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- 1 GZIP
- NAME
- gzip, gunzip, zcat - compress or expand files
-
- SYNOPSIS
- gzip [ -cdfhLrtvV19 ] [ name ... ]
- gunzip [ -cfhLrtvV ] [ name ... ]
- zcat [ -hLV ] [ name ... ]
-
- 2 DESCRIPTION
- Gzip reduces the size of the named files using Lempel-Ziv
- coding (LZ77). Whenever possible, each file is replaced by
- one with an extension *-z, while keeping the same ownership
- modes, access and modification times. If no files are
- specified, the standard input is compressed to the standard
- output. If the new file name is too long, gzip truncates it
- and keeps the original file name in the compressed file.
- Gzip will only attempt to compress regular files. In par-
- ticular, it will ignore devices.
-
- Compressed files can be restored to their original form
- using gzip -d or gunzip or zcat.
-
- gunzip takes a list of files on its command line and
- replaces each file whose name ends with -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 shorthand for .tar-z.
-
- 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. For pack, gunzip checks the uncompressed
- length. The compress format was not designed to allow con-
- sistency 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 gen-
- erally means that the standard uncompress does not check its
- input, and happily generates garbage output.
-
- 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.z format. 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 -z 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. Typi-
- cally, 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 expan-
- sion 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. gzip preserves the mode, ownership and timestamps of
- files when compressing or decompressing.
-
- 2 OPTIONS
- -c --stdout
- Write output on standard output; keep original files
- unchanged. If there are several input files, the out-
- put consists of a sequence of independently compressed
- members. To obtain better compression, concatenate all
- input files before compressing them.
-
- -d --decompress
- Decompress.
-
- -f --force
- Force compression even if the corresponding file already
- exists. If -f is not given, and when not running in the
- background, gzip prompts to verify whether an existing
- file should be overwritten.
-
- -h --help
- Display a help screen.
-
- -L --license
- Display the gzip license.
-
- -q --quiet
- Suppress all warnings.
-
- -r --recurse
- Travel the directory structure recursively. If any of
- the file names specified on the command line are direc-
- tories, gzip will descend into the directory and
- compress all the files it finds there (or decompress
- them in the case of gunzip ).
-
- -t --test
- Test. Check the compressed file integrity.
-
- -v --verbose
- Verbose. Display the name and percentage reduction for
- each file compressed.
-
- -V --version
- Version. Display the version number and compilation
- options.
-
- -# --fast --best
- Regulate the speed of compression using the specified
- digit #, where -1 or --fast indicates the fastest
- compression method (less compression) and -9 or --best
- indicates the slowest compression method (optimal
- compression). The default compression level is -5.
-
- 2 ENVIRONMENT
- The environment variable GZIP_OPT 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:
- GZIP_OPT == "-8 -v"
-
- 2 SEE ALSO
- compress, zip, unzip
-
- 2 DIAGNOSTICS
-
- Usage: gzip [-cdfhLrtvV19] [file ...]
- Invalid options were specified on the command line.
- file: not in gzip format
- The file specified to gunzip has not been
- compressed.
- file: Corrupt input. Use zcat to recover some data.
- The compressed file has been damaged. The data up to
- the point of failure can be recovered using
- define /user sys$output file.recover
- zcat file
- file: compressed with xx bits, can only handle yy bits
- File was compressed (using LZW) by a program that
- could deal with more bits than the decompress code
- on this machine. Recompress the file with gzip,
- which compresses better and uses less memory.
- file: already has z suffix -- no change
- The file is assumed to be already compressed.
- Rename the file and try again or use zcat.
- file already exists; do you wish to overwrite (y or n)?
- Respond "y" if you want the output file to be
- replaced; "n" if not.
- gunzip: corrupt input
- A SIGSEGV violation was detected which usually means
- that the input file has been corrupted.
- xx.x%
- Percentage of the input saved by compression.
- (Relevant only for -v.)
- -- not a regular file or directory: ignored
- When the input file is not a regular file or direc-
- tory, it is left unaltered.
-
- 2 CAVEATS
- On VMS:
- - upper case options need quotes: gzip "-V".
- - restoration of timestamps and version numbers is not supported
- - If a compressed file already exists, gzip -f overwrites it, it
- does not create a new version.
- - wildcards in file names are not supported.
- - multi-part gzip files are not supported.
- - gunzip does not preserver the input file format. You can use a
- separate utility to restore the original format.
- - gunzip and zcat can be used only if you have created the
- links to gzip as documented in makegzip.com. Otherwise
- you must use explicit parameters ("gzip -c" or "gzip -dc").
-
- 2 BUGS
- On VMS, files in VFC record format are not correctly handled by
- the C runtime library (the linefeed character is suppressed).
-