP
\fB\-\-lzma1\fR[\fB=\fIoptions\fR]
.PD 0
.TP
\fB\-\-lzma2\fR[\fB=\fIoptions\fR]
.PD
Add LZMA1 or LZMA2 filter to the filter chain.
These filters can be used only as the last filter in the chain.
.IP ""
LZMA1 is a legacy filter,
which is supported almost solely due to the legacy
.B .lzma
file format, which supports only LZMA1.
LZMA2 is an updated
version of LZMA1 to fix some practical issues of LZMA1.
The
.B .xz
format uses LZMA2 and doesn't support LZMA1 at all.
Compression speed and ratios of LZMA1 and LZMA2
are practically the same.
.IP ""
LZMA1 and LZMA2 share the same set of
.IR options :
.RS
.TP
.\" TRANSLATORS: Don't translate bold strings like B<preset>, B<dict>,
.\" B<mode>, B<nice>, B<fast>, or B<normal> because those are command line
.\" options. On the other hand, do translate the italic strings like
.\" I<preset>, I<size>, and I<mode>, because such italic strings are
.\" placeholders which a user replaces with an actual value.
.BI preset= preset
Reset all LZMA1 or LZMA2
.I options
to
.IR preset .
.I Preset
consist of an integer, which may be followed by single-letter
preset modifiers.
The integer can be from
.B 0
to
.BR 9 ,
matching the command line options
.B \-0
\&...\&
.BR \-9 .
The only supported modifier is currently
.BR e ,
which matches
.BR \-\-extreme .
If no
.B preset
is specified, the default values of LZMA1 or LZMA2
.I options
are taken from the preset
.BR 6 .
.TP
.BI dict= size
Dictionary (history buffer)
.I size
indicates how many bytes of the recently processed
uncompressed data is kept in memory.
The algorithm tries to find repeating byte sequences (matches) in
the uncompressed data, and replace them with references
to the data currently in the dictionary.
The bigger the dictionary, the higher is the chance
to find a match.
Thus, increasing dictionary
.I size
usually improves compression ratio, but
a dictionary bigger than the uncompressed file is waste of memory.
.IP ""
Typical dictionary
.I size
is from 64\ KiB to 64\ MiB.
The minimum is 4\ KiB.
The maximum for compression is currently 1.5\ GiB (1536\ MiB).
The decompressor already supports dictionaries up to
one byte less than 4\ GiB, which is the maximum for
the LZMA1 and LZMA2 stream formats.
.IP ""
Dictionary
.I size
and match finder
.RI ( mf )
together determine the memory usage of the LZMA1 or LZMA2 encoder.
The same (or bigger) dictionary
.I size
is required for decompressing that was used when compressing,
thus the memory usage of the decoder is determined
by the dictionary size used when compressing.
The
.B .xz
headers store the dictionary
.I size
either as
.RI "2^" n
or
.RI "2^" n " + 2^(" n "\-1),"
so these
.I sizes
are somewhat preferred for compression.
Other
.I sizes
will get rounded up when stored in the
.B .xz
headers.
.TP
.BI lc= lc
Specify the number of literal context bits.
The minimum is 0 and the maximum is 4; the default is 3.
In addition, the sum of
.I lc
and
.I lp
must not exceed 4.
.IP ""
All bytes that cannot be encoded as matches
are encoded as literals.
That is, literals are simply 8-bit bytes
that are encoded one at a time.
.IP ""
The literal coding makes an assumption that the highest
.I lc
bits of the previous uncompressed byte correlate
with the next byte.
For example, in typical English text, an upper-case letter is
often followed by a lower-case letter, and a lower-case
letter is usually followed by another lower-case letter.
In the US-ASCII character set, the highest three bits are 010
for upper-case letters and 011 for lower-case letters.
When
.I lc
is at least 3, the literal coding can take advantage of
this property in the uncompressed data.
.IP ""
The default value (3) is usually good.
If you want maximum compression, test
.BR lc=4 .
Sometimes it helps a little, and
sometimes it makes compression worse.
If it makes it worse, test
.B lc=2
too.
.TP
.BI lp= lp
Specify the number of literal position bits.
The minimum is 0 and the maximum is 4; the default is 0.
.IP ""
.I Lp
affects what kind of alignment in the uncompressed data is
assumed when encoding literals.
See
.I pb
below for more information about alignment.
.TP
.BI pb= pb
Specify the number of position bits.
The minimum is 0 and the maximum is 4; the default is 2.
.IP ""
.I Pb
affects what kind of alignment in the uncompressed data is
assumed in general.
The default means four-byte alignment
.RI (2^ pb =2^2=4),
which is often a good choice when there's no better guess.
.IP ""
When the alignment is known, setting
.I pb
accordingly may reduce the file size a little.
For example, with text files having one-byte
alignment (US-ASCII, ISO-8859-*, UTF-8), setting
.B pb=0
can improve compression slightly.
For UTF-16 text,
.B pb=1
is a good choice.
If the alignment is an odd number like 3 bytes,
.B pb=0
might be the best choice.
.IP ""
Even though the assumed alignment can be adjusted with
.I pb
and
.IR lp ,
LZMA1 and LZMA2 still slightly favor 16-byte alignment.
It might be worth taking into account when designing file formats
that are likely to be often compressed with LZMA1 or LZMA2.
.TP
.BI mf= mf
Match finder has a major effect on encoder speed,
memory usage, and compression ratio.
