Bzip2 linux 命令说明

Bzip2  linux 命令说明

参考资料

l         《鸟哥的 Linux 私房菜》之 “”

l         《Linux 基础教程(1) 操作系统基础》第14章

 

简介

 

[root@linux ~]# bzip2 [-cdz] 檔名

[root@linux ~]# bzcat 檔名.bz2

参数：

-c  ：将压缩的过程产生的数据输出到屏幕上！

-d  ：解压缩的参数

-z  ：压缩的参数

-#  ：与 gzip 同样的，都是在计算压缩比的参数， -9 最佳， -1 最快！

范例：

范例一：将刚刚的 /tmp/man.config 以 bzip2 压缩

[root@linux tmp]# bzip2 -z man.config

# 此时 man.config 会变成 man.config.bz2 ！

 

范例二：将范例一的档案内容读出来！

[root@linux tmp]# bzcat man.config.bz2

# 此时屏幕上会显示 man.config.bz2 解压缩之后的档案内容！！

 

范例三：将范例一的档案解压缩

[root@linux tmp]# bzip2 -d man.config.bz2

 

范例四：将范例三解开的 man.config 用最佳的压缩比压缩，并保留原本的档案

[root@linux tmp]# bzip2 -9 -c man.config > man.config.bz2

 

使用 compress 附档名自动建立为 .Z ，使用 gzip 附档名自动建立为 .gz 。这里的 bzip2 则是自动的将附档名建置为 .bz2 啰！所以当我们使用具有压缩功能的 bzip2 -z 时，那么刚刚的 man.config 就会自动的变成了 man.config.bz2 这个档名啰！

好了，那么如果我想要读取这个档案的内容呢？ 是否一定要解开？当然不需要啰！可以使用简便的 bzcat 这个指令来读取内容即可！例如上面的例子中， 我们可以使用 bzcat man.config.bz2 来读取数据而不需要解开！此外，当您要解开一个压缩档时， 这个档案的名称为 .bz, .bz2, .tbz, .tbz2 等等，那么就可以尝试使用 bzip2 来解看看啦！当然啰，也可以使用 bunzip2 这个指令来取代 bzip2 -d 啰。

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 ver-

       sion of itself, with the name "original_name.bz2".  Each compressed file has the same  modification  date,  permis-

       sions,  and,  when  possible,  ownership  as  the corresponding original, so that these properties can be correctly

       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 therefore point-

       less.

 

       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 com-

       pressed 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 stream can be

       decompressed correctly only by bzip2 version 0.9.0 or later.  Earlier versions of bzip2 will stop after decompress-

       ing 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  hundred  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 original.  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  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 decom-

              pression 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 verbosity

              level, spewing out lots of information which is primarily of interest for diagnostic 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  MEM-

              ORY  MANAGEMENT below.  The --fast and --best aliases are primarily for GNU gzip compatibility.  In particu-

              lar, --fast doesn’t make things significantly 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 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 impor-

       tant 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 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 >  recov-

       ered_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.

 

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 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 1.0.2 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 correctly decompress multiple concatenated compressed files.

       0.1pl2 cannot do this; it will stop after decompressing just the first file in the stream.

 

    

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