309 lines
13 KiB
Plaintext
309 lines
13 KiB
Plaintext
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XZ Utils
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========
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0. Overview
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1. Documentation
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1.1. Overall documentation
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1.2. Documentation for command-line tools
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1.3. Documentation for liblzma
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2. Version numbering
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3. Reporting bugs
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4. Translating the xz tool
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5. Other implementations of the .xz format
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6. Contact information
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0. Overview
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-----------
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XZ Utils provide a general-purpose data-compression library plus
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command-line tools. The native file format is the .xz format, but
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also the legacy .lzma format is supported. The .xz format supports
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multiple compression algorithms, which are called "filters" in the
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context of XZ Utils. The primary filter is currently LZMA2. With
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typical files, XZ Utils create about 30 % smaller files than gzip.
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To ease adapting support for the .xz format into existing applications
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and scripts, the API of liblzma is somewhat similar to the API of the
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popular zlib library. For the same reason, the command-line tool xz
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has a command-line syntax similar to that of gzip.
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When aiming for the highest compression ratio, the LZMA2 encoder uses
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a lot of CPU time and may use, depending on the settings, even
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hundreds of megabytes of RAM. However, in fast modes, the LZMA2 encoder
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competes with bzip2 in compression speed, RAM usage, and compression
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ratio.
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LZMA2 is reasonably fast to decompress. It is a little slower than
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gzip, but a lot faster than bzip2. Being fast to decompress means
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that the .xz format is especially nice when the same file will be
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decompressed very many times (usually on different computers), which
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is the case e.g. when distributing software packages. In such
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situations, it's not too bad if the compression takes some time,
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since that needs to be done only once to benefit many people.
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With some file types, combining (or "chaining") LZMA2 with an
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additional filter can improve the compression ratio. A filter chain may
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contain up to four filters, although usually only one or two are used.
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For example, putting a BCJ (Branch/Call/Jump) filter before LZMA2
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in the filter chain can improve compression ratio of executable files.
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Since the .xz format allows adding new filter IDs, it is possible that
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some day there will be a filter that is, for example, much faster to
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compress than LZMA2 (but probably with worse compression ratio).
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Similarly, it is possible that some day there is a filter that will
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compress better than LZMA2.
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XZ Utils doesn't support multithreaded compression or decompression
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yet. It has been planned though and taken into account when designing
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the .xz file format.
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1. Documentation
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----------------
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1.1. Overall documentation
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README This file
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INSTALL.generic Generic install instructions for those not familiar
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with packages using GNU Autotools
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INSTALL Installation instructions specific to XZ Utils
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PACKAGERS Information to packagers of XZ Utils
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COPYING XZ Utils copyright and license information
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COPYING.GPLv2 GNU General Public License version 2
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COPYING.GPLv3 GNU General Public License version 3
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COPYING.LGPLv2.1 GNU Lesser General Public License version 2.1
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AUTHORS The main authors of XZ Utils
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THANKS Incomplete list of people who have helped making
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this software
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NEWS User-visible changes between XZ Utils releases
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ChangeLog Detailed list of changes (commit log)
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TODO Known bugs and some sort of to-do list
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Note that only some of the above files are included in binary
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packages.
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1.2. Documentation for command-line tools
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The command-line tools are documented as man pages. In source code
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releases (and possibly also in some binary packages), the man pages
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are also provided in plain text (ASCII only) and PDF formats in the
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directory "doc/man" to make the man pages more accessible to those
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whose operating system doesn't provide an easy way to view man pages.
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1.3. Documentation for liblzma
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The liblzma API headers include short docs about each function
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and data type as Doxygen tags. These docs should be quite OK as
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a quick reference.
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I have planned to write a bunch of very well documented example
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programs, which (due to comments) should work as a tutorial to
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various features of liblzma. No such example programs have been
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written yet.
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For now, if you have never used liblzma, libbzip2, or zlib, I
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recommend learning the *basics* of the zlib API. Once you know that,
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it should be easier to learn liblzma.
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http://zlib.net/manual.html
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http://zlib.net/zlib_how.html
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2. Version numbering
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--------------------
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The version number format of XZ Utils is X.Y.ZS:
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- X is the major version. When this is incremented, the library
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API and ABI break.
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- Y is the minor version. It is incremented when new features
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are added without breaking the existing API or ABI. An even Y
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indicates a stable release and an odd Y indicates unstable
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(alpha or beta version).
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- Z is the revision. This has a different meaning for stable and
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unstable releases:
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* Stable: Z is incremented when bugs get fixed without adding
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any new features. This is intended to be convenient for
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downstream distributors that want bug fixes but don't want
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any new features to minimize the risk of introducing new bugs.
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* Unstable: Z is just a counter. API or ABI of features added
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in earlier unstable releases having the same X.Y may break.
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- S indicates stability of the release. It is missing from the
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stable releases, where Y is an even number. When Y is odd, S
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is either "alpha" or "beta" to make it very clear that such
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versions are not stable releases. The same X.Y.Z combination is
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not used for more than one stability level, i.e. after X.Y.Zalpha,
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the next version can be X.Y.(Z+1)beta but not X.Y.Zbeta.
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3. Reporting bugs
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-----------------
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Naturally it is easiest for me if you already know what causes the
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unexpected behavior. Even better if you have a patch to propose.
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However, quite often the reason for unexpected behavior is unknown,
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so here are a few things to do before sending a bug report:
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1. Try to create a small example how to reproduce the issue.