Usually Hash Chain match finders are faster than Binary Tree
match finders.
The default depends on the
.IR preset :
0 uses
.BR hc3 ,
1\(en3
use
.BR hc4 ,
and the rest use
.BR bt4 .
.IP ""
The following match finders are supported.
The memory usage formulas below are rough approximations,
which are closest to the reality when
.I dict
is a power of two.
.RS
.TP
.B hc3
Hash Chain with 2- and 3-byte hashing
.br
Minimum value for
.IR nice :
3
.br
Memory usage:
.br
.I dict
* 7.5 (if
.I dict
<= 16 MiB);
.br
.I dict
* 5.5 + 64 MiB (if
.I dict
> 16 MiB)
.TP
.B hc4
Hash Chain with 2-, 3-, and 4-byte hashing
.br
Minimum value for
.IR nice :
4
.br
Memory usage:
.br
.I dict
* 7.5 (if
.I dict
<= 32 MiB);
.br
.I dict
* 6.5 (if
.I dict
> 32 MiB)
.TP
.B bt2
Binary Tree with 2-byte hashing
.br
Minimum value for
.IR nice :
2
.br
Memory usage:
.I dict
* 9.5
.TP
.B bt3
Binary Tree with 2- and 3-byte hashing
.br
Minimum value for
.IR nice :
3
.br
Memory usage:
.br
.I dict
* 11.5 (if
.I dict
<= 16 MiB);
.br
.I dict
* 9.5 + 64 MiB (if
.I dict
> 16 MiB)
.TP
.B bt4
Binary Tree with 2-, 3-, and 4-byte hashing
.br
Minimum value for
.IR nice :
4
.br
Memory usage:
.br
.I dict
* 11.5 (if
.I dict
<= 32 MiB);
.br
.I dict
* 10.5 (if
.I dict
> 32 MiB)
.RE
.TP
.BI mode= mode
Compression
.I mode
specifies the method to analyze
the data produced by the match finder.
Supported
.I modes
are
.B fast
and
.BR normal .
The default is
.B fast
for
.I presets
0\(en3 and
.B normal
for
.I presets
4\(en9.
.IP ""
Usually
.B fast
is used with Hash Chain match finders and
.B normal
with Binary Tree match finders.
This is also what the
.I presets
do.
.TP
.BI nice= nice
Specify what is considered to be a nice length for a match.
Once a match of at least
.I nice
bytes is found, the algorithm stops
looking for possibly better matches.
.IP ""
.I Nice
can be 2\(en273 bytes.
Higher values tend to give better compression ratio
at the expense of speed.
The default depends on the
.IR preset .
.TP
.BI depth= depth
Specify the maximum search depth in the match finder.
The default is the special value of 0,
which makes the compressor determine a reasonable
.I depth
from
.I mf
and
.IR nice .
.IP ""
Reasonable
.I depth
for Hash Chains is 4\(en100 and 16\(en1000 for Binary Trees.
Using very high values for
.I depth
can make the encoder extremely slow with some files.
Avoid setting the
.I depth
over 1000 unless you are prepared to interrupt
the compression in case it is taking far too long.
.RE
.IP ""
When decoding raw streams
.RB ( \-\-format=raw ),
LZMA2 needs only the dictionary
.IR size .
LZMA1 needs also
.IR lc ,
.IR lp ,
and
.IR pb .
.TP
\fB\-\-x86\fR[\fB=\fIoptions\fR]
.PD 0
.TP
\fB\-\-arm\fR[\fB=\fIoptions\fR]
.TP
\fB\-\-armthumb\fR[\fB=\fIoptions\fR]
.TP
\fB\-\-arm64\fR[\fB=\fIoptions\fR]
.TP
\fB\-\-powerpc\fR[\fB=\fIoptions\fR]
.TP
\fB\-\-ia64\fR[\fB=\fIoptions\fR]
.TP
\fB\-\-sparc\fR[\fB=\fIoptions\fR]
.TP
\fB\-\-riscv\fR[\fB=\fIoptions\fR]
.PD
Add a branch/call/jump (BCJ) filter to the filter chain.
These filters can be used only as a non-last filter
in the filter chain.
.IP ""
A BCJ filter converts relative addresses in
the machine code to their absolute counterparts.
This doesn't change the size of the data
but it increases redundancy,
which can help LZMA2 to produce 0\(en15\ % smaller
.B .xz
file.
The BCJ filters are always reversible,
so using a BCJ filter for wrong type of data
doesn't cause any data loss, although it may make
the compression ratio slightly worse.
The BCJ filters are very fast and
use an insignificant amount of memory.
.IP ""
These BCJ filters have known problems related to
the compression ratio:
.RS
.IP \(bu 3
Some types of files containing executable code
(for example, object files, static libraries, and Linux kernel modules)
have the addresses in the instructions filled with filler values.
These BCJ filters will still do the address conversion,
which will make the compression worse with these files.
.IP \(bu 3
If a BCJ filter is applied on an archive,
it is possible that it makes the compression ratio
worse than not using a BCJ filter.
For example, if there are similar or even identical executables
then filtering will likely make the files less similar
and thus compression is worse.
The contents of non-executable files in the same archive can matter too.
In practice one has to try with and without a BCJ filter to see
which is better in each situation.
.RE
.IP ""
Different instruction sets have different alignment:
the executable file must be aligned to a multiple of
this value in the input data to make the filter work.
.RS
.RS
.PP
.TS
tab(;);
l n l
l n l.
