bzip2(1) General Commands Manual bzip2(1)
NAME
bzip2, bunzip2 - a block-sorting file compressor, v1.0.6
bzcat - decompresses files to stdout
bzip2recover - recovers data from damaged bzip2 files
SYNOPSIS
bzip2 [ -cdfkqstvzVL123456789 ] [ filenames ... ]
bunzip2 [ -fkvsVL ] [ filenames ... ]
bzcat [ -s ] [ filenames ... ]
bzip2recover filename
DESCRIPTION
bzip2 compresses files using the Burrows-Wheeler block sorting text compression algorithm, and Huffman coding.
Compression is generally considerably better than that achieved by more conventional LZ77/LZ78-based compres‐
sors, and approaches the performance of the PPM family of statistical compressors.
The command-line options are deliberately very similar to those of GNU gzip, but they are not identical.
bzip2 expects a list of file names to accompany the command-line flags. Each file is replaced by a compressed
version of itself, with the name "original_name.bz2". Each compressed file has the same modification date,
permissions, and, when possible, ownership as the corresponding original, so that these properties can be cor‐
rectly restored at decompression time. File name handling is naive in the sense that there is no mechanism
for preserving original file names, permissions, ownerships or dates in filesystems which lack these concepts,
or have serious file name length restrictions, such as MS-DOS.
bzip2 and bunzip2 will by default not overwrite existing files. If you want this to happen, specify the -f
flag.
If no file names are specified, bzip2 compresses from standard input to standard output. In this case, bzip2
will decline to write compressed output to a terminal, as this would be entirely incomprehensible and there‐
fore pointless.
bunzip2 (or bzip2 -d) decompresses all specified files. Files which were not created by bzip2 will be
detected and ignored, and a warning issued. bzip2 attempts to guess the filename for the decompressed file
from that of the compressed file as follows:
filename.bz2 becomes filename
filename.bz becomes filename
filename.tbz2 becomes filename.tar
filename.tbz becomes filename.tar
anyothername becomes anyothername.out
If the file does not end in one of the recognised endings, .bz2, .bz, .tbz2 or .tbz, bzip2 complains that it
cannot guess the name of the original file, and uses the original name with .out appended.
As with compression, supplying no filenames causes decompression from standard input to standard output.
bunzip2 will correctly decompress a file which is the concatenation of two or more compressed files. The
result is the concatenation of the corresponding uncompressed files. Integrity testing (-t) of concatenated
compressed files is also supported.
You can also compress or decompress files to the standard output by giving the -c flag. Multiple files may be
compressed and decompressed like this. The resulting outputs are fed sequentially to stdout. Compression of
multiple files in this manner generates a stream containing multiple compressed file representations. Such a
As a self-check for your protection, bzip2 uses 32-bit CRCs to make sure that the decompressed version of a
file is identical to the original. This guards against corruption of the compressed data, and against unde‐
tected bugs in bzip2 (hopefully very unlikely). The chances of data corruption going undetected is micro‐
scopic, about one chance in four billion for each file processed. Be aware, though, that the check occurs
upon decompression, so it can only tell you that something is wrong. It can't help you recover the original
uncompressed data. You can use bzip2recover to try to recover data from damaged files.
Return values: 0 for a normal exit, 1 for environmental problems (file not found, invalid flags, I/O errors,
&c), 2 to indicate a corrupt compressed file, 3 for an internal consistency error (eg, bug) which caused bzip2
to panic.
OPTIONS
-c --stdout
Compress or decompress to standard output.
-d --decompress
Force decompression. bzip2, bunzip2 and bzcat are really the same program, and the decision about what
actions to take is done on the basis of which name is used. This flag overrides that mechanism, and
forces bzip2 to decompress.
-z --compress
The complement to -d: forces compression, regardless of the invocation name.
-t --test
Check integrity of the specified file(s), but don't decompress them. This really performs a trial
decompression and throws away the result.
-f --force
Force overwrite of output files. Normally, bzip2 will not overwrite existing output files. Also
forces bzip2 to break hard links to files, which it otherwise wouldn't do.
bzip2 normally declines to decompress files which don't have the correct magic header bytes. If forced
(-f), however, it will pass such files through unmodified. This is how GNU gzip behaves.
-k --keep
Keep (don't delete) input files during compression or decompression.
-s --small
Reduce memory usage, for compression, decompression and testing. Files are decompressed and tested
using a modified algorithm which only requires 2.5 bytes per block byte. This means any file can be
decompressed in 2300k of memory, albeit at about half the normal speed.
During compression, -s selects a block size of 200k, which limits memory use to around the same figure,
at the expense of your compression ratio. In short, if your machine is low on memory (8 megabytes or
less), use -s for everything. See MEMORY MANAGEMENT below.
-q --quiet
Suppress non-essential warning messages. Messages pertaining to I/O errors and other critical events
will not be suppressed.
-v --verbose
Verbose mode -- show the compression ratio for each file processed. Further -v's increase the ver‐
dle files with names beginning with a dash, for example: bzip2 -- -myfilename.
--repetitive-fast --repetitive-best
These flags are redundant in versions 0.9.5 and above. They provided some coarse control over the be‐
haviour of the sorting algorithm in earlier versions, which was sometimes useful. 0.9.5 and above have
an improved algorithm which renders these flags irrelevant.
