(bzip) or decompress (bunzip) files
bzip [ -cdfkqstvzVL123456789 ] [ filenames ... ]
bunzip [ -fkvsVL ] [ filenames ... ]
bzcat [ -s ] [ filenames ... ]
actions: bzip2 =
compress, bunzip2 =
decompress, bzcat =
decompress to stdout.
If no file names are given, bzip2 compresses or decompresses from standard input
to standard output.
You can combine short flags, so `-v -4' means the same as -v4 or -4v
-c --stdout --to-stdout
Write output on standard output.
-d --decompress --uncompress
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.
The complement to -d: force compression, regardless of the
Check integrity of the specified file(s), but don't decompress
them. This really performs a trial decompression and throws
away the result.
Force overwrite of output files. Normally, bzip2 will not over-
write 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
Keep (don't delete) input files during compression or decompression.
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 lim-
its 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 MAN-
Suppress non-essential warning messages. Messages pertaining to
I/O errors and other critical events will not be suppressed.
Verbose mode -- show the compression ratio for each file pro-
cessed. Further -v's increase the verbosity level, spewing out
lots of information which is primarily of interest for diagnos-
-L --license -V --version
Display the software version, license terms and conditions.
-1 (or --fast) to -9 (or --best)
Set the block size to 100 k, 200 k .. 900 k when compressing.
Has no effect when decompressing. See MEMORY MANAGEMENT below.
The --fast and --best aliases are primarily for GNU gzip compat-
ibility. In particular, --fast doesn't make things signifi-
cantly faster. And --best merely selects the default behaviour.
-- Treats all subsequent arguments as file names, even if they
start with a dash. This is so you can handle files with names
beginning with a dash, for example: bzip2 -- -myfilename.
These flags are redundant in versions 0.9.5 and above. They
provided some coarse control over the behaviour of the sorting
algorithm in earlier versions, which was sometimes useful.
0.9.5 and above have an improved algorithm which renders these
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
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 )
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
-5 4400k 2100k 1350k 845160
-6 5200k 2500k 1600k 838626
-7 6100k 2900k 1850k 834096
-8 6800k 3300k 2100k 828642
-9 7600k 3700k 2350k 828642
RECOVERING DATA FROM DAMAGED FILES
bzip2 compresses files in blocks, usually 900kbytes long. Each block is handled
independently. If a media or transmission error causes a multi-block .bz2 file
to become damaged, it may be possible to recover data from the undamaged blocks
in the file.
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 decompress those which are
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
compressing with a smaller block size.
The sorting phase of compression gathers together similar strings in the file.
Because of this, files containing very long runs of repeated symbols, like
"aabaabaabaab ..." (repeated several hundred times) may compress 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 previous 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.
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 1.0.4 of bzip2. Compressed data created by
this version is entirely forwards 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 1.0.3,
but with the following exception: 0.9.0 and above can correctly decompress
multiple concatenated 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.