Filter;Alignment;Notes
x86;1;32-bit or 64-bit x86
ARM;4;
ARM-Thumb;2;
ARM64;4;4096-byte alignment is best
PowerPC;4;Big endian only
IA-64;16;Itanium
SPARC;4;
RISC-V;2;
.TE
.RE
.RE
.IP ""
Since the BCJ-filtered data is usually compressed with LZMA2,
the compression ratio may be improved slightly if
the LZMA2 options are set to match the
alignment of the selected BCJ filter.
Examples:
.RS
.IP \(bu 3
IA-64 filter has 16-byte alignment so
.B pb=4,lp=4,lc=0
is good
with LZMA2 (2^4=16).
.IP \(bu 3
RISC-V code has 2-byte or 4-byte alignment
depending on whether the file contains
16-bit compressed instructions (the C extension).
When 16-bit instructions are used,
.B pb=2,lp=1,lc=3
or
.B pb=1,lp=1,lc=3
is good.
When 16-bit instructions aren't present,
.B pb=2,lp=2,lc=2
is the best.
.B readelf \-h
can be used to check if "RVC"
appears on the "Flags" line.
.IP \(bu 3
ARM64 is always 4-byte aligned so
.B pb=2,lp=2,lc=2
is the best.
.IP \(bu 3
The x86 filter is an exception.
It's usually good to stick to LZMA2's defaults
.RB ( pb=2,lp=0,lc=3 )
when compressing x86 executables.
.RE
.IP ""
All BCJ filters support the same
.IR options :
.RS
.TP
.BI start= offset
Specify the start
.I offset
that is used when converting between relative
and absolute addresses.
The
.I offset
must be a multiple of the alignment of the filter
(see the table above).
The default is zero.
In practice, the default is good; specifying a custom
.I offset
is almost never useful.
.RE
.TP
\fB\-\-delta\fR[\fB=\fIoptions\fR]
Add the Delta filter to the filter chain.
The Delta filter can be only used as a non-last filter
in the filter chain.
.IP ""
Currently only simple byte-wise delta calculation is supported.
It can be useful when compressing, for example, uncompressed bitmap images
or uncompressed PCM audio.
However, special purpose algorithms may give significantly better
results than Delta + LZMA2.
This is true especially with audio,
which compresses faster and better, for example, with
.BR flac (1).
.IP ""
Supported
.IR options :
.RS
.TP
.BI dist= distance
Specify the
.I distance
of the delta calculation in bytes.
.I distance
must be 1\(en256.
The default is 1.
.IP ""
For example, with
.B dist=2
and eight-byte input A1 B1 A2 B3 A3 B5 A4 B7, the output will be
A1 B1 01 02 01 02 01 02.
.RE
.
.SS "Other options"
.TP
.BR \-q ", " \-\-quiet
Suppress warnings and notices.
Specify this twice to suppress errors too.
This option has no effect on the exit status.
That is, even if a warning was suppressed,
the exit status to indicate a warning is still used.
.TP
.BR \-v ", " \-\-verbose
Be verbose.
If standard error is connected to a terminal,
.B xz
will display a progress indicator.
Specifying
.B \-\-verbose
twice will give even more verbose output.
.IP ""
The progress indicator shows the following information:
.RS
.IP \(bu 3
Completion percentage is shown
if the size of the input file is known.
That is, the percentage cannot be shown in pipes.
.IP \(bu 3
Amount of compressed data produced (compressing)
or consumed (decompressing).
.IP \(bu 3
Amount of uncompressed data consumed (compressing)
or produced (decompressing).
.IP \(bu 3
Compression ratio, which is calculated by dividing
the amount of compressed data processed so far by
the amount of uncompressed data processed so far.
.IP \(bu 3
Compression or decompression speed.
This is measured as the amount of uncompressed data consumed
(compression) or produced (decompression) per second.
It is shown after a few seconds have passed since
.B xz
started processing the file.
.IP \(bu 3
Elapsed time in the format M:SS or H:MM:SS.
.IP \(bu 3
Estimated remaining time is shown
only when the size of the input file is
known and a couple of seconds have already passed since
.B xz
started processing the file.
The time is shown in a less precise format which
never has any colons, for example, 2 min 30 s.
.RE
.IP ""
When standard error is not a terminal,
.B \-\-verbose
will make
.B xz
print the filename, compressed size, uncompressed size,
compression ratio, and possibly also the speed and elapsed time
on a single line to standard error after compressing or
decompressing the file.
The speed and elapsed time are included only when
the operation took at least a few seconds.
If the operation didn't finish, for example, due to user interruption,
also the completion percentage is printed
if the size of the input file is known.
.TP
.BR \-Q ", " \-\-no\-warn
Don't set the exit status to 2
even if a condition worth a warning was detected.
This option doesn't affect the verbosity level, thus both
.B \-\-quiet
and
.B \-\-no\-warn
have to be used to not display warnings and
to not alter the exit status.
.TP
.B \-\-robot
Print messages in a machine-parsable format.
This is intended to ease writing frontends that want to use
.B xz
instead of liblzma, which may be the case with various scripts.
The output with this option enabled is meant to be stable across
.B xz
releases.
See the section
.B "ROBOT MODE"
for details.
.TP
.B \-\-info\-memory
Display, in human-readable format, how much physical memory (RAM)
and how many processor threads
.B xz
thinks the system has and the memory usage limits for compression
and decompression, and exit successfully.
