180 lines
7.6 KiB
Plaintext
180 lines
7.6 KiB
Plaintext
1 GZIP
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NAME
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gzip, gunzip, zcat - compress or expand files
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SYNOPSIS
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gzip [ -cdfhLrtvV19 ] [ name ... ]
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gunzip [ -cfhLrtvV ] [ name ... ]
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zcat [ -hLV ] [ name ... ]
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2 DESCRIPTION
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Gzip reduces the size of the named files using Lempel-Ziv
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coding (LZ77). Whenever possible, each file is replaced by
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one with an extension *-z, while keeping the same ownership
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modes, access and modification times. If no files are
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specified, the standard input is compressed to the standard
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output. If the new file name is too long, gzip truncates it
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and keeps the original file name in the compressed file.
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Gzip will only attempt to compress regular files. In par-
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ticular, it will ignore devices.
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Compressed files can be restored to their original form
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using gzip -d or gunzip or zcat.
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gunzip takes a list of files on its command line and
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replaces each file whose name ends with -z and which begins
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with the correct magic number with an uncompressed file
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without the original extension. gunzip also recognizes the
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special extensions .tgz and .taz as shorthand for .tar-z.
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gunzip can currently decompress files created by gzip, zip,
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compress or pack. The detection of the input format is
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automatic. When using the first two formats, gunzip checks
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a 32 bit CRC. For pack, gunzip checks the uncompressed
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length. The compress format was not designed to allow con-
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sistency checks. However gunzip is sometimes able to detect
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a bad .Z file. If you get an error when uncompressing a .Z
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file, do not assume that the .Z file is correct simply
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because the standard uncompress does not complain. This gen-
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erally means that the standard uncompress does not check its
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input, and happily generates garbage output.
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Files created by zip can be uncompressed by gzip only if
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they have a single member compressed with the 'deflation'
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method. This feature is only intended to help conversion of
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tar.zip files to the tar.z format. To extract zip files with
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several members, use unzip instead of gunzip.
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zcat is identical to gunzip -c. zcat uncompresses either a
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list of files on the command line or its standard input and
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writes the uncompressed data on standard output. zcat will
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uncompress files that have the correct magic number whether
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they have a -z suffix or not.
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Gzip uses the Lempel-Ziv algorithm used in zip and PKZIP.
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The amount of compression obtained depends on the size of
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the input and the distribution of common substrings. Typi-
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cally, text such as source code or English is reduced by
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60-70%. Compression is generally much better than that
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achieved by LZW (as used in compress), Huffman coding (as
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used in pack), or adaptive Huffman coding (compact).
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Compression is always performed, even if the compressed file
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is slightly larger than the original. The worst case expan-
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sion is a few bytes for the gzip file header, plus 5 bytes
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every 32K block, or an expansion ratio of 0.015% for large
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files. gzip preserves the mode, ownership and timestamps of
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files when compressing or decompressing.
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2 OPTIONS
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-c --stdout
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Write output on standard output; keep original files
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unchanged. If there are several input files, the out-
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put consists of a sequence of independently compressed
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members. To obtain better compression, concatenate all
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input files before compressing them.
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-d --decompress
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Decompress.
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-f --force
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Force compression even if the corresponding file already
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exists. If -f is not given, and when not running in the
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background, gzip prompts to verify whether an existing
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file should be overwritten.
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-h --help
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Display a help screen.
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-L --license
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Display the gzip license.
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-q --quiet
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Suppress all warnings.
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-r --recurse
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Travel the directory structure recursively. If any of
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the file names specified on the command line are direc-
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tories, gzip will descend into the directory and
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compress all the files it finds there (or decompress
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them in the case of gunzip ).
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-t --test
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Test. Check the compressed file integrity.
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-v --verbose
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Verbose. Display the name and percentage reduction for
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each file compressed.
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-V --version
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Version. Display the version number and compilation
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options.
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-# --fast --best
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Regulate the speed of compression using the specified
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digit #, where -1 or --fast indicates the fastest
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compression method (less compression) and -9 or --best
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indicates the slowest compression method (optimal
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compression). The default compression level is -5.
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2 ENVIRONMENT
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The environment variable GZIP_OPT can hold a set of default
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options for gzip. These options are interpreted first and
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can be overwritten by explicit command line parameters. For
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example:
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GZIP_OPT == "-8 -v"
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2 SEE ALSO
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compress, zip, unzip
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2 DIAGNOSTICS
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Usage: gzip [-cdfhLrtvV19] [file ...]
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Invalid options were specified on the command line.
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file: not in gzip format
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The file specified to gunzip has not been
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compressed.
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file: Corrupt input. Use zcat to recover some data.
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The compressed file has been damaged. The data up to
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the point of failure can be recovered using
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define /user sys$output file.recover
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zcat file
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file: compressed with xx bits, can only handle yy bits
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File was compressed (using LZW) by a program that
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could deal with more bits than the decompress code
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on this machine. Recompress the file with gzip,
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which compresses better and uses less memory.
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file: already has z suffix -- no change
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The file is assumed to be already compressed.
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Rename the file and try again or use zcat.
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file already exists; do you wish to overwrite (y or n)?
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Respond "y" if you want the output file to be
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replaced; "n" if not.
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gunzip: corrupt input
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A SIGSEGV violation was detected which usually means
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that the input file has been corrupted.
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xx.x%
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Percentage of the input saved by compression.
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(Relevant only for -v.)
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-- not a regular file or directory: ignored
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When the input file is not a regular file or direc-
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tory, it is left unaltered.
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2 CAVEATS
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On VMS:
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- upper case options need quotes: gzip "-V".
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- restoration of timestamps and version numbers is not supported
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- If a compressed file already exists, gzip -f overwrites it, it
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does not create a new version.
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- wildcards in file names are not supported.
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- multi-part gzip files are not supported.
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- gunzip does not preserver the input file format. You can use a
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separate utility to restore the original format.
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- gunzip and zcat can be used only if you have created the
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links to gzip as documented in makegzip.com. Otherwise
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you must use explicit parameters ("gzip -c" or "gzip -dc").
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2 BUGS
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On VMS, files in VFC record format are not correctly handled by
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the C runtime library (the linefeed character is suppressed).
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