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Compression

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Introduction

While fist widely used compression program was created for Unix (tar and compress) the field was stale and  it was DOS which provided strong impetus for the development of sophisticated compression programs. Many talented authors competed with each other until the dust settles and winners emerged.

The first widely popular DOS compression program was pkzip by late Phil Katz who create the company PKWARE.  Starting from pkzip 2.0 (released in 1993) it used  so called "deflating" a lossless data compression algorithm based on a combination of the LZ77 algorithm and Huffman coding. The resulting file format has since become ubiquitous in DOS and later Windows as well as on BBS and later the Internet -- almost all files with the .ZIP  (or .zip) extension are in PKZIP 2.x format. Utilities to read and write these files are available on all common platforms. It was later specified in RFC 1951  and several OSes like Windows 2000 and XP are able to work with such files natively. On April 24, 2007, PKWARE announced the release of SecureZIP Standard Version 11 as freeware, available on www.securezip.com. Competing programs included Rahul Dhesi's ZOO, Dean W. Cooper's DWC, and LHarc by Haruhiko Okomura and Haruyasu Yoshizaki. Gzip is an attempt to replicate part of functionality of pkzip in Unix environment. gzip  uses Lempel-Ziv coding (LZ77).

GZIP competes with bzip2 (slower and more CPU hungry,  but with better compression), xz, Info zip  package (zip and unzip) and rar.  Gzip is weaker as the for compression ratio but has other advantages like high speed and wide availability. It  is still it is more or less adequate and despite being obsolete from the compression ration standpoint is widely used.

By default applying gzip to the file leads to replacing the original file fo the archive with the extension .gz. it keeps while keeping the same ownership modes, access and modification times. If no files are specified or if a file name is "-", the standard input is compressed to the standard output. gzip  will only attempt to compress regular files. In particular, it will ignore symbolic links.

If the new file name is too long for its file system, gzip  truncates it. gzip  attempts to truncate only the parts of the file name longer than 3 characters. (A part is delimited by dots.) If the name consists of small parts only, the longest parts are truncated. For example, if file names are limited to 14 characters, gzip.msdos.exe is compressed to gzi.msd.exe.gz. Names are not truncated on systems which do not have a limit on file name length.

By default, gzip  keeps the original file name and timestamp in the compressed file. These are used when decompressing the file with the `-N' option. This is useful when the compressed file name was truncated or when the time stamp was not preserved after a file transfer. However, due to limitations in the current gzip  file format, fractional seconds are discarded. Also, time stamps must fall within the range 1970-01-01 00:00:00 through 2106-02-07 06:28:15 UTC, and hosts whose operating systems use 32-bit time stamps are further restricted to time stamps no later than 2038-01-19 03:14:07 UTC. The upper bounds assume the typical case where leap seconds are ignored.

Compressed files can be restored to their original form using `gzip -d' or gunzip  or zcat. If the original name saved in the compressed file is not suitable for its file system, a new name is constructed from the original one to make it legal.

gunzip  takes a list of files on its command line and replaces each file whose name ends with `.gz', `.z', `.Z', `-gz', `-z' or `_z' and which begins with the correct magic number with an uncompressed file without the original extension. gunzip  also recognizes the special extensions `.tgz' and `.taz' as shorthands for `.tar.gz' and `.tar.Z' respectively. When compressing, gzip  uses the `.tgz' extension if necessary instead of truncating a file with a `.tar' extension.

gunzip  can currently decompress files created by gzip, zip, compress  or pack. The detection of the input format is automatic. When using the first two formats, gunzip  checks a 32 bit CRC (cyclic redundancy check). For pack, gunzip  checks the uncompressed length. The compress  format was not designed to allow consistency checks. However gunzip  is sometimes able to detect a bad `.Z' file. If you get an error when uncompressing a `.Z' file, do not assume that the `.Z' file is correct simply because the standard uncompress  does not complain. This generally means that the standard uncompress  does not check its input, and happily generates garbage output. The SCO `compress -H' format (lzh  compression method) does not include a CRC but also allows some consistency checks.