.TP
.BR \-h ", " \-\-help
Display a help message describing the most commonly used options,
and exit successfully.
.TP
.BR \-H ", " \-\-long\-help
Display a help message describing all features of
.BR xz ,
and exit successfully
.TP
.BR \-V ", " \-\-version
Display the version number of
.B xz
and liblzma in human readable format.
To get machine-parsable output, specify
.B \-\-robot
before
.BR \-\-version .
.
.SH "ROBOT MODE"
The robot mode is activated with the
.B \-\-robot
option.
It makes the output of
.B xz
easier to parse by other programs.
Currently
.B \-\-robot
is supported only together with
.BR \-\-list ,
.BR \-\-filters\-help ,
.BR \-\-info\-memory ,
and
.BR \-\-version .
It will be supported for compression and
decompression in the future.
.
.SS "List mode"
.B "xz \-\-robot \-\-list"
uses tab-separated output.
The first column of every line has a string
that indicates the type of the information found on that line:
.TP
.\" TRANSLATORS: The bold strings B<name>, B<file>, B<stream>, B<block>,
.\" B<summary>, and B<totals> are produced by the xz tool for scripts to
.\" parse, thus the untranslated strings must be included in the translated
.\" man page. It may be useful to provide a translated string in parenthesis
.\" without bold, for example: "B<name> (nimi)"
.B name
This is always the first line when starting to list a file.
The second column on the line is the filename.
.TP
.B file
This line contains overall information about the
.B .xz
file.
This line is always printed after the
.B name
line.
.TP
.B stream
This line type is used only when
.B \-\-verbose
was specified.
There are as many
.B stream
lines as there are streams in the
.B .xz
file.
.TP
.B block
This line type is used only when
.B \-\-verbose
was specified.
There are as many
.B block
lines as there are blocks in the
.B .xz
file.
The
.B block
lines are shown after all the
.B stream
lines; different line types are not interleaved.
.TP
.B summary
This line type is used only when
.B \-\-verbose
was specified twice.
This line is printed after all
.B block
lines.
Like the
.B file
line, the
.B summary
line contains overall information about the
.B .xz
file.
.TP
.B totals
This line is always the very last line of the list output.
It shows the total counts and sizes.
.PP
The columns of the
.B file
lines:
.PD 0
.RS
.IP 2. 4
Number of streams in the file
.IP 3. 4
Total number of blocks in the stream(s)
.IP 4. 4
Compressed size of the file
.IP 5. 4
Uncompressed size of the file
.IP 6. 4
Compression ratio, for example,
.BR 0.123 .
If ratio is over 9.999, three dashes
.RB ( \-\-\- )
are displayed instead of the ratio.
.IP 7. 4
Comma-separated list of integrity check names.
The following strings are used for the known check types:
.\" TRANSLATORS: Don't translate the bold strings B<None>, B<CRC32>,
.\" B<CRC64>, B<SHA-256>, or B<Unknown-> here. In robot mode, xz produces
.\" them in untranslated form for scripts to parse.
.BR None ,
.BR CRC32 ,
.BR CRC64 ,
and
.BR SHA\-256 .
For unknown check types,
.BI Unknown\- N
is used, where
.I N
is the Check ID as a decimal number (one or two digits).
.IP 8. 4
Total size of stream padding in the file
.RE
.PD
.PP
The columns of the
.B stream
lines:
.PD 0
.RS
.IP 2. 4
Stream number (the first stream is 1)
.IP 3. 4
Number of blocks in the stream
.IP 4. 4
Compressed start offset
.IP 5. 4
Uncompressed start offset
.IP 6. 4
Compressed size (does not include stream padding)
.IP 7. 4
Uncompressed size
.IP 8. 4
Compression ratio
.IP 9. 4
Name of the integrity check
.IP 10. 4
Size of stream padding
.RE
.PD
.PP
The columns of the
.B block
lines:
.PD 0
.RS
.IP 2. 4
Number of the stream containing this block
.IP 3. 4
Block number relative to the beginning of the stream
(the first block is 1)
.IP 4. 4
Block number relative to the beginning of the file
.IP 5. 4
Compressed start offset relative to the beginning of the file
.IP 6. 4
Uncompressed start offset relative to the beginning of the file
.IP 7. 4
Total compressed size of the block (includes headers)
.IP 8. 4
Uncompressed size
.IP 9. 4
Compression ratio
.IP 10. 4
Name of the integrity check
.RE
.PD
.PP
If
.B \-\-verbose
was specified twice, additional columns are included on the
.B block
lines.
These are not displayed with a single
.BR \-\-verbose ,
because getting this information requires many seeks
and can thus be slow:
.PD 0
.RS
.IP 11. 4
Value of the integrity check in hexadecimal
.IP 12. 4
Block header size
.IP 13. 4
Block flags:
.B c
indicates that compressed size is present, and
.B u
indicates that uncompressed size is present.
If the flag is not set, a dash
.RB ( \- )
is shown instead to keep the string length fixed.
New flags may be added to the end of the string in the future.
.IP 14. 4
Size of the actual compressed data in the block (this excludes
the block header, block padding, and check fields)
.IP 15. 4
Amount of memory (in bytes) required to decompress
this block with this
.B xz
version
.IP 16. 4
Filter chain.
Note that most of the options used at compression time
cannot be known, because only the options
that are needed for decompression are stored in the
.B .xz
headers.