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2. Compile XZ Utils with debugging code using configure switches
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--enable-debug and, if possible, --disable-shared. If you are
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using GCC, use CFLAGS='-O0 -ggdb3'. Don't strip the resulting
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binaries.
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3. Turn on core dumps. The exact command depends on your shell;
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for example in GNU bash it is done with "ulimit -c unlimited",
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and in tcsh with "limit coredumpsize unlimited".
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4. Try to reproduce the suspected bug. If you get "assertion failed"
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message, be sure to include the complete message in your bug
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report. If the application leaves a coredump, get a backtrace
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using gdb:
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$ gdb /path/to/app-binary # Load the app to the debugger.
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(gdb) core core # Open the coredump.
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(gdb) bt # Print the backtrace. Copy & paste to bug report.
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(gdb) quit # Quit gdb.
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Report your bug via email or IRC (see Contact information below).
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Don't send core dump files or any executables. If you have a small
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example file(s) (total size less than 256 KiB), please include
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it/them as an attachment. If you have bigger test files, put them
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online somewhere and include a URL to the file(s) in the bug report.
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Always include the exact version number of XZ Utils in the bug report.
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If you are using a snapshot from the git repository, use "git describe"
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to get the exact snapshot version. If you are using XZ Utils shipped
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in an operating system distribution, mention the distribution name,
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distribution version, and exact xz package version; if you cannot
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repeat the bug with the code compiled from unpatched source code,
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you probably need to report a bug to your distribution's bug tracking
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system.
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4. Translating the xz tool
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--------------------------
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The messages from the xz tool have been translated into a few
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languages. Before starting to translate into a new language, ask
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the author whether someone else hasn't already started working on it.
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Test your translation. Testing includes comparing the translated
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output to the original English version by running the same commands
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in both your target locale and with LC_ALL=C. Ask someone to
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proof-read and test the translation.
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Testing can be done e.g. by installing xz into a temporary directory:
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./configure --disable-shared --prefix=/tmp/xz-test
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# <Edit the .po file in the po directory.>
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make -C po update-po
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make install
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bash debug/translation.bash | less
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bash debug/translation.bash | less -S # For --list outputs
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Repeat the above as needed (no need to re-run configure though).
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Note especially the following:
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- The output of --help and --long-help must look nice on
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an 80-column terminal. It's OK to add extra lines if needed.
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- In contrast, don't add extra lines to error messages and such.
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They are often preceded with e.g. a filename on the same line,
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so you have no way to predict where to put a \n. Let the terminal
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do the wrapping even if it looks ugly. Adding new lines will be
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even uglier in the generic case even if it looks nice in a few
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limited examples.
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- Be careful with column alignment in tables and table-like output
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(--list, --list --verbose --verbose, --info-memory, --help, and
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--long-help):
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* All descriptions of options in --help should start in the
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same column (but it doesn't need to be the same column as
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in the English messages; just be consistent if you change it).
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Check that both --help and --long-help look OK, since they
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share several strings.
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* --list --verbose and --info-memory print lines that have
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the format "Description: %s". If you need a longer
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description, you can put extra space between the colon
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and %s. Then you may need to add extra space to other
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strings too so that the result as a whole looks good (all
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values start at the same column).
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* The columns of the actual tables in --list --verbose --verbose
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should be aligned properly. Abbreviate if necessary. It might
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be good to keep at least 2 or 3 spaces between column headings
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and avoid spaces in the headings so that the columns stand out
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better, but this is a matter of opinion. Do what you think
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looks best.
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- Be careful to put a period at the end of a sentence when the
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original version has it, and don't put it when the original
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doesn't have it. Similarly, be careful with \n characters
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at the beginning and end of the strings.
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- Read the TRANSLATORS comments that have been extracted from the
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source code and included in xz.pot. If they suggest testing the
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translation with some type of command, do it. If testing needs
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input files, use e.g. tests/files/good-*.xz.
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- When updating the translation, read the fuzzy (modified) strings
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carefully, and don't mark them as updated before you actually
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have updated them. Reading through the unchanged messages can be
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good too; sometimes you may find a better wording for them.
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- If you find language problems in the original English strings,
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feel free to suggest improvements. Ask if something is unclear.
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- The translated messages should be understandable (sometimes this
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may be a problem with the original English messages too). Don't
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make a direct word-by-word translation from English especially if
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the result doesn't sound good in your language.
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In short, take your time and pay attention to the details. Making
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a good translation is not a quick and trivial thing to do. The
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translated xz should look as polished as the English version.
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5. Other implementations of the .xz format
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------------------------------------------
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7-Zip and the p7zip port of 7-Zip support the .xz format starting
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from the version 9.00alpha.
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http://7-zip.org/
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http://p7zip.sourceforge.net/
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XZ Embedded is a limited implementation written for use in the Linux
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kernel, but it is also suitable for other embedded use.
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https://tukaani.org/xz/embedded.html
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6. Contact information
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----------------------
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If you have questions, bug reports, patches etc. related to XZ Utils,
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contact Lasse Collin <lasse.collin@tukaani.org> (in Finnish or English).
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I'm sometimes slow at replying. If you haven't got a reply within two
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weeks, assume that your email has got lost and resend it or use IRC.
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You can find me also from #tukaani on Freenode; my nick is Larhzu.
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The channel tends to be pretty quiet, so just ask your question and
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someone may wake up.
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