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xdelta (1)
>> xdelta (1) ( Linux man: Команды и прикладные программы пользовательского уровня )
NAME
xdelta - Invoke Xdelta
SYNOPSIS
xdeltasubcommand
[
option...
] [
operand...
]
DESCRIPTION
Xdelta provides the ability to generate deltas between a pair of files
and later apply those deltas. It operates similar to the
diff
and
patch
commands, but works on binary files and does not produce a human
readable output.
Xdelta has three subcommands, delta, patch, and info. Delta accepts
two file versions and produces a delta, while patch accepts the
original file version and delta and produces the second version. The
info command prints useful information about a delta. Each subcommand
will be detailed seperately.
Gzip processing
Attempting to compute a delta between compressed input files usually
results in poor compression. This is because small differences
between the original contents causes changes in the compression of
whole blocks of data. To simplify things, Xdelta implements a special
case for
gzip(1)
compressed files. If any version input to the delta command is
recognized as having gzip compression, it will be automatically
decompressed into a temporary location prior to comparison. This
temporary location is either the value of the
TMPDIR
environment variable, if set, otherwise "/tmp".
The Xdelta patch header contains a flag indicating that the
reconstructed version should be recompressed after applying the
patch. In general, this allows Xdelta to operate transparently on
gzip compressed inputs.
There is one potential problem when automatically processing gzip
compressed files, which is that the recompressed content does not
always match byte-for-byte with the original compressed content. The
uncompressed content still matches, but if there is an external
integrity check such as cryptographic signature verification, it may
fail. To prevent this from happening, the --pristine option disables
automatic gzip processing.
MD5 integrity check
By default, Xdelta always verifies the MD5 checksum of the files it
reconstructs. This prevents you from supplying an incorrect input
during patch, which would result in corrupt output. Because of this
feature, you can feel confident that patch has produced valid results.
The --noverify option disables MD5 verification, but this is only
recommended for performance testing.
Compressed patch format
Xdelta uses a fairly simple encoding for its delta, then applies zlib
compression to the result. You should not have to post-compress an
Xdelta delta.
Delta
The delta subcommand has the following synopsis:
xdeltadelta
[
option...
]
fromfiletofilepatchout
Computes a delta from
fromfile
to
tofile
and writes it to
patchout
Applies
patchin
to
fromfile
and produces a reconstructed version of
tofile.
If fromfile was omitted, Xdelta attempts to use the original fromfile
name, which is stored in the delta. The from file must be identical
to the one used to create the delta. If its length or MD5 checksum
differs, patch will abort with an error message.
If tofile was omitted, Xdelta attempts to use the original tofile
name, which is also stored in the delta. If the original tofile name
already exists, a unique filename extension will be added to avoid
destroying any existing data.
Info
The info subcommand has the following synopsis:
xdeltainfopatchinfo
Prints information about
patchinfo
and the version it reconstructs, including file names, lengths, and
MD5 checksums.
Options
-0..9
Set the zlib compression level. Zero indicates no compression. Nine
indicates maximum compression.
-h, --help
Print a short help message and exit.
-q, --quiet
Quiet. Surpresses several warning messages.
-v, --version
Print the Xdelta version number and exit.
-V, --verbose
Verbose. Prints a bit of extra information.
-n, --noverify
No verify. Turns off MD5 checksum verification of the input and
output files.
-m=SIZE, --maxmem=SIZE
Set an upper bound on the size of an in-memory page cache. For
example, --maxmem=32M will use a 32 megabyte page cache.
-s=BLOCK_SIZE
Set the block size, unless it was hard coded (20% speed improvement).
Should be a power of 2.
-p, --pristine
Disable the automatic decompression of gzipped inputs, to prevent
unexpected differences in the re-compressed content.