.RE
.PD
.PP
The columns of the
.B summary
lines:
.PD 0
.RS
.IP 2. 4
Amount of memory (in bytes) required to decompress
this file with this
.B xz
version
.IP 3. 4
.B yes
or
.B no
indicating if all block headers have both compressed size and
uncompressed size stored in them
.PP
.I Since
.B xz
.I 5.1.2alpha:
.IP 4. 4
Minimum
.B xz
version required to decompress the file
.RE
.PD
.PP
The columns of the
.B totals
line:
.PD 0
.RS
.IP 2. 4
Number of streams
.IP 3. 4
Number of blocks
.IP 4. 4
Compressed size
.IP 5. 4
Uncompressed size
.IP 6. 4
Average compression ratio
.IP 7. 4
Comma-separated list of integrity check names
that were present in the files
.IP 8. 4
Stream padding size
.IP 9. 4
Number of files.
This is here to
keep the order of the earlier columns the same as on
.B file
lines.
.PD
.RE
.PP
If
.B \-\-verbose
was specified twice, additional columns are included on the
.B totals
line:
.PD 0
.RS
.IP 10. 4
Maximum amount of memory (in bytes) required to decompress
the files with this
.B xz
version
.IP 11. 4
.B yes
or
.B no
indicating if all block headers have both compressed size and
uncompressed size stored in them
.PP
.I Since
.B xz
.I 5.1.2alpha:
.IP 12. 4
Minimum
.B xz
version required to decompress the file
.RE
.PD
.PP
Future versions may add new line types and
new columns can be added to the existing line types,
but the existing columns won't be changed.
.
.SS "Filters help"
.B "xz \-\-robot \-\-filters-help"
prints the supported filters in the following format:
.PP
\fIfilter\fB:\fIoption\fB=<\fIvalue\fB>,\fIoption\fB=<\fIvalue\fB>\fR...
.TP
.I filter
Name of the filter
.TP
.I option
Name of a filter specific option
.TP
.I value
Numeric
.I value
ranges appear as
\fB<\fImin\fB\-\fImax\fB>\fR.
String
.I value
choices are shown within
.B "< >"
and separated by a
.B |
character.
.PP
Each filter is printed on its own line.
.
.SS "Memory limit information"
.B "xz \-\-robot \-\-info\-memory"
prints a single line with multiple tab-separated columns:
.IP 1. 4
Total amount of physical memory (RAM) in bytes.
.IP 2. 4
Memory usage limit for compression in bytes
.RB ( \-\-memlimit\-compress ).
A special value of
.B 0
indicates the default setting
which for single-threaded mode is the same as no limit.
.IP 3. 4
Memory usage limit for decompression in bytes
.RB ( \-\-memlimit\-decompress ).
A special value of
.B 0
indicates the default setting
which for single-threaded mode is the same as no limit.
.IP 4. 4
Since
.B xz
5.3.4alpha:
Memory usage for multi-threaded decompression in bytes
.RB ( \-\-memlimit\-mt\-decompress ).
This is never zero because a system-specific default value
shown in the column 5
is used if no limit has been specified explicitly.
This is also never greater than the value in the column 3
even if a larger value has been specified with
.BR \-\-memlimit\-mt\-decompress .
.IP 5. 4
Since
.B xz
5.3.4alpha:
A system-specific default memory usage limit
that is used to limit the number of threads
when compressing with an automatic number of threads
.RB ( \-\-threads=0 )
and no memory usage limit has been specified
.RB ( \-\-memlimit\-compress ).
This is also used as the default value for
.BR \-\-memlimit\-mt\-decompress .
.IP 6. 4
Since
.B xz
5.3.4alpha:
Number of available processor threads.
.PP
In the future, the output of
.B "xz \-\-robot \-\-info\-memory"
may have more columns, but never more than a single line.
.
.SS Version
.B "xz \-\-robot \-\-version"
prints the version number of
.B xz
and liblzma in the following format:
.PP
.\" TRANSLATORS: Don't translate the uppercase XZ_VERSION or LIBLZMA_VERSION.
.BI XZ_VERSION= XYYYZZZS
.br
.BI LIBLZMA_VERSION= XYYYZZZS
.TP
.I X
Major version.
.TP
.I YYY
Minor version.
Even numbers are stable.
Odd numbers are alpha or beta versions.
.TP
.I ZZZ
Patch level for stable releases or
just a counter for development releases.
.TP
.I S
Stability.
0 is alpha, 1 is beta, and 2 is stable.
.I S
should be always 2 when
.I YYY
is even.
.PP
.I XYYYZZZS
are the same on both lines if
.B xz
and liblzma are from the same XZ Utils release.
.PP
Examples: 4.999.9beta is
.B 49990091
and
5.0.0 is
.BR 50000002 .
.
.SH "EXIT STATUS"
.TP
.B 0
All is good.
.TP
.B 1
An error occurred.
.TP
.B 2
Something worth a warning occurred,
but no actual errors occurred.
.PP
Notices (not warnings or errors) printed on standard error
don't affect the exit status.
.
.SH ENVIRONMENT
.B xz
parses space-separated lists of options
from the environment variables
.\" TRANSLATORS: Don't translate the uppercase XZ_DEFAULTS or XZ_OPT.
.\" They are names of environment variables.
.B XZ_DEFAULTS
and
.BR XZ_OPT ,
in this order, before parsing the options from the command line.
Note that only options are parsed from the environment variables;
all non-options are silently ignored.