Files created by zip  can be uncompressed by gzip  only if they have a single member compressed with the 'deflation' method. This feature is only intended to help conversion of tar.zip  files to the tar.gz  format. To extract a zip  file with a single member, use a command like `gunzip <foo.zip' or `gunzip -S .zip foo.zip'. To extract zip  files with several members, use unzip  instead of gunzip.

zcat  is identical to `gunzip -c'. zcat  uncompresses either a list of files on the command line or its standard input and writes the uncompressed data on standard output. zcat  will uncompress files that have the correct magic number whether they have a `.gz' suffix or not.

gzip  uses the Lempel-Ziv algorithm used in zip  and PKZIP. The amount of compression obtained depends on the size of the input and the distribution of common substrings. Typically, text such as source code or English is reduced by 60-70%. Compression is generally much better than that achieved by LZW (as used in compress), Huffman coding (as used in pack), or adaptive Huffman coding (compact).

Compression is always performed, even if the compressed file is slightly larger than the original. The worst case expansion is a few bytes for the gzip  file header, plus 5 bytes every 32K block, or an expansion ratio of 0.015% for large files. Note that the actual number of used disk blocks almost never increases. gzip  normally preserves the mode, ownership and time stamps of files when compressing or decompressing.

The gzip  file format is specified in P. Deutsch, gzip file format specification version 4.3, Internet RFC 1952 (May 1996). The zip  deflation format is specified in P. Deutsch, deflate Compressed Data Format Specification version 1.3, Internet RFC 1951 (May 1996).

Examples

 

Here are some realistic examples of running gzip.

Compress the ISO content into gz file:

gzip -rc /mnt > /tmp/iso.gz
You can time this operation is the speed of comression of ISO can serve as a poor man test of performace of the computer
time gzip -rc /mnt > /tmp/iso.gz 

From 11 Simple Gzip Examples RootUsers

 

This is the output of the command `gzip -h':

     gzip version-number
     usage: gzip [-cdfhlLnNrtvV19] [-S suffix] [file ...]
      -c --stdout      write on standard output, keep original files unchanged
      -d --decompress  decompress
      -f --force       force overwrite of output file and compress links
      -h --help        give this help
      -l --list        list compressed file contents
      -L --license     display software license
      -n --no-name     do not save or restore the original name and time stamp
      -N --name        save or restore the original name and time stamp
      -q --quiet       suppress all warnings
      -r --recursive   operate recursively on directories
      -S .suf  --suffix .suf     use suffix .suf on compressed files
      -t --test        test compressed file integrity
      -v --verbose     verbose mode
      -V --version     display version number
      -1 --fast        compress faster
      -9 --best        compress better
      file...          files to (de)compress. If none given, use standard input.
     Report bugs to <bug-gzip@gnu.org>.

This is the output of the command `gzip -v texinfo.tex':

     texinfo.tex:             69.7% -- replaced with texinfo.tex.gz

The following command will find all gzip  files in the current directory and subdirectories, and extract them in place without destroying the original:

     find . -name '*.gz' -print | sed 's/^\(.*\)[.]gz$/gunzip < "&" > "\1"/' | sh

The format for running the gzip  program is:

     gzip option ...

gzip  supports the following options:

`--stdout'
`--to-stdout'
`-c'
Write output on standard output; keep original files unchanged. If there are several input files, the output consists of a sequence of independently compressed members. To obtain better compression, concatenate all input files before compressing them.
 
`--decompress'
`--uncompress'
`-d'
Decompress.
 
`--force'
`-f'
Force compression or decompression even if the file has multiple links or the corresponding file already exists, or if the compressed data is read from or written to a terminal. If the input data is not in a format recognized by gzip, and if the option `--stdout' is also given, copy the input data without change to the standard output: let zcat  behave as cat. If `-f' is not given, and when not running in the background, gzip  prompts to verify whether an existing file should be overwritten.
 
`--help'
`-h'
Print an informative help message describing the options then quit.
 
`--list'
`-l'
For each compressed file, list the following fields:
          compressed size: size of the compressed file
          uncompressed size: size of the uncompressed file
          ratio: compression ratio (0.0% if unknown)
          uncompressed_name: name of the uncompressed file
     

The uncompressed size is given as `-1' for files not in gzip  format, such as compressed `.Z' files. To get the uncompressed size for such a file, you can use:

          zcat file.Z | wc -c
     

In combination with the `--verbose' option, the following fields are also displayed:

          method: compression method (deflate,compress,lzh,pack)
          crc: the 32-bit CRC of the uncompressed data
          date & time: time stamp for the uncompressed file
     

The crc is given as ffffffff for a file not in gzip format.

With `--verbose', the size totals and compression ratio for all files is also displayed, unless some sizes are unknown. With `--quiet', the title and totals lines are not displayed.

