GZIPST
GZIP(1) USER COMMANDS GZIP(1)
NAME
gzip, gunzip, zcat - compress or expand files
SYNOPSIS
gzip [ -cdfhLrtvV19 ] [ name ... ]
gunzip [ -cfhLrtvV ] [ name ... ]
zcat [ -hLV ] [ name ... ]
DESCRIPTION
Gzip reduces the size of the named files using Lempel-Ziv
coding (LZ77). Whenever possible, each file is replaced by
one with the 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 symbolic links.
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 or .Z and which
begins with the correct magic number with an uncompressed
file without the original extension. gunzip is able to
extract files compressed with old versions of compress (3.0
and above) and pkzip files which contain a single member
compressed with the deflation algorithm. gunzip chooses
automatically the appropriate extraction algorithm, depend-
ing on the compression method. The uncompressed file will
have the mode, ownership and timestamps of the compressed
file.
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).
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
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removal of the damaged member. Better compression can be
usually obtained if all members are decompressed then
recompressed in a single step.
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 file has multiple links
or the corresponding .z file already exists. If -f is
not given, and when not running in the background, gzip
prompts to verify whether an existing .z file should be
overwritten.
-h --help
Display a help screen.
-L --license
Display the gzip license.
-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.
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SEE ALSO
pack(1), compact(1), compress(1), zip(1)
DIAGNOSTICS
Exit status is normally 0; if an error occurs, exit status
is 1.
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: 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.
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, (e.g. a symbolic link, socket, FIFO, device
file), it is left unaltered.
-- has xx other links: unchanged
The input file has links; it is left unchanged. See
ln(1) for more information. Use the -f flag to force
compression of multiply-linked files.
BUGS
The .z extension is already used by pack(1).
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