129 lines
4.8 KiB
Plaintext
129 lines
4.8 KiB
Plaintext
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XZ Utils FAQ
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============
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Q: What do the letters XZ mean?
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A: Nothing. They are just two letters, which come from the file format
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suffix .xz. The .xz suffix was selected, because it seemed to be
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pretty much unused. It is no deeper meaning.
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Q: What are LZMA and LZMA2?
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A: LZMA stands for Lempel-Ziv-Markov chain-Algorithm. It is the name
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of the compression algorithm designed by Igor Pavlov for 7-Zip.
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LZMA is based on LZ77 and range encoding.
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LZMA2 is an updated version of the original LZMA to fix a couple of
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practical issues. In context of XZ Utils, LZMA is called LZMA1 to
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emphasize that LZMA is not the same thing as LZMA2. LZMA2 is the
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primary compression algorithm in the .xz file format.
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Q: There are many LZMA related projects. How does XZ Utils relate to them?
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A: 7-Zip and LZMA SDK are the original projects. LZMA SDK is roughly
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a subset of the 7-Zip source tree.
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p7zip is 7-Zip's command line tools ported to POSIX-like systems.
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LZMA Utils provide a gzip-like lzma tool for POSIX-like systems.
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LZMA Utils are based on LZMA SDK. XZ Utils are the successor to
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LZMA Utils.
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There are several other projects using LZMA. Most are more or less
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based on LZMA SDK.
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Q: Do XZ Utils support the .7z format?
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A: No. Use 7-Zip (Windows) or p7zip (POSIX-like systems) to handle .7z
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files.
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Q: I have many .tar.7z files. Can I convert them to .tar.xz without
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spending hours recompressing the data?
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A: In the "extra" directory, there is a script named 7z2lzma.bash which
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is able to convert some .7z files to the .lzma format (not .xz). It
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needs the 7za (or 7z) command from p7zip. The script may silently
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produce corrupt output if certain assumptions are not met, so
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decompress the resulting .lzma file and compare it against the
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original before deleting the original file!
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Q: I have many .lzma files. Can I quickly convert them to the .xz format?
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A: For now, no. Since XZ Utils supports the .lzma format, it's usually
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not too bad to keep the old files in the old format. If you want to
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do the conversion anyway, you need to decompress the .lzma files and
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then recompress to the .xz format.
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Technically, there is a way to make the conversion relatively fast
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(roughly twice the time that normal decompression takes). Writing
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such a tool would take quite a bit time though, and would probably
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be useful to only a few people. If you really want such a conversion
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tool, contact Lasse Collin and offer some money.
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Q: I have installed xz, but my tar doesn't recognize .tar.xz files.
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How can I extract .tar.xz files?
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A: xz -dc foo.tar.xz | tar xf -
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Q: Can I recover parts of a broken .xz file (e.g. corrupted CD-R)?
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A: It may be possible if the file consists of multiple blocks, which
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typically is not the case if the file was created in single-threaded
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mode. There is no recovery program yet.
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Q: Is (some part of) XZ Utils patented?
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A: Lasse Collin is not aware of any patents that could affect XZ Utils.
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However, due to nature of software patents, it's not possible to
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guarantee that XZ Utils isn't affected by any third party patent(s).
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Q: Where can I find documentation about the file format and algorithms?
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A: The .xz format is documented in xz-file-format.txt. It is a container
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format only, and doesn't include descriptions of any non-trivial
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filters.
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Documenting LZMA and LZMA2 is planned, but for now, there is no other
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documentation that the source code. Before you begin, you should know
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the basics of LZ77 and range coding algorithms. LZMA is based on LZ77,
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but LZMA is *a lot* more complex. Range coding is used to compress
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the final bitstream like Huffman coding is used in Deflate.
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Q: I cannot find BCJ and BCJ2 filters. Don't they exist in liblzma?
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A: BCJ filter is called "x86" in liblzma. BCJ2 is not included,
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because it requires using more than one encoded output stream.
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Q: How do I build a program that needs liblzmadec (lzmadec.h)?
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A: liblzmadec is part of LZMA Utils. XZ Utils has liblzma, but no
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liblzmadec. The code using liblzmadec should be ported to use
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liblzma instead. If you cannot or don't want to do that, download
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LZMA Utils from <http://tukaani.org/lzma/>.
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Q: The default build of liblzma is too big. How can I make it smaller?
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A: Give --enable-small to the configure script. Use also appropriate
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--enable or --disable options to include only those filter encoders
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and decoders and integrity checks that you actually need. Use
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CFLAGS=-Os (with GCC) or equivalent to tell your compiler to optimize
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for size. See INSTALL for information about configure options.
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If the result is still too big, take a look at XZ Embedded. It is
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a separate project, which provides a limited but significantly
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smaller XZ decoder implementation than XZ Utils.
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