MEMORY MANAGEMENT
bzip2 compresses large files in blocks. The block size affects both the compression ratio achieved, and the
amount of memory needed for compression and decompression. The flags -1 through -9 specify the block size to
be 100,000 bytes through 900,000 bytes (the default) respectively. At decompression time, the block size used
for compression is read from the header of the compressed file, and bunzip2 then allocates itself just enough
memory to decompress the file. Since block sizes are stored in compressed files, it follows that the flags -1
to -9 are irrelevant to and so ignored during decompression.
Compression and decompression requirements, in bytes, can be estimated as:
Compression: 400k + ( 8 x block size )
Decompression: 100k + ( 4 x block size ), or
100k + ( 2.5 x block size )
Larger block sizes give rapidly diminishing marginal returns. Most of the compression comes from the first
two or three hundred k of block size, a fact worth bearing in mind when using bzip2 on small machines. It is
also important to appreciate that the decompression memory requirement is set at compression time by the
choice of block size.
For files compressed with the default 900k block size, bunzip2 will require about 3700 kbytes to decompress.
To support decompression of any file on a 4 megabyte machine, bunzip2 has an option to decompress using
approximately half this amount of memory, about 2300 kbytes. Decompression speed is also halved, so you
should use this option only where necessary. The relevant flag is -s.
In general, try and use the largest block size memory constraints allow, since that maximises the compression
achieved. Compression and decompression speed are virtually unaffected by block size.
Another significant point applies to files which fit in a single block -- that means most files you'd
encounter using a large block size. The amount of real memory touched is proportional to the size of the
file, since the file is smaller than a block. For example, compressing a file 20,000 bytes long with the flag
-9 will cause the compressor to allocate around 7600k of memory, but only touch 400k + 20000 * 8 = 560 kbytes
of it. Similarly, the decompressor will allocate 3700k but only touch 100k + 20000 * 4 = 180 kbytes.
Here is a table which summarises the maximum memory usage for different block sizes. Also recorded is the
total compressed size for 14 files of the Calgary Text Compression Corpus totalling 3,141,622 bytes. This
column gives some feel for how compression varies with block size. These figures tend to understate the
advantage of larger block sizes for larger files, since the Corpus is dominated by smaller files.
Compress Decompress Decompress Corpus
Flag usage usage -s usage Size
-1 1200k 500k 350k 914704
-2 2000k 900k 600k 877703
-3 2800k 1300k 850k 860338
-4 3600k 1700k 1100k 846899
The compressed representation of each block is delimited by a 48-bit pattern, which makes it possible to find
the block boundaries with reasonable certainty. Each block also carries its own 32-bit CRC, so damaged blocks
can be distinguished from undamaged ones.
bzip2recover is a simple program whose purpose is to search for blocks in .bz2 files, and write each block out
into its own .bz2 file. You can then use bzip2 -t to test the integrity of the resulting files, and decom‐
press those which are undamaged.
bzip2recover takes a single argument, the name of the damaged file, and writes a number of files
"rec00001file.bz2", "rec00002file.bz2", etc, containing the extracted blocks. The output filenames are
designed so that the use of wildcards in subsequent processing -- for example, "bzip2 -dc rec*file.bz2 >
recovered_data" -- processes the files in the correct order.
bzip2recover should be of most use dealing with large .bz2 files, as these will contain many blocks. It is
clearly futile to use it on damaged single-block files, since a damaged block cannot be recovered. If
you wish to minimise any potential data loss through media or transmission errors, you might consider com‐
pressing with a smaller block size.
PERFORMANCE NOTES
The sorting phase of compression gathers together similar strings in the file. Because of this, files con‐
taining very long runs of repeated symbols, like "aabaabaabaab ..." (repeated several hundred times) may com‐
press more slowly than normal. Versions 0.9.5 and above fare much better than previous versions in this
respect. The ratio between worst-case and average-case compression time is in the region of 10:1. For previ‐
ous versions, this figure was more like 100:1. You can use the -vvvv option to monitor progress in great
detail, if you want.
Decompression speed is unaffected by these phenomena.
bzip2 usually allocates several megabytes of memory to operate in, and then charges all over it in a fairly
random fashion. This means that performance, both for compressing and decompressing, is largely determined by
the speed at which your machine can service cache misses. Because of this, small changes to the code to
reduce the miss rate have been observed to give disproportionately large performance improvements. I imagine
bzip2 will perform best on machines with very large caches.
CAVEATS
I/O error messages are not as helpful as they could be. bzip2 tries hard to detect I/O errors and exit
cleanly, but the details of what the problem is sometimes seem rather misleading.
This manual page pertains to version 1.0.6 of bzip2. Compressed data created by this version is entirely for‐
wards and backwards compatible with the previous public releases, versions 0.1pl2, 0.9.0, 0.9.5, 1.0.0, 1.0.1,
1.0.2 and above, but with the following exception: 0.9.0 and above can correctly decompress multiple concate‐
nated compressed files. 0.1pl2 cannot do this; it will stop after decompressing just the first file in the
stream.
bzip2recover versions prior to 1.0.2 used 32-bit integers to represent bit positions in compressed files, so
they could not handle compressed files more than 512 megabytes long. Versions 1.0.2 and above use 64-bit ints
on some platforms which support them (GNU supported targets, and Windows). To establish whether or not
bzip2recover was built with such a limitation, run it without arguments. In any event you can build yourself
an unlimited version if you can recompile it with MaybeUInt64 set to be an unsigned 64-bit integer.
to improve the worst-case compression performance. Donna Robinson XMLised the documentation. The bz* scripts
are derived from those of GNU gzip. Many people sent patches, helped with portability problems, lent
machines, gave advice and were generally helpful.
bzip2(1)
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