Parsing is done with
.BR getopt_long (3)
which is used also for the command line arguments.
.PP
.B Warning:
By setting these environment variables,
one is effectively modifying programs and scripts that run
.BR xz .
Most of the time it is safe to set memory usage limits, number of threads,
and compression options via the environment variables.
However, some options can break scripts.
An obvious example is
.B \-\-help
which makes
.B xz
show the help text instead of compressing or decompressing a file.
More subtle examples are
.B \-\-quiet
and
.BR \-\-verbose .
In many cases it works well to enable the progress indicator using
.BR \-\-verbose ,
but in some situations the extra messages create problems.
The verbosity level also affects the behavior of
.BR \-\-list .
.TP
.B XZ_DEFAULTS
User-specific or system-wide default options.
Typically this is set in a shell initialization script to enable
.BR xz 's
memory usage limiter by default or set the default number of threads.
Excluding shell initialization scripts
and similar special cases, scripts should never set or unset
.BR XZ_DEFAULTS .
.TP
.B XZ_OPT
This is for passing options to
.B xz
when it is not possible to set the options directly on the
.B xz
command line.
This is the case when
.B xz
is run by a script or tool, for example, GNU
.BR tar (1):
.RS
.RS
.PP
.nf
.ft CR
XZ_OPT=\-2v tar caf foo.tar.xz foo
.ft R
.fi
.RE
.RE
.IP ""
Scripts may use
.BR XZ_OPT ,
for example, to set script-specific default compression options.
It is still recommended to allow users to override
.B XZ_OPT
if that is reasonable.
For example, in
.BR sh (1)
scripts one may use something like this:
.RS
.RS
.PP
.nf
.ft CR
XZ_OPT=${XZ_OPT\-"\-7e"}
export XZ_OPT
.ft R
.fi
.RE
.RE
.
.SH "LZMA UTILS COMPATIBILITY"
The command line syntax of
.B xz
is practically a superset of
.BR lzma ,
.BR unlzma ,
and
.B lzcat
as found from LZMA Utils 4.32.x.
In most cases, it is possible to replace
LZMA Utils with XZ Utils without breaking existing scripts.
There are some incompatibilities though,
which may sometimes cause problems.
.
.SS "Compression preset levels"
The numbering of the compression level presets is not identical in
.B xz
and LZMA Utils.
The most important difference is how dictionary sizes
are mapped to different presets.
Dictionary size is roughly equal to the decompressor memory usage.
.RS
.PP
.TS
tab(;);
c c c
c n n.
Level;xz;LZMA Utils
\-0;256 KiB;N/A
\-1;1 MiB;64 KiB
\-2;2 MiB;1 MiB
\-3;4 MiB;512 KiB
\-4;4 MiB;1 MiB
\-5;8 MiB;2 MiB
\-6;8 MiB;4 MiB
\-7;16 MiB;8 MiB
\-8;32 MiB;16 MiB
\-9;64 MiB;32 MiB
.TE
.RE
.PP
The dictionary size differences affect
the compressor memory usage too,
but there are some other differences between
LZMA Utils and XZ Utils, which
make the difference even bigger:
.RS
.PP
.TS
tab(;);
c c c
c n n.
Level;xz;LZMA Utils 4.32.x
\-0;3 MiB;N/A
\-1;9 MiB;2 MiB
\-2;17 MiB;12 MiB
\-3;32 MiB;12 MiB
\-4;48 MiB;16 MiB
\-5;94 MiB;26 MiB
\-6;94 MiB;45 MiB
\-7;186 MiB;83 MiB
\-8;370 MiB;159 MiB
\-9;674 MiB;311 MiB
.TE
.RE
.PP
The default preset level in LZMA Utils is
.B \-7
while in XZ Utils it is
.BR \-6 ,
so both use an 8 MiB dictionary by default.
.
.SS "Streamed vs. non-streamed .lzma files"
The uncompressed size of the file can be stored in the
.B .lzma
header.
LZMA Utils does that when compressing regular files.
The alternative is to mark that uncompressed size is unknown
and use end-of-payload marker to indicate
where the decompressor should stop.
LZMA Utils uses this method when uncompressed size isn't known,
which is the case, for example, in pipes.
.PP
.B xz
supports decompressing
.B .lzma
files with or without end-of-payload marker, but all
.B .lzma
files created by
.B xz
will use end-of-payload marker and have uncompressed size
marked as unknown in the
.B .lzma
header.
This may be a problem in some uncommon situations.
For example, a
.B .lzma
decompressor in an embedded device might work
only with files that have known uncompressed size.
If you hit this problem, you need to use LZMA Utils
or LZMA SDK to create
.B .lzma
files with known uncompressed size.
.
.SS "Unsupported .lzma files"
The
.B .lzma
format allows
.I lc
values up to 8, and
.I lp
values up to 4.
LZMA Utils can decompress files with any
.I lc
and
.IR lp ,
but always creates files with
.B lc=3
and
.BR lp=0 .
Creating files with other
.I lc
and
.I lp
is possible with
.B xz
and with LZMA SDK.
.PP
The implementation of the LZMA1 filter in liblzma
requires that the sum of
.I lc
and
.I lp
must not exceed 4.
Thus,
.B .lzma
files, which exceed this limitation, cannot be decompressed with
.BR xz .