The gzip  format represents the input size modulo 2^32, so the uncompressed size and compression ratio are listed incorrectly for uncompressed files 4 GB and larger. To work around this problem, you can use the following command to discover a large uncompressed file's true size:

          zcat file.gz | wc -c
     

 
`--license'
`-L'
Display the gzip  license then quit.
 
`--no-name'
`-n'
When compressing, do not save the original file name and time stamp by default. (The original name is always saved if the name had to be truncated.) When decompressing, do not restore the original file name if present (remove only the gzip  suffix from the compressed file name) and do not restore the original time stamp if present (copy it from the compressed file). This option is the default when decompressing.
 
`--name'
`-N'
When compressing, always save the original file name and time stamp; this is the default. When decompressing, restore the original file name and time stamp if present. This option is useful on systems which have a limit on file name length or when the time stamp has been lost after a file transfer.
 
`--quiet'
`-q'
Suppress all warning messages.
 
`--recursive'
`-r'
Travel the directory structure recursively. If any of the file names specified on the command line are directories, gzip  will descend into the directory and compress all the files it finds there (or decompress them in the case of gunzip).
 
`--suffix suf'
`-S suf'
Use suffix `suf' instead of `.gz'. Any suffix can be given, but suffixes other than `.z' and `.gz' should be avoided to avoid confusion when files are transferred to other systems. A null suffix forces gunzip to try decompression on all given files regardless of suffix, as in:
          gunzip -S "" *        (*.* for MSDOS)
     

Previous versions of gzip used the `.z' suffix. This was changed to avoid a conflict with pack.
 

`--test'
`-t'
Test. Check the compressed file integrity.
 
`--verbose'
`-v'
Verbose. Display the name and percentage reduction for each file compressed.
 
`--version'
`-V'
Version. Display the version number and compilation options, then quit.
 
`--fast'
`--best'
`-n'
Regulate the speed of compression using the specified digit n, where `-1' or `--fast' indicates the fastest compression method (less compression) and `--best' or `-9' indicates the slowest compression method (optimal compression). The default compression level is `-6' (that is, biased towards high compression at expense of speed).

Multiple compressed files can be concatenated. In this case, gunzip  will extract all members at once. If one member is damaged, other members might still be recovered after removal of the damaged member. Better compression can be usually obtained if all members are decompressed and then recompressed in a single step.

This is an example of concatenating gzip  files:

     gzip -c file1  > foo.gz
     gzip -c file2 >> foo.gz

Then

     gunzip -c foo

is equivalent to

     cat file1 file2

In case of damage to one member of a `.gz' file, other members can still be recovered (if the damaged member is removed). However, you can get better compression by compressing all members at once:

     cat file1 file2 | gzip > foo.gz

compresses better than

     gzip -c file1 file2 > foo.gz

If you want to recompress concatenated files to get better compression, do:

     zcat old.gz | gzip > new.gz

If a compressed file consists of several members, the uncompressed size and CRC reported by the `--list' option applies to the last member only. If you need the uncompressed size for all members, you can use:

     zcat file.gz | wc -c

If you wish to create a single archive file with multiple members so that members can later be extracted independently, use an archiver such as tar  or zip. GNU tar  supports the `-z' option to invoke gzip  transparently. gzip  is designed as a complement to tar, not as a replacement.

The environment variable GZIP  can hold a set of default options for gzip. These options are interpreted first and can be overwritten by explicit command line parameters. For example:

     for sh:    GZIP="-8v --name"; export GZIP
     for csh:   setenv GZIP "-8v --name"
     for MSDOS: set GZIP=-8v --name

When writing compressed data to a tape, it is generally necessary to pad the output with zeroes up to a block boundary. When the data is read and the whole block is passed to gunzip  for decompression, gunzip  detects that there is extra trailing garbage after the compressed data and emits a warning by default if the garbage contains nonzero bytes. You have to use the `--quiet' option to suppress the warning. This option can be set in the GZIP  environment variable, as in:

     for sh:    GZIP="-q"  tar -xfz --block-compress /dev/rst0
     for csh:   (setenv GZIP "-q"; tar -xfz --block-compress /dev/rst0)

In the above example, gzip  is invoked implicitly by the `-z' option of GNU tar. Make sure that the same block size (`-b' option of tar) is used for reading and writing compressed data on tapes. (This example assumes you are using the GNU version of tar.)



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