bzip2 manpage

Search topic Section

bzip2(1)		    General Commands Manual		      bzip2(1)

       bzip2, bunzip2 - a block-sorting file compressor, v1.0.6
       bzcat - decompresses files to stdout
       bzip2recover - recovers data from damaged bzip2 files

       bzip2 [ -cdfkqstvzVL123456789 ] [ filenames ...	]
       bunzip2 [ -fkvsVL ] [ filenames ...  ]
       bzcat [ -s ] [ filenames ...  ]
       bzip2recover filename

       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

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

       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

       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

       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.

       -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

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

       -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.

       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

	      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

       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.

       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

       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 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 above, 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 tar-
       gets, 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  May-
       beUInt64 set to be an unsigned 64-bit integer.

       Julian Seward, jsewardbzip.org.


       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.   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.