.PP
LZMA Utils creates only
.B .lzma
files which have a dictionary size of
.RI "2^" n
(a power of 2) but accepts files with any dictionary size.
liblzma accepts only
.B .lzma
files which have a dictionary size of
.RI "2^" n
or
.RI "2^" n " + 2^(" n "\-1)."
This is to decrease false positives when detecting
.B .lzma
files.
.PP
These limitations shouldn't be a problem in practice,
since practically all
.B .lzma
files have been compressed with settings that liblzma will accept.
.
.SS "Trailing garbage"
When decompressing,
LZMA Utils silently ignore everything after the first
.B .lzma
stream.
In most situations, this is a bug.
This also means that LZMA Utils
don't support decompressing concatenated
.B .lzma
files.
.PP
If there is data left after the first
.B .lzma
stream,
.B xz
considers the file to be corrupt unless
.B \-\-single\-stream
was used.
This may break obscure scripts which have
assumed that trailing garbage is ignored.
.
.SH NOTES
.
.SS "Compressed output may vary"
The exact compressed output produced from
the same uncompressed input file
may vary between XZ Utils versions even if
compression options are identical.
This is because the encoder can be improved
(faster or better compression)
without affecting the file format.
The output can vary even between different
builds of the same XZ Utils version,
if different build options are used.
.PP
The above means that once
.B \-\-rsyncable
has been implemented,
the resulting files won't necessarily be rsyncable
unless both old and new files have been compressed
with the same xz version.
This problem can be fixed if a part of the encoder
implementation is frozen to keep rsyncable output
stable across xz versions.
.
.SS "Embedded .xz decompressors"
Embedded
.B .xz
decompressor implementations like XZ Embedded don't necessarily
support files created with integrity
.I check
types other than
.B none
and
.BR crc32 .
Since the default is
.BR \-\-check=crc64 ,
you must use
.B \-\-check=none
or
.B \-\-check=crc32
when creating files for embedded systems.
.PP
Outside embedded systems, all
.B .xz
format decompressors support all the
.I check
types, or at least are able to decompress
the file without verifying the
integrity check if the particular
.I check
is not supported.
.PP
XZ Embedded supports BCJ filters,
but only with the default start offset.
.
.SH EXAMPLES
.
.SS Basics
Compress the file
.I foo
into
.I foo.xz
using the default compression level
.RB ( \-6 ),
and remove
.I foo
if compression is successful:
.RS
.PP
.nf
.ft CR
xz foo
.ft R
.fi
.RE
.PP
Decompress
.I bar.xz
into
.I bar
and don't remove
.I bar.xz
even if decompression is successful:
.RS
.PP
.nf
.ft CR
xz \-dk bar.xz
.ft R
.fi
.RE
.PP
Create
.I baz.tar.xz
with the preset
.B \-4e
.RB ( "\-4 \-\-extreme" ),
which is slower than the default
.BR \-6 ,
but needs less memory for compression and decompression (48\ MiB
and 5\ MiB, respectively):
.RS
.PP
.nf
.ft CR
tar cf \- baz | xz \-4e > baz.tar.xz
.ft R
.fi
.RE
.PP
A mix of compressed and uncompressed files can be decompressed
to standard output with a single command:
.RS
.PP
.nf
.ft CR
xz \-dcf a.txt b.txt.xz c.txt d.txt.lzma > abcd.txt
.ft R
.fi
.RE
.
.SS "Parallel compression of many files"
On GNU and *BSD,
.BR find (1)
and
.BR xargs (1)
can be used to parallelize compression of many files:
.RS
.PP
.nf
.ft CR
find . \-type f \e! \-name '*.xz' \-print0 \e
    | xargs \-0r \-P4 \-n16 xz \-T1
.ft R
.fi
.RE
.PP
The
.B \-P
option to
.BR xargs (1)
sets the number of parallel
.B xz
processes.
The best value for the
.B \-n
option depends on how many files there are to be compressed.
If there are only a couple of files,
the value should probably be 1;
with tens of thousands of files,
100 or even more may be appropriate to reduce the number of
.B xz
processes that
.BR xargs (1)
will eventually create.
.PP
The option
.B \-T1
for
.B xz
is there to force it to single-threaded mode, because
.BR xargs (1)
is used to control the amount of parallelization.
.
.SS "Robot mode"
Calculate how many bytes have been saved in total
after compressing multiple files:
.RS
.PP
.nf
.ft CR
xz \-\-robot \-\-list *.xz | awk '/^totals/{print $5\-$4}'
.ft R
.fi
.RE
.PP
A script may want to know that it is using new enough
.BR xz .
The following
.BR sh (1)
script checks that the version number of the
.B xz
tool is at least 5.0.0.
This method is compatible with old beta versions,
which didn't support the
.B \-\-robot
option:
.RS
.PP
.nf
.ft CR
if ! eval "$(xz \-\-robot \-\-version 2> /dev/null)" ||
        [ "$XZ_VERSION" \-lt 50000002 ]; then
    echo "Your xz is too old."
fi
unset XZ_VERSION LIBLZMA_VERSION
.ft R
.fi
.RE
.PP
Set a memory usage limit for decompression using
.BR XZ_OPT ,
but if a limit has already been set, don't increase it:
.RS
.PP
.nf
.ft CR
NEWLIM=$((123 << 20))\ \ # 123 MiB
OLDLIM=$(xz \-\-robot \-\-info\-memory | cut \-f3)
if [ $OLDLIM \-eq 0 \-o $OLDLIM \-gt $NEWLIM ]; then
    XZ_OPT="$XZ_OPT \-\-memlimit\-decompress=$NEWLIM"
    export XZ_OPT
fi
.ft R
.fi
.RE
.
