Skip site navigation (1)Skip section navigation (2)

FreeBSD Man Pages

Man Page or Keyword Search:
Man Architecture
Apropos Keyword Search (all sections) Output format
home | help
bzip2(1)							      bzip2(1)

NAME
       bzip2, bunzip2 -	a block-sorting	file compressor, v1.0.2
       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 compressors, 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 correctly restored at	decom-
       pression	 time.	File name handling is naive in the sense that there is
       no mechanism for	preserving original file  names,  permissions,	owner-
       ships  or dates in filesystems which lack these concepts, or have seri-
       ous 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
       therefore 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  concate-
       nated 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.  Com-
       pression	of multiple files in this manner generates a stream containing
       multiple	 compressed file representations.  Such	a stream can be	decom-
       pressed correctly only by bzip2 version 0.9.0 or	later.	 Earlier  ver-
       sions  of  bzip2	 will  stop  after decompressing the first file	in the
       stream.

       bzcat (or bzip2 -dc) decompresses all specified files to	 the  standard
       output.

       bzip2  will  read  arguments  from  the environment variables BZIP2 and
       BZIP, in	that order, and	will process them before  any  arguments  read
       from  the  command line.	 This gives a convenient way to	supply default
       arguments.

       Compression is  always  performed,  even	 if  the  compressed  file  is
       slightly	 larger	 than the original.  Files of less than	about one hun-
       dred bytes tend to get larger, since the	compression  mechanism	has  a
       constant	 overhead  in  the region of 50	bytes.	Random data (including
       the output of most file compressors) is coded at	about  8.05  bits  per
       byte, giving an expansion of around 0.5%.

       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	origi-
       nal.   This  guards  against  corruption	 of  the  compressed data, and
       against undetected  bugs	 in  bzip2  (hopefully	very  unlikely).   The
       chances	of  data corruption going undetected is	microscopic, 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 some-
       thing 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 com-
       pressed 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 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
	      behaves.

       -k --keep
	      Keep (don't delete) input	files during compression or decompres-
	      sion.

       -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 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-
	      AGEMENT 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  pro-
	      cessed.	Further	-v's increase the verbosity level, spewing out
	      lots of information which	is primarily of	interest for  diagnos-
	      tic purposes.

       -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.

       --repetitive-fast --repetitive-best
	      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
	      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  compres-
       sion  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
	    -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 rea-
       sonable certainty.  Each	block also carries its own 32-bit CRC, so dam-
       aged 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 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 dam-
       aged single-block  files,  since	 a damaged  block  cannot   be	recov-
       ered.   If  you	wish to	minimise any potential data loss through media
       or  transmission	errors,	you might consider compressing with a  smaller
       block size.

PERFORMANCE NOTES
       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	deter-
       mined  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 improve-
       ments.  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 1.0.2 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
       and 1.0.1, but with the following exception: 0.9.0 and above  can  cor-
       rectly  decompress multiple concatenated	compressed files.  0.1pl2 can-
       not do this; it will stop after decompressing just the  first  file  in
       the stream.

       bzip2recover versions prior to this one,	1.0.2, used 32-bit integers to
       represent bit positions in compressed files, so	it  could  not	handle
       compressed files	more than 512 megabytes	long.  Version 1.0.2 and above
       uses 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.

AUTHOR
       Julian Seward, jseward@acm.org.

       http://sources.redhat.com/bzip2

       The ideas embodied in bzip2 are due to (at least) the following people:
       Michael	Burrows	 and  David Wheeler (for the block sorting transforma-
       tion), David Wheeler (again, for	the Huffman coder), Peter Fenwick (for
       the  structured	coding	model  in  the original	bzip, and many refine-
       ments), and Alistair Moffat, Radford  Neal  and	Ian  Witten  (for  the
       arithmetic  coder  in the original bzip).  I am much indebted for their
       help, support and advice.  See the manual in  the  source  distribution
       for pointers to sources of documentation.  Christian von	Roques encour-
       aged me to look for faster sorting algorithms, so as to speed  up  com-
       pression.  Bela Lubkin encouraged me to improve the worst-case compres-
       sion performance.  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)

NAME | SYNOPSIS | DESCRIPTION | OPTIONS | MEMORY MANAGEMENT | RECOVERING DATA FROM DAMAGED FILES | PERFORMANCE NOTES | CAVEATS | AUTHOR

Want to link to this manual page? Use this URL:
<https://www.freebsd.org/cgi/man.cgi?query=bzip2&sektion=1&manpath=FreeBSD+4.8-RELEASE>

home | help