.SS "Custom compressor filter chains"
The simplest use for custom filter chains is
customizing a LZMA2 preset.
This can be useful,
because the presets cover only a subset of the
potentially useful combinations of compression settings.
.PP
The CompCPU columns of the tables
from the descriptions of the options
.BR "\-0" " ... " "\-9"
and
.B \-\-extreme
are useful when customizing LZMA2 presets.
Here are the relevant parts collected from those two tables:
.RS
.PP
.TS
tab(;);
c c
n n.
Preset;CompCPU
\-0;0
\-1;1
\-2;2
\-3;3
\-4;4
\-5;5
\-6;6
\-5e;7
\-6e;8
.TE
.RE
.PP
If you know that a file requires
somewhat big dictionary (for example, 32\ MiB) to compress well,
but you want to compress it quicker than
.B "xz \-8"
would do, a preset with a low CompCPU value (for example, 1)
can be modified to use a bigger dictionary:
.RS
.PP
.nf
.ft CR
xz \-\-lzma2=preset=1,dict=32MiB foo.tar
.ft R
.fi
.RE
.PP
With certain files, the above command may be faster than
.B "xz \-6"
while compressing significantly better.
However, it must be emphasized that only some files benefit from
a big dictionary while keeping the CompCPU value low.
The most obvious situation,
where a big dictionary can help a lot,
is an archive containing very similar files
of at least a few megabytes each.
The dictionary size has to be significantly bigger
than any individual file to allow LZMA2 to take
full advantage of the similarities between consecutive files.
.PP
If very high compressor and decompressor memory usage is fine,
and the file being compressed is
at least several hundred megabytes, it may be useful
to use an even bigger dictionary than the 64 MiB that
.B "xz \-9"
would use:
.RS
.PP
.nf
.ft CR
xz \-vv \-\-lzma2=dict=192MiB big_foo.tar
.ft R
.fi
.RE
.PP
Using
.B \-vv
.RB ( "\-\-verbose \-\-verbose" )
like in the above example can be useful
to see the memory requirements
of the compressor and decompressor.
Remember that using a dictionary bigger than
the size of the uncompressed file is waste of memory,
so the above command isn't useful for small files.
.PP
Sometimes the compression time doesn't matter,
but the decompressor memory usage has to be kept low, for example,
to make it possible to decompress the file on an embedded system.
The following command uses
.B \-6e
.RB ( "\-6 \-\-extreme" )
as a base and sets the dictionary to only 64\ KiB.
The resulting file can be decompressed with XZ Embedded
(that's why there is
.BR \-\-check=crc32 )
using about 100\ KiB of memory.
.RS
.PP
.nf
.ft CR
xz \-\-check=crc32 \-\-lzma2=preset=6e,dict=64KiB foo
.ft R
.fi
.RE
.PP
If you want to squeeze out as many bytes as possible,
adjusting the number of literal context bits
.RI ( lc )
and number of position bits
.RI ( pb )
can sometimes help.
Adjusting the number of literal position bits
.RI ( lp )
might help too, but usually
.I lc
and
.I pb
are more important.
For example, a source code archive contains mostly US-ASCII text,
so something like the following might give
slightly (like 0.1\ %) smaller file than
.B "xz \-6e"
(try also without
.BR lc=4 ):
.RS
.PP
.nf
.ft CR
xz \-\-lzma2=preset=6e,pb=0,lc=4 source_code.tar
.ft R
.fi
.RE
.PP
Using another filter together with LZMA2 can improve
compression with certain file types.
For example, to compress a x86-32 or x86-64 shared library
using the x86 BCJ filter:
.RS
.PP
.nf
.ft CR
xz \-\-x86 \-\-lzma2 libfoo.so
.ft R
.fi
.RE
.PP
Note that the order of the filter options is significant.
If
.B \-\-x86
is specified after
.BR \-\-lzma2 ,
.B xz
will give an error,
because there cannot be any filter after LZMA2,
and also because the x86 BCJ filter cannot be used
as the last filter in the chain.
.PP
The Delta filter together with LZMA2
can give good results with bitmap images.
It should usually beat PNG,
which has a few more advanced filters than simple
delta but uses Deflate for the actual compression.
.PP
The image has to be saved in uncompressed format,
for example, as uncompressed TIFF.
The distance parameter of the Delta filter is set
to match the number of bytes per pixel in the image.
For example, 24-bit RGB bitmap needs
.BR dist=3 ,
and it is also good to pass
.B pb=0
to LZMA2 to accommodate the three-byte alignment:
.RS
.PP
.nf
.ft CR
xz \-\-delta=dist=3 \-\-lzma2=pb=0 foo.tiff
.ft R
.fi
.RE
.PP
If multiple images have been put into a single archive (for example,
.BR .tar ),
the Delta filter will work on that too as long as all images
have the same number of bytes per pixel.
.
.SH "SEE ALSO"
.BR xzdec (1),
.BR xzdiff (1),
.BR xzgrep (1),
.BR xzless (1),
.BR xzmore (1),
.BR gzip (1),
.BR bzip2 (1),
.BR 7z (1)
.PP
XZ Utils: <https://tukaani.org/xz/>
.br
XZ Embedded: <https://tukaani.org/xz/embedded.html>
.br
LZMA SDK: <https://7-zip.org/sdk.html>